The LHC Long-Lived Particle Community

Timelapse of the moving of CMS during LS1. (Video: CERN)

The LHC and CERN

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator.

Different perspectives of the Globe of Innovation
Different perspectives of the Globe of Science and Innovation (Image: CERN)

The European Organization for Nuclear Research, is a European research organization that operates the largest particle physics laboratory in the world.

Searching for long-lived particles at the LHC

LHC LLP extras

27 May/19

The LHC LLP Community Initiative

The LHC LLP Community


LLP schematic

Schematic of the variety of challenging, atypical experimental signatures that can result from BSM LLPs in the detectors at the LHC. Shown is a cross-sectional plane in azimuthal angle, φ, of a general purpose detector such as ATLAS or CMS. Credit: Heather Russell

Particles in the Standard Model (SM) have lifetimes spanning an enormous range of magnitudes, from the Z boson (tau ~2x10^(-25) s) through to the proton (tau >~ 10^(34) years) and electron (stable).

Similarly, models beyond the SM (BSM) typically predict new particles with a variety of lifetimes. In particular, new weak-scale particles can easily have long lifetimes for several reasons, including approximate symmetries that stabilize the long-lived particle (LLP), small couplings between the LLP and lighter states, and suppressed phase space available for decays. For particles moving close to the speed of light, this can lead to macroscopic, detectable displacements between the production and decay points of an unstable particle for ctau >~ 10µm.

The experimental signatures of LLPs are varied and, by nature, are often very different from signals of SM processes. For example, LLP signatures can include tracks with unusual ionization and propagation properties; small, localized deposits of energy inside of the calorimeters without associated tracks; stopped particles that decay out of time with collisions; displaced vertices in the inner detector or muon spectrometer; and disappearing, appearing, and kinked tracks.

Because the long-lived particles of the SM have masses <~ 5 GeV and have well-understood experimental signatures, the unusual signatures of BSM LLPs offer excellent prospects for the discovery of new physics at particle colliders. At the same time, standard reconstruction algorithms may reject events or objects containing LLPs precisely because of their unusual nature, and dedicated searches are needed to uncover LLP signals. These atypical signatures can also resemble noise, pile-up, or mis-reconstructed objects in the detector; due to the rarity of such mis-reconstructions, Monte Carlo (MC) simulations may not accurately model backgrounds for LLP searches, and dedicated methods are needed to do so.

Although small compared to the large number of searches for prompt decays of new particles, many searches for LLPs at the ATLAS, CMS, and LHCb experiments at the Large Hadron Collider (LHC) have already been performed. Existing LLP searches have necessitated the development of novel methods for identifying signals of LLPs, and measuring and suppressing the relevant backgrounds. Indeed, in several scenarios searches for LLPs have sensitivities that greatly exceed the search for similar, promptly decaying new particles (for example, this is true for directly produced staus in supersymmetry). The excellent sensitivity of these searches, together with the lack of a definitive signal in any prompt channels at the LHC, have focused attention on other types of LLP signatures that are not currently covered. These include low-mass LLPs that do not pass trigger or selection thresholds of current searches, high multiplicities of LLPs produced in dark-sector showers, or unusual LLP production and decay modes that are not covered by current methods. Given the excellent  sensitivity of LHC detectors to LLPs, along with the potentially large production cross sections of LLPs and the enormous amount of data expected to be collected when the LHC switches to high-luminosity running in the 2020s, it is imperative that the space of LLP signatures be explored as thoroughly as possible to ensure that no signals are missed. This is particularly important now, at the end of LHC Run 2, as decisions are currently being made about detector upgrades for Phase 2 of the LHC, and design decisions should be made to ensure that sensitivity to LLPs is retained or possibly improved through high-luminosity running. Indeed, upgrades to the detectors may improve the sensitivity to LLPs compared to the conditions in Runs 1 and 2 of the LHC.

The growing theoretical and experimental interest in LLPs has been mirrored by an increased activity in proposals for LLP searches, new experimental analyses, and meetings to communicate results and discuss new ideas. Workshops focused on LLPs at the University of Massachusetts, Amherst ("LHC Searches for Long-Lived BSM Particles: Theory Meets Experiment",\%2Dparticles-theory-meets-experiment) in November of 2015; Fermilab; and KITP (UCSB) ("Experimental Challenges for the LHC Run II", in May of 2016, among others, highlighted the need for a community-wide effort to map the current space of both theoretical models for LLPs and the atypical experimental signatures that could be evidence of LLPs, assess the coverage of current experimental methods to these models, and identify areas where new searches are required. Additionally, the work presented in these meetings underscored the importance of presenting the results of experimental searches in a manner that allows for their application to different models, and generated new ideas for designing analyses with the goal of minimizing model dependence. Such largely model-independent presentation makes current searches more powerful by increasing their applicability to new scenarios, while reducing redundancies in searches and ensuring that gaps in coverage are identified and addressed. This task extends beyond the purview of any particular theoretical model or experiment, and requires an effort across collaborations to address the needs of the LLP community and illuminate a path forward.

This flurry of activity eventually coalesced in the establishment of a more central and regular platform -- the LHC LLP Community -- for experimentalists at the LHC and those in the theoretical and phenomenological communities to exchange ideas about LLP searches to ensure the full discovery potential of the LHC. This began with a mini-workshop at CERN in May of 2016 ("LHC Long-Lived Particle Mini-Workshop", and has continued with workshops in April of 2017 at CERN ("Searches for long-lived particles at the LHC: First workshop of the LHC LLP Community",, October of 2017 at ICTP Trieste ("Searches for long-lived particles at the LHC: Second workshop of the LHC LLP Community",, May of 2018 again at CERN, "Searching for long-lived particles at the LHC: Third workshop of the LHC LLP Community",, at Nikhef, in Amsterdam, in October of 2018, "Searching for long-lived particles at the LHC: Fourth workshop of the LHC LLP Community",, again at CERN in May of 2019, "Searching for LLPs at the LHC: Fifth Workshop of LHC LLP Community (, and in Ghent, Belgium, in November of 2019, "Searching for LLPs at the LHC: Sixth Workshop of LHC LLP Community, (

This is the work undertaken by the LHC LLP Community.  Welcome.