# Future Circular Collider Feasibility Study Report
## Volume 1: Physics and Experiments

M. Benedikt, F. Zimmermann, B. Auchmann, W. Bartmann, J. P. Burnet, C.

Carli, A. Chancé, P. Craievich, M. Giovannozzi, C. Grojean, et al.

► To cite this version:

M. Benedikt, F. Zimmermann, B. Auchmann, W. Bartmann, J. P. Burnet, et al. Future Circular Collider Feasibility Study Report Volume 1: Physics and Experiments. Eur.Phys.J.C, 2025, 85 (12), pp.1468. 10.1140/epjc/s10052-025-15077-x. hal-05390104

HAL Id: hal-05390104

https://hal.science/hal-05390104v1

Submitted on 21 Jan 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Future Circular Collider Feasibility Study Report

## Volume 1 Physics, Experiments, Detectors

M. Benedikt¹, F. Zimmermann¹, B. Auchmann¹,², W. Bartmann¹, J. P. Burnet¹, C. Carli¹, A. Chancé³, P. Craievich², M. Giovannozzi¹, C. Grojean⁴,⁵, J. Gutleber¹, K. Hanke¹, André Henriques¹, P. Janot¹, C. Lourenço¹, M. Mangano¹, T. Otto¹, J. Poole¹, S. Rajagopalan⁶, T. Raubenheimer⁷, E. Todesco¹, L. Ulrici¹, T. Watson¹, G. Wilkinson¹,⁸, P. Azzi⁹, G. Bernardi¹⁰,¹¹,¹², A. Blondel¹⁰,¹³,¹⁴, M. Boscolo¹⁵, D. d'Enterria¹, M. Dam¹⁶, J. de Blas¹⁷, B. Francois¹, A. Freitas¹⁸, G. Ganis¹, J. Keintzel¹, M. Klute¹⁹, M. McCullough¹, P. F. Monni¹, F. Palla²⁰, E. Perez¹, M.-A. Pleier⁶, W. Riegler¹, F. Sefkow⁴, M. Selvaggi¹, A. Abada¹⁰,²¹,²², M. Abbrescia²³,²⁴, H. Abdolmaleki²⁵,²⁶, S. H. Abidi⁶, A. Abramov¹, C. Adam¹⁰,²⁷,²⁸, M. Ady¹, P. R. Adžić²⁹, I. Agapov⁴, D. Aguglia¹, I. Ahmed³⁰, M. Aiba², G. Aielli³¹,³², T. Akan³³, N. Akchurin³⁴, D. Akturk³⁵, M. Al-Thakeel¹,³⁶,³⁷, G. L. Alberghi³⁶, J. Alcaraz Maestre³⁸, M. Aleksa¹, R. Aleksan³, F. Alharthi¹⁰,²¹,³⁹, J. Alimena⁴, A. Alimenti⁴⁰, S. Alioli⁴¹,⁴², L. Alix¹,¹⁰,²⁷, B. C. Allanach⁴³, L. Allwicher⁴, A. A. Altintas⁴⁴, M. Altınlı⁴⁴,⁴⁵, M. Alviggi⁴⁶,⁴⁷, G. Ambrosio⁴⁸, Y. Amhis¹⁰,²¹,²², A. Amiri⁴⁹,⁵⁰, G. Ammirabile²⁰, T. Andeen⁵¹, K. D. J. André¹, J. Andrea¹⁰,⁵²,⁵³, A. Andreazza⁵⁴,⁵⁵, M. Andreini¹, T. Andriollo⁵⁶, L. Angel⁵⁷, M. Angelucci¹⁵, S. Antusch⁵⁸, M. N. Anwar²³,⁵⁹, L. Apolinário⁶⁰, G. Apollinari⁴⁸, R. B. Appleby⁶¹,⁶², A. Apresyan⁴⁸, Aram Apyan⁶³, Armen Apyan⁶⁴, A. Arbey¹⁰,⁶⁵,⁶⁶, B. Argiento⁴⁶,⁴⁷, V. Ari⁶⁷, S. Arias⁶⁸, B. Arias Alonso¹, O. Arnaez¹⁰,²⁷,²⁸, R. Arnaldi⁶⁹, F. Arneodo⁷⁰, H. Arnold⁷¹, P. Arrutia Sota¹, M. E. Ascioti⁷²,⁷³, K. A. Assamagan⁶, S. Aumiller⁷⁴, G. Aydın⁷⁵, K. Azizi⁴⁹,⁷⁶, N. Bacchetta⁹, A. Bacci⁵⁴, B. Bai⁷⁷, Y. Bai⁷⁸, L. Balconi⁵⁴,⁵⁵, G. Baldinelli⁷²,⁷³, B. Balhan¹, A. H. Ball¹,⁷⁹, A. Ballarino¹, S. Banerjee⁸⁰, S. Banik²,⁸¹, D. P. Barber⁴,⁸², M. B. Barbero¹⁰,⁸³,⁸⁴, D. Barducci²⁰,⁸⁵, D. Barna⁸⁶, G. G. Barnaföldi⁸⁶, M. J. Barnes¹, A. J. Barr⁸, R. Bartek⁸⁷, H. Bartosik¹, S. A. Bass⁸⁸, U. Bassler¹⁰,⁸⁹,⁹⁰, M. J. Basso⁹¹,⁹², A. Bastianin⁵⁵,⁹³, P. Bataillard⁹⁴, M. Battistin¹, J. Bauche¹, L. Baudin¹, J. Baudot¹⁰,⁵²,⁵³, B. Baudouy³, L. Bauerdick⁴⁸, C. Bayındır⁹⁵,⁹⁶, H. P. Beck⁹⁷, F. Bedeschi²⁰, C. Bee⁷¹, M. Begel⁶, M. Behtouei¹⁵, L. Bellagamba³⁶, N. Bellegarde¹, E. Belli¹,⁹⁸, E. Bellingeri⁹⁹, S. Belomestnykh⁴⁸, A. D. Benaglia⁴¹, G. Bencivenni¹⁵, J. Bendavid¹, M. Benmergui¹⁰⁰, M. Benoit¹⁰¹, D. Benvenuti¹,²⁰, T. Bergauer¹⁰², N. Bernachot¹⁰³, J. Bernardi¹⁰⁴, Q. Berthet¹⁴,¹⁰⁵,¹⁰⁶, S. Bertoni¹⁰⁷, C. Bertulani¹⁰⁸, M. I. Besana², A. Besson¹⁰,⁵²,⁵³, M. Bettelini¹⁰⁹, S. Bettoni², S. Beuvier¹¹⁰,*, P. C. Bhat⁴⁸, S. Bhattacharya¹¹¹, J. Bhom¹¹², M. E. Biagini¹⁵, A. Bibet-Chevalier¹¹³, M. Bicrel¹¹⁴, M. Biglietti¹¹⁵, G. M. Bilei⁷², B. Bilki¹¹⁶,¹¹⁷, K. Bisgaard Christensen¹, T. Biswas¹¹⁸, F. Blanc¹¹⁹, F. Blekman⁴,¹²⁰,¹²¹, J. Blümlein⁴, D. Boccanfuso⁴⁶,¹²², A. Bogomyagkov¹²³, P. Boillon¹¹³, P. Boivin¹⁰⁶, M. J. Boland¹²⁴, S. Bologna¹²⁵, O. Bolukbasi⁴⁴, R. Bonnet¹⁰⁷, J. Borburgh¹, F. Bordry¹, P. Borges de Sousa¹, G. Borghello¹, L. Borriello⁴⁶, D. Bortoletto⁸, L. Bottura¹, G. Boudoul¹⁰,⁶⁵,⁶⁶, V. Boudry¹⁰,⁸⁹,⁹⁰, R. Boughezal¹²⁶, D. Bourilkov¹²⁷, M. Boyd⁹¹,¹²⁸, D. Boye⁶, G. Bozzi¹²⁹,¹³⁰, V. Braccini⁹⁹, C. Bracco¹, B. Bradu¹, A. Braghieri¹³¹, S. Braibant³⁶,³⁷, J. Bramante¹³², G. C. Branco¹³³, R. Brenner¹³⁴, N. Brisa¹⁰⁷, D. Britzger¹³⁵, G. Broggi¹,⁹⁸, L. Bromiley¹, E. Brost⁶, Q. Bruant³, R. Bruce¹, E. Bründermann¹⁹, L. Brunetti¹⁰,²¹,²⁸, O. Brüning¹, O. Brunner¹, X. Buffat¹, E. Bulyak¹³⁶, A. Burdyko⁵⁴,¹³⁷, H. Burkhardt¹,¹³⁸, P. N. Burrows¹³⁹, S. Busatto⁵⁴,⁹⁸, S. Buschaert⁹⁴, D. Buttazzo²⁰, A. Butterworth¹, D. Butti¹, G. Cacciapaglia¹⁴⁰,¹⁴¹,¹⁴², Y. Cai⁷, B. Caiffi¹⁴³, V. Cairo¹, O. Cakir⁶⁷, P. Calafiura¹⁴⁴, R. Calaga¹, S. Calatroni¹, D. G. Caldwell¹⁴⁵, A. Çalışkan¹⁴⁶, C. Calpini¹⁴⁷, M. Calviani¹, E. Camacho-Pérez¹⁴⁸, P. Camarri³¹,³², L. Caminada²,⁸¹, M. Campajola⁴⁶,⁴⁷, A. C. Canbay⁶⁷, K. Canderan¹, S. Candido¹, F. Canelli⁸¹, A. Canepa⁴⁸, S. Cantarella¹⁵, K. B. Cantún-Avila¹⁴⁸, L. Capriotti¹⁴⁹,¹⁵⁰, A. Caram¹⁵¹, A. Carbone⁵⁴, J. M. Carceller¹, G. Carini⁶, F. Carlier¹, C. M. Carloni Calame¹³¹, F. Carra¹, C. Cartannaz⁹⁴, S. Casenove¹, G. Catalano¹⁵², V. Cavaliere⁶, C. Cazzaniga¹⁵³, C. Cecchi⁷²,⁷³, F. G. Celiberto¹⁵⁴, M. Cepeda³⁸, F. Cerutti¹, F. Cetorelli⁴¹,⁴², G. Chachamis⁶⁰, Y. Chae⁴, F. Chagnet¹⁵⁵, I. Chaikovska¹⁰,²¹,²², M. Chalhoub⁹⁴, M. Chamizo-Llatas⁶, M. Champagne¹⁵⁶, H. Chanal¹⁰,¹⁵⁷,¹⁵⁸, G. Chapelier¹¹³, P. Charitos¹, C. Charles¹¹⁰, T. K. Charles¹⁵⁹, C. Charlot¹⁰,⁸⁹,⁹⁰, S. Chatterjee⁴, A. Chaudhuri¹⁶⁰, R. Chehab¹⁰,²¹,²², S. V. Chekanov¹⁶¹, H. Chen⁶, T. Chesne¹¹⁰, F. Chiapponi³⁶,³⁷, G. Chiarello¹⁶²,¹⁶³, M. Chiesa¹³¹, P. Chiggiato¹, Ph. Chomaz³, M. Chorowski¹⁶⁴, J. P. Chou¹⁶⁵, M. Chrzaszcz¹¹², W. Chung¹⁶⁶, S. Ciarlantini⁹,¹⁶⁷, A. Ciarma¹⁵, D. Cieri¹³⁵, A. K. Ciftci¹⁶⁸, R. Ciftci¹⁶⁹, R. Cimino¹⁵, F. Cirotto⁴⁶,⁴⁷, M. Ciuchini¹¹⁵, M. Cobal¹⁷⁰,¹⁷¹, A. Coccaro¹⁴³, R. Coelho Lopes De Sa¹⁷², J. A. Coleman-Smith¹, F. Collamati¹⁷³,
C. Colldelram¹⁷⁴, P. Collier¹, P. Collins¹, J. Collot¹⁰,¹⁷⁶,¹⁷⁶, M. Colmenero¹, L. Colnot¹⁵², G. Coloretti⁸¹,
D. Contardo¹⁰,⁶⁵,⁶⁶, E. Conte¹⁰,⁵²,⁵³, F. A. Conventi⁴⁶,¹⁷⁷, A. Cook¹, L. Cooley¹⁷⁸,¹⁷⁹, A. S. Cornell¹⁸⁰, C. Cornella¹,
G. Cornette¹¹⁰, I. Corredoira¹⁸¹, P. Costa Pinto¹, F. Coudere³, J. Coupard¹, S. Coussy⁹⁴, G. Crawford¹⁸²,
R. Crescenzi¹⁸³, I. Crespo Garrido¹,¹⁸⁴, T. Critchley¹,¹⁴, A. Crivellin⁸¹, T. Croci⁷², C. Cudré¹¹⁰, G. Cummings⁴⁸,
F. Cuna²³, R. Cunningham¹, B. Curé¹, E. Curtis¹⁸⁵, M. D'Alfonso¹⁸⁶, L. D'Aloia Schwartzentruber¹⁸⁷, G. D'Amen⁶,
B. D'Anzi²³,²⁴, A. D'Avanzo⁴⁶,⁴⁷, A. D'Onofrio⁴⁶, M. D'Onofrio¹⁸⁸, M. Da Col¹⁵², M. Da Rocha Rolo⁶⁹,
C. Dachauer¹⁸⁹, B. Dağlı³⁵, A. Dainese⁹, B. Dalena³, W. Dallapiazza¹⁹⁰, H. Damerau¹, V. Dao⁷¹, A. Das¹⁹¹,
M. S. Daugaard¹, S. Dauphin¹¹³, A. David¹, T. Davídek¹⁹², G. J. Davies¹⁸⁵, J. Davighi¹, S. Dawson⁶, A. de Cosa¹⁵³,
S. De Curtis¹⁹³, N. De Filippis²³,⁵⁹, E. De Lucia¹⁵, R. De Maria¹, E. De Matteis⁵⁴, A. De Roeck¹, A. De Santis¹⁵,
A. De Vita¹,⁹,¹⁶⁷, A. Deandrea¹⁰,⁶⁵,⁶⁶, C. J. Debono¹⁹⁴, M. Deeb¹⁰⁶, M. M. Defranchis¹, J. Degens¹⁸⁸, S. Deghaye¹,
V. Del Duca¹⁵, C. L. Del Pio⁶, A. Del Vecchio⁹⁸, D. Delikaris¹, A. Dell'Acqua¹, M. Della Pietra⁴⁶,⁴⁷,
M. Delmastro¹⁰,²⁷,²⁸, L. Delprat¹, E. Delugas¹⁵², Z. Demiragli¹⁹⁵, L. Deniau¹, D. Denisov⁶, H. Denizli¹⁹⁶,
A. Denner¹⁹⁷, A. Denot¹¹³, G. Deptuch⁶, A. Desai¹⁹⁸, H. Deveci¹, A. Di Canto⁶, A. Di Ciaccio³¹,³²,
L. Di Ciaccio¹⁰,²⁷,²⁸, D. Di Croce¹,¹¹⁹, C. Di Fraia⁴⁶,⁴⁷, B. Di Micco⁴⁰,¹¹⁵, R. Di Nardo⁴⁰,¹¹⁵, T. B. Dingley⁸,
F. Djama¹⁰,⁸³,⁸⁴, F. Djurabekova¹⁹⁹, D. Dockery⁴⁸, S. Doebert¹, D. Domange¹,²⁰⁰, M. Donegà¹⁵³, U. Dosselli⁹,
H. A. Dostmann¹,²⁰¹, J. A. Dragovich⁴⁸, I. Drebot⁵⁴, M. Drewes²⁰², T. A. du Pree²⁰³, Z. Duan²⁰⁴,
C. Duarte-Galvan²⁰⁵, O. Duboc²⁰⁶, I. Dubovyk²⁰⁷, M. Duda², P. Duda¹⁶⁴, H. Duran Yildiz⁶⁷, H. Durand¹¹⁰,
P. Durand¹¹⁰, G. Durieux²⁰², Y. Dutheil¹, I. Dutta⁴⁸, J. S. Dutta²⁰⁸, S. Dutta²⁰⁹, F. Duval¹, F. Eder¹, M. Eisterer¹⁰⁴,
Z. El Bitar¹⁰,⁵²,⁵³, A. El Saied²¹⁰, M. Elisei⁵⁴, J. Ellis¹,²¹¹, W. Elmetenawee²³, J. Elmsheuser⁶, V. Daniel Elvira⁴⁸,
S. C. Eno²¹², Y. Enomoto²¹³, B. A. Erdelyi⁹,¹⁶⁷, O. E. Eruteya¹⁴,²¹⁴, M. Escobar²¹⁵, O. Etisken²¹⁶, I. Eymard¹⁴⁷,
J. Eysermans¹⁸⁶, D. Falchieri³⁶, C. Falkenberg²⁰⁶, F. Fallavollita¹,¹³⁵, A. Afalou¹,¹⁰,²¹, J. Faltova¹⁹², J. Fanini¹,
L. Fanò⁷²,⁷³, K. Fanti¹¹⁰, R. Farinelli³⁶, M. Farino¹⁶⁶, S. Farinon¹⁴³, H. Fatehi⁴⁹, J. Fatterbert¹¹⁰, A. Faure²¹⁷,
A. Faus-Golfe¹⁰,²¹,²², G. Favia¹, L. Favilla⁴⁶,¹²², W. J. Fawcett⁴³, A. Federowicz⁴⁸, L. Feligioni¹⁰,⁸³,⁸⁴, L. Felsberger¹,
Y. Feng³⁴, A. Fernández Téllez²¹⁸, R. Ferrari¹³¹, L. Ferreira¹, F. Ferro¹⁴³, M. Fiascaris¹, C. Fiorio⁵⁵, S. A. Fleury¹,
L. Florez¹⁹⁰, M. Florio⁵⁵,¹⁵², A. Fondacci⁷², B. Fontimpe²¹⁵, K. Foraz¹, R. Fortunati², M. Fouaidy¹⁰,²¹,²²,
A. Foussat¹, A. Fowler¹, J. D. Fox²¹⁹, M. Francesconi⁴⁶, R. Franqueira Ximenes¹, F. Fransesini¹⁵, A. Frasca¹,¹⁸⁸,
J. A. Frost⁸, K. Furukawa²¹³, A. Gabrielli³⁶,³⁷, A. Gaddi¹, F. Gaede⁴, A. Gallén¹³⁴, R. Galler²²⁰,²²¹, E. Gallice¹¹⁰,
E. Gallo⁴,¹²⁰, H. Gamper¹, S. Ganjour³, S. Gao⁶, A. Garand¹⁵¹, C. Garaus²⁰⁶, D. Garcia¹, R. García Alía¹,
R. García Gil²²², C. M. Garcia Jaimes¹,¹¹⁹, H. Garcia Rodrigues²,²²³, C. Garion¹, M. Garlaschè¹, D. Garnier¹⁵⁵,
M. V. Garzelli¹²⁰, S. Gascon-Shotkin¹⁰,⁶⁵,⁶⁶, M. Gasior¹, G. Gaudino⁴⁶,¹²², G. Gaudio¹³¹, V. Gaur²²⁴,
K. Gautam⁸¹,¹²¹, V. Gawas¹, T. Gehrmann⁸¹, A. Gehrmann-De Ridder⁸¹,¹⁵³, K. Geiger¹, M. Genco¹⁵², F. Gerigk¹,
H. Gerwig¹, A. Ghribi¹,¹⁰,²²⁵, P. Giacomelli³⁶, S. Giagu⁹⁸,¹⁷³, E. Gianfelice⁴⁸, S. Giappichini¹⁹, D. Gibellieri¹,²²⁶,
F. Giffoni¹⁵², G. Gil da Silveira²²⁷, S. S. Gilhardoni¹, M. Giovannetti¹⁵, T. Girardet¹¹⁰, S. Girod¹,¹¹⁰, P. Giubellino⁶⁹,
P. Giubilato⁹,¹⁶⁷, F. Giuli³¹,³², M. Giuliani¹⁰⁷, E. L. Gkougkousis¹,⁸¹, S. Glukhov²²⁸, J. Gluza²⁰⁷, B. Goddard¹,
C. Goffing¹,¹⁹, D. Goldsworthy¹, T. Golling¹⁴, R. Gonçalo⁶⁰,²²⁹, V. P. Gonçalves⁵⁷,²³⁰, T. Gonçalves Da Silva²¹⁵,
J. Gonski⁷, R. Gonzalez Suarez¹³⁴, S. Gorgi Zadeh¹, S. Gori²³¹, E. Gorini¹⁶²,²³², L. Gouskos²³³,
M. Gouzevitch¹⁰,⁶⁵,⁶⁶, E. Granados¹, F. Grancagnolo¹⁶², S. Grancagnolo¹⁶²,²³², A. Grassellino⁴⁸, A. Grau¹⁹,
E. Graverini²⁰,⁸⁵,¹¹⁹, F. G. Gravili¹⁶²,²³², H. M. Gray¹⁴⁴,²³⁴, M. Grazzini⁸¹, Mario Greco⁴⁰,¹¹⁵, Michela Greco⁶⁹,²³⁵,
A. Greljo⁵⁸, J.-L. Grenard¹, A. V. Gritsan²³⁶, R. Gröber⁹,¹⁶⁷, A. Grudiev¹, E. Gschwendtner¹, J. Gu²³⁷,
D. Guadagnoli²⁸,¹⁴⁰,²³⁸, G. Guerrieri¹, A. Guiavarch²¹⁰, G. Guillermo Canton¹,²³⁹, M. Guinchard¹,
Y. O. Günaydin²⁴⁰, K. Gurcel¹⁰⁰, L. X. Gutierrez Guerrero²⁴¹,²⁴², D. Gutiérrez Rueda¹, A. Gutiérrez-Rodríguez²⁴³,
V. Guzey¹⁹⁹,²⁴⁴, C. Haber¹⁴⁴, T. Hacheney²⁴⁵, B. Hacışahinoğlu⁴⁴, K. Hahn¹²⁶, J. Hajer¹³³, T. Hakulinen¹,
J. C. Hammersley²⁴⁶, M. Hance²³¹, J. B. Hansen¹⁶, B. Härer¹⁹, E. Hauzinger²²⁰, M. Haviernik¹⁹², B. Hegner¹,
C. Helsens¹¹⁹, Ana Henriques¹, C. Hernalsteens¹, H. Hernández-Arellano²¹⁸, R. J. Hernández-Pinto²⁰⁵,
M. A. Hernández-Ruíz²⁴³, J. Hernández-Sánchez²¹⁸, J. W. Heron¹, L. M. Herrmann¹, R. Hirosky²⁴⁷,
J. F. Hirschauer⁴⁸, J. D. Hobbs⁷¹, K. Hock⁶, S. Höhe⁴⁸, M. Hofer¹, G. Hoffstaetter⁶,²⁴⁸, W. Höfle¹,
M. Hohlmann²⁴⁹, F. Holdener²⁵⁰, B. Holzer¹, C. G. Honorato²¹⁸, H. Hoorani²⁵¹, A. Houver¹¹⁰, E. Howling¹,⁸,¹³⁹,
X. Huang⁷, F. Hug²⁵², B. Humann¹, P. Hunchak¹²⁴, Y. Husein¹, A. Hussain¹,²⁵³, G. Iadarola¹, G. Iakovidis⁶,
G. Iaselli²³,⁵⁹, P. Iengo⁴⁶, A. Ilg⁸¹, M. Iodice¹¹⁵, A. O. M. Iorio⁴⁶,⁴⁷, V. Ippolito¹⁷³, U. Iriso¹⁷⁴, J. Isaacson⁴⁸,
G. Isidori⁸¹, R. Islam²⁵⁴, A. Istepanyan¹¹⁰, S. Izquierdo Bermudez¹, V. Izzo⁴⁶, P. D. Jackson¹⁹⁸, R. Jafari¹,⁴⁹,
S. S. Jagabathuni¹,¹⁴, S. Jana²⁵⁵,²⁵⁶, C. Järmyr Eriksson¹, P. Jausserand¹⁵⁵, M. Jensen²⁵⁷, J. M. Jimenez¹,
F. R. Joaquim¹³³, O. R. Jones¹, J. Joos¹¹³, E. Jourd'huy¹⁰,²⁵⁸, E. Jourdan²¹⁵, J. M. Jowett¹,²⁵⁹, A. Jueid²⁶⁰,
A. W. Jung²⁰⁸, M. Kagan⁷, I. Kahraman⁶⁷, V. Kain¹, J. Kalinowski²⁶¹, J. F. Kamenik²⁶²,²⁶³, A. Kanso²⁶⁴, T. Kar²⁶⁵,
S. O. Kara²⁶⁶, H. Karadeniz²⁶⁷, B. Karmakar²⁰⁷, S. R. Karmarkar²⁰⁸, V. Karpati²⁶⁸, I. Karpov¹, M. Karppinen¹,
P. Karst¹⁰,⁸³,⁸⁴, S. Kartal⁴⁴, V. V. Kashikhin⁴⁸, U. Kaya⁶⁷, A. Kehagias¹,²⁶⁹, M. Kennouche¹, M. Kenzie⁴³,
M. Kerréveur-Lavaud⁵⁶, R. Kersevan¹,²⁷⁰, V. Keus¹⁹⁹,²⁷¹, H. Khanpour²⁵,²⁷²,²⁷³, V. V. Khoze²⁷⁴, V. A. Khoze²⁷⁴,
P. Kicsiny¹, R. Kieffer¹, C. Kiel¹¹⁹, J. Kieseler¹⁹, A. Kilic²⁷⁵, B. Kilminster⁸¹, S. Kim²⁷⁶, Z. Kirca²⁷⁵, M. Klein¹⁸⁸,*,
A. Klimentov⁶, V. Klyukhin¹²³,²⁷⁷, M. Knecht¹⁴⁰,²⁷⁸,²⁷⁹, B. Kniehl¹²⁰, P. Ko²⁸⁰, S. Ko¹, F. Kocak²⁷⁵, T. Koffas²⁸¹,
C. Kokkinos²⁸²,²⁸³, K. Kołodziej²⁰⁷, K. Kong²⁸⁴, P. Kontaxakis¹⁴, I. A. Koop¹²³, P. Kopciewicz¹, P. Koppenburg²⁰³,
M. Koratzinos¹,², K. Kordas²⁸⁵, A. Korsun¹⁰,²¹,²², O. Kortner¹³⁵, S. Kortner¹³⁵, B. Korzh¹⁴, T. Koseki²¹³, J. Kosse²,
P. Kostka¹,¹⁸⁸, S. Kostoglou¹, A. V. Kotwal⁸⁸, G. Kozlov¹,²⁷⁷, I. Kozsar¹, T. Kramer¹, P. Krkotić¹, H. Kroha¹³⁵,
K. Kröninger²⁴⁵, S. Kuday¹,⁶⁷, G. Kuhlmann²⁸⁶, O. Kuhlmann¹,²⁸⁷, M. Kuhn²⁸⁸, A. Kulesza²⁸⁹, M. Kumar²⁹⁰,
F. Kurian⁶, A. Kurtulus¹,¹⁵³, T. H. Kwok⁸¹, S. La Mendola¹, M. Lackner¹⁰⁴,²⁹¹, T. Ładziński¹, D. Lafarge¹, P.
Laïdouni¹, G. Lamanna¹⁰,²⁷,²⁸, N. Lamas³⁰, G. Landsberg²³³, C. Lange², D. J. Lange¹⁶⁶, A. Langner¹,
A. J. Lankford²⁹², L. Lari⁶, M. S. Larson²⁹³, K. Lasocha¹, A. Latina¹, S. Lauciani¹⁵, M. Laufenberg¹¹⁰,
G. Lavezzari¹, L. Lavezzi⁶⁹, L. Lavezzo¹, M. Le Garrec¹,¹⁰,²⁷, A. Le Jeune¹⁰⁷, Ph. Lebrun¹,²⁹⁴, Y. Léchevin¹,
A. Lechner¹, E. Lecointe¹¹⁰, J. S. H. Lee²⁹⁵, S. W. Lee²⁹⁶, S. J. Lee²⁸⁰,²⁹⁷, T. Lefevre¹, C. Leggett¹⁴⁴, T. Lehtinen²⁹⁸,
S. Leone²⁰, C. Leonidopoulos²⁹⁹, S. Leontsinis⁸¹, G. Leprince-Maillère³⁰⁰, G. Lerner¹, O. Leroy¹⁰,⁸³,⁸⁴, T. Lesiak¹¹²,
P. Levai⁸⁶, A. Leveratto⁹⁹, R. Levi¹⁵⁵, A. Li⁶, S. Li³⁰¹,³⁰², D. Liberati³⁰³, G. L. Lichtenstein⁵⁷, M. Liepe²⁴⁸,
Z. Ligeti¹⁴⁴, H. Lin³⁰⁴, S. Linda¹⁴⁷, E. Lipeles³⁰⁵, Z. Liu³⁰⁶, S. M. Liuzzo³⁰⁷, T. Loeliger²⁸⁸,
A. Loeschcke Centeno³⁰⁸, A. Lorenzetti⁸¹, C. Lorin³, R. Losito¹, M. Louka²³,³⁰⁹, M. L. Loureiro García¹⁸⁴,
I. Low¹²⁶,¹⁶¹, K. Lubonis¹⁵⁵, M. T. Lucchini⁴¹,⁴², V. Lukashenko⁸¹, G. Luminati¹⁵, A. J. G. Lunt¹,³¹⁰,
A. Lusiani²⁰,³¹¹, M. Luzum³¹², H. Ma⁶, A. Maas³¹³, E. Macchia¹,⁹⁸,¹⁷⁳, A. Macchiolo⁸¹, G. E. Machinet²⁶⁴,
R. Madar¹⁰,¹⁵⁷,¹⁵⁸, T. Madlener⁴, C. Madrid³⁴, A. Magalotti⁴⁰, M. Maggiora⁶⁹,²³⁵, A.-M. Magnan¹⁸⁵,
M. A. Mahmoud³¹⁴, Y. Mahmoud³¹⁵,³¹⁶, F. Mahmoudi¹,¹⁰,⁶⁵, H. Mainaud Durand¹, J. Maitre¹¹³, Y. Makhloufi¹⁴,
B. Malaescu¹⁰,¹³,³¹⁷, A. Malagoli⁹⁹, C. H. Malan¹¹³, M. Malekhosseini⁴⁹, A. Maloizel¹,¹¹,¹², S. Malvezzi⁴¹,
A. Malzac¹⁵¹, G. Manco¹³¹, L. S. Mandacarú Guerra¹⁶⁶, P. Manfrinetti⁹⁹,³¹⁸, E. Manoni⁷², J. Mans³⁰⁶,
L. Mantani³¹⁹, S. Manzoni¹, L. Marafatto¹⁷⁰, C. Marcel¹, T. Marcel¹¹⁴, R. Marchevski¹¹⁹, G. Marchiori¹⁰,¹¹,¹²,
F. Mariani⁵⁴,⁹⁸, V. Mariani⁷²,⁷³, S. Marin¹, C. Marinas³¹⁹, V. Marinozzi⁴⁸, S. Mariotto⁵⁴,⁵⁵, C. Marquis¹¹⁰,
J. Martelain³²⁰, G. Martelli⁷²,⁷³, A. Martens¹⁰,²¹,²², I. Martin-Melero¹, V. I. Martinez Outschoorn¹⁷²,
F. Martinez²¹⁸, C. M. Jardim³⁸, L. Marzola³²¹,³²², S. Masciocchi²⁵⁹,²⁶⁵, A. Mashal²⁵, A. Masi¹, I. Masina¹⁴⁹,¹⁵⁰,
P. Mastrapasqua²⁰², V. Mateu³²³, S. Mattiazzo⁹,¹⁶⁷, M. Maugis¹⁰⁷, V. Maura²¹¹, D. Mauree¹⁴⁷,
G. H. I. Maury-Cuna³²⁴, A. Mayoux¹, E. Mazzeo¹, S. Mazzoni¹, M. Meena¹⁰,⁵²,⁵³, E. Meftah¹⁴, Andrew Mehta¹⁸⁸,
Ankita Mehta¹, B. Mele¹⁷³, R. Mena-Andrade¹, M. Mentink¹, D. Mergelkuhl¹, V. Mertinger²⁶⁸, L. Mether¹,
S. Meylan¹¹⁰, T. Michel¹⁰⁷, T. Michlmayr², M. Migliorati⁹⁸,¹⁷³, A. Milanese¹, C. Milardi¹⁵, G. Milhano⁶⁰,
M. Minty⁶, C. Mirabelli³²⁵, T. Miralles¹⁰,¹⁵⁷,¹⁵⁸, L. Miralles Verge¹, D. Mirarchi¹, K. Mirbaghestan⁸¹,
N. Mirian⁴,³²⁶, V. A. Mitsou³¹⁹, D. S. Mitzel²⁴⁵, M. Mlynarikova¹, S. Möbius⁹⁷, M. Mohammadi Najafabadi¹,²⁵,
G. B. Mohanty³²⁷, R. N. Mohapatra²¹², S. Moneta⁷², E. Monnier¹⁰,⁸³,⁸⁴, S. Monteil¹⁰,¹⁵⁷,¹⁵⁸, I. León Monzón²⁰⁵,
F. Moortgat¹,³²⁸, N. Morange¹⁰,²¹,²², M. Moretti¹⁴⁹,¹⁵⁰, S. Moretti⁷⁹, T. Mori¹,²¹³, I. Morozov¹²³, A. Morozzi⁷²,
M. Morrone¹, A. Moscariello¹⁴, F. Moscatelli⁷²,³²⁹, I. Moulin²¹⁷, N. Mounet¹, A. Mueller³³⁰, A.-S. Müller¹⁹,
B. O. Müller²⁸⁶, J. Mundet²²², E. Musa¹,⁴, V. Musat¹,⁸, R. Musenich¹⁴³, E. Musumeci³¹⁹, M. Mylona¹,
V. V. Mytrochenko¹⁰,²¹,¹³⁶, B. Nachman¹⁴⁴, S. Nagaitsev⁶, T. Nakamoto²¹³, M. Napsuciale³²⁴, M. Nardecchia⁹⁸,¹⁷³,
G. Nardini³³¹, G. Narváez-Arango³³², S. Naseem⁷⁰, A. Natochii⁶, A. Navascues Cornago¹, B. Naydenov¹, G. Nergiz¹,
A. V. Nesterenko²⁷⁷, C. Neubüser³³³, H. B. Newman³³⁴, F. Niccoli¹,³³⁵, O. Nicrosini¹³¹, U. Niedermayer²²⁸,
G. Niehues¹⁹, J. Nielsen¹, G. Nigrelli¹,⁹⁸,¹⁷³, S. Nikitin¹²³, I. B. Nikolaev¹²³, A. Nisati¹⁷³, N. Nitika¹⁷⁰,¹⁷¹, J. M. No³³⁶,
M. Nonis¹, Y. Nosochkov⁷, A. Novokhatski¹,⁷, J. M. O’Callaghan³³⁷, S. A. Ochoa-Oregon²⁰⁵, K. Ohmi²⁰⁴,²¹³,
K. Oide¹,¹⁴,²¹³, V. A. Okorokov¹²³, C. Oleari⁴¹,⁴², D. Oliveira Damazio¹,⁶, Y. Onel¹¹⁷, A. Onofre³³⁸,³³⁹,³⁴⁰,
P. Osland³⁴¹, Y. M. Oviedo-Torres³⁴²,³⁴³,³⁴⁴, A. Ozansoy⁶⁷, F. Ozaydin⁹⁵,³⁴⁵, K. Ozdemir³⁴⁶, A. Ozturk¹,
M. A. Pérez de León²⁰⁵, S. Pacetti⁷²,⁷³, H. Pacey⁸, J. Paciello¹¹³, C. E. Pagliarone³⁴⁷,³⁴⁸, A. Paillex¹¹⁰,
H. F. Pais da Silva¹, A. Pampaloni¹⁴³, C. Pancotti¹⁵², M. Pandurović³⁴⁹, O. Panella⁷², G. Panizzo¹⁷⁰,¹⁷¹,
C. Pantouvakis⁹,¹⁶⁷, L. Panwar¹⁰,¹³,³¹⁷, P. Paolucci⁴⁶, Y. Papa¹¹⁰, A. Papaefstathiou³⁵⁰, Y. Papaphilippou¹,
A. Paramonov¹⁶¹, A. Pareti¹³¹,³⁵¹, B. Parker⁶, V. Parma¹, F. Parodi¹⁴³,³¹⁸, M. Parodi¹, B. Paroli⁵⁴,⁵⁵,
J. A. Parsons³⁵², D. Passarelli⁴⁸, D. Passeri⁷²,⁷³, B. Pattnaik³¹⁹, A. Patwa³⁵³, C. Paus¹⁸⁶, F. Pauss¹⁵³, F. Peauger¹,
I. Pedraza²¹⁸, R. Pedro⁶⁰, J. Pekkanen¹, G. Peon¹, A. Perez¹¹⁴, F. Pérez¹⁷⁴, J. C. Perez¹, J. M. Pérez³⁸,
R. Perez-Ramos¹⁴⁰,¹⁴¹,³⁵⁴, G. Pérez Segurana¹, A. Perillo Marcone¹, S. Perna⁴⁶,⁴⁷, K. Peters⁴, S. Petracca⁴⁶,⁵⁵,
A. R. Petri⁵⁴, F. Petriello¹²⁶, A. Petrovic¹, L. Pezzotti³⁶, G. Piacquadio⁷¹, G. Piazza¹⁸³, A. Piccini¹, F. Piccinini¹³¹,
A. Pich³¹⁹, T. Pieloni¹¹⁹, J. Pierlot¹, A. D. Pilkington⁶¹, M. Pillet³²⁵, M. Pinamonti¹⁷⁰,¹⁷¹, N. Pinto²³⁶, L. Pintucci¹⁷⁰,¹⁷¹, F. Pinzauti¹, K. Piotrzkowski²⁷², C. Pira¹⁵, M. Pitt¹, R. Pittau¹⁷, S. Pittet¹, P. Placidi⁷²,⁷³, W. Placzek³⁵⁶, S. Plätzer³¹³,³⁵⁷, E. Ploerer⁸¹,¹²¹, H. Podlech³⁵⁸,³⁵⁹, F. Poirier¹⁰,²⁷,²⁸, G. Polesello¹³¹, M. Poli Lener¹⁵, J. Polinski¹⁶⁴, Z. Polonsky⁸¹, N. Pompeo⁴⁰, M. Pont¹⁷⁴, G. Alexandru-Popeneciu³⁶⁰, W. Porod¹⁹⁷, L. Porta¹, L. Portales³, T. Portaluri³⁰⁸, M. A. C. Potenza⁵⁵, C. Prasse²⁸⁶, E. Premat¹⁸⁷, M. Presilla¹⁹, S. Prestemon¹⁴⁴, A. Price³⁵⁶, M. Primavera¹⁶², R. Principe¹, M. Prioli⁵⁴, F. M. Procacci²³, E. Proserpio⁵⁴,¹³⁷, A. Provino⁹⁹,³¹⁸, C. Pueyo¹, T. Puig³⁰, N. Pukhaeva²⁷⁷, S. Pulawski²⁰⁷, G. Punzi²⁰,⁸⁵, A. Pyarelal³⁶¹, J. Qian³⁰⁴, H. Quack³⁶², F. S. Queiroz⁵⁷, G. Quintas-Neves³⁰⁰, H. Rafique⁷⁹, J.-Y. Raguin², J. Raidal³²¹, M. Raidal³²¹, P. Raimondi⁴⁸, A. Rajabi⁴, S. Ramírez-Uribe²⁰⁵, S. Randles¹⁸⁸, T. Rao⁶, C. Ø. Rasmussen⁶, A. Ratkus³⁶³, P. N. Ratoff⁶²,³⁶⁴, P. Razis³⁶⁵,³⁶⁶, P. Rebello Teles¹,³⁶⁷, M. N. Rebelo¹³³, M. Reboud¹⁰,²¹,²², S. Redaelli¹, C. Regazzoni¹¹⁰, L. Reichenbach¹,³⁶⁸, M. Reissig¹⁹, E. Renou¹¹⁰, A. Rentería-Olivo³¹⁹, J. Reuter⁴, S. Rey¹¹⁰, A. Ribon¹, D. Ricci¹, M. Rignanese⁹,¹⁶⁷, S. Rimjaem³⁶⁹, R. A. Rimmer³⁷⁰, R. Rinaldesi¹, L. Rinolfi¹,²⁹⁴, O. Rios¹, G. Ripellino¹³⁴, B. Rivas³⁷¹, A. Rivetti⁶⁹, T. Robens³⁷², F. Robert¹⁸⁷, E. Robutti¹⁴³, C. Roderick¹, G. Rodrigo³¹⁹, M. Rodríguez-Cahuantzi²¹⁸, L. Röhrig¹⁵⁷,¹⁵⁸,²⁴⁵, M. Roig³⁷³, F. Rojat¹¹³, J. Rojo²⁰³,³⁷⁴, J. Roloff²³³, P. Roloff¹, A. Romanenko⁴⁸, A. Romero Francia¹, H. Romeyer³⁷⁵, N. Rompotis¹⁸⁸, N. Rongieras¹⁰⁷, G. Rosaz¹, K. Roslon³⁷⁶, M. Rossetti Conti⁵⁴, A. Rossi⁷²,⁷³, E. Rossi⁴⁶,⁴⁷, L. Rossi⁵⁴,⁵⁵, A. N. Rossia⁹,¹⁶⁷, S. Rostami⁴⁹, G. Roy¹, B. Rubik⁴⁸, I. Ruehl¹, A. Ruiz-Jimeno³⁷⁷, R. Ruprecht¹⁹, J. P. Rutherfoord³⁶¹, L. Rygaard⁴, M. S. Ryu²⁹⁶, L. Sabato¹,¹¹⁹, G. Sadowski¹⁰,⁵²,⁵³, D. Saez de Jauregui¹⁹,³⁷⁸, M. Sahin³⁷⁹, A. Sailer¹, M. Saito³⁸⁰, P. Saiz¹, G. P. Salam³⁸¹,³⁸², R. Salerno¹⁰,⁸⁹,⁹⁰, T. Salmi²⁹⁸, B. Salvachua¹, J. P. T. Salvesen¹,⁸,¹³⁹, B. Salvi³⁰⁰, D. Sampsonidis²⁸⁵, Y. Villamizar¹⁴⁰,¹⁴¹,¹⁴², C. Sandoval³³², S. Sanfilippo², E. Santopinto¹⁴³, R. Santoro⁵⁴,¹³⁷, X. Sarasola¹¹⁹, L. Sarperi²⁸⁸, I. H. Sarpün³⁸³, S. Sasikumar¹, M. Sauvain³⁸⁴, A. Savoy-Navarro³,¹⁰, R. Sawada³⁸⁰, G. Sborlini³⁸⁵, J. Scamardella⁴⁶,⁴⁷, M. Schaer², M. Schaumann¹,⁴, M. Schenk¹, C. Scheuerlein¹, C. Schiavi¹⁴³,³¹⁸, A. Schloegelhofer¹, D. Schoerling¹, A. Schöning²⁶⁵, S. Schramm¹⁴, D. Schulte¹, P. Schwaller²⁵²,³⁸⁶, A. Schwartzman⁷, Ph. Schwemling³, R. Schwienhorst³⁸⁷, A. Sciandra⁶, L. Scibile¹, I. Scimemi³⁸⁸, E. Scomparin⁶⁹, C. Sebastiani¹, B. Seeber³⁸⁹, J. T. Seeman⁷, M. Seidel²,¹¹⁹, S. Seidel⁸², J. Seixas³⁴⁰,³⁹⁰,³⁹¹, N. Selimovic⁶, C. Senatore¹⁴, A. Senol¹⁹⁶, N. Serra⁸¹, A. Seryi³⁷⁰, A. Sfyrla¹⁴, Pramond Sharma³⁹², Punit Sharma⁶, C. J. Sharp¹, L. Shchutska¹¹⁹, V. Shiltsev³⁹³, M. Siano⁵⁴,⁵⁵, R. Sierra¹, E. Silva⁴⁰, R. C. Silva⁵⁷,³⁴⁴, L. Silvestrini¹⁷³, F. Simon¹⁹, G. Simonetti¹, R. Simoniello¹, B. K. Singh³⁹⁴, S. Singh⁶, B. Singhal⁸⁷, A. Siodmok¹,³⁵⁶, Y. Sirois¹⁰,⁸⁹,⁹⁰, E. Sirtori¹⁵², B. Sitar³⁹⁵, D. Sittard¹, E. Sitti¹⁵³, T. Sjöstrand⁶⁸, P. Skands³⁹⁶, L. Skinnari²⁹³, K. Skoufaris¹, K. Skovpen³²⁸, M. Skrzypek¹¹², P. Slavich¹⁴⁰,¹⁴¹,¹⁴², V. Slokenbergs³⁴, V. Smaluk⁶, J. Smiesko¹,³⁹⁷, S. S. Snyder⁶, E. Solano¹⁷⁴, P. Sollander¹, O. V. Solovyanov¹,¹⁰,¹⁵⁷, M. Son³⁹⁸, F. Sonnemann¹, R. Soos¹,¹⁰,²¹, F. Sopkova¹⁹², T. Sorais³⁹⁹, M. Sorbi⁵⁴,⁵⁵, S. Sorti⁵⁴,⁵⁵, R. Soualah⁴⁰⁰, M. Souayah¹, L. Spallino¹⁵, S. Spanier⁴⁰¹, P. Spiller²⁵⁹, M. Spira², D. Stagnara¹⁰⁷, M. Stallmann¹⁹⁰, D. Standen¹, J. L. Stanyard¹, B. Stapf¹, G. H. Stark²³¹, M. Statera⁵⁴, C. Staudinger¹,²⁰⁶, B. A. Stefanek³¹⁹, G. Streicher⁴⁰², N. P. Strohmaier², R. Stroynowski¹¹¹, S. Stucci⁶, G. Stupakov⁷, S. Su³⁶¹, A. Sublet¹, K. Sugita²⁵⁹, M. K. Sullivan⁷, S. Sultansoy³⁵, D. Sutherland¹⁸², I. Syratchev¹, R. Szafron⁶, A. Sznajder⁴⁰³, W. Tachon⁴⁰⁴, N. D. Tagdulang⁴⁸,¹⁷⁴,³³⁷, N. A. Tahir²⁵⁹, Y. Takahashi¹²⁷, J. Tamazirt¹⁰,²¹,²², S. Tang⁶, Y. Tanimoto²¹³, I. Tapan²⁷⁵, G. F. Tassielli²³,⁴⁰⁵, A. M. Teixeira¹⁰,¹⁵⁷,¹⁵⁸, V. I. Telnov¹²³, H. H. J. Ten Kate¹,⁴⁰⁶, V. Teotia⁶, J. ter Hoeve²⁹⁹, A. Thabuis¹, G. T. Telles³⁰, A. Tishelman-Charny⁶, S. Tissandier¹¹³, S. Tizchang²⁵,⁴⁰⁷, J.-P. Tock¹, B. Todd¹, L. Toffolin¹,¹⁷⁰,⁴⁰⁸, A. Tolosa-Delgado¹, R. Tomás García¹, T. Tomasini⁴⁰⁹, G. Tonelli²⁰,⁸⁵, T. Tong⁴¹⁰, F. Toral³⁸, T. Torims¹,³⁶³, L. Torino¹⁷⁴, K. Torokhtii⁴⁰, R. Torre¹⁴³, E. Torrence⁴¹¹, R. Torres⁶²,¹⁸⁸, T. Mitsuhashi²¹³, A. Tracogna¹⁵², O. Traver¹⁷⁴, D. Treille¹, A. Tricoli⁶, P. Trubacova¹, E. Tsesmelis¹, G. Tsipolitis²⁶⁹, V. Tsulaia¹⁴⁴, B. Tuchming³, C. G. Tully¹⁶⁶, I. Turk Cakir⁶⁷, C. Turrioni⁷², J. Tynan¹¹⁰, F. P. Ucci¹³¹,³⁵¹, S. Udongwo⁴¹², C. S. Ün²⁷⁵, A. Unnervik¹, A. Upegui¹⁰⁵,¹⁰⁶, J. P. Uribe-Ramírez²⁰⁵, J. Uythoven¹, R. Vaglio⁴⁷,⁹⁹, F. Valchkova-Georgieva⁴¹³, P. Valente¹⁷³, R. U. Valente¹⁷³, A.-M. Valente-Feliciano³⁷⁰, G. Valentino¹,¹⁹⁴, C. A. Valerio-Lizarraga²⁰⁵,³²⁴, S. Valette¹, J. W. F. Valle³¹⁹, L. Valle¹, N. Valle¹³¹, N. Vallis¹,²,¹¹⁹, G. Vallone¹⁴⁴, P. van Gemmeren¹⁶¹, W. Van Goethem¹, P. van Hees⁶⁸, U. van Rienen⁴¹², L. van Riesen-Haupt¹,¹¹⁹, P. Van Trappen¹, M. Vande Voorde⁴¹⁴,⁴¹⁵, A. L. Vanel¹, E. W. Varnes³⁶¹, J.-L. Vay¹⁴⁴, F. Veit²⁸⁶, I. Veliscek⁶, R. Veness¹, A. Ventura¹⁶²,²³⁶, S. Verdú-Andrés⁶, M. Verducci²⁰,⁸⁵, C. B. Verhaaren⁴¹⁶, C. Vernieri⁷, A. P. Verweij¹, J.-F. Vian⁴¹⁷, A. Vicini⁵⁴,⁵⁵, N. Vignaroli¹⁶²,²³², S. Vignetti¹⁵², M. C. Villeneuve²²⁰, I. Vivarelli³⁶,³⁷, E. Voevodina¹,¹³⁵, D. M. Vogt⁴¹⁸, B. Voirin⁴¹⁹, S. Voiriot¹¹⁰, J. Voiron¹⁴⁷, P. Vojtyla¹, V. Völkl¹, L. von Freeden¹, Z. Vostrel¹,⁴²⁰, N. Voumard¹, E. Vryonidou⁶¹, V. Vysotsky¹²³, R. Wallny¹⁵³, L.-T. Wang⁴²¹, Y. Wang¹⁰,²¹,²², R. Wanzenberg⁴, B. F. L. Ward⁴²², N. Wardle¹⁸⁵, Z. Wąs¹¹², L. Watrelot¹, A. T. Watson⁴²³, M. F. Watson⁴²³, M. S. Weber⁹⁷, C. P. Welsch⁶²,¹⁸⁸, M. Wendt¹,⁶, J. Wenninger¹, B. Weyer¹, G. White⁴²⁴,
S. White³⁰⁷, B. Wicki<fnref n="1" />, M. Widorski<fnref n="1" />, U. A. Wiedemann<fnref n="1" />, A. R. Wiederhold⁶¹, A. Wiedl<fnref n="19" />, H.-U. Wienands¹⁶¹, A. Wieser¹⁵³, C. Wiesner<fnref n="1" />, H. Wilkens<fnref n="1" />, D. Willi⁴²⁵, P. H. Williams⁶²,⁴²⁶, S. L. Williams<fnref n="43" />, A. Winter⁴²³, R. B. Wittwer⁸¹, D. Wollmann<fnref n="1" />, Y. Wu¹¹⁹, Z. Wu<fnref n="10" />,<fnref n="27" />,<fnref n="28" />, J. Xiao<fnref n="10" />,⁶⁵,⁶⁶, K. Xie³⁸⁷, S. Xie<fnref n="48" />,³³⁴, M. Yalvac<fnref n="33" />, F. Yaman⁴²⁶,⁴²⁷, W.-M. Yao¹⁴⁴, M. Yeresko<fnref n="10" />,¹⁵⁷,¹⁵⁸, A. Yilmaz¹⁹⁶, H. D. Yoo²⁷⁶, T. You²¹¹, F. Yu²⁵²,³⁸⁶, S. S. Yu⁸⁷, T.-T. Yu⁴¹¹, S. Yue<fnref n="1" />, A. Zaborowska<fnref n="1" />, M. Zahnd¹¹⁰, C. Zamantzas<fnref n="1" />, G. Zanderighi⁷⁴,¹³⁵, C. Zannini<fnref n="1" />, R. Zanzottera⁵⁴,⁵⁵, P. Zaro¹⁰⁷, R. Zennaro<fnref n="2" />, M. Zerlauth<fnref n="1" />, H. Zhang²⁰⁴, J. Zhang¹⁶¹, Y. Zhang²⁰⁴, Z. Zhang<fnref n="10" />,<fnref n="21" />,²⁰⁴, Y. Zhao<fnref n="1" />, Y.-M. Zhong⁴²⁸, B. Zhou³⁰⁴, D. Zhou²¹³, J. Zhu³⁰⁴, G. Zick³⁷³, M. A. Zielinski<fnref n="1" />, E. Zimmermann¹¹⁰, A. Zingaretti<fnref n="9" />,¹⁶⁷, J. Zinn-Justin<fnref n="3" />, A. V. Zlobin<fnref n="48" />, M. Zobov<fnref n="15" />, F. Zomer<fnref n="10" />,<fnref n="21" />,<fnref n="22" />, S. Zorzetti<fnref n="48" />, X. Zuo<fnref n="19" />, J. Zurita³¹⁹, V. V. Zutshi³⁹³, M. Zykova<fnref n="2" />, FCC Collaboration<fnref n="1" />,<fnref n="8" />

<footnote n="1">¹ CERN, European Organization for Nuclear Research, Geneva, Switzerland</footnote>

<footnote n="2">² PSI, Paul Scherrer Institute, Villigen, Switzerland</footnote>

<footnote n="3">³ CEA/Irfu, Commissariat à l'Energie Atomique et aux Energies Alternatives, Institut de recherche sur les lois fondamentales de l'Univers,
Saclay, France</footnote>

<footnote n="4">⁴ DESY, Deutsches Elektronen-Synchrotron, Hamburg, Germany</footnote>

<footnote n="5">⁵ Humboldt-Universität zu Berlin, Berlin, Germany</footnote>

<footnote n="6">⁶ BNL, Brookhaven National Laboratory, Upton, NY, USA</footnote>

<footnote n="7">⁷ SLAC National Accelerator Laboratory, Menlo Park, CA, USA</footnote>

<footnote n="8">⁸ University of Oxford, Oxford, UK</footnote>

<footnote n="9">⁹ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Padua, Italy</footnote>

<footnote n="10">¹⁰ CNRS/IN2P3, Centre National de la Recherche Scientifique, Institut National de Physique Nucléaire et de Physique des Particules,
Paris, France</footnote>

<footnote n="11">¹¹ APC, Laboratoire AstroParticule et Cosmologie, Paris, France</footnote>

<footnote n="12">¹² Université Paris Cité, Paris, France</footnote>

<footnote n="13">¹³ LPNHE, Laboratoire de Physique Nucléaire et de Hautes Énergies, Paris, France</footnote>

<footnote n="14">¹⁴ UNIGE, Université de Genève, Geneva, Switzerland</footnote>

<footnote n="15">¹⁵ INFN, Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Italy</footnote>

<footnote n="16">¹⁶ NBI, Niels Bohr Institute, Copenhagen, Denmark</footnote>

<footnote n="17">¹⁷ Universidad de Granada, Granada, Spain</footnote>

<footnote n="18">¹⁸ University of Pittsburgh, Pittsburgh, PA, USA</footnote>

<footnote n="19">¹⁹ KIT, Karlsruher Institut für Technologie, Karlsruhe, Germany</footnote>

<footnote n="20">²⁰ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Pisa, Italy</footnote>

<footnote n="21">²¹ IJCLab, Laboratoire de Physique des 2 Infinis Irène Joliot Curie, Orsay, France</footnote>

<footnote n="22">²² Université Paris-Saclay et Université Paris-Cité, Paris, France</footnote>

<footnote n="23">²³ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Bari, Italy</footnote>

<footnote n="24">²⁴ Università di Bari, Bari, Italy</footnote>

<footnote n="25">²⁵ IPM, Institute for Research in Fundamental Science, Tehran, Iran</footnote>

<footnote n="26">²⁶ Malayer University, Malayer, Iran</footnote>

<footnote n="27">²⁷ LAPP, Laboratoire d'Annecy de Physique des Particules, Annecy, France</footnote>

<footnote n="28">²⁸ Université Savoie Mont Blanc, Annecy, France</footnote>

<footnote n="29">²⁹ University of Belgrade, Belgrade, Serbia</footnote>

<footnote n="30">³⁰ ICMAB/CISC, Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas, Barcelona, Spain</footnote>

<footnote n="31">³¹ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tor Vergata, Rome, Italy</footnote>

<footnote n="32">³² Università Roma Tor Vergata, Rome, Italy</footnote>

<footnote n="33">³³ Yozgat Bozok Üniversitesi, Yozgat, Türkiye</footnote>

<footnote n="34">³⁴ Texas Tech University, Lubbock, TX, USA</footnote>

<footnote n="35">³⁵ TOBB ETU, TOBB Ekonomi ve Teknoloji Üniversitesi, Ankara, Türkiye</footnote>

<footnote n="36">³⁶ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Bologna, Italy</footnote>

<footnote n="37">³⁷ Università di Bologna, Bologna, Italy</footnote>

<footnote n="38">³⁸ CIEMAT, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Madrid, Spain</footnote>

<footnote n="39">³⁹ KACST, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia</footnote>

<footnote n="40">⁴⁰ Università Roma Tre, Rome, Italy</footnote>

<footnote n="41">⁴¹ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Milano-Bicocca, Milan, Italy</footnote>

<footnote n="42">⁴² Università di Milano-Bicocca, Milan, Italy</footnote>

<footnote n="43">⁴³ University of Cambridge, Cambridge, UK</footnote>

<footnote n="44">⁴⁴ İstanbul Üniversitesi, İstanbul, Türkiye</footnote>

<footnote n="45">⁴⁵ Eskişehir Teknik Üniversitesi, Eskişehir, Türkiye</footnote>

<footnote n="46">⁴⁶ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Naples, Italy</footnote>

<footnote n="47">⁴⁷ Università di Napoli Federico II, Naples, Italy</footnote>

<footnote n="48">⁴⁸ FNAL, Fermi National Accelerator Laboratory, Batavia, IL, USA</footnote>

<footnote n="49">⁴⁹ University of Tehran, Tehran, Iran</footnote>

<footnote n="50">⁵⁰ FUM, Ferdowsi University of Mashhad, Mashhad, Iran</footnote>

<footnote n="51">⁵¹ University of Texas Austin, Austin, TX, USA</footnote>

<footnote n="52">⁵² IPHC, Institut Pluridisciplinaire Hubert Curien, Strasbourg, France</footnote>
53 Université de Strasbourg, Strasbourg, France

54 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Milano, Milan, Italy

55 Università di Milano, Milan, Italy

56 PIBG, Pôle Invertébrés du Basin Genevois, Geneva, Switzerland

57 UFRN, Universidade Federal do Rio Grande do Norte, Natal, Brazil

58 UNIBAS, University of Basel, Basel, Switzerland

59 Politecnico di Bari, Bari, Italy

60 LIP, Laboratório de Instrumentação e Física Experimental de Partículas, Lisbon, Portugal

61 University of Manchester, Manchester, UK

62 CI, Cockcroft Institute, Warrington, UK

63 Brandeis University, Waltham, MA, USA

64 A. Alikhanyan National Laboratory, Yerevan, Armenia

65 IP2I, Institut de Physique des 2 Infinis de Lyon, Lyon, France

66 Université Claude Bernard Lyon 1, Lyon, France

67 Ankara Üniversitesi, Ankara, Türkiye

68 Lund University, Lund, Sweden

69 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Turin, Italy

70 New York University Abu Dhabi, Abu Dhabi, United Arab Emirates

71 Stony Brook University, Stony Brook, NY, USA

72 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Perugia, Italy

73 Università di Perugia, Perugia, Italy

74 Technische Universität München, Munich, Germany

75 Hatay Mustafa Kemal Üniversitesi, Hatay, Türkiye

76 Doğuş Üniversitesi, Istanbul, Türkiye

77 Harbin Institute of Technology, Harbin, People's Republic of China

78 University of Wisconsin-Madison, Madison, WI, USA

79 RAL, Rutherford Appleton Laboratory, Science and Technology Facilities Council, Didcot, UK

80 IMSc, Institute of Mathematical Sciences, Chennai, Chennai, India

81 Universität Zürich, Zurich, Switzerland

82 University of New Mexico, Albuquerque, NM, USA

83 CPPM, Centre de Physique des Particules de Marseille, Marseille, France

84 Aix-Marseille Université, Marseille, France

85 Università di Pisa, Pisa, Italy

86 HUN-REN Wigner Research Centre for Physics, Budapest, Hungary

87 Catholic University of America, Washington, DC, USA

88 Duke University, Durham, NC, USA

89 LLR, Laboratoire Leprince-Ringuet, Palaiseau, France

90 École Polytechnique, Institut Polytechnique de Paris, Palaiseau, France

91 TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics, Vancouver, Canada

92 Simon Fraser University, Burnaby, Canada

93 FEEM, Fondazione Ente Nazionale Idrocarburi (ENI) Enrico Mattei, Milan, Italy

94 BRGM, Bureau de Recherches Géologiques et Minières, Orléans, France

95 Işık Üniversitesi, Istanbul, Türkiye

96 İstanbul Teknik Üniversitesi, Istanbul, Türkiye

97 UNIBE, University of Bern, Bern, Switzerland

98 Università di Roma la Sapienza, Rome, Italy

99 CNR-SPIN, Consiglio Nazionale delle Ricerche, Genoa, Italy

100 Expert naturaliste et entomologiste, Rumilly, France

101 ORNL, Oak Ridge National Laboratory, Oak Ridge, TN, USA

102 HEPHY, Institut für Hochenergiephysik, Vienna, Austria

103 Geos, Bureau d'ingénieurs conseils en géotechnique, génie civil, hydraulique et environnement, Geneva, Switzerland

104 TUWIEN, Technische Universität Wien, Vienna, Austria

105 HEPIA, Haute École du Paysage, d'Ingénierie et d'Architecture de Genève, Geneva, Switzerland

106 HES-SO University of Applied Sciences and Arts Western Switzerland, Geneva, Switzerland

107 SETEC ALS, Société d'ingénierie conseil en infrastructures de transport, génie civil et environnement, Lyon, France

108 East Texas A&M University, Commerce, TX, USA

109 Amberg Engineering Ltd, Regensdorf-Watt, Switzerland

110 ECOTEC Environnement SA, Bureau d'études et de conseil en environnement, Geneva, Switzerland

111 Southern Methodist University, Dallas, TX, USA

112 IFJ PAN, Institute of Nuclear Physics, Polish Academy of Sciences, Kraków, Poland

113 Cerema, établissement public pour l'élaboration, le déploiement et l'évaluation de politiques publiques d'aménagement et de transport, Lyon, France

114 Rendel Ltd, Engineering Design Consultancy Firm, London, UK

115 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tre, Rome, Italy

116 İstanbul Beykent Üniversitesi, Istanbul, Türkiye

117 University of Iowa, Iowa City, IA, USA

<figure><img src="image_2.png" /></figure>
118 Indian Institute of Technology Kanpur, Kanpur, India

119 EPFL, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

120 Fakultät für Mathematik, Informatik und Naturwissenschaften, Universität Hamburg, Hamburg, Germany

121 VUB, Vrije Universiteit Brussel, Brussels, Belgium

122 Scuola Superiore Meridionale, Naples, Italy

123 Affiliated with an Institute Formerly Covered by a Cooperation Agreement with CERN, Geneva, Switzerland

124 University of Saskatchewan and the Canadian Light Source, Saskatoon, Canada

125 University of Bristol, Bristol, UK

126 Northwestern University, Evanston, IL, USA

127 University of Florida, Gainesville, FL, USA

128 York University, Toronto, Canada

129 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, Cagliari, Italy

130 Università di Cagliari, Cagliari, Italy

131 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy

132 Queen's University, Kingston, Canada

133 CFTP-IST, Centro de Física Téorica de Partículas, Instituto Superior Tecnico, Universidade de Lisboa, Lisbon, Portugal

134 Uppsala University, Uppsala, Sweden

135 MPP, Max-Planck-Institut für Physik Garching, Garching, Germany

136 NSC KIPT, National Science Center Kharkiv Institute of Physics and Technology, Kharkiv, Ukraine

137 Università degli Studi dell'Insubria, Como, Italy

138 Albert-Ludwigs-Universität Freiburg, Freiburg im Breisgau, Germany

139 JAI, John Adams Institute for Accelerator Science, University of Oxford, Oxford, UK

140 CNRS/INP, Centre National de la Recherche Scientifique, Institut de Physique, Paris, France

141 LPTHE, Laboratoire de Physique Théorique et Hautes Energies, Paris, France

142 Sorbonne Université, Paris, France

143 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Genova, Genoa, Italy

144 LBNL, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

145 IIT, Instituto Italiano di Tecnologia, Genoa, Italy

146 Gümüşhane Üniversitesi, Gümüşhane, Türkiye

147 WSP Ingénieurs Conseils SA, Geneva, Switzerland

148 UADY, Autonomous University of Yucatan, Mérida, Mexico

149 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, Ferrara, Italy

150 Università di Ferrara, Ferrara, Italy

151 MARCELEON, Cabinet d'ingénierie juridique et foncière, Chambéry, France

152 CSIL (Economic Research Institute), Milan, Italy

153 ETHZ, Swiss Federal Institute of Technology Zurich, Zurich, Switzerland

154 UAH, Universidad de Alcalá Madrid, Alcalá de Henares, Spain

155 CIA, Conseil Ingénierie Acoustique, Lyon, France

156 Evinerude, Bureau d'études environnementales, Lyon, France

157 LPCA, Laboratoire de Physique de Clermont Auvergne, Clermont-Ferrand, France

158 Université Clermont Auvergne, Clermont-Ferrand, France

159 ANSTO, Australian Synchrotron, Melbourne, Australia

160 Brahmananda Keshab Chandra College, Kolkata, India

161 ANL, Argonne National Laboratory, Lemont, IL, USA

162 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Lecce, Lecce, Italy

163 Università di Palermo, Palermo, Italy

164 Wrocław University of Science and Technology, Wrocław, Poland

165 Rutgers University, New Brunswick, NJ, USA

166 Princeton University, Princeton, NJ, USA

167 Università di Padova, Padua, Italy

168 IUE, İzmir Ekonomi Üniversitesi, İzmir, Türkiye

169 Ege Üniversitesi, İzmir, Türkiye

170 INFN, Istituto Nazionale di Fisica Nucleare, Gruppo Collegato di Udine, Udine, Italy

171 Università di Udine, Udine, Italy

172 University of Massachusetts Amherst, Amherst, MA, USA

173 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Rome, Italy

174 CELLS/ALBA, Consortium for the Construction, Equipment and Exploitation of the Synchrotron Light Laboratory, Cerdanyola del Vallès, Spain

175 LPSC, Laboratoire de Physique Subatomique et de Cosmologie, Grenoble, France

176 Université Grenoble Alpes, Grenoble, France

177 Università degli Studi di Napoli Parthenope, Naples, Italy

178 National High Magnetic Field Laboratory, Tallahassee, FL, USA

179 Florida State University, Tallahassee, FL, USA

180 University of Johannesburg, Johannesburg, South Africa

181 IGFAE, Instituto Galego de Fisica de Altas Enerxías, Universidade de Santiago de Compostela, Santiago de Compostela, Spain

182 University of Glasgow, Glasgow, UK
183 LSE, London School of Economics, London, UK

184 Universidade de Santiago de Compostela, Santiago de Compostela, Spain

185 Imperial College London, London, UK

186 MIT, Massachusetts Institute of Technology, Cambridge, MA, USA

187 CETU, Centre d'Etude des Tunnels, Lyon, France

188 University of Liverpool, Liverpool, UK

189 Linde Kryotechnik AG, Pfungen, Switzerland

190 ILF Consulting Engineers, Zurich, Switzerland

191 Hokkaido University, Sapporo, Japan

192 CUNI, Charles University, Prague, Czech Republic

193 INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Florence, Italy

194 University of Malta, Msida, Malta

195 BU, Boston University, Boston, MA, USA

196 IBU, Bolu Abant İzzet Baysal ÜnİVERSİTESİ, Bolu, Türkiye

197 Julius-Maximilians-Universität Würzburg, Würzburg, Germany

198 University of Adelaide, Adelaide, Australia

199 HIP, Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland

200 ULB, Université Libre de Bruxelles, Brussels, Belgium

201 IMA, Institut für Maschinenelemente, Universität Stuttgart, Stuttgart, Germany

202 CP3, Centre de Cosmologie, de Physique des Particules et de Phénoménologie, Université Catholique de Louvain, Louvain-la-Neuve, Belgium

203 NIKHEF, Nationaal Instituut Voor Subatomaire Fysica, Amsterdam, Netherlands

204 IHEP, Chinese Academy of Sciences, Beijing, People's Republic of China

205 UAS, Universidad Autónoma de Sinaloa, Culiacán, Mexico

206 BOKU, Universität für Bodenkultur Wien, Vienna, Austria

207 University of Silesia in Katowice, Katowice, Poland

208 Purdue University, West Lafayette, IN, USA

209 University of Delhi, Delhi, India

210 Ginger BURGEAP, bureau d'études en environnement, Lyon, France

211 King's College London, London, UK

212 University of Maryland, College Park, MD, USA

213 KEK, High Energy Accelerator Research Organization, Tsukuba, Japan

214 Geoenergy, Reservoir Geology and Basin Analysis Group, Geneva, Switzerland

215 SETEC International, Société d'ingénierie en charge des transports et des infrastructures, Lyon, France

216 KKU, Kırıkkale Üniversitesi, Kırıkkale, Türkiye

217 SETEC LERM, Société d'ingénierie conseil en matériaux de construction, Lyon, France

218 BUAP, Benemérita Universidad Autónoma de Puebla, Puebla, Mexico

219 Stanford University, Stanford, CA, USA

220 MUL, Montanuniversität Leoben, Lehrstuhl für Subsurface Engineering, Geotechnik und Unterirdisches Bauen, Leoben, Austria

221 MUL-ZaB, Underground Research Center, Zentrum am Berg, Leoben, Austria

222 IFAE, Institut de Física d'Altes Energies, Barcelona, Spain

223 FHNW, University of Applied Sciences Northwestern Switzerland, Windisch, Switzerland

224 UPES, University of Petroleum and Energy Studies, Dehradun, India

225 GANIL, Grand Accélérateur National d'Ions Lourds, Caen, France

226 Université Caen Normandie, Caen, France

227 UFRGS, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil

228 Technische Universität Darmstadt, Darmstadt, Germany

229 Universidade de Coimbra, Coimbra, Portugal

230 UFPel, Universidade Federal de Pelotas, Pelotas, Brazil

231 University of California Santa Cruz, Santa Cruz, CA, USA

232 Università del Salento, Lecce, Italy

233 Brown University, Providence, RI, USA

234 University of California Berkeley, Berkeley, CA, USA

235 Università di Torino, Turin, Italy

236 Johns Hopkins University, Baltimore, MD, USA

237 Fudan University, Shanghai, People's Republic of China

238 LAPTh, Laboratoire d'Annecy-le-Vieux de Physique Théorique, Annecy-le-Vieux, France

239 Dongguan University of Technology, Dongguan, People's Republic of China

240 Kahramanmaraş Sütçü İmam Üniversitesi, Kahramanmaraş, Türkiye

241 UNACH, Universidad Autónoma de Chiapas, Tuxtla Gutiérrez, Mexico

242 MCTP, Mesoamerican Centre for Theoretical Physics, Tuxtla Gutiérrez, Mexico

243 UAZ, Universidad Autónoma de Zacatecas, Zacatecas, Mexico

244 University of Jyväskylä, Jyväskylä, Finland

245 Technische Universität Dortmund, Dortmund, Germany

246 Overleaf, London, UK

247 University of Virginia, Charlottesville, VA, USA

248 Cornell University, Ithaca, NY, USA

<figure><img src="image_3.png" /></figure>
249 FIT, Florida Institute of Technology, Melbourne, FL, USA

250 Shirokuma GmbH, Zurich, Switzerland

251 National Centre for Physics, Islamabad, Pakistan

252 Johannes Gutenberg Universität Mainz, Mainz, Germany

253 PAEC, Pakistan Atomic Energy Commission, Islamabad, Pakistan

254 Mathabhanga College, Mathabhanga, India

255 Harish-Chandra Research Institute, Prayagraj, India

256 MPIK, Max-Planck-Institut für Kernphysik Heidelberg, Heidelberg, Germany

257 European Spallation Source ERIC, Lund, Sweden

258 Centre de calcul de l'IN2P3, Villeurbanne, France

259 GSI, Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

260 IBS, Institute for Basic Science, Center for Theoretical Physics of the Universe, Daejeon, Republic of Korea

261 University of Warsaw, Warsaw, Poland

262 University of Ljubljana, Ljubljana, Slovenia

263 Jozef Stefan Institute, Ljubljana, Slovenia

264 Microhumus, Bureau d'étude et d'ingénierie spécialisé dans la gestion des sols dégradés, Nancy, France

265 Fakultät für Physik und Astronomie, Universität Heidelberg, Heidelberg, Germany

266 Niğde Ömer Halisdemir Üniversitesi, Niğde, Türkiye

267 Giresun Üniversitesi, Giresun, Türkiye

268 University of Miskolc, Miskolc, Hungary

269 NTUA, National Technical University of Athens, Athens, Greece

270 Transmutex SA, Geneva, Switzerland

271 DIAS, Dublin Institute for Advanced Studies, School of Theoretical Physics, Dublin, Ireland

272 AGH, University of Science and Technology, Kraków, Poland

273 University of Science and Technology of Mazandaran, Behshahr, Iran

274 IPPP, Institute for Particle Physics Phenomenology, Durham University, Durham, UK

275 Bursa Uludağ Üniversitesi, Bursa, Türkiye

276 YU, Yonsei University, Seoul, Republic of Korea

277 Affiliated with an International Laboratory Covered by a Cooperation Agreement with CERN, Geneva, Switzerland

278 CPT, Centre de Physique Théorique, Marseille, France

279 Aix-Marseille Université et Université du Sud Toulon Var, Marseille, France

280 KIAS, Korea Institute for Advanced Study, Seoul, Republic of Korea

281 Carleton University, Ottawa, Canada

282 FEAC Engineering P.C., Patras, Greece

283 UPATRAS, University of Patras, Patras, Greece

284 University of Kansas, Lawrence, KS, USA

285 AUTH, Aristotle University of Thessaloniki, Thessaloniki, Greece

286 IML, Fraunhofer-Institut für Materialfluss und Logistik, Dortmund, Germany

287 RWTH Aachen, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany

288 ZHAW, Zurich University of Applied Sciences, Winterthur, Switzerland

289 Universität Münster, Münster, Germany

290 University of the Witwatersrand, Johannesburg, South Africa

291 Fachhochschule Technikum Wien, Vienna, Austria

292 University of California Irvine, Irvine, CA, USA

293 Northeastern University, Boston, MA, USA

294 ESI, European Scientific Institute, Archamps, France

295 UOS, University of Seoul, Seoul, Republic of Korea

296 KNU Kyungpook National University, Daegu, Republic of Korea

297 KU, Korea University, Seoul, Republic of Korea

298 Tampere University, Tampere, Finland

299 University of Edinburgh, Edinburgh, UK

300 BG Ingénieurs Conseils, Lausanne, Switzerland

301 T.-D. Lee Institute, Shanghai, People's Republic of China

302 Shanghai Jiao Tong University, Shanghai, People's Republic of China

303 CNR, Consiglio Nazionale delle Ricerche, Rome, Italy

304 University of Michigan, Ann Arbor, MI, USA

305 University of Pennsylvania, Philadelphia, PA, USA

306 University of Minnesota, Minneapolis, MN, USA

307 ESRF, European Synchrotron Radiation Facility, Grenoble, France

308 SUSSEX, University of Sussex, Brighton, UK

309 Università di Bari Aldo Moro, Bari, Italy

310 University of Bath, Bath, UK

311 Scuola Normale Superiore di Pisa, Pisa, Italy

312 Universidade de São Paulo, São Paulo, Brazil

313 Universität Graz, Graz, Austria

314 Center for High Energy Physics, Fayoum University, Fayoum, Egypt
315 Center of theoretical physics, British University in Egypt, Cairo, Egypt

316 Cairo University, Cairo, Egypt

317 Sorbonne Université et Université Paris Cité, Paris, France

318 Università di Genova, Genoa, Italy

319 IFIC-CSIC/UV, Instituto de Física Corpuscular, Consejo Superior de Investigaciones Científicas/Universidad de Valencia, Valencia, Spain

320 Service de géologie, sols et déchets du canton de Genève, Geneva, Switzerland

321 NICPB, National Institute for Chemical Physics and Biophysics, Tallinn, Estonia

322 UT, University of Tartu, Tartu, Estonia

323 Universidad de Salamanca, Salamanca, Spain

324 UGTO, Universidad de Guanajuato, Guanajuato, Mexico

325 Edaphos Engineering, Geneva, Switzerland

326 Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

327 Tata Institute of Fundamental Research Mumbai, Mumbai, India

328 Universiteit Gent, Ghent, Belgium

329 CNR-IOM, Consiglio Nazionale delle Ricerche, Trieste, Italy

330 JKU, Johannes Kepler Universität Linz, Linz, Austria

331 University of Stavanger, Stavanger, Norway

332 Universidad Nacional de Colombia, Bogotá, Colombia

333 Trento Institute for Fundamental Physics and Applications, Trento, Italy

334 Caltech, California Institute of Technology, Pasadena, CA, USA

335 Università dalla Calabria, Rende, Italy

336 IFT, Instituto de Física Teórica, Universidad Autónoma de Madrid, Madrid, Spain

337 UPC, Universitat Politècnica de Catalunya, Barcelona, Spain

338 Departamento de Física, Universidade do Minho, Braga, Portugal

339 Centro de Física das Universidades do Minho e do Porto, Porto, Portugal

340 LaPMET, Laboratory of Physics for Materials and Emergent Technologies, Porto, Portugal

341 University of Bergen, Bergen, Norway

342 SAPHIR, Instituto Milenio de Física Subatómica en la Frontera de Altas Energías, Santiago, Chile

343 Universidad Andres Bello, Santiago, Chile

344 IIP, International Institute of Physics, Natal, Brazil

345 Tokyo International University, Tokyo, Japan

346 İzmir Bakırçay Üniversitesi, İzmir, Türkiye

347 INFN, Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Gran Sasso, Assergi, Italy

348 Università degli Studi di Cassino e del Lazio Meridionale, Cassino, Italy

349 Vinča Institute of Nuclear Sciences, Belgrade, Serbia

350 Kennesaw State University, Kennesaw, GA, USA

351 Università di Pavia, Pavia, Italy

352 Columbia University, New York, NY, USA

353 DOE, Department of Energy of the United States of America, Washington, DC, USA

354 IPSA, Institut Polytechnique des Sciences Avancées, Ivry-sur-Seine, France

355 Università degli Studi del Sannio, Benevento, Italy

356 UJ, Jagiellonian University, Kraków, Poland

357 Universität Wien, Vienna, Austria

358 Goethe-Universität Frankfurt, Institut für Angewandte Physik, Frankfurt am Main, Germany

359 HFFH, Helmholtz Forschungsakademie Hessen für FAIR, Frankfurt am Main, Germany

360 INCDTIM, National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania

361 University of Arizona, Tucson, AZ, USA

362 Technische Universität Dresden, Dresden, Germany

363 RTU, Riga Technical University, Riga, Latvia

364 Lancaster University, Lancaster, UK

365 University of Cyprus, Nicosia, Cyprus

366 Cosmos Open University, Nicosia, Cyprus

367 CBPF, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil

368 Universität Bonn, Bonn, Germany

369 CMU, Chiang Mai University, Chiang Mai, Thailand

370 JLAB, Thomas Jefferson National Accelerator Facility, Newport News, VA, USA

371 ESPOL, Escuela Superior Politécnica del Litoral, Guayaquil, Ecuador

372 IRB, Rudjer Boskovic Institute, Zagreb, Croatia

373 Air Liquide Advanced Technologies, Grenoble, France

374 VU Amsterdam, Amsterdam, Netherlands

375 INGÉROP, Groupe d'ingénierie et de conseil en mobilité durable, transition énergétique et cadre de vie, Lyon, France

376 Warsaw University of Technology, Warsaw, Poland

377 IFCA, Instituto de Física de Cantabria, Santander, Spain

378 Institut für Beschleunigerphysik und Technologie, Eggenstein-Leopoldshafen, Germany

379 Uşak Üniversitesi, Uşak, Türkiye

380 ICEPP, International Center for Elementary Particle Physics, University of Tokyo, Tokyo, Japan

<figure><img src="image_4.png" /></figure>
381 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK

382 All Souls College, University of Oxford, Oxford, UK

383 Akdeniz Üniversitesi, Antalya, Türkiye

384 Latitude Durable SARL, Geneva, Switzerland

385 USAL, Universidad de Salamanca, Salamanca, Spain

386 PRISMA+ Cluster of Excellence, Mainz, Germany

387 Michigan State University, East Lansing, MI, USA

388 Universidad Complutense Madrid, Madrid, Spain

389 scMetrology SARL, Lausanne, Switzerland

390 IST, Instituto Superior Tecnico, Universidade de Lisboa, Lisbon, Portugal

391 CeFEMA, Center of Physics and Engineering of Advanced Materials, Lisbon, Portugal

392 Indian Institute of Science Education and Research Mohali, Mohali, India

393 NIU, Northern Illinois University, DeKalb, IL, USA

394 Banaras Hindu University, Varanasi, India

395 Comenius University, Bratislava, Slovakia

396 Monash University, Melbourne, Australia

397 Slovak Academy of Sciences, Bratislava, Slovakia

398 KAIST, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea

399 Amberg Engineering Chambéry, Chambéry, France

400 Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates

401 University of Tennessee, Knoxville, TN, USA

402 WIFO, Österreichisches Institut für Wirtschaftsforschung, Vienna, Austria

403 Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil

404 Mélica, NATURA SCOP, Études et expertises environnementales, Aubenas, France

405 Università LUM, Casamassima, Casamassima, Italy

406 University of Twente, Enschede, Netherlands

407 Arak University, Arak, Iran

408 Università di Trieste, Trieste, Italy

409 ForestAllia, Cabinet de gestion et d'expertise forestières, Besançon, France

410 Universität Siegen, Siegen, Germany

411 University of Oregon, Eugene, OR, USA

412 Universität Rostock, Rostock, Germany

413 CEGELEC SA, Geneva, Switzerland

414 KTH, Royal Institute of Technology, Stockholm, Stockholm, Sweden

415 OKC, Oskar Klein Centre for Cosmoparticle Physics, Stockholm, Sweden

416 Brigham Young University, Provo, UT, USA

417 Expert foncier et agricole, Chadrat, France

418 ITSM, Institut für Thermische Strömungsmaschinen und Maschinenlaboratorium, Universität Stuttgart, Stuttgart, Germany

419 École Normale Supérieure de Lyon, Lyon, France

420 CTU, Czech Technical University, Prague, Czech Republic

421 University of Chicago, Chicago, IL, USA

422 Baylor University, Waco, TX, USA

423 University of Birmingham, Birmingham, UK

424 University of Southampton, Southampton, UK

425 Swisstopo, Federal Office of Topography, Wabern, Switzerland

426 Daresbury Laboratory, Science and Technology Facilities Council, Daresbury, UK

427 IZTECH, İzmir Yüksek Teknoloji Enstitüsü, İzmir, Türkiye

428 City University of Hong Kong, Hong Kong, Hong Kong

* Deceased

Received: 4 June 2025 / Accepted: 14 November 2025

© CERN for the benefit of the FCC collaboration 2025

Study Leader: M. Benedikt

Deputy Study Leader: F. Zimmermann

Editors: B. Auchmann, W. Bartmann, J. P. Burnet, C. Carli, A. Chancé, P. Craievich, M. Giovannozzi, C. Grojean, J. Gutleber, K. Hanke, A. Henriques, P. Janot, C. Lourenço, M. Mangano, T. Otto, J. Poole, S. Rajagopalan, T. Raubenheimer, E. Todesco, L. Ulrici, T. Watson, G. Wilkinson

Volume 1 Chapter Coordinators: P. Azzi, G. Bernardi, A. Blondel, M. Boscolo, D. d'Enterria, M. Dam, J. de Blas, B. Francois, A. Freitas, G. Ganis, J. Keintzel, M. Klute, M. McCullough, P. F. Monni, F. Palla, E. Perez, M.-A. Pleier, W. Riegler, F. Sefkow, M. Selvaggi.

<footnote>ª e-mail: FCC Head. Office@cern.ch</footnote>
**Abstract** Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study's focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

## Contents

Note from the Editors

Preface from CERN's Director-General

Preface from the FCC Collaboration Board Chair

1 Overview

1.1 FCC-ee: a great Higgs factory, and so much more

1.2 FCC-hh: the energy-frontier collider with the broadest exploration potential

2 Specificities of the FCC physics case

2.1 The impact of FCC in particle physics

2.2 Characterisation of the Higgs boson: role of EW measurements and of FCC-hh

2.2.1 Impact of Z-pole measurements in Higgs couplings determination

2.2.2 Impact of diboson measurements in Higgs couplings determination

2.2.3 Complementarity between Z-pole and higher-energy runs

2.2.4 Complementarity and synergy between FCC-ee and FCC-hh

2.3 Discovery landscape

2.3.1 BSM exploration potential

2.3.2 Tera-Z sensitivity to heavy new physics

2.3.3 Flavour deconstruction at FCC-ee

2.3.4 Heavy neutral leptons

2.3.5 Dark matter and dark sectors

2.3.6 Axion-like particles

2.3.7 Exotic decays of the Higgs and Z bosons

2.3.8 Other new physics searches

2.3.9 Complementarity and synergy between FCC-ee and FCC-hh

2.4 Selected topics in flavour physics

2.4.1 Lepton universality tests in $\tau$ decays

2.4.2 Lepton flavour violating $\tau$ decays

2.4.3 Rare b-hadron decays with $\tau^+\tau^-$ pairs in the final state

2.4.4 Charged-current b-hadron decays with a $\tau\nu$ pair in the final state

2.4.5 Rare b- and c-hadron decays to di-neutrino final states

2.4.6 Rare decays and CPV studies with neutrals

2.4.7 On-shell Z, W, and Higgs flavour-changing decays

2.4.8 Combined studies and synergy between the flavour and EW programmes
2.5 FCC-hh specificities compared to high-energy lepton colliders

2.5.1 Generalities

2.5.2 Resonance searches

2.5.3 Pair production of new particles

2.6 Physics reach of alternative FCC-hh $\sqrt{s}$ options

2.6.1 Higgs properties

2.6.2 WIMP DM search

2.6.3 High-mass reach

2.7 A forward-physics facility at FCC-hh

2.7.1 Neutrino physics

2.7.2 BSM sensitivity

2.7.3 QCD and hadronic structure

2.7.4 Summary

3 Theoretical calculations

3.1 Electroweak corrections

3.2 QCD precision calculations

3.2.1 QCD studies in $Z/\gamma^* \to$ jets

3.2.2 QCD aspects of Higgs physics

3.2.3 QCD modelling of the top-quark threshold

3.3 Monte Carlo event generators

3.3.1 QCD aspects

3.3.2 QED aspects

3.4 Organisation and support of future activities to improve theoretical precision

4 Detector requirements

4.1 Introduction

4.2 Brief overview of the current detector concepts

4.2.1 The IDEA detector concept

4.2.2 The CLD detector concept

4.2.3 The ALLEGRO detector concept

4.3 Measurement of the tracks of charged particles

4.3.1 Track momentum resolution: the measurement of the Higgs boson mass

4.3.2 Track momentum resolution: the Z width and the stability of the momentum scale

4.3.3 Angular resolutions

4.3.4 Number of tracker layers and highly-displaced vertices

4.3.5 Work ahead

4.3.6 Preliminary conclusions

4.4 Requirements for the vertex detector

4.4.1 Heavy-flavour tagging and the Higgs boson coupling to charm quarks

4.4.2 Reconstruction of vertices and the measurement of the $B \to K^*\tau\tau$ branching fraction

4.4.3 Vertex resolutions and heavy-flavour electroweak precision observables

4.4.4 Alignment, overall scale of the detector, and the measurement of the tau lifetime

4.4.5 Preliminary conclusions

4.5 Requirements for charged hadron particle identification

4.5.1 Strange tagging and the Higgs boson coupling to strange (and charmed) quarks

4.5.2 Separation of $K^\pm$ from $\pi^\pm$ and measurement of $b \to s\bar{\nu}\bar{\nu}$

4.5.3 Separation of $K^\pm$ from $\pi^\pm$ and measurement of $B_s \to D_s K$

4.5.4 Work ahead

4.5.5 Preliminary conclusions

4.6 Requirements for electromagnetic calorimetry

4.6.1 Energy resolution and monophoton final states

4.6.2 Energy resolution and decays of heavy flavoured hadrons into photons or $\pi^0$

4.6.3 Requirements on the geometrical acceptance for $e^+e^- \to \gamma\gamma$ and $\ell^+\ell^- \to \ell\ell$ events at Z-pole energies

4.6.4 Sensitivity to charged lepton flavour violation: $Z \to \mu e$ and $\tau \to \mu\gamma$
<table>
  <tbody>
    <tr>
      <td>4.6.5</td>
      <td>Bremsstrahlung recovery</td>
    </tr>
    <tr>
      <td>4.6.6</td>
      <td>Prompt decays of ALPs a → γγ</td>
    </tr>
    <tr>
      <td>4.6.7</td>
      <td>Requirements from π<sup>0</sup> → γγ reconstruction in τ decays</td>
    </tr>
    <tr>
      <td>4.6.8</td>
      <td>Work ahead</td>
    </tr>
    <tr>
      <td>4.6.9</td>
      <td>Preliminary conclusions</td>
    </tr>
    <tr>
      <td>4.7</td>
      <td>Requirements for the hadron calorimeter</td>
    </tr>
    <tr>
      <td>4.7.1</td>
      <td>Reconstruction of Higgs boson hadronic final states</td>
    </tr>
    <tr>
      <td>4.7.2</td>
      <td>Measurement of the Higgs boson invisible width</td>
    </tr>
    <tr>
      <td>4.7.3</td>
      <td>Search for heavy neutral leptons</td>
    </tr>
    <tr>
      <td>4.7.4</td>
      <td>Work ahead</td>
    </tr>
    <tr>
      <td>4.7.5</td>
      <td>Preliminary conclusions</td>
    </tr>
    <tr>
      <td>4.8</td>
      <td>Requirements for the muon detector</td>
    </tr>
    <tr>
      <td>4.9</td>
      <td>Precise timing measurements</td>
    </tr>
    <tr>
      <td>4.9.1</td>
      <td>Time-of-flight measurements</td>
    </tr>
    <tr>
      <td>4.9.2</td>
      <td>Time measurements very close to the IP</td>
    </tr>
    <tr>
      <td>4.9.3</td>
      <td>Time measurements in the calorimeters</td>
    </tr>
    <tr>
      <td>4.10</td>
      <td>Selected studies with full simulation</td>
    </tr>
    <tr>
      <td>4.10.1</td>
      <td>Machine-learning event reconstruction</td>
    </tr>
    <tr>
      <td>4.10.2</td>
      <td>Jet flavour tagging</td>
    </tr>
    <tr>
      <td>4.10.3</td>
      <td>Higgs boson mass determination</td>
    </tr>
    <tr>
      <td>4.10.4</td>
      <td>Tau polarisation</td>
    </tr>
    <tr>
      <td>4.10.5</td>
      <td>Summary of detector requirements</td>
    </tr>
    <tr>
      <td>4.11</td>
      <td>Outlook</td>
    </tr>
    <tr>
      <td>5</td>
      <td>Machine-detector interface</td>
    </tr>
    <tr>
      <td>5.1</td>
      <td>Interaction region layout</td>
    </tr>
    <tr>
      <td>5.2</td>
      <td>Integration and alignment</td>
    </tr>
    <tr>
      <td>5.3</td>
      <td>Maintenance and detector opening</td>
    </tr>
    <tr>
      <td>5.4</td>
      <td>Beam-induced backgrounds in the detectors</td>
    </tr>
    <tr>
      <td>5.5</td>
      <td>Implementation tests and prototyping</td>
    </tr>
    <tr>
      <td>5.6</td>
      <td>Outlook</td>
    </tr>
    <tr>
      <td>6</td>
      <td>Detector concepts and systems</td>
    </tr>
    <tr>
      <td>6.1</td>
      <td>Detector concepts</td>
    </tr>
    <tr>
      <td>6.2</td>
      <td>The CLD and ILD detector concepts</td>
    </tr>
    <tr>
      <td>6.3</td>
      <td>The IDEA detector concept</td>
    </tr>
    <tr>
      <td>6.4</td>
      <td>The ALLEGRO detector concept</td>
    </tr>
    <tr>
      <td>6.5</td>
      <td>Vertex detectors</td>
    </tr>
    <tr>
      <td>6.6</td>
      <td>Main tracking systems</td>
    </tr>
    <tr>
      <td>6.6.1</td>
      <td>Silicon tracker</td>
    </tr>
    <tr>
      <td>6.6.2</td>
      <td>Drift chamber</td>
    </tr>
    <tr>
      <td>6.6.3</td>
      <td>Straw tracker</td>
    </tr>
    <tr>
      <td>6.6.4</td>
      <td>Time projection chamber</td>
    </tr>
    <tr>
      <td>6.7</td>
      <td>Particle identification</td>
    </tr>
    <tr>
      <td>6.7.1</td>
      <td>Precision timing detector</td>
    </tr>
    <tr>
      <td>6.7.2</td>
      <td>Compact RICH detector</td>
    </tr>
    <tr>
      <td>6.8</td>
      <td>Electromagnetic calorimeters</td>
    </tr>
    <tr>
      <td>6.8.1</td>
      <td>Silicon pads, MAPS, scintillator strips</td>
    </tr>
    <tr>
      <td>6.8.2</td>
      <td>Noble liquid</td>
    </tr>
    <tr>
      <td>6.8.3</td>
      <td>Segmented crystals with dual-readout</td>
    </tr>
    <tr>
      <td>6.8.4</td>
      <td>The GRAiNITA ECAL</td>
    </tr>
    <tr>
      <td>6.9</td>
      <td>Hadron calorimeters</td>
    </tr>
    <tr>
      <td>6.9.1</td>
      <td>Scintillator tiles</td>
    </tr>
    <tr>
      <td>6.9.2</td>
      <td>Gaseous detectors</td>
    </tr>
    <tr>
      <td>6.9.3</td>
      <td>Dual readout</td>
    </tr>
  </tbody>
</table>

<figure><img src="image_5.png" /></figure>
<table>
  <tbody>
    <tr>
      <td>6.10</td>
      <td>Coil</td>
    </tr>
    <tr>
      <td>6.11</td>
      <td>Cryostat</td>
    </tr>
    <tr>
      <td>6.12</td>
      <td>Muon system</td>
    </tr>
    <tr>
      <td>6.13</td>
      <td>Luminosity measurement</td>
    </tr>
    <tr>
      <td>6.13.1</td>
      <td>LumiCal design</td>
    </tr>
    <tr>
      <td>6.14</td>
      <td>Outlook</td>
    </tr>
    <tr>
      <td>7</td>
      <td>FCC cavern infrastructure</td>
    </tr>
    <tr>
      <td>7.1</td>
      <td>Introduction</td>
    </tr>
    <tr>
      <td>7.2</td>
      <td>The FCC-hh reference detector</td>
    </tr>
    <tr>
      <td>7.3</td>
      <td>FCC cavern infrastructure</td>
    </tr>
    <tr>
      <td>8</td>
      <td>Software and computing</td>
    </tr>
    <tr>
      <td>8.1</td>
      <td>The FCC software ecosystem</td>
    </tr>
    <tr>
      <td>8.2</td>
      <td>The main components of KEY4HEP</td>
    </tr>
    <tr>
      <td>8.3</td>
      <td>The event data model EDM4HEP</td>
    </tr>
    <tr>
      <td>8.3.1</td>
      <td>A specific use case: LEP data in EDM4HEP</td>
    </tr>
    <tr>
      <td>8.4</td>
      <td>Integration of event generators</td>
    </tr>
    <tr>
      <td>8.4.1</td>
      <td>Event generators as packages</td>
    </tr>
    <tr>
      <td>8.4.2</td>
      <td>Common data formats</td>
    </tr>
    <tr>
      <td>8.4.3</td>
      <td>Configuration</td>
    </tr>
    <tr>
      <td>8.5</td>
      <td>Parametrised simulation</td>
    </tr>
    <tr>
      <td>8.6</td>
      <td>Full simulation</td>
    </tr>
    <tr>
      <td>8.6.1</td>
      <td>Detector description and simulation strategy</td>
    </tr>
    <tr>
      <td>8.6.2</td>
      <td>Sub-detector models</td>
    </tr>
    <tr>
      <td>8.6.3</td>
      <td>Full detector models</td>
    </tr>
    <tr>
      <td>8.7</td>
      <td>Digitisation</td>
    </tr>
    <tr>
      <td>8.8</td>
      <td>Reconstruction</td>
    </tr>
    <tr>
      <td>8.9</td>
      <td>Analysis tools</td>
    </tr>
    <tr>
      <td>8.10</td>
      <td>Visualisation</td>
    </tr>
    <tr>
      <td>8.11</td>
      <td>Computing resources</td>
    </tr>
    <tr>
      <td>8.11.1</td>
      <td>Resource modelling</td>
    </tr>
    <tr>
      <td>8.11.2</td>
      <td>Modelling resources for FCC-ee</td>
    </tr>
    <tr>
      <td>8.11.3</td>
      <td>Minimal baseline working scenario</td>
    </tr>
    <tr>
      <td>8.11.4</td>
      <td>Optimising resources: software</td>
    </tr>
    <tr>
      <td>8.11.5</td>
      <td>Optimising resources: analysis techniques</td>
    </tr>
    <tr>
      <td>8.11.6</td>
      <td>Optimising resources: workload and data management</td>
    </tr>
    <tr>
      <td>8.11.7</td>
      <td>Increasing available resources: pledged resources</td>
    </tr>
    <tr>
      <td>8.11.8</td>
      <td>Increasing available resources: opportunistic resources</td>
    </tr>
    <tr>
      <td>8.11.9</td>
      <td>Projecting to the Z run</td>
    </tr>
    <tr>
      <td>8.12</td>
      <td>Human resources: status and needs</td>
    </tr>
    <tr>
      <td>8.13</td>
      <td>Outlook</td>
    </tr>
    <tr>
      <td>9</td>
      <td>Energy calibration, polarisation, monochromatisation</td>
    </tr>
    <tr>
      <td>9.1</td>
      <td>Overview</td>
    </tr>
    <tr>
      <td>9.2</td>
      <td>Input from the experiments</td>
    </tr>
    <tr>
      <td>9.2.1</td>
      <td>The crossing angle α</td>
    </tr>
    <tr>
      <td>9.2.2</td>
      <td>The longitudinal boost and the collision-energy spread</td>
    </tr>
    <tr>
      <td>9.2.3</td>
      <td>Relative √s determination in the Z-resonance scan</td>
    </tr>
    <tr>
      <td>9.2.4</td>
      <td>Absolute √s determination</td>
    </tr>
    <tr>
      <td>9.3</td>
      <td>Expected precision on EW observables from the collision energy and its spread</td>
    </tr>
    <tr>
      <td>9.4</td>
      <td>Prospects for monochromatisation and the measurement of the electron Yukawa coupling</td>
    </tr>
    <tr>
      <td></td>
      <td>The electron Yukawa via resonant Higgs production at 125 GeV</td>
    </tr>
    <tr>
      <td></td>
      <td>FCC-ee monochromatisation</td>
    </tr>
    <tr>
      <td>9.5</td>
      <td>Outlook</td>
    </tr>
    <tr>
      <td>10</td>
      <td>Community building</td>
    </tr>
  </tbody>
</table>
<table><tr><td>11 Outlook</td></tr><tr><td>References</td></tr></table>

## Note from the Editors

One of the recommendations of the 2020 update of the European Strategy for Particle Physics was that “Europe, together with its international partners, should investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an electron-positron Higgs and electroweak factory as a possible first stage.

In June 2021, the CERN Council launched the FCC Feasibility Study to be completed by 2025, in time for the next update of the European Strategy for Particle Physics. The study results are made publicly available through this FCC Feasibility Study Report, as input to the European Particle Physics Strategy update process, initiated by the CERN Council in March 2024. The studies presented in this FCC Feasibility Study Report do not imply any commitment by the CERN Member or Associate Member States to build the Future Circular Collider.

This report and the assumptions contained in it do not prejudice further territorial feasibility analysis by the Host States, France and Switzerland, as well as the outcome of their respective public debate and concertation processes, and future decisions of their relevant authorities.

## Preface from CERN's Director-General

In 2021, in response to the 2020 update of the European Strategy for Particle Physics, the CERN Council initiated the Future Circular Collider (FCC) Feasibility Study.

This report summarises an immense amount of work carried out by the international FCC collaboration over several years. It covers, inter alia, physics objectives and potential, geology, civil engineering, technical infrastructure, territorial implementation, environmental aspects, R&D needs for the accelerators and detectors, socio-economic benefits and cost. It constitutes important input for the ongoing update of the European Strategy for Particle Physics.

The Feasibility Study required engagement with a broad range of stakeholders. In particular, throughout the Study, CERN has been accompanied by its two Host States, France and Switzerland, and has been working with entities at local, regional and national level. I am very grateful to the Host State authorities and teams for their invaluable help. Furthermore, significant sections of the Study were supported by the European Union under the Horizon 2020 and Horizon Europe framework programmes. The Study also greatly benefited from contributions from accelerator laboratories and universities from across Europe, such as the Swiss Accelerator Research and Technology (CHART) initiative, and from the Americas, Asia, Africa and Australia.

The proposed FCC integrated programme consists of two possible stages: an electron-positron collider serving as a Higgs-boson, electroweak and top-quark factory running at different centre-of-mass energies, followed at a later stage by a proton-proton collider operating at an unprecedented collision energy of around 100 TeV. The complementary physics programmes of each stage match the physics priorities expressed in the 2020 update of the European Strategy for Particle Physics.

A major achievement of the Feasibility Study is the choice of placement of the collider ring and the entire infrastructure, including the surface sites and the access shafts, which was developed and optimised over several years following the principle “avoid, reduce, compensate”. Sustainability studies have assessed energy efficiency, land use, water and resource management, and socio-economic impact, ensuring that the FCC is designed in accordance with the latest environmental and societal standards.

I would like to thank all contributors to this report for their hard work and commitment, which allowed the outstanding results presented here to be achieved.

Fabiola Gianotti

CERN, Director-General

<figure><img src="image_6.png" /></figure>
Preface from the FCC Collaboration Board Chair

Building on the earlier Future Circular Collider (FCC) Conceptual Design Study conducted between 2014 and 2018, the FCC Feasibility Study (2021–2025) has been undertaken by a robust international collaboration, now comprising over 160 institutes worldwide. The FCC “integrated programme”, developed in the framework of the Feasibility Study, consists of an initial electron-positron collider, the FCC-ee, which could be followed by a proton-proton collider, the FCC-hh. This staging takes into account the physics priorities as formulated in the updates of the European Strategy for Particle Physics of 2012 and 2020, as well as the relative technology readiness and costs of the FCC-ee and FCC-hh.

Over the years, I have closely followed the steady progress of the study, representing the FCC collaboration at the inter-
national steering committee and participating in annual FCC Week meetings, which include sessions of the International
Collaboration Board. The commitment and enthusiasm of the members of the collaboration has always been impressive.
The collective effort is clearly visible. Participation by students and early-career researchers is increasing. There is a shared
determination and momentum to move forward.

The strong international collaboration around the FCC and its global network provide a solid foundation for the future of this project. The FCC community continues to grow, with increasing engagement from new institutes and partners worldwide. This broad support will be essential as the project enters its next phase.

The FCC Feasibility Study demonstrates not only the technical viability of the project, but also the strength of the inter-
national community that supports it. As we move towards the next step in the decision-making phase, this collective effort is
key to showing a possible path forward. The FCC promises far-reaching scientific opportunities and long-term benefits for
innovation, training, and global collaboration in science and technology.

Philippe Chomaz

CEA, Chair of the FCC International Collaboration Board

1 Overview

The particle physics landscape has been profoundly influenced by the discovery of a Higgs boson with a mass around 125 GeV at the LHC [1,2]. The long-predicted matrix of particles and interactions of the Standard Model (SM) is now complete, and this consistent and predictive theory has so far been successful at describing all phenomena accessible to collider experiments. After almost 15 years of LHC operation, the remarkable precision measurements and the many exploratory searches have demonstrated its validity and excluded signs of new physics across an order of magnitude of energies around the TeV scale. This notwithstanding, several fundamental experimental facts remain unexplained in the current framework, such as the abundance of matter over antimatter, the evidence for dark matter, or the non-zero neutrino masses, and many theoretical issues definitively also require physics beyond the present Standard Model (BSM), altogether calling for intensified collider exploration. For the first time since the Fermi theory, however, the field of possible explanations to these unanswered questions provides little guidance for what form this exploration may take.

The confirmation by the LHC of the SM predictions up to the TeV range requires a different approach to addressing these
open questions. Solutions could exist at even higher energy, at the price of either an *unnatural* value of the weak scale or an
ingenious but still elusive structure. Instead, radically new physics scenarios have recently been devised, which often include
light and feebly coupled structures [3–5]. Neither the mass scale (from meV to ZeV) of this new physics nor the intensity of
the couplings to the SM (from 1 to 10<sup>-12</sup> or less) are known, thus calling for a new, broad, and powerful tool of exploration.
To experimentally push the limits of the unknown as far as possible with a real chance of discovery, this tool must be able to
address the following goals in the broadest and deepest possible way.

- Map the properties of the Higgs and electroweak (EW) gauge bosons, pinning down their interactions with accuracies order(s) of magnitude better than today, and acquiring sensitivity to, e.g., the processes that led to the formation of today's Higgs vacuum field during the time span between 10<sup>-12</sup> and 10<sup>-10</sup> s after the Big Bang.

- Sharpen our knowledge of already identified particle physics phenomena with a comprehensive and accurate campaign of precision electroweak, QCD, flavour, Higgs, and top measurements, sensitive to tiny deviations from the predicted Standard Model behaviour and probing energy scales far beyond the direct kinematic reach. Such a campaign requires running at the intensity frontier with large collected event samples, exquisitely precise experimental conditions and theoretical calculations, as well as a maximal amount of synergies within the programme.
- Improve by orders of magnitude the sensitivity to rare and elusive phenomena at low energies, including the possible discovery of light particles with very small couplings (e.g., massive neutrinos, and/or axion-like particles). In particular, the search for dark matter should seek to reveal, or conclusively exclude, dark sector candidates belonging to broad classes of models.

- Improve, by at least an order of magnitude, the direct discovery reach for new particles at the energy frontier.

The entirely new context that led to these ambitious objectives calls for the highest integrated luminosities at the electroweak and Higgs scales, and for parton-parton collision centre-of-mass energies an order of magnitude above the TeV scale, already extensively probed by the LHC (and further so by the HL-LHC) direct searches. The 2021 update of the European Strategy for Particle Physics (ESPPU) [6] and the U.S. Community Study on the Future of Particle Physics (Snowmass '21) [7] now require an $e^{+}e^{-}$ Higgs factory with the highest priority, to complete and deepen the trailblazing Higgs boson measurements performed at the LHC and HL-LHC. In addition, the 2021 ESPPU and the 2023 U.S. P5 report have both highlighted the long-term vision [6,8] to operate a 10 TeV parton centre-of-mass energy (pCM) collider following this $e^{+}e^{-}$ programme. Specifically, Europe's longer-term ambition [6] is to operate a proton-proton collider at the highest achievable energy, sensitive to energy scales an order of magnitude higher than those reached at the LHC. By providing considerable advances in sensitivity, precision, and energy far above the TeV scale, the Future Circular Collider (FCC) matches the present landscape and the above requirements to near perfection. Indeed, the FCC starts with a luminosity-frontier $e^{+}e^{-}$ machine spanning centre-of-mass energies from below the Z pole to beyond the top-pair production threshold (FCC-ee), later evolving to an energy-frontier hadron collider (FCC-hh). Both machines are strongly motivated in their own right, as shown in the rest of this section. In particular, the full programme will provide the best possible sensitivity to new physics in the Higgs sector: with almost three million Higgs bosons, FCC-ee will offer in a few years model-independent measurements of the Higgs boson mass, width, and couplings to Z, W, $\tau$, b, c, order-of-magnitude more precise than today, to probe the possible BSM origin of Electroweak Symmetry Breaking. Ultimately, FCC-hh will produce 20 billion Higgs bosons and, in combination with FCC-ee, provide the most complete and precise set of measurements of the Higgs self-coupling, top Yukawa coupling and of other rare or invisible modes probing in new directions the processes that led to the formation of today's Higgs vacuum field. Efforts to understand, analyse, and document the broad physics potential of such an ambitious programme started in 2012 [9] and culminated in the FCC Conceptual Design Report (CDR) in 2018 [10–12].

From the scale of Fermi interactions to that of the W and Z bosons, from the EW precision measurements to the observation of the top quark and of the Higgs boson, precision has always provided the route to new discoveries; the FCC will be no exception in this respect. Any deviation from the SM predictions, interpreted as the manifestation of higher-dimensional operators, will point to a new energy scale that will be explored directly later on. Furthermore, correlations among several deviations will be instrumental in characterising the structure of new physics and to reveal its exact or approximate global symmetries. In that regard, the fact that the SM features some a priori 'accidental' selection rules (lepton and baryon number conservation, custodial symmetry, suppression of flavour-changing neutral currents, smallness of the mixing among the different quarks, collective suppression of CP violation effects) that generic new physics scenarios do not share, is a welcome virtue and a promise for future discoveries or deeper understanding. The expected FCC precision is such that much will be learned, regardless of the outcome: theories beyond the SM will be very much constrained, even with a null result, and will further guide new models from these precision measurements. When a new physics signal is eventually observed, these precision measurements (whether they agree with or deviate from the SM) will be precious in establishing the nature of the signal.

The presently proposed schedule of the FCC programme has 15 years of FCC-ee operation followed by 25 years of FCC-hh operation, interleaved with a shutdown of 10 years to dismantle the lepton collider and install the hadron collider in the tunnel, for a grand-total of 50 years, a similar duration to that of the current LEP + LHC programme (1989–2041). This sequence optimises the overall investment and its science value for several fundamental reasons: (i) the powerful physics complementarity of the two machines, leading to the broadest exploration potential; (ii) the synergy of the infrastructure, which leads to a considerable cost saving and reduces the financial burden; (iii) the implementation schedule, which opens a time window of at least 25 years for the development of the critical technology of high-field magnets for the hadron collider, reducing the financial and technological risks; and (iv) the duration of the programme and its strategic importance, which makes it conceivable to obtain the upfront funding for the common infrastructure. The recyclable infrastructure and the large integrated luminosities also significantly improve the global sustainability of the particle physics worldwide endeavour over the 21st century, by minimising the operation time, the electricity consumption, the cost, and the carbon emissions for a given scientific outcome [13,14].
**Fig. 1** The FCC-ee baseline design luminosity, summed over 4 IPs, displayed as a function of the centre-of-mass energy, from the Z pole to the $t\bar{t}$ threshold and beyond (red curve). The luminosity typically achievable by linear $e^+e^-$ Higgs factories with a single IP in their baseline design between 250 and 380 GeV is also indicated (dash-dotted oval in the lower-right corner of the figure)

<figure><img src="image_7.png" /><figcaption>**Table 1** The baseline FCC-ee operation model with four interaction points, showing the centre-of-mass energies, design instantaneous luminosities for each IP, and integrated luminosity per year summed over 4 IPs. The integrated luminosity values correspond to 185 days of physics per year and a 75% operational efficiency (i.e., $1.2 \times 10^7$ sec-</figcaption></figure>

onds per year) [15], in the Z, WW, ZH, and $t\bar{t}$ baseline sequence. The last two rows indicate the total integrated luminosity and number of events expected to be produced in the four detectors. The number of WW events includes all $\sqrt{s}$ values from 157.5 GeV up

<table><thead><tr><th>Working point</th><th>Z pole</th><th>WW thresh.</th><th>ZH</th><th>t<span style="text-decoration: overline;">t</span></th><th></th></tr></thead><tbody><tr><td>√s (GeV)</td><td>88, 91, 94</td><td>157, 163</td><td>240</td><td>340–350</td><td>365</td></tr><tr><td>Lumi/IP (10<sup>34</sup> cm<sup>-2</sup>s<sup>-1</sup>)</td><td>140</td><td>20</td><td>7.5</td><td>1.8</td><td>1.4</td></tr><tr><td>Lumi/year (ab<sup>-1</sup>)</td><td>68</td><td>9.6</td><td>3.6</td><td>0.83</td><td>0.67</td></tr><tr><td>Run time (year)</td><td>4</td><td>2</td><td>3</td><td>1</td><td>4</td></tr><tr><td>Integrated lumi. (ab<sup>-1</sup>)</td><td>205</td><td>19.2</td><td>10.8</td><td>0.42</td><td>2.70</td></tr><tr><td>Number of events</td><td>6 × 10<sup>12</sup> Z</td><td>2.4 × 10<sup>8</sup> WW</td><td>2.2 × 10<sup>6</sup> ZH<br>+<br>65k WW → H</td><td>2 × 10<sup>6</sup> t<span style="text-decoration: overline;">t</span><br>+ 370k ZH<br>+ 92k WW → H</td><td></td></tr></tbody></table>

## 1.1 FCC-ee: a great Higgs factory, and so much more

With its high luminosity, its clean experimental conditions, its multiple interaction regions, and a range of energies that cover the four heaviest elementary particles known today, FCC-ee offers a most comprehensive physics exploration programme as a Higgs, electroweak, QCD, flavour, and top factory, with high potential for discoveries. The baseline plan, confirmed by the feasibility study mid-term review recommendations, considers operating detectors at four interaction points (IPs), spanning the $e^+e^-$ centre-of-mass energies around the Z pole, the WW threshold, the ZH production maximum, up to the $t\bar{t}$ threshold and just above. The current values for the luminosities expected at these energies are displayed in Fig. 1 and the envisioned 15-year experimental programme is summarised in Table 1, together with the numbers of events expected at each energy.

The currently envisioned working hypotheses for the operation model [16], i.e. for the overall sequence and the duration of each step, will be continuously optimised in the coming years. An example of a baseline sequence is displayed in Fig. 2, with the Z, WW, and ZH runs assumed to happen in this chronological order. In reality, however, there will be quasi-total flexibility in the choice of the running sequence. (See inset below: ‘A quasi total flexibility’.) This flexibility will accommodate the likely requests of the user community and will also fit the possible need for runs at different energies in view of complementary measurements.
<figure><img src="image_8.png" /><figcaption>**Fig. 2** Baseline operation model for FCC-ee with four interaction points, showing the integrated luminosity at the Z pole (green), the WW threshold (blue), the Higgs factory (red), and the t\bar{t} threshold (orange) as a function of time. In this baseline model, the sequence of e2vents follows the increase in collision energy, but there is quasi-total flexibil-</figcaption></figure>

ity in the sequence all the way to 240 GeV (see inset). The integrated luminosities delivered during the first two years at the Z pole and the first year at the t\bar{t} threshold are half of the annual design value. The hatched area indicates the shutdown time needed to prepare for the higher energy runs at the t\bar{t} threshold and beyond

## A quasi-total flexibility

In Ref. [17] it was deemed essential to establish the technical and financial feasibility of scheduling a Z pole run after the ZH run or the WW threshold run, ideally with a first Z pole run during the initial period of FCC-ee operation. Indeed, the Z pole run, while extremely fertile in physics opportunities, is undoubtedly the most ambitious and demanding part of the programme from all perspectives (accelerator, energy calibration, detectors systematic biases, theory calculations). It will be extremely challenging to achieve all the goals of the Z pole run during the first four years of the collider operation.

This requirement from physics was compounded by a recommendation from the mid-term review committees to consolidate (and ideally simplify) the design of the RF system to allow efficient energy-staging, as well as to reduce complexity, risk, and cost; and to study options to avoid the 1-cell/2-cell RF cavity reconfiguration between Z and ZH/WW running, in order to simplify the SRF system implementation and to improve flexibility in the physics programme. The new versatile RF system designed accordingly (see Ref. [18]) enables a quasi-total flexibility to choose the running sequence. For example, it would allow for short (few weeks) initial Z pole and WW threshold runs, to commission the collider and the detectors, to establish the resonant depolarisation procedures for centre-of-mass energy calibration, etc. The ZH run could then proceed early, before going back to the Z pole and the WW threshold, both now at full luminosity, with fully functional resonant depolarisation and a complete understanding of the collider.

It will ultimately be up to the experimental collaborations and the relevant scientific committees to optimise the time flexibility and to tailor the FCC-ee operation scenario according to a number of factors that cannot be mastered today. For example, external events such as the FCC-hh magnet readiness or the CERN financial situation may call for a change in the overall duration of the FCC-ee running time. Meanwhile, new avenues will be explored during the next phase of the FCC study, between April 2025 and the end of 2027, towards increasing the luminosities at all energies, as it would be highly beneficial across the whole programme and would make a qualitative difference in a number of cases.

The original motivation for an e⁺e⁻ circular collider was to create a high-luminosity Higgs Factory, operating at $\sqrt{s} = 240$ GeV in the LEP/LHC tunnel [19–21]. Choosing to build it in the 80–100 km tunnel that would ultimately be hosting a 100 TeV hadron collider was cardinal in making this machine unique on the Higgs factory market. To start with, such a tunnel enables a highly versatile Higgs Factory that extends much beyond the study of the Higgs boson alone, for three main reasons:

1. A circumference of 80–100 km is required for an e⁺e⁻ circular collider to reach (for the first time in e⁺e⁻ collisions) the top-pair production threshold, making the FCC-ee energy range optimal to sharpen and challenge our current physics knowledge by studying all SM particles. In particular, enabling precise top quark measurements is essential for the overall FCC electroweak and Higgs precision physics programme.

2. For a given power and centre-of-mass energy, the luminosity of a circular collider is roughly proportional to the ring circumference. A circumference of 80–100 km makes FCC-ee the e⁺e⁻ electroweak, Higgs, and top factory project with the highest luminosity proposed to date, able to produce, when summing all the data collected at the 4 IPs, $6 \times 10^{12}$ Z bosons, $2.4 \times 10^8$ W pairs, almost $3 \times 10^6$ Higgs bosons, and $2 \times 10^6$ top pairs, in as little as 15 years. (See also inset below: 'Rationale for the operation model').

<figure><img src="image_9.png" /></figure>
## Rationale for the operation model

(From the mid-term review) It would also be interesting to add more information in the final Feasibility report about the optimisation of the durations of the various energy runs ($Z$, $WW$, $ZH$, $t\bar{t}$) as well as a more detailed and quantitative discussion demonstrating the importance of the $t\bar{t}$ running.

The baseline set of centre-of-mass energies and integrated luminosities is found to be the most sensible way to distribute luminosity within the 15 years overall running constraint. It is deemed sufficient to establish a minimal, yet remarkable, physics outcome for such a collider, with the smallest possible set of centre-of-mass energies that enables a study of all particles of the SM with a real chance of discovery.

- At the Z pole ($\sqrt{s} = 91.2$ GeV), at least $5 \times 10^{12}$ Z (in 2 years) enables otherwise unreachable flavour (b, c, $\tau$) physics, studies of QCD and hadronisation, the search for rare or forbidden decays, and the exploration of the dark sector. Together with the runs at $\sqrt{s} \simeq 88$ and 94 GeV, the Z pole data yield 50 to 1000-fold improved measurements of the Z line-shape and many electroweak precision observables (EWPO), including the Z invisible width. An integrated luminosity of at least $40 \text{ ab}^{-1}$ at the two energies, just below and just above the Z pole (1 year each), was deemed appropriate for a direct and statistically limited determination of $\alpha_{QED}(m_Z)$ and $\alpha_S(m_Z)$, which otherwise would greatly limit the new physics interpretation of all measurements of $\sin^2\theta_W^{\text{eff}}$. One of the findings of the study has been the realisation of how rich the run at and around the Z pole is for the FCC-ee physics outcome.

- An integrated luminosity of at least $20 \text{ ab}^{-1}$ in 2 years around the $W^+W^-$ production threshold, evenly shared between $\sqrt{s} = 157.5$ and 162.5 GeV, is needed for the measurement of the W mass (and decay width) with a statistical precision commensurate with the expected precision of the centre-of-mass energy determination. These data are also important for, in particular, the determination of the number of neutrino species and for an independent measurement of the strong coupling constant.

- An integrated luminosity of at least $10 \text{ ab}^{-1}$ in 3 years at $\sqrt{s} = 240$ GeV provides model-independent measurements of the Higgs boson couplings from a combination of its branching fractions and of the total ZH production cross section. In particular, a per-mil precision can be reached on the coupling of the Higgs boson to the Z, which greatly constrains new physics coupled to the Higgs boson.

- A short run to scan the $t\bar{t}$ threshold ($\sqrt{s} = 340-350$ GeV) allows the measurement of the top-quark mass, a fundamental parameter of the Standard Model, with a precision of $\mathcal{O}(10 \text{ MeV})$, for which hadron colliders cannot compete. Because the prediction of EWPO's is, in various ways, sensitive to $m_{\text{top}}$, the discovery power of this EWPO exploration is limited by the uncertainty on $m_{\text{top}}$. For example, matching the precision of the SM predictions from the EWPO measurements to the 180 keV (resp. 4 keV) statistical uncertainty on the W mass (resp. Z width) requires a 20 MeV (resp. 15 MeV) knowledge of $m_{\text{top}}$. It is only when the mass of the top quark mass is measured with this precision that the FCC-ee EWPO measurements give their best sensitivity, typically increasing the reach in new physics energy scale by 60%.

- The ZH cross section dependence on $\sqrt{s}$ provides sensitivity to the Higgs boson self-coupling when data at 365 GeV are available, allowing a 5$\sigma$ discovery of this coupling to be contemplated. More importantly, the per-cent measurement of the top EW couplings (i) matches the EWPO ppm precision at the Z pole and the WW threshold; and (ii) keeps the theoretical uncertainties on the top Yukawa coupling determination at FCC-hh at the per-cent level, a pre-requisite for the model-independent determination of the Higgs boson self-coupling.

3. A circumference of 80–100 km is also required for FCC-ee to offer an excellent control of the centre-of-mass energy, not only at the Z pole but also at the WW threshold, with the use of resonant depolarisation [22,23]. At the Z pole, the centre-of-mass energy scale should be known to 1 ppm or better, with a point-to-point residual uncertainty of 28 keV and a per-mil level spread. These parameters are key inputs to the electroweak precision programme [24]. Details are given in Sect. 9. The resulting precisions on the Z mass and width, the forward-backward asymmetry of muon pairs around the Z pole, the electromagnetic coupling constant $\alpha_{QED}(m_Z)$, as well as on the W mass, are listed in the first rows of Table 2.

Several other fundamental aspects add to the uniqueness of FCC-ee.

1. One of the findings of the FCC feasibility study has been the realisation of how rich the intensity-frontier multi-Tera-Z run is for the FCC-ee physics outcome. It promises comprehensive measurements of the Z lineshape and many EWPOs with at least fifty-fold improved precision, as well as direct and precise determinations of the $\alpha_{QED}(m_Z)$ [25,26] and $\alpha_S(m_Z)$ interaction couplings (Table 2), which would otherwise be dominant parametric uncertainties for virtually all precision SM calculations. The comparison of these data with commensurately accurate SM predictions is a way to reveal the existence of new physics (or to severely constrain its properties) through virtual loops or mixing: a factor of 50 in precision corresponds to a factor of 7 in energy scale, representing a step towards discovery (Sect. 2.3 and Refs. [27,28]) similar to that from LHC to FCC-hh. This Z pole run offers more than ‘just higher precision’. It also enables otherwise unreachable flavour (b, c, $\tau$) physics, studies of QCD and hadronisation, the search for rare or forbidden decays, the exploration of the dark sector, and significantly increases sensitivity to new feebly interacting particles, altogether further increasing the prospects for discovery.

2. A feature unique to circular colliders is the ability to serve several IPs. Another finding of the FCC Feasibility Study has been the demonstration of the importance of operating four detectors (instead of only two in the 2014–2018 Conceptual Design Study), which led to an optimisation of the ring layout with a new four-fold periodicity (further increasing the synergies with FCC-hh). In a configuration with 4 IPs, the FCC science value for the investment is maximised in multiple ways. (See inset below: ‘The benefits of four interaction points’.)

3. Finally, FCC-ee stands out among the Higgs Factory projects for its opportunity to access the Higgs boson coupling to electrons [29–31], through the resonant production process $e^+e^- \to H$ at $\sqrt{s} = 125$ GeV [32]. This measurement relies on the combination of the high luminosity, the possibility of operating four detectors, the continuous ppm centre-of-mass
energy control, and the ability for centre-of-mass monochromatisation [33]. If proven feasible, this would be one of the toughest challenges for FCC-ee.

As a Higgs factory, FCC-ee has all the advantages of an $e^+e^-$ collider running in the vicinity of the ZH cross section maximum: the measurement of this cross section by counting events with an identified Z boson (and for which the mass recoiling against the Z gathers around the Higgs boson mass [34], independently of the Higgs boson properties details) provides a precise and model-independent determination of $\kappa_Z$, the Higgs boson coupling to the Z. This absolute measurement can then be used as a 'standard candle' by all other measurements, including those made at hadron or muon colliders. The position of the Z recoil mass peak also provides an accurate measurement of the Higgs boson mass from the precise knowledge of the centre-of-mass energy. In combination with the measurement of the rate of ZH events with a $H \to ZZ^*$ decay, proportional to $\kappa_Z^4/\Gamma_H$, a model-independent determination of the total width $\Gamma_H$ can be obtained. The analysis of the other decays provides a set of model-independent partial width and coupling measurements.

## The benefits of four interaction points

(From the mid-term review) *The Scientific Advisory Committee recommends to construct 4 IPs for FCC-ee from the beginning. The Scientific Policy Committee finds the proposal to include more than two IPs an attractive option. However, we expect, for the final Feasibility report, arguments focusing not only on the naïve luminosity gain but also on the overall physics optimisation.*

**Detector diversity** The diverse set of demanding physics measurements and searches at FCC-ee leads to many different challenging detector requirements, which cannot be simultaneously satisfied by only one or even two detectors. Instead, four FCC-ee interaction points allow for a broader diversity of detector solutions to be explored and, therefore, a wider community with diverse interests and skills to join the FCC-ee project, with a better perspective of optimally covering all FCC-ee physics opportunities. Detector diversity also allows cross-checks of results across experiments: a single experiment is quite vulnerable to unforeseen effects, and consistency between two experiments can still happen by chance. Such bad luck is much less likely with four diverse detectors. (See remark about redundancy below.)

**Improved sustainability** Operating FCC-ee with 4 IPs provides a net gain of luminosity (typically up to a factor 1.7 with respect to the 2 IP option) for a given electricity consumption. The physics outcome of 15 years operation with 4 IPs would require 25 years with just 2 IPs. Running with four experiments would decrease the FCC-ee operation carbon emissions in the same proportions. Folding in the construction carbon budget of the tunnel the additional two IPs and two detectors, it is found that operating FCC-ee with 4 IPs would reduce the total carbon footprint by ~15% for the same physics outcome as with 2 IPs, both with today's figures and with those expected in the construction and operation times of FCC-ee.

**Reduced and more robust systematic uncertainties** In order to fully benefit from the considerable FCC-ee event samples, especially at the Z pole, most of the work will focus on reducing the systematic uncertainties to match the statistical precision. As a matter of fact, many key measurements are potentially affected by detector-related systematic biases. These biases are uncorrelated between experiments, so that the related systematic uncertainties scale down as $1/\sqrt{N_{\text{experiments}}}$. Precision is also about redundancy: measuring the same quantity in several experiments can reveal sources of errors that would have been overlooked otherwise. The illustrative example closest to FCC-ee comes from the LEP measurement of the Z mass with the 1991 data, when a large discrepancy of ~20±5 MeV was noticed between the measurements from L3 and OPAL, on the one hand, and the measurements from ALEPH and DELPHI, on the other. The investigation that followed identified and solved the origin of the issue, but it could have remained unnoticed for a long time (or forever) had there been only ALEPH and DELPHI (or only L3 and OPAL) around the ring.

**Collider monitoring** With the large collision rate at the Z pole, each detector will act as sophisticated beam instrumentation by allowing quick and high-quality measurements of many beam parameters: positions and sizes of the luminous regions; longitudinal and transverse boosts of the collision at the four IPs (beam energy difference, centre-of-mass energy spread, crossing angle); the dependence of these parameters on the exact position of the collision within each bunch; and other information we have not thought of yet. These measurements at four different points of the collider provide a huge amount of information on the beam properties, which is useful for collider performance optimisation and, more specifically, on the 'energy model', which is the cornerstone of the precision programme at the Z pole, at the WW threshold, and even more so at the Higgs boson resonance ($\sqrt{s} = 125$ GeV).

**Time flexibility** The additional flexibility offered by two additional interaction points could be a game changer for the FCC-ee scientific outcome. For example, reaching the $5\sigma$ discovery level for the Higgs self-coupling, contemplating a first measurement of the electron Yukawa coupling, or securing a thorough search for sterile neutrinos, would simply be missed opportunities with only two IPs (a little over $2\sigma$ significance expected). The larger luminosity expected with four IPs allows for a different time allocation to the different centre-of-mass energies, to be optimised when the time comes, by the FCC-ee user community.

**Community building** The FCC-hh will require the participation of an even larger community of users than the LHC. The large collaborations operating the four detectors at FCC-ee would raise the overall profile of the worldwide high-energy frontier community to a level more appropriate to support the next high-energy proton collider.

**Integrated luminosity gain** Many FCC-ee measurements are statistically limited, either directly (e.g., the direct determination of $\alpha_{QED}(m_Z^2)$ from the muon forward-backward asymmetry measurements at $\sqrt{s} = 88$ and 94 GeV) or because their experimental systematic uncertainties would continue to improve with additional data, as is the case for many critical observables (e.g., the Z width, the effective mixing angle, etc.). Any increase of integrated luminosity is also welcome for searches for feebly interacting particles or for rare/forbidden processes. Therefore, these measurements and searches immediately benefit from four interaction points.

The FCC-ee best-measured Higgs boson couplings, $\kappa_Z$ and $\kappa_W$, with a precision of 0.1% and 0.23%, after 8 years of operation at $\sqrt{s} \ge 240$ GeV, respectively, would require half a century to be reached with linear $e^+e^-$ Higgs factories running either at 250 and 500 GeV or at 380 and 1500 GeV, making FCC-ee the most time- and cost-effective, as well as the most sustainable, Higgs factory of all proposed options [13, 14]. More details on the physics programme are given in Sect. 2.

The many new opportunities of physics measurements and searches at FCC-ee create at least as many challenges. Reaching experimental and theoretical systematic uncertainties commensurate with the statistical precision of the many measurements feasible at FCC-ee requires a careful study of the detector concepts, possibly of the mode of operation, and of theoretical developments.

The FCC-ee physics programme presents a number of key theoretical challenges [37]. Generally speaking, the aim is either to provide the tools to compare experimental observations to theoretical predictions at a level of precision similar or better than the (statistical) experimental uncertainties (Table 2), or to identify the additional calculations, tools, observables, or
<caption>**Table 2** Experimental (statistical and systematic) precision expected for a selection of measurements accessible at FCC-ee, compared with the present world-average precision [35]. Some of the FCC-ee experimental systematic uncertainties (4th column) are initial estimates from early 2021 [36] and others have been improved and consolidated during the Feasibility Study. A goal of further studies will be to improve</caption>

them down to the level of the statistical uncertainties (3rd column) with new ideas and innovative methods. This set of measurements, together with those of the Higgs boson properties, achieves indirect sensitivity to new physics up to a scale Λ of 100 TeV in an Effective Field Theory (EFT) description with dimension-6 operators (Sect. 2) and possibly much higher in specific new physics (non-decoupling) models

<table><thead><tr><th rowspan="2">Observable</th><th colspan="3">Present</th><th rowspan="2">FCC-ee Stat.</th><th rowspan="2">FCC-ee Syst.</th><th rowspan="2">Comment and leading uncertainty</th></tr><tr><th>Value</th><th>±</th><th>Uncertainty</th></tr></thead><tbody><tr><td>m<sub>Z</sub> (keV)</td><td>91,187,600</td><td>±</td><td>2000</td><td>4</td><td>100</td><td>From Z line shape scan<br>Beam energy calibration</td></tr><tr><td>Γ<sub>Z</sub> (keV)</td><td>2,495,500</td><td>±</td><td>2300</td><td>4</td><td>12</td><td>From Z line shape scan<br>Beam energy calibration</td></tr><tr><td>sin<sup>2</sup> θ<sub>W</sub><sup>eff</sup> (×10<sup>6</sup>)</td><td>231,480</td><td>±</td><td>160</td><td>1.2</td><td>1.2</td><td>From A<sub>FB</sub><sup>μμ</sup> at Z peak<br>Beam energy calibration</td></tr><tr><td>1/α<sub>QED</sub>(m<sub>Z</sub><sup>2</sup>) (×10<sup>3</sup>)</td><td>128,952</td><td>±</td><td>14</td><td>3.9<br>0.8</td><td>Small<br>tbc</td><td>From A<sub>FB</sub><sup>μμ</sup> off peak<br>From A<sub>FB</sub><sup>μμ</sup> on peak<br>QED&amp;EW uncert. dominate</td></tr><tr><td>R<sub>ℓ</sub><sup>Z</sup> (×10<sup>3</sup>)</td><td>20,767</td><td>±</td><td>25</td><td>0.05</td><td>0.05</td><td>Ratio of hadrons to leptons<br>Acceptance for leptons</td></tr><tr><td>α<sub>S</sub>(m<sub>Z</sub><sup>2</sup>) (×10<sup>4</sup>)</td><td>1196</td><td>±</td><td>30</td><td>0.1</td><td>1</td><td>Combined R<sub>ℓ</sub><sup>Z</sup>, Γ<sub>tot</sub><sup>Z</sup>, σ<sub>had</sub><sup>0</sup> fit</td></tr><tr><td>σ<sub>had</sub><sup>0</sup> (×10<sup>3</sup>) (nb)</td><td>41,480.2</td><td>±</td><td>32.5</td><td>0.03</td><td>0.8</td><td>Peak hadronic cross section<br>Luminosity measurement</td></tr><tr><td>N<sub>ν</sub> (×10<sup>3</sup>)</td><td>2996.3</td><td>±</td><td>7.4</td><td>0.09</td><td>0.12</td><td>Z peak cross sections<br>Luminosity measurement</td></tr><tr><td>R<sub>b</sub> (×10<sup>6</sup>)</td><td>216,290</td><td>±</td><td>660</td><td>0.25</td><td>0.3</td><td>Ratio of b¯b to hadrons</td></tr><tr><td>A<sub>FB</sub><sup>b,0</sup> (×10<sup>4</sup>)</td><td>992</td><td>±</td><td>16</td><td>0.04</td><td>0.04</td><td>b-quark asymmetry at Z pole<br>From jet charge</td></tr><tr><td>A<sub>FB</sub><sup>pol.τ</sup> (×10<sup>4</sup>)</td><td>1498</td><td>±</td><td>49</td><td>0.07</td><td>0.2</td><td>τ polarisation asymmetry<br>τ decay physics</td></tr><tr><td>τ lifetime (fs)</td><td>290.3</td><td>±</td><td>0.5</td><td>0.001</td><td>0.005</td><td>ISR, τ mass</td></tr><tr><td>τ mass (MeV)</td><td>1776.93</td><td>±</td><td>0.09</td><td>0.002</td><td>0.02</td><td>estimator bias, ISR, FSR</td></tr><tr><td>τ leptonic (μνμντ) BR (%)</td><td>17.38</td><td>±</td><td>0.04</td><td>0.00007</td><td>0.003</td><td>PID, π<sup>0</sup> efficiency</td></tr><tr><td>m<sub>W</sub> (MeV)</td><td>80,360.2</td><td>±</td><td>9.9</td><td>0.18</td><td>0.16</td><td>From WW threshold scan<br>Beam energy calibration</td></tr><tr><td>Γ<sub>W</sub> (MeV)</td><td>2085</td><td>±</td><td>42</td><td>0.27</td><td>0.2</td><td>From WW threshold scan<br>Beam energy calibration</td></tr><tr><td>α<sub>S</sub>(m<sub>W</sub><sup>2</sup>) (×10<sup>4</sup>)</td><td>1010</td><td>±</td><td>270</td><td>2</td><td>2</td><td>Combined R<sub>ℓ</sub><sup>W</sup>, Γ<sub>tot</sub><sup>W</sup> fit</td></tr><tr><td>N<sub>ν</sub> (×10<sup>3</sup>)</td><td>2920</td><td>±</td><td>50</td><td>0.5</td><td>Small</td><td>Ratio of inv. to leptonic<br>in radiative Z returns</td></tr><tr><td>m<sub>top</sub> (MeV)</td><td>172,570</td><td>±</td><td>290</td><td>4.2</td><td>4.9</td><td>From t<span style="text-decoration: overline">t</span> threshold scan<br>QCD uncert. dominate</td></tr><tr><td>Γ<sub>top</sub> (MeV)</td><td>1420</td><td>±</td><td>190</td><td>10</td><td>6</td><td>From t<span style="text-decoration: overline">t</span> threshold scan<br>QCD uncert. dominate</td></tr><tr><td>λ<sub>top</sub>/λ<sub>top</sub><sup>SM</sup></td><td>1.2</td><td>±</td><td>0.3</td><td>0.015</td><td>0.015</td><td>From t<span style="text-decoration: overline">t</span> threshold scan<br>QCD uncert. dominate</td></tr><tr><td>ttZ couplings</td><td></td><td>±</td><td>30%</td><td>0.5–1.5 %</td><td>Small</td><td>From √s = 365 GeV run</td></tr></tbody></table>
experimental inputs that are required to achieve this level of precision. Another essential line of research to be followed jointly by theorists and experimenters is to identify observables, or ratios of observables, for which experimental and/or theoretical uncertainties can be reduced. Finally, both for motivational purposes and for prioritisation, the relative impact of the various measurements on the search for new physics should be evaluated. The theoretical work motivated by the FCC programme can be organised as follows.

- Calculation of QED (mostly), EW, and QCD corrections to (differential) cross sections, needed to convert experimental measurements to so-called 'pseudo-observables': couplings, masses, partial widths, asymmetries, etc., without altering significantly the possible new-physics contributions in the original measurements. Appropriately accurate event generators are essential for the implementation of these effects in the experimental procedures.

- Calculation of the pseudo-observables with the precision required in the framework of the Standard Model so as to take full advantage of the experimental precision.

- Identification of the limiting issues, such as the questions related to the definition of parameters, in particular the treatment of quark masses and, more generally, QCD objects.

- Investigation of the sensitivity of the proposed (or new) experimental observables to the effect of new physics in a number of important, specific scenarios. This essential work must be done at an early stage, before the project is fully designed, since it potentially affects the priorities for detector concepts and for the running plan.

A community of theorists has already risen to the precision challenges, especially at the Z peak [38]. An evaluation of the options for the path ahead can be found in Refs. [37,39,40], and is developed in Sect. 3.

A complete set of detector specifications and their impact on the physics measurements is another main deliverable of the feasibility study. The performance of a number of options for the various detector components of possible future detectors: calorimeters [41], tracking and vertex detectors [42], muon detectors [43], luminometers [44], and particle identification devices [45] are being studied to understand how they could meet the requirements. More details are given in Sect. 6. The following gives a brief idea of the type of requirements that are under study.

- **Higgs and top physics** A set of requirements have been established by past linear collider studies for Higgs and top physics at 240 and 340-365 GeV, arising in particular from the desired performance of particle-flow reconstruction, flavour tagging, or lepton momentum measurement. They need to be adapted to take into account the cleaner FCC-ee experimental environment, the smaller beam-pipe radius, and the smaller maximum operating energy [46]. Additional requirements arise, related to the need for a more accurate Higgs boson mass determination [34] prior to the s-channel Higgs boson production run, and for a more accurate ZH cross section measurement, to enable a determination of the Higgs self-coupling. The s-channel Higgs boson production also leads to demanding requirements on the centre-of-mass energy monochromatisation, while keeping a high luminosity and calling for the most sensitive analysis to separate Higgs boson decays from the huge background of $e^{+}e^{-}$ annihilation events [32].

- **Tera-Z challenges** The high luminosity and large event rate at the Z pole (over 100 kHz), to which simulated data need to be added, turn into considerable challenges for data taking, storage and processing. The Z lineshape determination, which is based on cross section measurements as a function of the centre-of-mass energy for hadronic and leptonic decays of the Z, requires accurate mechanical construction and in-situ alignment of the luminometer and detector end-caps, in view of acquiring precise knowledge of the central tracker and calorimeter acceptance, for the dilepton and diphoton events (and, to a lesser extent, for hadronic events). The point-to-point centre-of-mass-energy uncertainties of the resonance scan, which can be verified in particular by means of the muon pair mass and boost reconstruction, are most relevant for the Z width and the forward-backward asymmetry measurements. The accuracy of the mass reconstruction needed sets stringent constraints on the stability of the momentum reconstruction over time and scan points [24]. With a statistical precision of 4 keV for both the Z mass and the Z width, the centre-of-mass determination is critical for the physics output of the Tera-Z run [22,47].

- **Physics with W pairs** The sub-MeV precision on the W mass expected from a scan of the WW production threshold seems, a priori, more demanding on the centre-of-mass energy calibration, and on the theoretical understanding of the cross section, than on the detector itself. At higher energies, the requirements on jet angular calibrations and the possible dependence of the precise knowledge of the composition of hadrons in these jets, however, should be revisited. Similarly, requirements on lepton angle, energy reconstruction, and energy scale, should be established [48], although they are probably covered by the requirements from the Z run.
- **Flavour challenges** The formidable progress of the performance of vertexing devices in the past two decades, paired with the five times smaller beam-pipe diameter with respect to LEP, leads to much improved b- and c-tagging performance. The resulting expected statistical uncertainties on the flavour EWPOs, such as $R_{b,c}$ or $A_{FB}^{b,c}$, allow for much larger improvements (by up to a factor of 2000) with respect to the LEP measurements of other EWPOs. In addition, the rich FCC-ee flavour programme can be fully exploited only if the detector is equipped with hadron identification covering the effective momentum range at the Z resonance [49] and electromagnetic calorimetry with a superb energy resolution [50].

- **Tau physics** The $\tau$ measurements listed in Table 2 (lifetime, mass, and branching fractions) have the potential to achieve a determination of the Fermi constant at the level of a few $10^{-5}$. These measurements provide some of the most demanding detector requirements on momentum resolution (for the mass resolution), on the knowledge of the vertex detector dimensions (for the tau lifetime), and on $e/\mu/\pi$ separation over the whole momentum range (for the leptonic branching ratios). The $\tau$-based EWPOs, $R_{\tau}^{Z}$, $A_{FB}^{\text{pol},\tau}$, and $P_{\tau}$, as well as the detailed study of the hadronic spectral functions, require fine granularity and high efficiency in the tracker and electromagnetic calorimeter [51].

- **Feebly interacting particles** The current heavy neutral lepton (HNL) search strategy is based on a rather conservative signal selection, requiring in particular an HNL decay less than 1 m away from the interaction point. With the planned integrated luminosity, the discovery region extends to the physical regions favoured by the see-saw models of neutrino masses. It would be possible to extend it by detecting HNL decays further away from the IP, e.g., by making use of the large amount of cavern space surrounding the detector [52]. Other feebly interacting particles, decaying into non-pointing or delayed photons, pose a different set of challenges, which could possibly benefit from a high-precision timing detector. See Sect. 2.3.4 for details.

Integrating all these (initial) detector requirements will be a considerable challenge, commensurate with the broad set of measurement opportunities and with the discovery potential offered by FCC-ee. A refined list of detector requirements is being derived from a number of case studies inspired by the set of physics benchmark measurements at FCC-ee (encompassing those presented in Table 2), as described in Refs. [53,54]. The four FCC interaction points will not be too many to cover the resulting variety of detector requirements and physics opportunities. More details are given in Sect. 4.

The experimental environment at FCC-ee, where the incoming beams cross typically a few million times before interacting, is gentle and clean compared to hadron colliders, muon colliders and even linear $e^{+}e^{-}$ colliders. The main challenges have been extensively studied by the Machine Detector Interface group [55]:

- The design of the $e^{+}$ and $e^{-}$ rings is asymmetric around each interaction region, in order to eliminate the need for strong bending magnets upstream of the collision points and thus minimise the synchrotron radiation background in the detectors.

- The large number of bunches and the low level of beamstrahlung radiation result in a low rate of pile-up events, typically $2 \times 10^{-3}$ at the Z pole and less than $10^{-2}$ at the top energies.

- The strong focusing optics requires a short free space (2.2 m) between the last beam elements. A compensating solenoid and quadrupole assembly that fits in a forward dead cone of no more than 100 mrad, has been designed and a complete mock-up is being prepared.

- The luminosity monitors situated in front of this assembly could be adapted to the geometry with two beam pipes crossing at an angle of 30 mrad.

- The small beam transverse dimensions allow for a beam pipe of 10 mm radius and for the first layer of the vertex detector to be installed very close to the vacuum chamber.

- In the current design with compensating solenoids, the detector solenoid magnetic field must be limited to 2 T when operating at the Z pole, to avoid a blow up of the vertical beam emittance and a resulting loss of luminosity. Alternative designs with non-local compensation are being studied.

These conditions are favourable on several accounts. The 100 mrad low angle dead cone offers the possibility to extend the detector coverage down to a polar angle of $\cos \theta \simeq 0.99$, while the 10 mm radius beam pipe should be a prime starting point for high efficiency b- and c-flavour tagging against light quarks and gluons. The 2 T magnetic field limit is not a significant handicap since the momentum of the produced partons is typically distributed around 50 GeV and does not exceed 183 GeV. If needed, an increase of the magnetic field to 3 T or more can be envisioned at higher energies, without loss of luminosity. More details about the challenges pertaining to the machine-detector interface and beam backgrounds are given in Sect. 5.

The work for the particle physicists is now clearly cut out: design the experimental setup and prepare the theoretical tools that can, demonstrably, fully exploit the capabilities of FCC-ee. Experimentation at FCC-ee is both relatively easy and
extremely demanding. The experimental conditions are clean, with essentially no pile-up, a well-defined and controllable centre-of-mass energy, and benign beamstrahlung and synchrotron radiation effects. The sophistication arises from the very richness of the programme. Matching the experimental and theoretical accuracy to the statistical precision, and the detector configuration to the variety of channels and discovery cases, will be the real challenge.

## 1.2 FCC-hh: the energy-frontier collider with the broadest exploration potential

The physics landscape emerging from the FCC-ee, complemented by the HL-LHC exploration, will significantly raise the bar for the performance requirement of subsequent colliders. Several challenges remain open.

- The extension of Higgs boson property measurements to several processes driven by smaller effective couplings (e.g., rare decays such as $H \to \gamma\gamma$, $\mu^+\mu^-$, $Z\gamma$), and to processes requiring higher energy (e.g., $t\bar{t}H$ and HH production).

- The extension of studies of EW dynamics to a regime well beyond the scale of the EW symmetry breaking.

- The direct exploration of the multi-TeV energy scale, to identify and directly study the sources of possible deviations found in the FCC-ee precision measurements, and to systematically extend the mass reach for a broad spectrum of BSM theories, particularly those whose impact on low-energy EW and Higgs observables is suppressed and those whose primary production processes require initial-state gluons.

The 100 TeV FCC-hh provides the most complete and effective facility to tackle these challenges. The study of the FCC-hh physics potential is very mature; it occupied the first phase of the efforts towards the FCC CDR, thanks to a world-wide effort whose results have been documented in extensive reports [56,57], partly summarised in Volumes 1 and 3 of the CDR [10,12]. Some of the key results are recalled here. Recent preliminary studies of physics performance at different collider energies, in the 80–120 TeV range, have been done in the context of this feasibility study, and are reported in Sect. 2.6. Section 2.7 outlines the opportunities offered by a forward-physics facility, which would expand the landscape of FCC-hh physics measurements presented in the CDR.

On the Higgs boson side, the data sample produced by FCC-hh of over 20 billion Higgs bosons will bring the absolute determination of the couplings to muons, to photons, to the top quark, and to $Z\gamma$ below the per-cent level. The large production yield of Higgs bosons at large transverse momentum allows measurements to be performed in kinematic regions with optimal signal-to-background ratios and reduced experimental systematic uncertainties. The possibility to accurately measure the ratio of those couplings to couplings precisely measured at the FCC-ee (e.g., the HZZ* and $t\bar{t}Z$ couplings), enables the FCC-hh to deliver sub-percent absolute measurements of those couplings, which are beyond the reach of lepton colliders. The large HH production rate, furthermore, will bring the uncertainty on the Higgs self-coupling below the 5% level, even with conservative estimates of the experimental systematic uncertainties. The studies of Higgs boson production in very-high $Q^2$ processes, whether in direct, associated, or vector boson fusion/scattering production, test the existence of higher-dimensional effective operators in ways that are complementary to what is accessible even at the highest-energy lepton colliders.

The direct search for new particles extends the mass reach to the several tens of TeV range, in particular around 40 TeV for s-channel produced EW or coloured resonances, and in the 10–20 TeV range for pair-produced strongly-interacting particles, such as squarks, stops, vector-like top partners, and gluinos. Weakly interacting particles can be probed up to 1.5–5 TeV, depending on their weak multiplet assignment, allowing in particular the theoretically limited spectrum of possible WIMP dark-matter candidates to be covered.

Likewise, the search for partners of the Higgs boson extends to the multi-TeV region, a sensitivity that, together with the precise determination of the Higgs boson self-coupling, constrains extensions of the Higgs sector leading to a strong first-order EW phase transition in the early universe.

Last but not least, the FCC-hh is a unique facility to extend the study of the less known manifestation of the SM dynamics, namely the behaviour of high-density and high-temperature strongly interacting plasmas, through the analysis of data collected in central lead-lead collisions at nucleon-nucleon collision energies up to $\sqrt{s} \approx 40$ TeV [58]. Despite the significant progress made over the last decades at the SPS, at RHIC, and at the LHC regarding our understanding of the properties of the hot QCD medium created in high-energy heavy-ion collisions, grounded on increasingly precise and diverse experimental measurements, much remains unknown about the physics behind this most exotic area of QCD. Details of the concrete physics potential of heavy-ion running at the FCC-hh are provided in Refs. [57,58]. Aside from its intrinsic and undisputed scientific value, this part of the physics programme, exclusively accessible at hadron colliders, offers the opportunity to enlarge the community of scientists attracted to the FCC, providing them the sole facility that can continue and extend the study of the
high-density and high-temperature QCD medium produced in high-energy heavy ion collisions and the associated collective QCD phenomena, presently being actively pursued at the LHC.

The precision programme of the known elements of the SM should be complemented by a direct exploration of the unknown. Exploration requires breadth. To understand the properties of the visible Universe, large-scale structure surveys are performed, gravitational waves with pulsar timing and much more are sought. In the same vein, to understand the microverse this must be explored with as many different microscopes as possible. The FCC-ee will directly explore the evasive and weakly-coupled light new-physics frontier, while the FCC-hh will offer a direct way to scan the new energy frontier, above the TeV scale. Together, the FCC-ee and the FCC-hh complement each other in this exploration of the unknown in a more comprehensive way than any other energy frontier projects presently under consideration.

# 2 Specificities of the FCC physics case

## 2.1 The impact of FCC in particle physics

Particle physics is the science of studying fundamental processes on the smallest accessible scales. The last 60 years of particle physics have led to a radical overhaul of the understanding of the Universe, culminating in the landmark discovery of the Higgs boson in 2012. In this context, FCC may be viewed as a general-purpose particle physics facility, aimed at completing the extensive testing of the SM, exploring remaining open questions, and finding leads towards understanding the fundamental origins of the Universe. The combination of the most powerful $e^{+}e^{-}$ Higgs and electroweak factory with the ultimate 100 TeV-class hadron collider is unique in its capacity to carry out a new phase of exploration in new regimes of precision and energy, and in a controlled and repeatable experimental environment. More specifically, FCC will shed light on the many fundamental open questions about how the Universe came to be.

**What is the origin of the Higgs boson?** The Higgs boson discovered at LHC, with a mass of $125.20 \pm 0.11$ GeV, is, so far, consistent with the SM version of Electroweak Symmetry Breaking (EWSB), which predicts the full Higgs phenomenology with no free parameter. These predictions must be tested with the highest possible precision with FCC-ee and FCC-hh. Indeed, there are many reasons to believe that the underlying microscopic physics, from which the Higgs boson emerges, is associated with some of the deepest mysteries in particle physics. One such mystery concerns the origin of the Higgs boson mass and the naturalness puzzle [59,60], which FCC is uniquely placed to address by a combination of precision Higgs coupling measurements at the per-mil level and electroweak precision measurements improved by a factor of 50 or much more with respect to the current precision, with direct high-energy exploration to comprehensively probe symmetry-based explanations for the electroweak hierarchy.

**What is the origin of the presence of matter in the Universe?** The predominance of matter over antimatter, a.k.a. the baryon asymmetry of the universe (BAU), calls for both C and CP violation, as well as an out-of-equilibrium epoch in cosmological history. The SM EWSB does not suffice to predict a significant BAU, but particle physics can propose two hypotheses to accommodate this observation. A first possibility is that the Higgs potential is modified via a change of the Higgs self-coupling, leading to a first-order electroweak phase transition (EWPT). The FCC measurement of the Higgs boson self-coupling will reach a level of precision deemed adequate to clarify if the EWPT played a role in setting an out-of-equilibrium phase, a necessary condition for creating the observed matter-antimatter asymmetry. Another possibility is that the Higgs boson couples to neutrinos, which would generate a fermion-number-violating Majorana mass term, leading in turn to leptogenesis and, possibly, to the right amount of BAU. With its very high luminosity, FCC is best suited for the direct observation of the resulting heavy neutral leptons (HNL).

**What is the origin of mass and flavour?** The Higgs mechanism is responsible for generating mass, but the reason for the pattern of Yukawa couplings is yet to be understood. The FCC unique exploration potential of flavour physics and of new symmetries and forces would lead to a deeper understanding of approximate conservation laws, such as baryon and lepton number conservation (or the absence thereof in case of Majorana neutrinos); would probe the limits of lepton flavour universality and violation; and could reveal new selection rules governing the fundamental laws of nature. A crucial step in this endeavour is assessing the Yukawa couplings of the second and, partially, of the first fermion families, as they are an essential input to any model of flavour. The measurements of the muon and top Yukawa couplings at FCC-hh can be compared to the tau and charm Yukawa coupling measurements at FCC-ee with sub-per-cent precision. A first measurement of the strange Yukawa coupling is also possible at FCC-ee. Finally, an FCC-ee run at the s-channel Higgs resonance uniquely offers sensitivity to the electron Yukawa coupling, tantalisingly close to its SM value.
**What is the nature of dark matter?** The unprecedented luminosity and detector environment of FCC-ee make it uniquely sensitive to exploring weakly-coupled dark sectors. Dark matter candidates, such as heavy axions, dark photons, etc., provide long-lived particle signatures. If dark matter is a doublet or triplet weakly interacting massive particle (WIMP), FCC-hh will cover the entire parameter space up to the upper mass limit for a thermal relic. A range of complementary detector facilities could also be envisioned to extend the FCC capabilities for neutrino physics, long-lived particles, and forward physics.

**What lies beyond the Standard Model?** An impressive number of electroweak precision observables (EWPOs) will be measured by FCC-ee, with accuracies ranging from a factor 50 (e.g., for the W mass) to a factor 1000 (e.g., for $R_b$) better than today (Table 2). This new precision regime will allow the effect of further weakly-coupled heavy particles to be detected up to masses of several tens of TeV (and much more in the non-decoupling configuration). Alternatively, this precision brings sensitivity to particles mixing with the SM fermions, as HNL are expected to do in the Seesaw model (as shown, for example, in Fig. 29). The SM, possibly extended with additional light degrees of freedom, might be a low-energy effective field theory (EFT) approximation of a microscopic, ultraviolet (UV) theory from which it originates. The FCC-ee will improve the sensitivity to all EFT Wilson coefficients by one to several order(s) of magnitude, while FCC-hh can further extend the reach of direct and indirect exploration in a complementary way. The combined FCC programme is the most powerful general survey of this uncharted EFT territory and of the existence of new particles beyond the known fermions and bosons.

**Relation with exotic astrophysical and cosmological signals** Stochastic gravitational waves from cosmological phase transitions or astrophysical signatures of high-energy gamma rays are examples of phenomena that can arise due to exotic new physics which is, by contrast, directly accessible only to a facility offering the highest possible energy, such as FCC. Examples include confining new physics in a dark sector, decaying or co-annihilating TeV-scale WIMPs, or a modified EWPT. More precise top quark and Higgs boson mass measurements would also pin down more precisely the quantitative assessment of the SM vacuum metastability issue and of its interpretations.

As an all-in-one-facility, FCC provides the versatility, redundancy, and cross-correlations necessary to identify the fundamental origins of new phenomena. In general, its varied range of precise measurements of particle properties and interaction dynamics promises a set of guaranteed results with an unrivalled breadth with respect to other potential future facilities.

This section presents the unique role of FCC to address these fundamental questions, with a special focus on: the combined characterisation of the Higgs boson at FCC-ee and FCC-hh (Sect. 2.2); the direct discovery potential resulting from the FCC-ee clean environment, high integrated luminosity, and large acceptance rates of the detectors (Sect. 2.3); and the excellent opportunities in flavour physics (Sect. 2.4). The advantages of FCC-hh over multi-TeV lepton colliders to complement FCC-ee are presented in Sect. 2.5. Finally, the physics reach of FCC-hh as a function of its centre-of-mass energy, and the possibility of a forward-physics facility at FCC-hh, are briefly discussed in Sects. 2.6 and 2.7.

## 2.2 Characterisation of the Higgs boson: role of EW measurements and of FCC-hh

The Higgs boson is certainly a central piece of the current understanding of the fundamental structure of matter and physics laws. Its discovery in 2012, and the rich measurement programme over the past 13 years, have been successful milestones in high-energy physics that, maybe more importantly, enabled the formulation of relevant questions about the nature of new physics beyond the weak scale, in very concrete and often quantitative terms. As already shown in various studies [61–63], FCC-ee offers fantastic opportunities for learning more about the Higgs boson, beyond what can be achieved at HL-LHC (Table 3) or at other proposed Higgs factories [14].

In a record time, FCC-ee will bring the Higgs programme into a sub-percent precision area, allowing the classical as well as the quantum mechanical effects of new physics on the Higgs couplings to be probed and, from the pattern of the deviations, new selection rules distinguishing different models to be discovered. Concrete examples proving the relevance of the sub-percent precision target can be found in Table 5 of Ref. [64], where several BSM models beyond the reach for direct discovery at HL-LHC are considered, and for which per-cent level deviations in the Higgs couplings are predicted. A sub-per-cent precision is a clear necessary target to expose such deviations. As mentioned in Sect. 1.1, the Higgs precision programme at 240 GeV (365 GeV) can be achieved within three (eight) years of operation with FCC-ee, while other colliders considered at CERN would need half a century to reach a similar precision [14].

These early phenomenological projections have been confirmed during the Feasibility Study by new detailed experimental studies, with different detector set-ups [65–67], presented in this volume. Further directions in the Higgs precision programme also need to be more systematically investigated beyond what was done so far, in particular in the context of specific flavour scenarios or considering BSM sources of CP violation. This document, instead, emphasises the benefit of the interplay between
<caption>**Table 3** Expected 68% CL relative precision of the $\kappa$ parameters (Higgs couplings relative to the SM) and of the Higgs boson total decay width $\Gamma_H$, together with the corresponding 95% CL upper limits on the untagged (undetected events), $B_{\text{unt}}$, and invisible, $B_{\text{inv}}$, branching ratios at HL-LHC, FCC-ee (combined with HL-LHC), and the FCC integrated programme. For the HL-LHC numbers, a $|\kappa_V| \le 1$ constraint is applied (denoted with an asterisk), since no direct access to $\Gamma_H$ is possible at hadron colliders; this restriction is lifted in the combination with FCC-ee.</caption>

The ‘-’ indicates that a particular parameter has been fixed to the SM value, due to lack of sensitivity. From Ref. [61], updated with 4 IPs, the baseline luminosities of Table 1, and the most recent versions of the data analysis. For some of the entries, the $\kappa$ precision starts being limited by the projected SM parametric uncertainties, e.g. in $m_b$ [40]. For these entries, the precision obtained by neglecting such parametric uncertainties is also reported (separated by a /)

<table><thead><tr><th>Coupling</th><th>HL-LHC</th><th>FCC-ee</th><th>FCC-ee + FCC-hh</th></tr></thead><tbody><tr><td>κ<sub>Z</sub> (%)</td><td>1.3*</td><td>0.10</td><td>0.10</td></tr><tr><td>κ<sub>W</sub> (%)</td><td>1.5*</td><td>0.29</td><td>0.25</td></tr><tr><td>κ<sub>b</sub> (%)</td><td>2.5*</td><td>0.38 / 0.49</td><td>0.33 / 0.45</td></tr><tr><td>κ<sub>g</sub> (%)</td><td>2*</td><td>0.49 / 0.54</td><td>0.41 / 0.44</td></tr><tr><td>κ<sub>τ</sub> (%)</td><td>1.6*</td><td>0.46</td><td>0.40</td></tr><tr><td>κ<sub>c</sub> (%)</td><td>–</td><td>0.70 / 0.87</td><td>0.68 / 0.85</td></tr><tr><td>κ<sub>γ</sub> (%)</td><td>1.6*</td><td>1.1</td><td>0.30</td></tr><tr><td>κ<sub>Zγ</sub> (%)</td><td>10*</td><td>4.3</td><td>0.67</td></tr><tr><td>κ<sub>t</sub> (%)</td><td>3.2*</td><td>3.1</td><td>0.75</td></tr><tr><td>κ<sub>μ</sub> (%)</td><td>4.4*</td><td>3.3</td><td>0.42</td></tr><tr><td>|κ<sub>s</sub>| (%)</td><td>–</td><td>+29<br>-67</td><td>+29<br>-67</td></tr><tr><td>Γ<sub>H</sub> (%)</td><td>–</td><td>0.78</td><td>0.69</td></tr><tr><td>B<sub>inv</sub> (&lt;, 95% CL)</td><td>1.9 × 10<sup>-2</sup> *</td><td>5 × 10<sup>-4</sup></td><td>2.3 × 10<sup>-4</sup></td></tr><tr><td>B<sub>unt</sub> (&lt;, 95% CL)</td><td>4 × 10<sup>-2</sup> *</td><td>6.8 × 10<sup>-3</sup></td><td>6.7 × 10<sup>-3</sup></td></tr></tbody></table>

Higgs and electroweak measurements, a specificity of FCC-ee that was not discussed in detail in the FCC CDR [10,11] and has been studied afterwards [62,63].

The interpretation of current Higgs boson measurements at LHC is so far not hindered by the limited precision of the electroweak measurements at LEP and SLC. With FCC-ee targeting an order-of-magnitude improvement in the precision of Higgs boson properties in the main channels, the current (experimental and theoretical) precision on electroweak quantities would become a limitation. The Z-pole run of FCC-ee is instrumental in avoiding contamination from electroweak coupling uncertainties in the Higgs boson characterisation. If the electroweak symmetry is linearly realised in the SM fields, the interplay between the Higgs and electroweak sectors is even deeper [68]. Indeed, $e^+e^- \rightarrow W^+W^-$ production is then sensitive to some of the same new-physics effects as Higgs boson production and decay processes, making both types of measurements complementary.

The Standard Model Effective Field Theory (SMEFT) framework is adopted, truncated to operators of dimension six [69, 70]. The SMEFT is an appropriate framework for enumerating and quantifying the different possibilities in a relatively model-independent way. It assumes that new physics arises at a scale $\Lambda$, significantly above the electroweak scale, with the Higgs boson embedded in a $SU(2)_L$ doublet. The current status of the global SMEFT fit is shown in Fig. 3, adapted from Ref. [63]. In this figure, the results of the fit for the different dimension-six operators affecting at leading order either the electroweak processes (including anomalous triple gauge couplings, aTGCs, and boson-fermion couplings, Vff) or the Higgs processes, or both simultaneously, are projected on a more physically meaningful set of effective couplings capturing the effects of new physics. More details can be found in Ref. [63].

The interplay between Higgs and electroweak measurements is illustrated in Fig. 4, which shows the expected precision in the effective coupling determination. The correlations are displayed as internal lines of variable thickness and are visibly reduced when including Z-pole data at FCC-ee (dark blue) on top of the current electroweak measurements (light blue). The importance of Z-pole measurements is summarised below, followed by a discussion of the importance of the diboson process for Higgs physics.

The SMEFT results discussed in this section were obtained from fits with only linear effects in $\Lambda^{-2}$, consistently with the dimension-6 expansion. To estimate the theory uncertainty associated with the neglected higher-order terms in the EFT expansion, Fig. 5 shows the ratio of the bounds on the Wilson coefficients obtained in the linear case to those including quadratic contributions from dimension-6 operators, which are formally of the same order as dimension-8 contributions.
<figure><img src="image_10.png" /><figcaption>**Fig. 3** Results of a global SMEFT fit to HL-LHC (grey) and FCC-ee (yellow, up to 240 GeV, and blue, up to 365 GeV) data, interpreted in terms of the 68% probability sensitivity to Higgs and electroweak effective couplings. Adapted from Ref. [63]</figcaption></figure>

These results derive from the HL-LHC+FCC-ee SMEFT fit of Ref. [71]. All the displayed operator coefficients can be accessed at FCC-ee, with the exception of the operators in the top-left quadrant, which enter in top-quark measurements at HL-LHC but do not contribute to FCC-ee observables at tree level. So, in almost all cases, the results of the linear fit are unchanged after the addition of quadratic effects. This observation can be used as an indication that the uncertainty associated to effects of $O(\Lambda^{-4})$ is well controlled by FCC-ee precision measurements and is expected to be small. Another way to gauge the usefulness of the SMEFT analysis is to compare the bound on the scale of new physics inferred from the fit of the dimension-6 Wilson coefficients to the actual limits set in explicit BSM models, as will be reported in the next section. Of course, this does not exclude that (non-decoupling) new physics could still exist at much lower scales.

### 2.2.1 Impact of Z-pole measurements in Higgs couplings determination

The importance of having precise Z-pole measurements for the extraction of Higgs couplings from future $e^{+}e^{-}$ Higgs data is clear, given that the Zee vertex also enters in ZH production. A precise-enough extraction of the Zee vertex is therefore needed to avoid extra uncertainties from new physics effects in these interactions. The left-handed and right-handed Zee couplings can be extracted from the $Z \to e^{+}e^{-}$ partial decay width and the left-right asymmetry parameter $A_{e}$, determined at FCC-ee from the hadronic-to-leptonic cross-section ratios, the dilepton forward-backward asymmetries, and the tau polarisation forward-backward asymmetry, measured at the Z pole. This Zee coupling measurement also helps mitigating the (centre-of-mass-energy growing) new-physics effects in the HZee interactions, directly correlated with the new-physics contributions to the Zee vertex in the SMEFT formalism.
<figure><img src="image_11.png" /></figure>

**Fig. 4** Correlations between the determination of different EW/aTGC/Higgs interactions from a fit to future projections at HL-LHC and FCC-ee within the SMEFT framework. The bars on the exterior of the circle indicate the expected 68% CL sensitivity in the determination of corresponding coupling (with different scales for the different sectors). The light/dark blue lines in the interior of the

circle show the results excluding/including the FCC-ee programme of EW measurements at the Z pole. The Z-pole run is essential to isolate the Higgs measurements (there is no dark blue line connecting the Higgs and EW sectors) and to ensure that the extraction of the Higgs couplings is not hindered by the uncertainties on the EW coupling determination. Adapted from Refs. [62,63]

The impact of Z-pole measurements in Higgs coupling fits was studied in Ref. [62] and, more recently, in Ref. [63]. As a result of the reduced correlations between the HZZ and Zee couplings, displayed in Fig. 4, and of the corresponding reduction of the uncertainty in the (H)Zee interactions, the precision of the extraction of the Higgs couplings is improved, as illustrated in Fig. 6. In this figure, the deterioration in the precision of the Higgs coupling extraction (with respect to an extraction with perfectly known Z couplings, dubbed *perfect EW* in the figure) is shown in two configurations: one with the future FCC-ee Z-pole measurements, for which the Higgs coupling precision is close to the perfect EW case; and the other, limited to the precision of LEP/SLC Z-pole measurements, for which a deterioration of up to a factor of two in precision would be observed
**Fig. 5** Impact of quadratic contributions from dimension-6 operators, i.e., at $O(\Lambda^{-4})$, in the SMEFT fit of Ref. [71]. A $U(2)_Q \times U(2)_u \times U(3)_d \times (U(1)_L \times U(1)_e)^3$ flavour symmetry is assumed. For each of the different Wilson coefficients considered, the blue star indicates the ratio of the 68% CL bound obtained in the pure dimension-6 (linear) fit to that including quadratic dimension-six contributions. The difference between the two fits is minimal for most operators entering in FCC-ee observables at tree level, i.e., leaving aside the purely hadronic top-quark operators in the top-left quadrant. The FCC-ee fit includes EWPOs at the Z-pole, light fermion-pair production, Higgs boson production (ZH+ VBF), diboson production, and top pair production

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

**Fig. 6** Impact of Z-pole measurements on the Higgs coupling, Higgs decay width, and anomalous triple gauge-boson coupling ($\delta g_{1,Z}$ and $\delta\kappa_\gamma$) precision from the SMEFT fit interpretation. The ratio to the precision that would be obtained with infinitely precise Z-pole measurements

is shown for fits that either exclude ('T' bars) or include (solid bars) the projected FCC-ee Z-pole run measurements, with the Higgs programme at 240 GeV (yellow bars) and the additional data at 365 GeV (blue bars). The colour code is the same as in Fig. 3. Adapted from Ref. [63]

(for the Higgs programme at 240 GeV). Data at 365 GeV already significantly alleviate the impact of Z-pole measurements on most couplings, though a 30–40% precision improvement is still observed for the Higgs couplings to the Z and W bosons.

Not all the $6 \times 10^{12}$ Z bosons expected during the Z-pole run are needed to reach the precision on the Zee vertex required to fully mitigate the EW contamination in the Higgs couplings determination. An estimate of the number of events needed to reduce such contamination to a level that does not hinder the Higgs couplings extraction is displayed in Fig. 7, which shows the deterioration in the precision of the most precise HZZ and Hbb couplings with respect to the full statistics of the FCC-ee Z-pole run, as a function of the number of events. From the perspective of Higgs couplings, a few $10^9$ Z's, equivalent to less than a day of operation at the Z pole, are enough to saturate the Higgs coupling precision from the 240 and 365 GeV runs.
<figure><img src="image_14.png" /><figcaption>**Fig. 7** Estimated deterioration in the precision of the HZZ (left) and Hbb (right) couplings as a function of the number of Z's produced at FCC-ee, with respect to the baseline of 6 × 10¹² events, for the FCC-ee programme up to 240 GeV (yellow) and up to 365 GeV (blue)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 8** Relative precision of the determination of the Higgs couplings and aTGCs in the SMFET fit, as a function of the selection efficiency assumed in the $e^+e^- \to W^+W^-$ study: 1% (light shades), 45% (solid</figcaption></figure>

bars, chosen as default value), and 100% (triangles). The sensitivity comes mainly from the high-energy runs and the dependency at the WW threshold remains small. From Ref. [62]

### 2.2.2 Impact of diboson measurements in Higgs couplings determination

Together with gauge invariance, the assumption that electroweak symmetry is linearly realised in SMEFT implies a series of correlations between the new-physics effects on the electroweak properties of the SM particles, for example the aforementioned correlation between contributions to the Zee and HZee interactions. Another correlation relates modifications of the Higgs couplings to anomalous triple-gauge couplings (aTGCs): two out of the three aTGCs ($\delta g_{1,Z}$ and $\delta \kappa_{\gamma}$) are closely related to the Higgs couplings to vector bosons.

As shown in Ref. [72] for existing measurements and in Refs. [62,63] for FCC-ee projections, diboson and Higgs measurements are indeed sensitive to the same types of new-physics effects, so that diboson measurements can help constrain Higgs couplings (and vice-versa). In the current SMEFT fits, all angular information available from the $e^+e^- \to W^+W^-$ process is encoded in optimal observables, which bring a significant improvement [62] with respect to the fits performed in the FCC CDR [10,11], where only the (binned) W angular distribution was exploited to constrain the aTGCs. The inclusion of the full diboson kinematic information in the SMEFT fit also constrains all other possible interactions that can modify this process. The achieved relative precision on the Higgs couplings to gauge bosons and on the aTGCs is displayed in Fig. 8, for three assumed values of the selection efficiency for the $e^+e^- \to W^+W^-$ events: 1%, 45% (default value in the following), and 100%. A detailed experimental study of the projected precision is required to assess the ultimate sensitivity of the diboson measurements.

### 2.2.3 Complementarity between Z-pole and higher-energy runs

Electroweak precision physics at the Z pole is typically thought to mainly constrain the $\hat{S}$ and $\hat{T}$ oblique parameters characterising the gauge 2-point functions, and not be as sensitive as higher energy runs for the $\hat{W}$ and $\hat{Y}$ parameters, the aTGCs, the Higgs
<figure><img src="image_16.png" /><figcaption>**Fig. 9** The 95% CL projected sensitivities at FCC-ee to four-fermion, gauge, and Higgs dimension-6 operators, in terms of the effective scale of new physics Λ (in TeV), from measurements of Z-pole observables (right part of the table) and of higher-energies observables (left part of the table). The rightmost two columns show the above- and on-pole combinations of all observables. Adapted from Ref. [74]</figcaption></figure>

self-energy and self-coupling, Higgs-vector-vector interactions, and a variety of four-fermion operators that, at tree-level, only enter off-pole observables [73]. All these interactions, however, enter at higher-orders in the Z-pole observable theoretical predictions. The ultra-high precision provided by the Tera-Z event samples can overcome the extra loop-suppression factor of these operators and actually provide relevant constraints, competitive and complementary to higher-energy measurements where these interactions enter at leading order [74,75].

This statement is illustrated in Fig. 9, which shows the sensitivity (in terms of effective scale of new physics Λ) to dimension-6 SMEFT operators entering in the four-fermion, gauge, and Higgs sectors, from the corresponding constraints in Z-pole observables on the one hand, and in higher-energy (denoted ‘above-pole’) observables on the other. Besides often being better, the on-pole bounds also constrain different directions in the SMEFT space, as illustrated in Fig. 10 for two specific examples. As a consequence, the combination of on-pole and above-pole data very substantially reduces the overall volume of parameter space and, accordingly, tightens the constraints on specific models. The scope of the Tera-Z electroweak precision programme of FCC-ee is, therefore, far wider and more general than a naïve extrapolation of the already amazingly accurate SM confirmation from LEP data would suggest.

### 2.2.4 Complementarity and synergy between FCC-ee and FCC-hh

As shown in Table 3, the precision of many Higgs boson couplings is expected to improve by about one order of magnitude with respect to the (partial and not assumption-free) knowledge that will be available at the end of the HL-LHC era. The precision of several couplings, such as Hμμ, Htt, Hγγ or HZγ, will however remain dominated by the HL-LHC knowledge, even if they will only be made assumption-free by the FCC-ee absolute determination of the HZZ coupling. The few per-cent HL-LHC precision will be reduced by an order of magnitude at FCC-hh, by measuring ratios of branching ratios, free of systematic uncertainties and normalised by the FCC-ee precise coupling determination [10]. The successive improvements of
<figure><img src="image_17.png" /></figure>

**Fig. 10** Projected 68% CL sensitivities at FCC-ee, from measurements at the Z pole (orange) and in higher-energy runs (blue), in the planes of Higgs self-energy vs. self-coupling (left) and of the $\tilde{W}$ oblique parameter vs. an operator modifying anomalous triple-gauge couplings (right). The overall combined sensitivities (green) are much more constraining than the individual bounds. In the right plot, $S_{2W}$

and $S_{3W}$ are the coefficients of the dimension-6 operators $O_{2W} = -1/2 (D^\mu W^I_{\mu\nu}) (D_\rho W^{I\rho\nu})$ and $O_{3W} = \epsilon_{ijk} W^I_{\mu\nu} W^{J\rho\nu} W^K_{\rho\mu}$, both normalised with a scale of 1 TeV. In the left plot, the region between the dashed orange lines corresponds to linearised on-pole constraints in the SMEFT operator coefficient and the solid black line corresponds to a weak quadruplet scalar model. Adapted from Ref. [74]

<figure><img src="image_18.png" /></figure>

**Fig. 11** Higgs coupling precision improvements with the global SMEFT fit from HL-LHC to FCC-hh, benefitting from the FCC-ee accurate and absolute coupling determination to optimally exploit the hadron collider measurements

the Higgs boson coupling precision when going from HL-LHC to FCC-hh, benefitting on the way from the FCC-ee absolute and accurate coupling determination, are shown in Fig. 11.

The FCC-ee role in the top Yukawa coupling determination is twofold. On the one hand, and as for all other Higgs couplings, the HZZ absolute determination at 240 GeV enables a model-independent determination of the top Yukawa coupling from the HL-LHC data. On the other hand, the determination of the $Z\bar{t}$ couplings from $e^+e^- \to t\bar{t}$ at 365 GeV [76,77] can be used to normalise the measurement of the $t\bar{t}H$ cross section at FCC-hh to that of the $t\bar{t}Z$ cross section, and to reduce the uncertainty in the top Yukawa coupling down to the per-cent level [78], as illustrated in the left panel of Fig. 12.

In this discussion, it is assumed that contributions from, e.g., dipole or four-fermion interactions in $e^+e^- \to t\bar{t}$ or $pp \to t\bar{t}X$ are negligible. The contributions of these four-fermion interactions to the $t\bar{t}H$ cross section at FCC-hh could be constrained from $t\bar{t}$ measurements. To constrain the contributions of the eett four-fermion interactions to the $e^+e^- \to t\bar{t}$ cross section, extra handles might need to be identified, during the next phase of the study, to lift the approximate flat directions that could appear in a global top-quark analysis.

The Higgs boson self-coupling, $\kappa_\lambda$, will start being probed at HL-LHC, with an uncertainty that was estimated at the time of the 2020 ESPPU to be about 50%, in a fit where only deformations of $\kappa_\lambda$ are allowed. Since then, improved analysis techniques and the inclusion of additional decay channels have reduced this uncertainty to 26% [80]. The larger acceptance of the upgraded HL-LHC trackers is likely to bring it well below 25%. In $e^+e^- \to t\bar{t}$ collisions, one-loop radiative corrections, that involve the Higgs self-coupling, to the Higgs boson production cross sections, are proportional to $\kappa_\lambda$. These corrections amount to several per-cent in the SM at 240 GeV and strongly depend on the centre-of-mass energy [81]. With this energy dependence,
<figure><img src="image_19.png" /><figcaption>**Fig. 12** Results from “toy” fits to simulated data to illustrate the benefit of FCC-ee measurements in the FCC-hh determinations of the top Yukawa coupling (left) and of the Higgs self-coupling (right). In these fits, the total HH → bγγ cross section is used as a proxy to discuss the interplay between the two couplings, following the parameterisation of Ref. [79]. Simplifying assumptions are made on how the top EW cou-</figcaption></figure>

plings and the Higgs couplings to bosons and fermions are measured with a standalone FCC-hh. The yellow (grey) areas show the 68% and 95% probability contours obtained assuming that the FCC-ee measurements are (are not) used in the coupling extraction from the FCC-hh data

<figure><img src="image_20.png" /><figcaption>**Fig. 13** The projected Higgs self-coupling relative precision from the SMEFT global fit, as a function of the integrated luminosities of the 240 and 365 GeV runs, with FCC-ee alone (left) and FCC-ee combined with HL-LHC (right). The 4IP and HL-4IP dots represent the FCC-ee baseline scenario and an hypothetical scenario (described in the text) with doubled integrated luminosities at 240 and 365 GeV, respectively</figcaption></figure>

the sub-percent precision of the FCC-ee Higgs cross-section measurements at 240 and 365 GeV suffice to disentangle the effect of new physics in the HZZ coupling and in $κ_λ$, and allows a stand-alone determination of $κ_λ$ with a precision of 28%, as shown in the left panel of Fig. 13. Besides being quantitatively competitive with HL-LHC projections, this precision is also qualitatively distinct, as it is achieved within an SMEFT framework that accounts for a broad variety of potential new physics effects.

The combination of FCC-ee with HL-LHC therefore lifts the assumption made for the sole HL-LHC extraction, and significantly improves the overall precision on $κ_λ$ to 18%, as shown in the right panel of Fig. 13. If deemed important, it would be possible to double the FCC-ee integrated luminosity at 240 and 365 GeV, and reach a precision close to 15% on $κ_λ$, in a high-luminosity scenario dubbed HL-4IP in Fig. 13. The doubling of the integrated luminosity can be achieved either by running twice as long at these centre-of-mass energies, or by doubling the instantaneous luminosity, with a larger number of colliding bunches and/or a smaller vertical $β^*$ (along with a smaller vertical emittance), but without any hardware modifications to the collider, as explained in Ref. [82].

Recent studies [83,84] have shown that other new physics interactions, entering at the same order in perturbation theory, but absent from the SMEFT framework considered in this report, could marginally affect the $κ_λ$ extraction from single-Higgs processes. In rare and extreme cases, the value of $κ_λ$ extracted from single production at FCC-ee data could differ from that with pair production at HL-LHC, a difference that would allow this new physics to be identified and constrained. More pragmatically, a global fit is needed to bound these other interactions and to mitigate their impact in the determination of
<figure><img src="image_21.png" /><figcaption>**Fig. 14** Improvement in the determination of $\kappa_\lambda$ at FCC-ee (the darker colours are for the HL-4IP optimised scenario) and, subsequently, at FCC-hh</figcaption></figure>

$\kappa_\lambda$. The more operators are considered, the more observables are needed in the global fit. Some operators currently weakly bounded by experimental data, e.g., the four-fermion eett operator, will require further investigation in the next phase of the study.

This subtlety is anecdotal in the grand vision of the FCC integrated project. Indeed, the Higgs self-coupling will be uniquely and unambiguously probed at FCC-hh via Higgs boson pair (HH) production. Current estimates, combining the $b\bar{b}\gamma\gamma$, $b\bar{b}\tau^+\tau^-$, $b\bar{b}ZZ$, and 4 b decay channels, suggest that a precise determination, with an uncertainty as small as 3.4%, would be within the reach of a 100 TeV pp collider [85], and probably better by a factor of two with progress similar to those made in the HL-LHC projections [80] and in recent FCC-hh studies [86,87]. The role of the different FCC stages in the determination of the Higgs self-coupling is summarised in Fig. 14.

As for HL-LHC, this FCC-hh precision refers to an exclusive determination of the Higgs self-coupling, i.e., only considering deformations of $\kappa_\lambda$. Other new physics interactions, however, could modify the HH production or decay rates and it is important to keep their uncertainties under control. While an inclusive analysis taking into account all these effects has not yet been undertaken, an illustration of the impact of such effects is displayed in the right panel of Fig. 12. This figure shows how the uncertainty in the top Yukawa coupling modifies the $\kappa_\lambda$ precision from the measurement of the total $gg \to HH \to b\bar{b}\gamma\gamma$ cross section. The top Yukawa coupling enters with several powers in the different diagrams, contributing to $\sigma(\text{gg} \to \text{HH})$. As noted above, the precise determination of the top Yukawa coupling at FCC-hh relies on the measurement of the $\sigma(\text{pp} \to t\bar{t}H)/\sigma(\text{pp} \to t\bar{t}Z)$ ratio, where the $t\bar{t}Z$ coupling itself should be allowed to vary within its experimental uncertainty, rather than being fixed to its assumed SM value. The plots in Fig. 12 show the impact of using, or not, the direct FCC-ee measurement of the $t\bar{t}Z$ coupling in the extraction of the top Yukawa coupling at FCC-hh (left panel) and the consequent impact on the $\kappa_\lambda$ determination (right panel).

Finally, a concern was expressed about the sensitivity of the $\kappa_\lambda$ determination at HL-LHC if its true value were about twice the SM value ($\kappa_\lambda = 2$), for which the destructive interference between the box and triangle diagrams of the HH production by gluon fusion in pp collisions would significantly decrease the cross section. While the HL-LHC relative precision would actually not degrade for $\kappa_\lambda = 2$, the synergy with FCC-ee is, once more, remarkable here. Indeed, at FCC-ee the $e^+e^- \to ZH$ production cross section is a linear function of the true value of $\kappa_\lambda$: $\sigma_{ZH} = \sigma_0 \times (1+\alpha\kappa_\lambda)$, with $\alpha > 0$ (i.e., with a constructive interference). The FCC-ee relative precision on $\kappa_\lambda$, therefore, evolves as $1/\kappa_\lambda$ when the true value of $\kappa_\lambda$ increases. For $\kappa_\lambda = 2$, a stand-alone precision of 14% would therefore be achieved at FCC-ee with an explicit EFT fit similar to that of Figs. 13 and 14, improved to less than 10% in the high-luminosity scenario.

## 2.3 Discovery landscape

The discovery landscape for BSM physics at FCC-hh has been well documented in Ref. [57] and in the FCC CDR [10]. Here, the focus is mostly on the FCC-ee discovery landscape, for which many new results have been produced since the CDR. These results extend and refine the scope of BSM searches at FCC-ee, a notable example being the search for long-lived particles (LLPs) and heavy neutral leptons (HNLs). The complementarity of the FCC-ee and FCC-hh exploration potential is also discussed.
<figure><img src="image_22.png" /><figcaption>**Fig. 15** Scan of the parameter space for a real scalar singlet model, where all points shown have a first-order electroweak phase transition, plotted in the plane of the fractional change in the Higgs coupling to a pair of Z bosons relative to its SM value ('hZZ coupling') vs. the triple-Higgs coupling normalised to its SM value ('hhh coupling'). The space outside the two FCC-hh lines and above the FCC-ee line can be covered at those facilities. Adapted from Ref. [93] and scaled to baseline luminosities</figcaption></figure>

### 2.3.1 BSM exploration potential

Answers to the open questions about the Universe presented in Sect. 2.1 call for physics beyond the SM. In the absence of clear predictions as to the origin of this new physics, the capability and versatility of FCC-ee allow a uniquely broad range of possible BSM scenarios to be covered. In addition to probing the known elementary particles and fundamental forces with the highest precision, it can survey uncharted territory both directly towards ultra-weak couplings and indirectly at very high energies, up to 100 TeV, for so-far unknown particles and interactions.

At the centre of many mysteries lies the Higgs boson. Besides enabling a much sharper picture of the Higgs boson and testing its (non-)SM nature, the ZH run can also provide crucial model-independent sensitivity to its invisible decay mode(s), which probe the Higgs portal to dark sectors. Ultimately, an explanation for the origin of the Higgs mechanism itself is sought. From what underlying theory does the Higgs sector emerge? While this outstanding question is sufficient motivation in itself, a more fundamental description of the nature of electroweak symmetry breaking is, moreover, expected to be associated with new theoretical principles addressing the hierarchy problem, a naturalness strategy that has proven useful in the past and whose success or failure now would be of profound importance [88,89]. These theories typically extend the symmetries of the SM, for example in supersymmetric, composite, or extra-dimensional frameworks [90]. More recent proposals involving light new physics include novel types of cosmological dynamics [91]. Alternatively, something radically new altogether may manifest in direct searches, indirectly by measuring higher-dimensional operator coefficients or spectacularly in unforeseen types of exotic signatures. It is crucial to fully explore the Higgs boson as much as possible above the TeV scale.

A deeper understanding of the scalar sector of the SM can also show whether the early Universe underwent a first-order electroweak phase transition (FOEWPT) or not, knowledge that is crucial for understanding the matter-antimatter asymmetry. As an example, in the real scalar singlet model, a FOEWPT is correlated with a modification of the Higgs coupling to Z bosons in a way that can be explored almost entirely by FCC-ee (Fig. 15). Furthermore, a FOEWPT can also lead to gravitational wave (GW) signatures, which could be detectable by future GW observatories, such as LISA. Since it will be difficult to disentangle these signals from other astrophysical phenomena, precisely measuring the Higgs properties at colliders could have an important role to play in settling this important question [92]. The synergy between cosmology and particle physics has been fruitful in the past and may well lead to a more profound understanding of the nature of the electroweak phase transition [3].

The power of FCC-ee to probe the Higgs boson at much higher resolution than accessible today is that it enables peering further into the cloud of quantum fluctuations surrounding it. The envisioned precision indirectly opens a window onto physics at multi-TeV energies, around the level of current electroweak precision observables from LEP but uniquely sensitive to the Higgs boson. Given the number of possible BSM Feynman diagrams contributing virtually to these quantum fluctuations, there is a tremendous variety of possible new physics scenarios solving shortcomings of the SM that cannot be tested by other means. Similar to the past Fermi theory, the SMEFT is an EFT that maps out the path for continued exploration of what lies
far beyond. The precision achievable in the Higgs, EW, and flavour sectors singles out the FCC-ee as the most powerful tool for the indirect BSM exploration through EFT studies.

Non-decoupling particles get most of their mass from the Higgs mechanism, requiring a more general EFT framework than SMEFT, known as HEFT, to accurately capture their low-energy phenomenology [94]. Their coupling to the Higgs boson has a lower bound and unitarity requires their mass to have an upper limit. Their parameter space is therefore finite and can be comprehensively covered by FCC-ee, as detailed in Ref. [95]. An interesting open question – is the Higgs responsible for most of the mass of any other particles beyond the SM ones? – could then be basically settled.

A further qualitative leap in resolving power comes from studying the Z boson at the Z-pole run of FCC-ee. A Tera-Z factory, with five orders of magnitude more Z bosons than at LEP, is another truly exciting prospect in its breadth of SM physics and spectacular BSM sensitivity. While the measurements at a Higgs factory bring the knowledge of the Higgs sector up to the high-precision standards of LEP electroweak precision observables, a new era of ultra-high precision observables will begin with Tera-Z. This ultra-high precision gives FCC-ee an indirect sensitivity to scales of several tens of TeV for weakly-coupled new physics, while the sensitivity for strongly coupled physics can reach 100 TeV. To consolidate this sensitivity, the statistical precision reached by the very large Tera-Z event samples must be matched by a new generation of challenging theoretical calculations at higher electroweak loop orders, the development of which is another benefit of the FCC-ee physics programme: pushing the boundaries of experiment also expands the frontiers of theory and the understanding of quantum field theory.

After the Z boson, the W boson can also be used as a precision tool at FCC-ee. Its mass is one of the most precisely measured parameters that can be calculated in the SM and is thus of utmost importance. The production of almost $5 \times 10^8$ W bosons during the planned runs at the WW threshold and above will provide tests of the SM and of a plethora of BSM models with a precision at least one order of magnitude better than today. The combination of multi-Z and W production will measure the gauge-boson sector with the sharpest cutting-edge precision, leading not only to an unprecedented overview at the microscopic scale of electroweak physics but also offering sensitivity to a far broader range of physics intricately linked across the SM, and SMEFT more generally, by precision quantum effects.

Going to its highest energy, FCC-ee can explore physics associated with the heaviest known particle, the top quark, with the $t\bar{t}$ run. Its mass plays a fundamental role in the prediction of SM processes and for the cosmological fate of the metastable vacuum if the SM were extended up to $10^{10}$ GeV or higher [96–99]. An improvement by more than an order of magnitude, achievable at FCC-ee [77], is necessary to perform the above-mentioned precision tests at the quantum level. This improvement goes hand in hand with significant progress in the experimental determination of the strong coupling constant [100], one of nature’s fundamental parameters. The large top quark mass suggests that it plays a unique role, not only in Higgs and flavour physics, but also in relation with new, so far unobserved, phenomena. Indeed, proposed solutions to important open questions motivate couplings of BSM physics to the third generation, which also happens to be among the least constrained sectors of the SM. Therefore, the precise measurements of the top quark mass and other top quark characteristics are crucial for BSM precision exploration.

As a heavy flavour factory, FCC-ee can also investigate the other crucial third generation particles, the tau lepton and the bottom quark. At the Z-pole run, a record number of ‘clean’ B mesons and $\tau$ leptons are expected to be produced, the rare decays of which offer exceptional insight into flavourful processes beyond several tens of TeV in energy. Exploring the flavour structure of the SM in new regimes can yield clues as to its origin and potentially reveal BSM symmetries and selection rules. In addition to testing BSM flavour models, it provides a valuable experimental probe of CP violation and lepton flavour violation/universality that can arise generically in BSM extensions.

Finally, following the proposals and approvals at LHC of ‘parasite’ detectors that enhance its physics potential, it is possible to also envision additional experiments at FCC at different locations [101–103]. Notably, the civil engineering provides FCC-ee with much bigger detector caverns (Sect. 7) than needed for a lepton collider, in order to use them later for FCC-hh. It would then be possible to, e.g., install instrumentation in the cavern walls to search for new long-lived particles [52, 104, 105].

Even in the absence of no-lose theorems for the discovery of BSM physics, whether in particle physics or anywhere else, the case for a general-purpose particle observatory, exploring phenomena at the smallest possible scales, provides the only guarantee to further improve the knowledge of the Universe and explore its fundamental origins. On both these fronts, FCC-ee is guaranteed to make significant progress.

### 2.3.2 Tera-Z sensitivity to heavy new physics

The extensive physics programme and unprecedented precision of FCC-ee renders it uniquely suited to a general exploration of the zeptoscale. In particular, the understanding of the role of the Tera-Z programme at FCC-ee in discovering evidence for BSM physics has evolved significantly in recent years. The picture that has emerged is that, if there is heavy new BSM
<figure><img src="image_23.png" /><figcaption>**Fig. 16** The reach of FCC-ee precision electroweak observables (hatched) compared to present day flavour, electroweak, and high energy collider constraints, for all operators consistent with U(2)⁵-symmetric</figcaption></figure>

flavour physics at the new physics matching scale. The bounds are 3σ single-parameter fits, obtained running from a scale of 3 TeV with full resummation of the logarithmic terms. Taken from Ref. [106]

physics coupled to the SM fields that modifies SM observables, Tera-Z is well placed to find traces of it. This reasoning has been developed in the context of concrete models concerned with the flavour puzzle [106] and the microscopic origins of the Higgs boson [107, 108]. Even if a broad and agnostic view of the motivation for the presence of new physics is taken, Tera-Z offers discovery potential for almost any scenario that leads to tree-level modifications of the SM [27, 28].

When considering the possibility of new physics at the TeV scale, it is incumbent upon any model-builder to account for all aspects relating to flavour. Since precision flavour observables have, for decades, provided a powerful indirect window onto physics at the shortest accessible distances, it is common that putative new physics scenarios fall foul of flavour constraints, requiring the mass scale of new states to greatly exceed the TeV scale. It is important, therefore, to identify and classify new physics scenarios that can be consistent with the present suite of experimental constraints. In Ref. [106] it was reported that new physics with an approximate U(2)⁵ flavour symmetry could exist at very low scales whilst remaining consistent with constraints. The high energy states in such a scenario necessarily give rise to modifications of the SM, with effects captured by families of dimension-6 SMEFT operators. The coefficients of these operators are shown in Fig. 16, alongside present-day constraints from flavour, precision EW, and high-energy collider probes.

It is clear that new physics states as low as the TeV scale are presently compatible with observations. On the other hand, projections from a precision EW programme at FCC-ee, driven primarily by the Tera-Z programme, are shown to probe significantly beyond this scale, as far as 10 TeV. Not only is the sensitivity of operators that enter electroweak precision at tree level enhanced to the tens of TeV scale, there are also many more third generation operators being constrained at loop level through RGE evolution. This observation would significantly impact the understanding of the new physics landscape at high energies and its interplay with flavour, providing opportunities to discover the effects of heavy new physics.

The previous discussion concerns the flavour structure of new physics scenarios. In contrast, Ref. [107] focuses specifically on the question of Higgs compositeness. Composite Higgs models are a class of scenarios in which the Higgs boson is a composite bound state of a new strongly-coupled dynamics at the TeV scale and a potential precursor of extra spatial dimensions. These models offer compelling answers to the question of the microscopic origins of the Higgs sector of the SM, in analogy with the way in which QCD explains the existence and properties of the pions. However, a significant challenge to such a scenario follows from the magnitude of the Higgs boson Yukawa coupling to the top quark. This large coupling is difficult to accommodate in the most basic realisations, but it can arise if the left and/or right-handed top quarks are also
<figure><img src="image_24.png" /><figcaption>**Fig. 17** The FCC-ee reach for probing Higgs and top compositeness through the precision EW programme, as compared to nearer-term HL-LHC and flavour measurements. Taken from Ref. [107]. Left, mixed, and right top partial compositeness are considered (left to right panels)</figcaption></figure>

partially composite. This observation thus brings flavour considerations to the fore when attempting to understand the origins of the Higgs sector.

Reference [107] considers the role that an FCC-ee precision EW programme would play in probing the possibility of partial top quark compositeness. A custodial symmetry is typically assumed of such models to evade current electroweak precision bounds from the *T* parameter, equivalent to a custodially-violating dimension-6 operator. However, the SM violation of custodial symmetry itself leads to a next-to-leading-log RG running into the *T* parameter that can no longer be so easily evaded at FCC-ee. This renders large swathes of parameter space accessible to FCC-ee, as shown in Fig. 17, where FCC-ee projections are compared to nearer-term HL-LHC and flavour projections. A huge increase in projected reach is observed, to compositeness scales of at least 25 TeV.

A final consideration, developed in Refs. [27,28], could be considered an 'informed agnostic' perspective on the nature of heavy new physics. The leading low energy effects of any heavy new physics scenarios with states whose mass does not depend significantly on the Higgs field can be captured in the dim-6 operators of the SMEFT. However, that does not conversely mean that any pattern of dim-6 SMEFT operators corresponds to a legitimate heavy new physics scenario. Thus, it is too simplistic to perform EFT fits in the context of future colliders without paying heed to concrete UV scenarios.

As an informed middle ground, Refs. [27,28] considered all possible new physics scenarios where any dim-6 SMEFT operators are generated at tree-level. The number of possibilities is large, but finite, as detailed in Table 4. If any such scenario exists and modifies SM properties in any sector of the SM at dim-6, the precision EW programme at FCC-ee has sensitivity, as shown in Figs. 18, 19, and 20.

As a matter of fact, FCC-ee can effectively probe virtually all heavy particles linearly coupled to the SM, with a sensitivity that reaches up to 100 TeV at tree-level and $O(1-10)$ TeV at the one-loop level for $O(1)$ couplings. This reach could extend even higher in the case of strongly-coupled new physics. Quantum RG effects are crucial. Any treatment that considers the SMEFT contributions of these states only at tree-level would overlook the majority of the sensitivity of the FCC-ee Tera-Z programme to their existence.

Reference [27] only considers the Tera-Z and WW runs. Recently, the SMEFit Collaboration studied the impact of the various runs combined and individually. Details can be found in Refs. [71, 110, 111], including the slightly different EW-input scheme, which leads to some minor differences with respect to the results of Ref. [27] for some models. Figure 21 demonstrates the exploratory power of the various runs to a variety of models in which the full one-loop matching is performed at the matching scale. Figures 22, 23, and 24 consider selected scalar, fermion, and vector scenarios with RG-evolved Wilson coefficients but not one-loop matching, as in Figs. 18, 19, and 20.

The picture that emerges from Figs. 21, 22, 23, and 24 is that the Tera-Z programme of FCC-ee is, indeed, extremely powerful in searching for new physics, extending well beyond the scales probed at HL-LHC, and is also highly complementary to the Higgs run. The latter offers loop-induced sensitivity to the Higgs trilinear coupling [81], which could probe custodially symmetric models, as shown for the $\Theta_1 + \Theta_3$ model in Fig. 21. This weak custodial quadruplet scalar model and the Higgs
<table><caption>**Table 4** Heavy scalars, fermions, and vectors that can contribute to SMEFT at dimension-6 with corresponding representations under SU(3)$_C \times$ SU(2)$_L \times$ U(1)$_Y$. Taken from Ref. [109]</caption><thead><tr><th></th><th>S</th><th>S<sub>1</sub></th><th>S<sub>2</sub></th><th>&phi;</th><th>&Xi;</th><th>&Xi;<sub>1</sub></th><th>&Theta;<sub>1</sub></th><th>&Theta;<sub>3</sub></th></tr></thead><tbody><tr><td rowspan="4">Scalar</td><td>(1, 1)<sub>0</sub></td><td>(1, 1)<sub>1</sub></td><td>(1, 1)<sub>2</sub></td><td>(1, 2)<sub>1/2</sub></td><td>(1, 3)<sub>0</sub></td><td>(1, 3)<sub>1</sub></td><td>(1, 4)<sub>1/2</sub></td><td>(1, 4)<sub>3/2</sub></td></tr><tr><td>&omega;<sub>1</sub></td><td>&omega;<sub>2</sub></td><td>&omega;<sub>4</sub></td><td>&Pi;<sub>1</sub></td><td>&Pi;<sub>7</sub></td><td>&zeta;</td><td></td><td></td></tr><tr><td>(3, 1)<sub>-1/3</sub></td><td>(3, 1)<sub>1/3</sub></td><td>(3, 1)<sub>-4/3</sub></td><td>(3, 2)<sub>1/6</sub></td><td>(3, 2)<sub>7/6</sub></td><td>(3, 3)<sub>-1/3</sub></td><td></td><td></td></tr><tr><td>&Omega;<sub>1</sub></td><td>&Omega;<sub>2</sub></td><td>&Omega;<sub>4</sub></td><td>&Upsilon;</td><td>&Phi;</td><td></td><td></td><td></td></tr><tr><td rowspan="2">Fermion</td><td>(6, 1)<sub>1/3</sub></td><td>(6, 1)<sub>-2/3</sub></td><td>(6, 1)<sub>4/3</sub></td><td>(6, 3)<sub>1/3</sub></td><td>(8, 2)<sub>1/2</sub></td><td></td><td></td><td></td></tr><tr><td>N</td><td>E</td><td>&Delta;<sub>1</sub></td><td>&Delta;<sub>3</sub></td><td>&Sigma;</td><td>&Sigma;<sub>1</sub></td><td></td><td></td></tr><tr><td rowspan="3">Vector</td><td>(1, 1)<sub>0</sub></td><td>(1, 1)<sub>-1</sub></td><td>(1, 2)<sub>-1/2</sub></td><td>(1, 2)<sub>-3/2</sub></td><td>(1, 3)<sub>0</sub></td><td>(1, 3)<sub>-1</sub></td><td></td><td></td></tr><tr><td>U</td><td>D</td><td>Q<sub>1</sub></td><td>Q<sub>5</sub></td><td>Q<sub>7</sub></td><td>T<sub>1</sub></td><td>T<sub>2</sub></td><td></td></tr><tr><td>(3, 1)<sub>2/3</sub></td><td>(3, 1)<sub>-1/3</sub></td><td>(3, 2)<sub>1/6</sub></td><td>(3, 2)<sub>-5/6</sub></td><td>(3, 2)<sub>7/6</sub></td><td>(3, 3)<sub>-1/3</sub></td><td>(3, 3)<sub>2/3</sub></td><td></td></tr><tr><td rowspan="4">Vector</td><td>B</td><td>B<sub>1</sub></td><td>W</td><td>W<sub>1</sub></td><td>G</td><td>G<sub>1</sub></td><td>H</td><td>L<sub>1</sub></td></tr><tr><td>(1, 1)<sub>0</sub></td><td>(1, 1)<sub>1</sub></td><td>(1, 3)<sub>0</sub></td><td>(1, 3)<sub>1</sub></td><td>(8, 1)<sub>0</sub></td><td>(8, 1)<sub>1</sub></td><td>(8, 3)<sub>0</sub></td><td>(1, 2)<sub>1/2</sub></td></tr><tr><td>&ell;<sub>3</sub></td><td>&ell;<sub>2</sub></td><td>&ell;<sub>5</sub></td><td>Q<sub>1</sub></td><td>Q<sub>5</sub></td><td>&chi;</td><td>&Upsilon;<sub>1</sub></td><td>&Upsilon;<sub>5</sub></td></tr><tr><td>(1, 2)<sub>-3/2</sub></td><td>(3, 1)<sub>2/3</sub></td><td>(3, 1)<sub>5/3</sub></td><td>(3, 2)<sub>1/6</sub></td><td>(3, 2)<sub>-5/6</sub></td><td>(3, 3)<sub>2/3</sub></td><td>(6, 2)<sub>1/6</sub></td><td>(6, 2)<sub>-5/6</sub></td></tr></tbody></table>

<figure><img src="image_25.png" /><figcaption>**Fig. 18** Projected bounds on the masses of new scalar fields (the bounds are $2\sigma$ fits, obtained running with the leading-logarithmic terms from a scale of 2 TeV, at which the couplings to the SM particles are assumed to be unity, down to the mass of the Z). 'Flavourless couplings' correspond to models in which couplings of the new states to fermions are absent, 'Universal couplings' correspond to equal couplings to all</figcaption></figure>

generations, and 'Third-gen only' to new physics coupled only to the third generation fermions. The vertical dashed lines separate fields that contribute to EWPOs at tree level (left), via one-loop RG evolution (middle), and via one-loop matching (right). From Ref. [27], scaled to baseline luminosities

trilinear coupling can, moreover, be probed on the Z pole, as shown on the left plot of Fig. 25, with the combination significantly extending the coverage of the model parameter space [74].

The Tera-Z run is crucial to definitively answer the fundamental question of whether any particles other than the SM ones obtain most of their mass from the Higgs mechanism. Such so-called 'loryon' candidates have a finite parameter space that can be almost entirely probed by current and future colliders, with a remaining open window for the elusive real singlet scalar model [95]. This window can potentially be closed by precision measurements at the Z pole, as shown on the right plot of Fig. 25, from Ref. [74], thus allowing this important question to be definitively settled. The high sensitivity of FCC-ee to the
<figure><img src="image_26.png" /><figcaption>**Fig. 20** Projected bounds on the masses of new fermion fields. Same as for Fig. 18</figcaption></figure>

<figure><img src="image_27.png" /></figure>

$\hat{W}$ and $\hat{Y}$ parameters, both at the Z pole and in higher energy runs, also enables constraining elusive BSM candidates, such as weakly interacting massive particles. The combination of on-pole and above-pole data, shown in Fig. 26, indeed gives a projected sensitivity beyond the indirect sensitivity reach of Drell–Yan searches at HL-LHC [74].

It will be critical to reduce theory uncertainties (Sect. 3) in order to capitalise on the full potential of the FCC-ee data. More specifically, moving from NLO to NNLO will be required to extract many driving observables from data in the electroweak and Higgs sectors, namely $m_Z$, $\Gamma_Z$, $\sin^2\theta_W^{\text{eff}}$, $m_W$, and the ZH production cross section (Fig. 10). Indeed, for a number of scenarios, the Tera-Z programme would probe the parameter space inaccessible to the runs at higher energies only in the context of improved theory uncertainties. An additional noteworthy aspect concerns the synergy with FCC-hh. Indirect probes at FCC-ee approach or exceed the 10 TeV scale. As a result, the only motivated successor to such a probe would be a collider capable of directly probing beyond that parameter space or one capable of directly producing the new states for which indirect evidence may have emerged at FCC-ee. In other words, a successor collider would necessarily have to be capable of directly probing beyond the 10 TeV scale. The only such plausible facility is FCC-hh, strengthening the combined FCC physics case.

It should be noted that exceptions from the exploratory power of Tera-Z can exist. It was recently shown in Ref. [115] that a class of anomaly-free Z' models could exist in which the charge assignments lead to a suppression of electroweak-precision-
<figure><img src="image_28.png" /><figcaption>**Fig. 21** Sensitivity (95% CL) to a selection of models from Table 4 with Wilson coefficients fully matched at one-loop (the bounds are obtained from a $\chi^2$ fit and the couplings of the new particle are set to unity at the matching scale, taken to be the mass of this heavy particle, the different operators are then run down to the Z boson mass with full numerical resummation of the logarithmic terms). In the case of new physics coupled to SM fermions, heavy scalar and fermions couple to the</figcaption></figure>

third generation only, while vectors couple to third-generation quarks and all generations of leptons. The starting scenario corresponds to all LEP/SLD and LHC constraints anticipated by the end of HL-LHC operation. Additional combinations of FCC-ee runs are then shown. Black markers denote anticipated constraints were the theory uncertainties to remain as at present. The $\Theta_1 + \Theta_3$ model corresponds to the custodial quadruplet model of Refs. [112–114]

<figure><img src="image_29.png" /><figcaption>**Fig. 22** As Fig. 21, but without the one-loop matched terms, for scalars</figcaption></figure>

sensitive operators at tree and one-loop level. Nonetheless, such a scenario would be strongly constrained above the Z-pole, both at FCC-ee and HL-LHC, demonstrating additional complementarity between runs and colliders.

The individual models considered are, themselves, not being proposed as well-motivated in any subjective sense. Rather, the collection of scenarios and the full family of Wilson coefficient patterns they populate can be taken as being qualitatively representative of the broad family of Wilson coefficients that could arise in UV scenarios. Thus the fact that the FCC-ee Tera-Z programme has sensitivity to this wide class of models, whether at tree-level or one-loop, is indicative of the fact that it will have sensitivity to generic UV scenarios. To argue otherwise would require the construction of an explicit counterexample
<figure><img src="image_30.png" /><figcaption>**Fig. 24** As Fig. 21, but without the one-loop matched terms, for vectors. The models *B* and *W* include flavour-universal couplings to leptons. See Ref. [110] for details</figcaption></figure>

<figure><img src="image_31.png" /></figure>

model. Within the class of models considered in Ref. [27], the two counterexample states are $\Omega_4$ and a special case for $G$, for which only the four top-right interaction is generated up to one loop. However, even BSM modifying four-top operators can be highly constrained through its next-to-leading-log running into the $T$ parameter at the $Z$ pole [106,107], as also seen in Figs. 22 and 24.

Finally, as another concrete UV scenario, supersymmetric models are a well-motivated approach to understanding the origin of the Higgs and addressing the hierarchy problem, where the BSM particles are, instead, typically expected to arise from new weakly-coupled dynamics. The Higgs couplings at one loop, in particular to gluons and photons, can probe the contributions of superpartners such as the stop, the coloured scalar partner of the top. Such indirect probes are complementary to more model-dependent direct searches and their sensitivity can reach around a TeV, as shown on the left plot of Fig. 27, taken from Ref. [116]. The right plot of Fig. 27, from Ref. [108], shows the projected Z branching ratio sensitivity at FCC-ee to the selectron and pure wino, together with the current direct search constraints at LHC.

In summary, through a diverse precision programme covering electroweak and Higgs physics, FCC-ee has demonstrable coverage of a very wide range of scenarios involving heavy new physics beyond the Standard Model.
<figure><img src="image_32.png" /><figcaption>**Fig. 25** FCC-ee sensitivity at 68% CL on the weak custodial quadruplet scalar (left) and real singlet scalar (right) models in the mass vs. coupling plane when combining Z pole data (orange) with ZH measure-</figcaption></figure>

ments (blue). For the real singlet case, the constraints exclude a first order phase transition (FOPT) for the mass region shown. See Ref. [74] for further information

<figure><img src="image_33.png" /><figcaption>**Fig. 27** Left: projected 2σ sensitivity of Higgs couplings to stops at FCC-ee in the parameter space of stop masses $m_{\tilde{\tau}_1}$ vs. $m_{\tilde{\tau}_0}$; from Ref. [116]. Right: FCC-ee Z branching ratio projected statistical and</figcaption></figure>

systematic 1σ uncertainties in the left-handed selectron mass vs. pure wino mass plane; from Ref. [108] scaled to the baseline luminosities