Search for long-lived neutral particles decaying into lepton jets in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

Several models of physics beyond the Standard Model predict neutral particles that decay into final states consisting of collimated jets of light leptons and hadrons (socalled "lepton jets"). These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors' linear dimensions. This paper presents the results of a search for lepton jets in proton-proton collisions at the centre-of-mass energy of √ s = 8 TeV in a sample of 20.3 fb −1 collected during 2012 with the ATLAS detector at the LHC. Limits on models predicting Higgs boson decays to neutral long-lived lepton jets are derived as a function of the particle's proper decay length. An extensively studied case is one in which the two sectors couple via the vector portal, in which a light hidden photon (dark photon, γ d ) mixes kinetically with the SM photon. If the hidden photon is the lightest state in the hidden sector, it decays back to SM particles with branching fractions that depend on its mass [6,8,9]. For the case in which the γ d kinetically mixes with hypercharge, one finds that , the kinetic mixing parameter, controls -1 - Keywords: Hadron-Hadron Scattering JHEP11(2014)088both the γ d decay branching fractions and lifetime. More generally, however, the branching fractions and lifetime are model-dependent and may depend on additional parameters.Due to their small mass, these particles are typically produced with a large boost and, due to their weak interactions, can have non-negligible lifetime. As a result one may expect, from dark photon decays, collimated jet-like structures containing pairs of electrons and/or muons and/or charged pions ("lepton jets", LJs) that can be produced far from the primary interaction vertex of the event (displaced LJs).Neutral particles which decay far from the interaction point into collimated final states represent a challenge both for the trigger and for the reconstruction capabilities of the LHC detectors. Collimated charged particles in the final state can be difficult to disentangle due to the limited granularity of the detector. Moreover, in the absence of information from the inner tracking system, it is necessary to use the muon spectrometer (MS) for the reconstruction of tracks which originate from a secondary decay far from the primary interaction vertex (IP).The high-resolution, high-granularity measurement capability of the ATLAS "air-core" MS is ideal for this type of search. In addition, the ATLAS inner tracking system can be used to define isolation criteria to significantly reduce, for decay vertices far from the interaction point, the otherwise overwhelming SM background from proton-proton collisions.The search for displaced LJs presented in this paper employs the full dataset collected by ATLAS during the 2012 run at √ s = 8 TeV, corresponding to an integrated luminosity of 20.3 fb −1 . Related searches for prompt LJs have been performed both at the Tevatron [10,11] and at the LHC [12][13][14][15]. Additional constraints on scenarios with hidden phot...

Recent Progress in Search for Dark Sector Signatures

“…The searches reviewed in this paper seek to further test the SM by looking for deviations in the associated neutral mesons decays [173][174][175][177][178][179][180], for anomalous production of the tightly collimated prompt [155,164] or displaced lepton pairs [158], and also by looking for the anomalous production of events with gauge bosons and at least two prompt leptons in the final state, e and µ lepton flavors [156,162,165]. As a signature-based search, specific models of new physics are only considered to benchmark the performance of the selection criteria.…”

“…A search for dark photon with mass above 210 MeV, was performed by the CDF [153]; the DØ collaboration focused on dark-photon with mass 1.4 GeV [152], and came up with more results for m D = 0.3, 0.9, and 1.3 GeV in [154]. The sensitivity of that analysis to models considered in the AT-LAS work is approximately two orders of magnitude lower [155,156,158].…”

“…Results from an 8 TeV ATLAS search for displaced muon lepton-jets as decay products of the dark photon with mass of 0.4 GeV are shown in [158]. The CMS results for displaced lepton-jets are shown in [159,160].…”

“…The range ϵ i 10 −4 -10 −3 , which satisfies all the constraints (for recent studies, see [193,209] and references therein) implies dark photons with very short lifetimes [149], which leads to the prompt decays of the dark photon [155,156,164] (see also for more details on the prompt lepton-jet search [162,165,166]). For sufficiently small values of ϵ i (less than 10 −4 ), the dark photon lifetime can be sizable, leading to so-called displaced decay vertices observable in the laboratory frame [158][159][160]. Therefore, most of the recent LHC searches focus on the simplest case G D = U(1) Y and try to investigate both options, which encapsulates the main features of dark sector phenomenology [130,143,161].…”

Displaced vertex searches for sterile neutrinos at future lepton colliders

“…This effect of a secondary vertex from the decays of the heavy neutrino, that is displaced from the primary vertex, yields an exotic signature and constitutes a powerful search channel for heavy neutrinos at particle colliders. This type of signature is being looked for by the ATLAS and CMS collaborations in their searches for long-lived neutral particles [10,11].…”