Data taken in 2011 with the ATLAS detector at the Large Hadron Collider have been used to search for physics beyond the Standard Model. Results are presented based on luminosities between 2 and 5 fb−1 of √�(s) = 7TeV protonproton collisions focusing on final states with long-lived particles. No evidence of new physics is found

Policicchio, A.

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DOI 10.1393/ncc/i2014-11663-0Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Searches for long-lived particles with the ATLAS detectorat the LHCA. Policicchio on behalf of the ATLAS CollaborationUniversita` della Calabria and INFN, Gruppo Collegato di Cosenza - Rende (CS), Italyricevuto l’1 Ottobre 2013Summary. — Data taken in 2011 with the ATLAS detector at the Large HadronCollider have been used to search for physics beyond the Standard Model. Resultsare presented based on luminosities between 2 and 5 fb−1 of√(s) = 7TeV proton-proton collisions focusing on final states with long-lived particles. No evidence ofnew physics is found.PACS 12.60.-i – Models beyond the Standard Model.1. – IntroductionThe Standard Model (SM) of particle physics has proven extremely successful, but de-spite its many successes still remains incomplete. New particles with long lifetimes (LLP,Long-Lived Particles) are predicted in many models of physics beyond the SM, such asHidden Valley (HV) scenarios or Supersymmetry (SUSY) with Gauge-Mediated Super-symmetry Breaking (GMSB), Anomaly-Mediated Supersymmetry Breaking (AMSB) orR-parity violation (RPV). These particles may be neutral and decay into SM and/orweakly interacting particles that escape the detection. This note presents the searcheswith ATLAS detector [1] for LLPs based on data collected during 2011 at√s = 7TeV.2. – Heavy stable charged particles2.1. Long-lived charginos in AMSB model . – The results presented are interpretedin the context of AMSB scenario. The chargino (χ±1 ) decays to a neutralino and a lowenergy pion, resulting in a disappearing track in the inner tracker with few associated hitsin the outer region of the tracking system, namely in the Transition Radiation Tracker(TRT). The selection cut flow is based on the number of hits in the TRT and the pT ofthe track, starting from events triggered by a single jet from initial state radiation. Thetransverse momentum spectrum of candidate tracks is found to be consistent with theexpectation from the SM background processes and constraints on chargino propertiesc© CERN on behalf of the ATLAS CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 38SEARCHES FOR LONG-LIVED PARTICLES WITH THE ATLAS DETECTOR AT THE LHC 39Fig. 1. – Left: constraint on the lifetime (τχ±1)-mass (mχ±1) parameter space in the search forlong-lived charginos. Right: Cross-section upper limits at 95% CL for gluino R-hadrons in thefull-detector search are shown. Both the expected limit with ±1, 2σ uncertainty bands andthe observed upper limit are given. In addition the theoretical prediction for the productioncross-section calculated at NLO+NLL and its uncertainty are drawn.are obtained based on 4.7 fb−1 of data [2]. For a mass difference between chargino andneutralino of ∼ 160 (170)MeV the chargino mass is excluded up to 103 (85)GeV at 95%CL. The constraint on the lifetime (τχ±1 ) - mass (mχ±1 ) parameter space is shown in fig. 1(left).2.2. R-hadrons and sleptons . – Many extensions of the SM include heavy, long-lived,charged particles that have speed β, significatively less than 1 and/or electric charge notequal to ±1e. Sleptons are predicted to be metastable in GMSB models. R-hadrons,bound states formed by squark and gluinos, are predicted to be metastable and theirelectric charge can change due to nuclear scattering in the detector. With lifetimesgreater than a few nanoseconds, these particles can travel distances larger than thetypical collider detector and appear as stable. The analysis strategy is driven by thelarge amount of energy lost via ionization (dE/dx) and/or through the long time-of-flight(TOF) needed to reach the outer detectors. The analyses fall into multiple topologiesand strategies: requiring tracks reconstructed in both the inner detectors and the muonsystem, as in the tracker+TOF analysis; requiring tracks reconstructed only in the innersilicon detectors, as in the tracker-only analysis; requiring tracks reconstructed only in themuon system, as in the muon-only analysis. Many different searches for R-hadrons andsleptons have been presented with ATLAS data collected at 7TeV. No significant excessis observed with 4.7 fb−1 of data [3]. Long-lived stau’s in the GMSB model considered,for tanβ = 5–20, are excluded at 95% CL for masses up to 300GeV, while directlyproduced long-lived sleptons, or sleptons decaying to long-lived ones, are excluded at95% CL up to a stau mass of 278GeV. Long-lived R-hadrons containing a gluino (stop,sbottom) are excluded for masses up to 985GeV (683GeV, 612GeV) at 95% CL, for ageneric interaction model. Cross-section upper limits at 95% CL for gluino R-hadrons inthe full-detector search are shown in fig. 1 (right).40 A. POLICICCHIO on behalf of the ATLAS COLLABORATIONFig. 2. – Left: Upper limits at 95% confidence level on σ × BR2 vs. the neutralino lifetime fordifferent combinations of squark and neutralino masses, based on the observation of zero eventsin a 4.4 fb−1 data sample. The shaded areas around these curves represent the ±1σ uncertaintybands on the expected limits. The horizontal lines show the cross sections calculated fromPROSPINO for squark masses of 700GeV and 1500GeV. The shaded regions around theselines represent the uncertainties on the cross sections. Right: Observed 95% upper limits on theprocess h→ πvπv vs the πv proper decay length, expressed as a multiple of the SM cross sectionfor Higgs production. Exclusion limits assume 100% branching ratio for the Higgs decaying toπv’s.3. – Displaced vertexes3.1. Displaced neutralino decay in RPV SUSY . – The results presented are interpretedin the context of an RPV scenario. The signature under consideration corresponds tothe displaced decay of the lightest supersymmetric particle, resulting in a muon andmany high-pT charged tracks originating from a single displaced vertex. The muon isused for triggering the events. A dedicated vertex reconstruction has been developed.No events passing the signal selection are observed and limits are set on the productioncross section for supersymmetric particles, multiplied by the square of the branchingratio for a neutralino to decay to charged hadrons and a muon, as a function of theneutralino lifetime. To allow these limits to be used in a variety of models, they arepresented, as shown in fig. 2 (left) [4], for a range of squark and neutralino masses:MH: medium-mass squark (700GeV), heavy neutralino(494GeV); ML: medium-masssquark (700GeV), light neutralino (108GeV); HH: heavy squark (1500GeV) and heavyneutralino (494GeV).3.2. Hidden sector neutral particles. – Decays far from the interaction point are sig-natures of long-lived neutral particles. ATLAS has performed a search based on 1.9 fb−1for pairs of back-to-back particles decaying to heavy fermions in the muon system [5].Such decays are signatures, for example, of Higgs decays to pairs of long-lived neutralparticles h→ πvπv, where πv is a pseudoscalar from a Hidden Sector weakly coupled tothe Standard Model sector. Limits have been set as a function of the proper decay lengthof the πv excluding the range 0.5–23m depending on the Higgs mass (120–140GeV) andon the πv mass (20–40GeV). Exclusion limits are shown in fig. 2 (right).SEARCHES FOR LONG-LIVED PARTICLES WITH THE ATLAS DETECTOR AT THE LHC 41Fig. 3. – The 95% upper limits on the σ×BR for the process H → γdγd +X as a function of thedark photon cτ for mH = 100GeV and with mH = 140GeV. The expected limit is shown asthe dashed curve and the solid curve shows the observed limit. The horizontal lines correspondto the Higgs boson SM cross sections at the two mass values.A specific model of a neutral long-lived, spinless, exotic particle which has a nonzerobranching fraction to dileptons has been used in the search for displaced clusters ofleptons (leptonjets). This scenario predicts up to two displaced dilepton vertices in thetracking volume per event. In the study, the model consists of a Higgs boson decayingto new hidden sector particles which finally produce two sets of collimated muon pairs:h → fd1 + fd1 , fd1 → fd2 + γd, γd → l+l− where fd1 is a fermion from the hiddensector, fd2 is a stable fermion from the hidden sector, and γd is a dark photon decayingto pairs of leptons. In 1.9 fb−1 of data [6] no events consistent with this Higgs bosondecay mode are observed. 95% CL upper limits on the cross section times branchingratio (σ × BR) has been evaluated for the process H → γdγd + X (X refers to the twostable fd2 fermions) with the γd mass set to 0.4GeV. The (σ×BR) is given as a functionof the γd mean lifetime, expressed as cτ for an Higgs mass of 100 and 140GeV. Theselimits are shown on fig. 3. Table I shows the ranges in which the γd cτ is excluded at the95% CL for H → γdγd + X branching ratios of 100% and 10%.Table I. – Ranges in which γd cτ is excluded at 95% CL for 100 and 140GeV Higgs masses,assuming 100% and 10% branching ratio of H → γdγd + X.Higgs boson mass Excluded cτ [mm] Excluded cτ [mm][GeV] BR(100%) BR(10%)100 1 ≤ cτ ≤ 670 5 ≤ cτ ≤ 159140 1 ≤ cτ ≤ 430 7 ≤ cτ ≤ 8242 A. POLICICCHIO on behalf of the ATLAS COLLABORATION4. – ConclusionsThe search for new phenomena beyond the Standard Model is a very active field atthe ATLAS experiment. Recent results for long-lived particle searches at ATLAS arepresented in this note. These searches take advantage of the unique detector signaturespredicted by the different models. No excess over the Standard Model expectation hasbeen observed and limits are placed on the various models. The reader is invited to readthe detailed papers on each of these analyses.REFERENCES[1] The ATLAS Collaboration, JINST, 3 (2008) S08003.[2] The ATLAS Collaboration, JHEP, 01 (2013) 131.[3] The ATLAS Collaboration, Phys. Lett. B, 720 (2013) 277.[4] The ATLAS Collaboration, Phys. Lett. B, 719 (2013) 280.[5] The ATLAS Collaboration, Phys. Rev. Lett., 108 (2012) 251801.[6] The ATLAS Collaboration, Phys. Lett. B, 721 (2013) 32.

Searches for long-lived particles with the ATLAS detector

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Searches for long-lived particles with the ATLAS detector at the LHC

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