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.

We study baryon number violation in R-parity violating supersymmetry with focus on ΔB = 2 processes which allow neutron-anti-neutron (n − \( \overline{n} \)) oscillations. We provide prospects for going beyond the present limits by means of a new search for n − \( \overline{n} \) oscillations. The motivation is the recently proposed n − \( \overline{n} \) oscillation experiment at the European Spallation Source in Lund, which is projected to be able to improve the current bound on the transition probability in the quasi-free regime by three orders of magnitude. We consider various processes giving rise to baryon number violation and extract the corresponding simplified models, including only the relevant superpartners and couplings. In terms of these models we determine the exclusion limits from LHC searches as well as from searches for flavor transitions, CP violation and di-nucleon decays. We find that, for certain regions of parameter space, the proposed n − \( \overline{n} \) experiment has a reach that goes beyond all other experiments, as it can probe gluino and squark masses in the multi-TeV range.

The goal of the HIBEAM/NNBAR program is to search for baryon number violation via the conversion or oscillation of neutrons into sterile neutrons and/or antineutrons at the European Spallation Source. A key experimental component of the program is the construction of an annihilation detector to directly observe the production of an antineutron following the oscillation. Design studies for the annihilation detector are presented. The predicted response of the detector models are studied using Geant4 simulations made with Monte Carlo simulations of the annihilation signal topology and cosmic ray backgrounds. Particle identification and sensitive discriminating observables, such as invariant mass and sphericity, are shown.

We consider a possible neutron–antineutron ($n-\overline{n}$) oscillation experiment at the PF1B instrument at Institut Laue Langevin. It can improve the best existing constraint on the transition rate and also allow the testing of the methods and instrumentation which would be needed for a later larger-scale experiment at ESS. The main gain factors over the most competitive experiment, performed earlier at PF1 instrument at ILL, are: a more intense neutron beam and a new operating mode based on coherent n and $\overline{n}$ mirror reflections. The installation of such an experiment would need a temporary replacement of the existing ballistic neutron guide by a specially designed $n/\overline{n}$ guide with a gradually increasing cross section and a specially selected coating as well as the development and construction of an advanced $\overline{n}$ annihilation detector with a high efficiency and low background. The overall gain factor could reach up to an order of magnitude and depends on the chosen experiment configuration.

The various RPV coefficients of the six-quark operators for the non-EW cases need to be re-scaled by a factor 1/2 for consistency: (Fromula presented.) This affects equations (4.2), (4.3), (4.4), (5.2), (5.4), (5.6), (5.7), (5.8), (6.3), and figures 4, 9, 10, 11, 12. The qualitative changes in the exclusion bounds are mild. We thank Dr. Chang Sub Shin for pointing out this mismatch.SCOPUS: er.jinfo:eu-repo/semantics/publishe