Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider

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.

Exploring color-octet scalar parameter space in minimal R-symmetric models

“…Therefore we do not take seriously the prospect of a long-lived scalar sgluon. Nevertheless, any model that predicts reasonably small mass splitting of left-and right-chiral states also predicts a pseudoscalar long-lived enough to decay with displaced vertices [35]. This is an exciting signature that degrades the efficacy of most searches that constrain the pseudoscalar and calls for searches for long-lived particles (LLPs).…”

Exploring 0νββ and leptogenesis in the alternative left-right model

“…The lightest charged Higgs boson in the ALRM is H AE 2 , which is longlived, so that limits on its mass are probed by searches for heavy stable charged particles. The H AE 2 bosons are pairproduced via the Drell-Yan mechanism, in proton-proton collisions at center-of-mass energies of 7 TeV [33,34], 8 TeV [35] and 13 TeV [36][37][38][39][40][41] and in electron-positron collisions at a center-of-mass energy of 183 GeV [42].…”

Sensitivity reach on heavy neutral leptons and τ -neutrino mixing |UτN|2 at the HL-LHC

“…We display the main constraints from EWPD [57][58][59][60][61] (brown dashed line), L3 [62][63][64] (pink dashed line), DELPHI [65] (blue dashed line), LEP2 [62][63][64] (red solid line), CMS-13 TeV trilepton [66] (black solid line), CMS-13 TeV same-sign dilepton [67] (purple solid line), ATLAS-13 TeV trilepton [68] M N ≳ 50 GeV and U 2 αN ≳ 10 −8 , the decay length of HNL is quite small such that we can simply take the decay of HNL as prompt in most of the parameter space for each lepton flavor. In contrast, the low mass HNL with tiny U 2 αN can easily generate the displaced vertex signature after it has been produced at colliders [32,[49][50][51][52][53][54][55][56], which is of immense interest in the upcoming LHC run.…”