Long-lived particles: theory and experimental probes

TL;DR

Long-lived particles (LLPs) are ubiquitous in both the SM and beyond, and their decays can occur at measurable times or positions within detectors, described by the exponential survival probability $P(t)=e^{-rac{t}{eta ext{ } au}}$ with $ au$ the rest-frame lifetime and $eta$ the velocity factor. The paper links lifetime to the decay width via $ Gamma=rac{ ext{hbar}}{ au}$ and outlines three general mechanisms that prolong lifetimes: heavy mediators in the decay, small couplings, or restricted final-state phase space. It surveys motivations for LLPs in dark matter frameworks, sterile neutrinos, and supersymmetry, and exhausts experimental detection strategies, including direct charged-track searches and indirect displaced-signature analyses. Finally, it maps the current LLP experimental landscape across the LHC, lepton colliders, fixed-target/beam-dump experiments, and astrophysical probes, highlighting trigger design, reconstruction challenges, and opportunities for dedicated LLP detectors.

Abstract

Long-lived particles (LLPs) are particles that are stable or that live long enough for their decays to be experimentally distinguishable in time or position from their production point. We provide an overview of the phenomenology and experimental signatures of LLPs, focusing on LLPs at the Large Hadron Collider (LHC). We explain what determines a particle's lifetime and we show that LLPs are ubiquitous both within the Standard Model and beyond. We survey the methods used to experimentally detect and characterize particles at collider-based experiments, and discuss how searches for LLPs present both experimental challenges and exciting new possibilities for detection. Finally, we situate LHC searches for LLPs within the broader experimental landscape with a brief overview of searches for LLPs at lower-energy experiments and a discussion of astrophysical and cosmological probes offering complementary insight into the physics of LLPs beyond the Standard Model.

Figures (7)

• Figure 1: A particle's mean decay position, $d$, is determined by its rest-frame lifetime $\tau$ and speed through $d=\beta\gamma c\tau$. Top panel: The probability for a particle to not have decayed by position $d$, for three different values of $\beta\gamma c\tau$. Middle: the probability for a particle to have decayed by position $d$. Bottom: the probability that the particle decays within a given detector volume, using the regions defined by Figure \ref{['fig:detector']}, where the "Prompt" region is defined by $d < 2$ mm.
• Figure 2: A selection of Standard Model particles in the mass-lifetime plane. Different markers indicate different categories of particles, while the shading indicates the year the particle was discovered. While the order in which particles are discovered depends on many factors, there is an apparent trend where the earliest particles to be discovered within a given species have relatively long lifetimes. This figure is inspired by earlier versions from Refs. Alimena:2019zriLee:2018pag.
• Figure 3: A transverse cross-section view of a simplified detector at the LHC, where the beam runs perpendicular to the page. Only the top half of the detector is shown. The inner tracker consists of silicon pixel and strip layers surrounded by an electromagnetic calorimeter, which is in turn surrounded by a hadronic calorimeter. Outside the calorimeters, a gaseous tracking detector serves as a muon system.
• Figure 4: Several examples of direct detection signatures from electrically charged LLPs. In the first panel, an LLP decays inside the tracker into a charged particle with very low momentum and a neutral particle, creating a disappearing track signature. In the middle panel, a heavy, charged LLP leaves more energy due to ionization energy loss in the tracker than a minimum ionizing particle with comparable momentum. The LLP decays in the calorimeter, but the details of its decay are not relevant to the $dE/dx$ signature. In the right panel, a charged LLP does not decay inside the detector and therefore leaves a track throughout the whole detector, which can be identified as originating from a heavy BSM particle due to $dE/dx$ or Time-of-Flight measurements in any of the detector subsystems.
• Figure 5: Several examples of signatures involving displaced objects in the tracker. In the first panel, a heavy, neutral LLP decays to a pair of charged particles inside the tracking detector layers. Tracks from the charged decay products can be reconstructed as displaced based off both their origin as well as their impact parameters, and a displaced vertex can be reconstructed. In the middle panel, a boosted LLP produces a pair of displaced tracks. The tracks are displaced in origin, and a displaced vertex can be reconstructed. However, as the decay products inherit the parent LLP's boost, they are collimated, and it may not be possible to use the track impact parameters to identify that the tracks are displaced as they will point back to the origin. In the right panel, pair-production of charged LLPs, each of which decays to a charged and neutral decay product, produces a signature with two displaced tracks which do not form a displaced vertex.