Searching for Long-Lived Particles at the LHC

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Long-lived particles: what are they and where could they come from?

By definition, a long-lived particle (LLP) is an unstable particle with a sizeable lifetime. The definition of sizeable, in turn, will depend on the experimental apparatus, as it should be long enough to allow the LLP to travel for an observable distance in a given detector before decaying. In the LHC experiments, LLPs could be produced in the proton-proton collisions and manifest themselves in the detectors via their late decay, leading to a signature of visible particles which do not originate from the interaction point.

For a particle to have a long lifetime, the decay must be suppressed, which can happen through two mechanisms: small couplings or a suppressed decay phase space. The former case can happen for example if the interaction violates an approximate symmetry which would otherwise forbid the decay to take place. The latter case can happen if the mass difference between the decaying particle and its decay products is small (a so-called compressed mass spectrum), or if the mediator of the decay is very massive. These mechanisms are at play even in the Standard Model (SM), which contains multiple long-lived particles, such as the B, D, K mesons or the muons. It is therefore not unexpected that they could also be at play in theories beyond the SM.

As high-energy collisions can lead to large relativistic boosts, if the LLPs are relatively light, the displacement can be enhanced in the laboratory frame with respect to the rest-frame lifetime.

Signatures of long-lived particles at the LHC

- Displaced vertices in the inner detector. More specific signatures can be defined by imposing additional requirements on the nature of the displaced vertices, eg at least two oppositely charged leptons, DV+muon, DV+electron, DV+jets and DV+Emiss - Displaced vertices in the muon system - Displaced lepton pairs: While the displaced vertex signatures aim attention at intersecting tracks, interpreting the intersection point as the position of a possible LLP decay, searches for displaced particle tracks probe the displacement of the particle tracks from the primary interaction poin using the transverse and longitudinal impact parameters d0 and dz - Displaced jets: displaced tracks relative to the interaction point, and tracks forming a displaced vertex; jet is generated by a neutral long-lived particle decay in one of the calorimeters; distinguishable from prompt jets, if they generate an exceptionally high ratio EH/EEM of energy - Displaced lepton jets: Lepton jets are a class of jets of highly collimated charged leptons, i.e. electrons and/or muons, which can be supplemented with pions - Non-pointing and delayed photons: flight direction and time-of-flight information provided by the electromagnetic calorimeter to detect non-pointing and delayed photons - Delayed jets: the delay is measured relative to the arrival time of a massless particle coming from the interaction point on the direct way. - Disappearing tracks: The absence of hits at higher radii suggests that the particle has decayed, but no charged decay product is observed - Kinked tracks: charged LLP decay product is observable as a track with a different direction. In this case, the tracks of the LLP and the charged decay product mimic the track of a particle that gets abruptly deflected. - Emerging jets: jets which become apparent gradually during their propagation from the interaction point through the detector. - Heavy stable charged particles: Unstable charged particles with a high mass, leading to a low velocity compared to the speed of light even at high momenta, and a lifetime that favours a decay outside of the detector volume are referred to as heavy stable charged particle ; atypical rate of energy loss dE/dx through ionisation. Moreover, the comparatively low velocity can be observed by measuring the time-of-flight (ToF). - Stopped particles: long-lived particles could come to rest within the detector, where they decay at a later time; their decay can be completely disconnected from the production. In particular, it can take place when there is no collision activity in the detector

Search strategies and unique difficulties

These are atypical signatures, so : - dedicated algorithms are often necessary to reconstruct the objects, as the 'conventional' algorithms to reconstruct particles in the detector and/or suppress noise often rely on some compatibility with a prompt decay from the interaction point - for the same reason, these searches might require dedicated triggering strategies - the main background sources are often different from more conventional searches which are dominated by SM processes with similar final states: in LLP searchs, the BG is more likely to originate from noise, pileup, or rare mis-reconstructions. These are unlikely to be well modelled by MC and thus data-driven BG estimation techniques are prevalent.

== New and proposed dedicated detectors


J. Alimena et al, Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider, Phys. G: Nucl. Part. Phys. 47 090501 (2020), arXiv:1903.04497

L. Lee, C. Ohm, A. Soffer and T. T. Yu, Collider Searches for Long-Lived Particles Beyond the Standard Model, Prog. Part. Nucl. Phys. 106, 210-255 (2019), arXiv:1810.12602

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