Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider
Authors: Alimena Juliette , Beacham James , Borsato Martino , Cheng Yangyang , Vidal Xabier Cid , Cottin Giovanna , Curtin David , De Roeck Albert , Desai Nishita , Evans Jared A , Knapen Simon , Kraml Sabine , Lessa Andre , Liu Zhen , Mehlhase Sascha , Ramsey-Musolf Michael J , Russell Heather , Shelton Jessie , Shuve Brian , Verducci Monica , Zurita Jose , Adams Todd , Adersberger Michael , Alpigiani Cristiano , Apresyan Artur , Bainbridge Robert John , Batozskaya Varvara , Beauchesne Hugues , Benato Lisa , Berlendis S , Bhal Eshwen , Blekman Freya , Borovilou Christina , Boyd Jamie , Brau Benjamin P , Bryngemark Lene , Buchmueller Oliver , Buschmann Malte , Buttinger William , Campanelli Mario , Cesarotti Cari , Chen Chunhui , Cheng Hsin-Chia , Cheong Sanha , Citron Matthew , Coccaro Andrea , Coco V , Conte Eric , Cormier Félix , Corpe Louie D , Craig Nathaniel , Cui Yanou , Dall’Occo Elena , Dallapiccola C , Darwish M R , Davoli Alessandro , Cosa Annapaola de , Simone Andrea De , Rose Luigi Delle , Deppisch Frank F , Dey Biplab , Diamond Miriam D , Dienes Keith R , Dildick Sven , Döbrich Babette , Drewes Marco , Eich Melanie , ElSawy M , Valle Alberto Escalante del , Facini Gabriel , Farina Marco , Feng Jonathan L , Fischer Oliver , Flaecher H U , Foldenauer Patrick , Freytsis Marat , Fuks Benjamin , Galon Iftah , Gershtein Yuri , Giagu Stefano , Giammanco Andrea , Gligorov Vladimir V , Golling Tobias , Grancagnolo Sergio , Gustavino Giuliano , Haas Andrew , Hahn Kristian , Hajer Jan , Hammad Ahmed , Heinrich Lukas , Heisig Jan , Helo J C , Hesketh Gavin , Hill Christopher S , Hirsch Martin , Hohlmann M , Holmes Tova , Hulsbergen W , Huth John , Ilten Philip , Jacques Thomas , Jayatilaka Bodhitha , Jeng Geng-Yuan , Johns K A , Kaji Toshiaki , Kasieczka Gregor , Kats Yevgeny , Kazana Malgorzata , Keller Henning , Khlopov Maxim Yu , Kling Felix , Kolberg Ted R , Kostiuk Igor , Kuwertz Emma Sian , Kvam Audrey , Landsberg Greg , Lanfranchi Gaia , Lara Iñaki , Ledovskoy Alexander , Linthorne Dylan , Liu Jia , Longarini Iacopo , Lowette Steven , Lubatti Henry , Lutz Margaret , Luo Jingyu , Mamuz̆ić Judita , Marinangeli Matthieu , Mariotti Alberto , Marlow Daniel , McCullough Matthew , McDermott Kevin , Mermod P , Milstead David , Mishra-Sharma Siddharth , Mitsou Vasiliki A , Berlingen Javier Montejo , Moortgat Filip , Morandini Alessandro , Morris Alice Polyxeni , Morse David Michael , Mrenna Stephen , Nachman Benjamin , Nemevs̆ek Miha , Nesti Fabrizio , Ohm Christian , Pascoli Silvia , Pedro Kevin , Peña Cristián , Rodriguez Karla Josefina Pena , Piedra Jónatan , Pinfold James L , Policicchio Antonio , Popara Goran , Prisciandaro Jessica , Proffitt Mason , Rauco Giorgia , Redi Federico , Reece Matthew , Hall Allison Reinsvold , Sfar H Rejeb , Renner Sophie , Robinson Dean , Roepe Amber , Ronzani Manfredi , Salvioni Ennio , Santra Arka , Sawada Ryu , Scholtz Jakub , Schuster Philip , Schwaller Pedro , Sebastiani Cristiano , Sekmen Sezen , Selvaggi Michele , Si Weinan , Soffi Livia , Stolarski Daniel , Stuart David , III John Stupak , Sung Kevin , Taylor Wendy , Templ Sebastian , Thomas Brooks , Torró-Pastor Emma , Trocino Daniele , Trojanowski Sebastian , Trovato Marco , Tsai Yuhsin , Tully C G , Vámi Tamás Álmos , Vasquez Juan Carlos , Sierra Carlos Vázquez , Vellidis K , Vermassen Basile , Vit Martina , Walker Devin G E , Wang Xiao-Ping , Watts Gordon , Xie Si , Yexley Melissa , Young Charles , Yu Jiang-Hao , Zalewski Piotr , Zhang Yongchao 
Published: September 2020

Abstract
Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments—as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER—to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity ‘dark showers’, highlighting opportunities for expanding the LHC reach for these signals.