The CompactLinear Collider:

CLIC

20th Lomonosov Conference, 22 August 2021
Aidan Robson, University of Glasgow
1CLICProject overview
u Status after European Strategyu Physics reach and recent studies
u OutlookuCompact Linear Collider:e+e– collisions up to 3TeV
http://clic.cern/2Collaborations
http://clic.cern/
CLIC accelerator collaboration~60 institutes from 28 countries

CLIC detector and physics (CLICdp)
30 institutes from 18 countries

CLIC accelerator studies:• CLIC accelerator design and development• (Construction and operation ofCLIC Test Facility, CTF3)

Focus of CLIC-specific studies on:• Physics prospects & simulation studies• Detector optimization + R&D for CLIC

including JINR Dubna

including: JINR, Dubna;
Budker Inst., Novosibirsk;
Inst. of Applied Physics (RAS)

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The Compact Linear CollideruA high-luminosity, multi-TeV electron–positron collideruPlanned for construction at CERN in three energy stages:
380GeV, focusing on precision
Higgs boson and top-quark
physicsu

1.5 and 3TeV, expanding Higgs
and top studies including Higgs
self-coupling, and opening higher
direct and indirect sensitivity to
Beyond Standard Model (BSM)uNominal physics programmelasts for 25–30 years; approvablein stagesuBenefit of linear machine:
length/energy staging plancan be updated in response todeveloping physics landscapeu20th Lomonosov Conference, Aug 2021

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CLIC History
u3-volume CDR 2012

AcceleratoruUpdated Staging Baseline 2016

Physics & Detectors

Strategy &Implementation

4 Yellow Reports 2018uSummary Report Physics PotentialProject
Implementation

Detector
Technologies

CLIC is now a mature project– technical timeline going
into European Strategy gave
readiness for construction
starting ~2026, with
first collisions ~2035

http://clic.cern/european-strategy
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CLIC at 380GeVuTwo-beam acceleration scheme

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Power extraction
and transfer

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CLIC at 3TeV

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Arrival timestabilised to50 fs

Accelerator challenges

GHzx2

High-current drive beam Produced at CLIC Test Facility
CTF3, now the ‘CERN Linear
bunched at 12 GHz
x3
28A Electron Accelerator for Research’
12 GHzfacility, CLEAR

Power transfer + mainbeam acceleration
Demonstrated 2-beam acceleration

Drive beam arrival
time stabilised toCLIC specificationof 50fs~100 MV/m gradient in main-beam cavities
Achieved in structures produced by different sources

Alignment & stability
The CLIC strategy:•Alignment; vibration damping;good beam measurement and feedback•

Tests in small accelerators of equipment
and algorithms (FACET at Stanford,ATF2 at KEK, CTF3, Light-sources)–> Key accelerator technologies
have been demonstrated
For more on accelerator, see talk from Steinar
35 at June joint Snowmass AF/EF session:Stapnes
https://indico.fnal.gov/event/43871/
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Aidan Robson8CLIC status after European Strategy
European Strategy for Particle Physics was updated in June 2020 after aseveral-year process:– prioritises an electron–positron Higgs factory as the next collider– articulates the ambition to operate a proton–proton collider at thehighest achievable energy– mandates a technical and financial feasibility study for a 100TeV collider– mandates intensified accelerator R&D, including on high-gradientstructuresu Over the next 5 years CERN will continue the investment in R&D for
key technologies related to CLICu–> CLIC is maintained, so that if in 2026 the feasibility study is not conclusive
for FCC then CLIC could be implemented in an expeditious way:
Project approval ~2028, Tunnel construction starting ~2030
CLIC is the least-expensive Higgs factory proposed for construction inEurope, and leads to unique physics potential at high energy runningu

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9CLIC status after European Strategy
ECFA Newsletter 5 (August 2020)

https://cds.cern.ch/record/2729018
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CLIC studies 2021–25
X-band technology:• Design and manufacturing of X-band structures and components• Study structures breakdown limits and optimization, operation andconditioning• Baseline verification and explore new ideas• Assembly and industry qualification
35• Structures for applications, FELs, medical, etc

Technical and experimental studies, design and parameters:•••••

Module studies (see some targets for development below)
Beamdynamics and parameters: Nanobeams (focus on beam-delivery),pushing multi TeV region (parameters and beam structure vs energy efficiency)
Tests in CLEAR (wakefields, instrumentation) and other facilities (e.g. ATF2)High efficiency klystrons
Injector studies suitable for X-band linacs (coll. with Frascati)

Application of X-band technology (examples):••••••20th Lomonosov Conference, Aug 2021

A compact FEL (CompactLight: EU Design Study 2018-21)
Compact Medical linacs (proton and electrons)
Inverse Compton Scattering Source (SmartLight)
Linearizers and deflectors in FELs (PSI, DESY, more)
1 GeV X-band linac at LNF
eSPS for light dark matter searches (within the PBC-project)Aidan Robson

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CLIC in the news
15 Sept. 2020
https://home.cern/news/news/knowledge-sharing/cern-and-lausanneuniversity-hospital-collaborate-pioneering-new-cancer

24 June 2021
https://home.cern/news/news/knowledge-sharing/clearstudy-paves-way-novel-electron-based-cancer-therapy

https://physicsworld.com/a/cern-accelerator-technology-to-underpinflash-radiotherapy-facility/

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12Physics processes and staging
u

2-fermion production e.g. qquWW productionu

Higgsstrahlung (HZ):– best at 240–380 GeV: “Higgs factory”

utt threshold: 350 GeVu

tt continuum: >365 GeVuuu

Double Higgsstrahlung (HHZ):– cross-section maximum ~600 GeV
Single and double Higgs in WW fusion(Hnene and HHnene):– cross-section rises with energy
Direct searches for new particles:
highest possible energy–> Best explored in several energy stages

Baseline staging scenarioemphasis is on getting tomulti-TeV collisions quickly
Polarised electron beam (–80%, +80%)
Ratio (50:50) at √s=380GeV ; (80:20) at √s=1.5 and 3TeV

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Aidan Robson13CLIC Detector Concept
Essential characteristics:
B-field: 4T
u Vertex detector with 3 double layers
u Silicon tracking system: 1.5m radiusu ECAL with 40 layers (22 X0)
u HCAL with 60 layers (7.5 l)
uPrecise timing forbackground suppression(bunch crossings 0.5ns apart)~10ns hit time-stamping in tracking
u 1ns accuracy for calorimeter hits
uCLICdp-Note-2017-001
arXiv:1812.07337+ Dedicated detector R&D programme, particularly on Vertex & Tracking
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Vertex and Tracking R&D
Stringent requirements for CLIC vertex & tracker detectors inspired broad
and integrated technology R&D programme
Benefit from rapid progress in Si industry and synergies with HL-LHC

Allpix2 simulationof HV-CMOS
pixel sensor

Highlights:
Full efficiency obtained from hybrid assemblies of 50μm thin sensors
that satisfy CLIC time-stamping requirements
u Sensor design with enhanced charge-sharing is underway to reach required spatial
resolution with thin sensors
u Good progress towards reducing detector mass with active-edge sensors
and through-Si interconnects
u Promising results from fully integrated technologies;
CLIC-specific fully integrated designs underway (CLICTD, CLIPS)
u Developed advanced simulation/analysis tools for detector performance
optimisationuuu

Feasibility of power-pulsing demonstrated; power consumption specification met
Feasibility of air cooling demonstrated in simulation & full vertex detector mockup
CLICpix + 50 μm sensor

C3PD+CLICpix2 glue ass.

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CLICTDAir cooling

ATLASPIX HV-CMOS

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Higgs coupling sensitivityuuuu

Extensive set of full GEANT-based
simulation studies including beam
backgrounds done for Higgs sector
Full simulation: imaging calorimetry
allows e.g. H->bb/cc/gg separation
Model-independent coupling extraction
arXiv:1812.01644
based on Eur. Phys. J. C 77, 475 (2017)
Sensitivities used as input for EFT fits

e+ Z
e–

timing/momentum cuts

ZH

Many
channels stu
diede+n
W H

e–Wn(These precisions are for unpolarised beams; baseline is on slide 13)
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Aidan Robson† : fast simulation*: extrapolated from 1.4TeV16Alternative run scenario
To illustrate the flexibility of the runplan: two modifications with respect tothe baseline staging:u

Increasing initial stage from 8 to 13yearsu–> Integrated luminosity at 380GeV
increases from 1ab–1 to 4ab–1

L [1034cm-2s-1]

1000

100FCC-ee
CEPCILCILC-up.
CLIC
CLIC-up

From arXiv:
2001.05278

CLIC baseline

CLIC longer first stage

CERN-ACC-2019-0051

uuDoubling bunch train repetition rate
at initial stage from 50Hz to 100 Hz–> modest increase in cost (~5%)
and power (from 170MW to 220MW)u

From European
Strategy Briefing BookEither scenario (longer 1st stage, orbaseline 1st+2nd stage) very competitive
u10

Proposed e+e– colliders give similar Higgs
performance at the initial stage “Higgs Factory”
u1100
Daniel Schulte,Granada symposium

1000–> look at what is unique to CLICEcm [GeV]

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Higgs self-coupling, and ZH at 3TeV
u High-energy running gives direct access to Higgs self-couplinge+dominates
at higher √scomplementary
at 1.5TeVe+Ze–ZHgHHHHe–nWHHHW gHHHn

H

1.4TeV3TeVs(HHnene)>3s EVIDENCE
Ds
s = 28%>5s OBSERVATION
Dss = 7.3%s(ZHH)

3.3s EVIDENCE

2.4s EVIDENCE

gHHH/gSM
HHH

1.4TeV:–29%, +67%
rate-only analysis

1.4 + 3TeV:–8%, +11%
differential analysis

Direct access to two processes that behave
differently with non-SM values of self-couplinguEur. Phys. J. C 80, 1010 (2020)

u Recently-completed high-energy studies
ZHH and ZH(qqbb) at 3 TeV toconfirm fast simulation /extrapolation– use of jet substructure– first use of b-tagging inboosted Higgs decays at CLICZH(qqbb)
at 3TeV

all-hadronic ZHH https://arxiv.org/abs/2008.05198

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Aidan Robson18Top-quark physics
u CLIC is unique among e+e– colliders byaccessing top-quark physics from the initialenergystage
uThresholdscan:uPair production:

First study of boosted
top production ine+e–Top cross-sections, bothpolarisations ~1%u

Top forward-backward
asymmetries ~3–4%
uStatistically optimal
observables for top EWK
couplings; more than one
energy stage allows global fituMass and width√s=3TeV
e+e– -> tt -> qqqqbb

https://arxiv.org/abs/2008.05526

u Optimisation of scan points including beamspectrum; here optimising on mass and width.
u Expected top-quark mass precision of 25MeV canbe improved by 25% without losing precision onwidth or Yukawa. https://arxiv.org/abs/2103.00522

e+e–g/Ztt

new

Electron beam polarisation
provides new observables

Top-quark physics at CLIC: JHEP11 (2019) 003
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Aidan Robson19BSM signature searches recent highlights
CLIC 3TeV, geY=1

u Dark matter using mono-photon signature at 3TeV,e+e– -> XXgu New study using ratio of electron beam polarisations
to reduce systematics
u Exclusions for simplified model with mediator Y andDM particle X
u For benchmark mediator of 3.5TeV, photon energy
spectrum discriminates different DM mediators &allows 1TeV DM particle mass measurement to ~1%
https://arxiv.org/abs/2103.06006

u Search for heavy neutrinos
u e+e– -> Nn -> qqln signature allows full reconstruction of N
u BDT separates signal from SM; beam backgrounds included
u cross-section limits converted to mass (mN) coupling (VIN) plane

LHC analysis: [1812.08750],
diff. assumption V eN = V mN ≠ V tN = 0

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BSM effects through global EFT fits
Standard Model

Scale of new decoupled physics

Strongly benefits from high-energy running

Universal EFT fit

102

10
1

Dimension-6
operators

Includes CLIC measurements of: u Higgs
u Topu WW+–u e e ->f fprecision reach of the Universal EFT fit

light shade: CLIC + LEP/SLD
HL-LHC (3/ab, S1) + LEP/SLD
HL-LHC (3/ab, S2) + LEP/SLD
solid shade: combined with HL-LHC(S2)HL-LHCpreliminary
CLIC Stage 1
blue line: individual reach
CLIC Stage 1+2
yellow mark: additional result
CLIC Stage 1+2+3
Smaller value corresponds
to higher scale L probed

10-1
10-2
10-3

cH

cWW

cBB

cHW

cHB

cGG×10cyf

arXiv:1812.02093 The CLIC potential for new physics, CERN-2018-009-M
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c3 WcWBcTc2 W ×10 c2 B×102effects grow with energy

2c6frome+e––>HH21

BSM indirect searches
Composite Higgs (or top) would appear
through SM-EFT operators – translate EFTlimits into characteristic coupling strength
g* of composite sector and mass m*uu

can be sensitive to new states–> excluded mass ranges

fe+e–

farXiv:1810.10993 - Di Luzio, Gröber, Panico
imist
ic

5-s discovery
regions (CLIC)

pess8

g*-ds/d(cosq ) in e+e– -> ff

10e.g. for n=3 Dirac fermion, m=2TeV
saturates DM relic mass density:
can be excluded by CLICci
isttimop

Other states 95% Exclusion ReachCLIC6 h, hZ, WW, tth eeff

4HL-LHCexclusion region(HL-LHC)

pe
optim

1020

isticim
ss2

Precision measurements e.g.

istic304050m* [TeV]
CLIC can discover compositeness up to~10TeV compositeness scale(~30 – ~50TeV in favourable conditions)– above what HL-LHC can exclude(  )(  )(  )(  )(  )(  )(  )(  )(  /)

-1.5TeV
3-TeV



[]

DF=Dirac Fermion, MF=Majorana Fermion, CS=Complex ScalarSU(3)xSU(2)xU(1) representation; different n-tuplet multiplicities

arXiv:1812.02093 The CLIC Potential for New Physics
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22Interpretations and full programme
e+Higgs + heavy singlet:
Complementarity of direct search
e–and indirect constraints
u0.100

sin2

0.050

nfH

HbbbbuContact interactions interpretations

nLHC 8 TeV Higgs couplings
LHC 300 fb

CMS 13 TeV36 fb-1-1LHC 3 ab-1

HL-LHC Higgs couplingss=mh/m

*0.010
CLIC 1.5 TeV, 1.5 ab-10.005

CLIC 0.38+1.5+3 TeV Higgs couplingss=m /m2h**2

CLIC 3 TeV, 3 ab-10.001
500

1000

1500m [GeV]

2000

2500

Precision Higgs couplings and self-coupling
Precision electroweak and top-quark analysis
Sensitivity to BSM effects in the SMEFT
Higgs and top compositenessBaryogenesis
Direct discoveries of new particles
Extra Higgs boson searches
Dark matter searches
Lepton and flavour violation
Neutrino properties
Hidden sector searches
Exotic Higgs boson decays

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CLIC reaches ~28TeV in OW, ~34TeV in OB

Many more studies inCERN Yellow Report:
The CLIC Potential for
New Physics (250 pages)
arXiv:1812.02093 CERN-2018-009-M
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Ongoing physics studies
Accelerator R&D continues –> CLIC physics remains very relevantu Growing interest in high-energy lepton collisions:u–> CLIC is by far the most advanced TeV-scale lepton collider considered,and the only one where detailed physics studies have been done.
uParticular areas of focus beyond Higgs physics:– importance of top-quark physics in e+e–
– importance of several energy stages in e+e–
– direct searches, in particular for elusive signatures– further and novel ways of constraining NP from precision measurements– importance of beam polarisation– new BSM scenarios of particular relevance to multi-TeV lepton collisions–> look at your favourite model at CLIC energies–> if new benchmark models are defined, e.g. during US Snowmass
exercise, then please help obtain sensitivities for them!

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Tools for CLIC sensitivity studies
A Delphes card for the CLICdet detector model iswell-documented and has already been extensively used:uWhizard settings for CLIC:

https://gitlab.cern.ch/CLICdp/DetectorSoftware/clic-whizard2-settings

CLICdet Delphes card description and validation:
https://arxiv.org/abs/1909.12728
Further information on the use of the CLICdet Delphes card
can be found here:

https://twiki.cern.ch/twiki/bin/view/CLIC/CLICdetDelphesInstructions

b-tagging working points
u jet reconstruction choices
u etc.
uDelphes jet pTvalidationin HZ events

If you are interested in using the full simulation or have questions on
Whizard and Delphes for CLICdet, you are very welcome to contact us:
clicdp-snowmass-samples-contacts@cern.chu

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Exploiting synergies
Priority: focusing on project synergies– detector concepts and software tools
Detector

CLICdet

CLICdet adaptedfor FCC-ee

ColliderSW nameSW status

ILD

ILC

iLCSoft

Full sim/reco

SiD

ILC

iLCSoft

Full sim/reco

CLICdet

CLIC

iLCSoft

Full sim/reco

CLD

FCC-eeiLCSoft

Full sim/reco

IDEA

FCC-eeFCC-SW

Fast sim/reco

IDEA

CEPCFCC-SW

Fast sim/reco

CEPCbaseline

CEPCiLCSoftbranch-off

Full sim/reco

SW futureKey4hep–> all moving to common framework
CLICdet

CLICdet adaptedfor muon collider

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CLIC perspective
CLIC is a mature project, ready to provide a Higgs factory andsubsequent multi-TeV lepton machineuprecision measurementsu sensitivity to elusive signaturesu extended energy/mass reachu

CERN is continuing investment in CLIC accelerator R&D for the next 5yearsu

So far, CLIC has provided the most detailed studies for high-energy
lepton collisions, where interest is increasingu

You are strongly encouraged to continue exploring synergies inaccelerator technology, detector technologies, and physics studies, among
the different collider options and to contribute to CLIC!
Thank you!
u

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