Search for Long-Lived Particles and Lepton-Jets with the ATLAS detector

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1 EPJ Web of Conferences 6, 177 (213) DOI:.51/epjconf/ Owne by the authors, publishe by EDP Sciences, 213 Search for Long-Live Particles an Lepton-Jets with the ALAS etector D. Salvatore 1,a On behalf of the ALAS Collaboration 1 University of Calabria, ponte P. Bucci, 8736 Rene (CS), Italy Abstract. Many extensions of the Stanar Moel inclue neutral weakly-couple particles that can be longlive. hese long-live particles occur in many moels, inclue gauge-meiate extensions of the Minimal Supersymmetric Moel (MSSM), MSSM with R-parity violation, inelastic ark matter an the Hien Valley scenario. Results are presente on the ALAS searches at the LHC for possible rare Higgs boson ecays to pair of neutral, long-live hien-sector particles that lea to final states containing collimate lepton jets or fermion anti-fermion pairs. No excess of events above the expecte backgroun has been observe on ata collecte in 211 at a center of mass energy of 7 ev an limits on the cross sections are set. 1 Introuction Recently, the prouction of a boson with a mass of about 125 GeV has been observe by the ALAS [1] an CMS [2] Collaborations. he observation is compatible with the prouction an ecay of the Stanar Moel (SM) Higgs boson at this mass. Strengthening or rejecting the SM Higgs boson hypothesis is currently of utmost importance an thus a search for non-sm Higgs boson ecays is of high interest. Several moels of physics beyon the SM preict the existence of Hien Sectors able to comunicate to the SM through several portals (Higgs, Z, loop of SUSY particles). A light Higgs boson can ecay to particles of the hien sector [3] such as the long-live pseuoscalar v-pion (π v ) or scalar hien fermions. hese Hien Valley (HV) particles can ecay back in the stanar sector to collimate jets of electrons or muons, lepton-jets (LJ) or fermion anti-fermion pairs. Lifetimes can be comparable to ALAS [4] imensions, leaing to isplace ecays far from the interaction point. 2 Prompt lepton-jets his analysis [5] is base on a moel where a pair of squarks is prouce an each of the squarks cascae ecays into ark-sector particles, incluing one or more ark-photons (γ ). he γ s ecay into pairs of leptons, forming LJs. Aitionally, ark-sector particles may raiate multiple γ s, increasing the lepton multiplicities an number of the LJs. he amount of raiation is etermine by the ark sector gauge coupling parameter α. Setting α =. results in a simple LJ with two har leptons. Larger values may prouce LJs with four, six, eight, or more prompt leptons from the ecay of overlapping γ s, albeit with reuce boost. he transverse momentum (p ) a aniela.salvatore@cern.ch of the leptons increases with γ mass, but ecreases with α. In this moel, α =.,.1, an.3, an γ masses (m γ ) of 15, 3, an 5 MeV are use. For m γ = 15 MeV, the γ is below the µ + µ threshol an can only ecay to electrons. With m γ 3 MeV, the γ ecays to electron an muon pairs. Aitionally, for m γ = 5 MeV, 2% of the ecays prouce pion pairs. his analysis consiers LJs in three signatures: single muon-jets with four or more muons, pairs of muon-jets each with two or more muons, an pairs of electron-jets each with two or more electrons. Events containing electron-jets (EJs) were selecte using single-electron triggers with an online p threshol of 2 or 22 GeV. o ensure proper moelling of the trigger acceptance, events were require to contain at least one reconstructe electron with p > 35 GeV, above which the trigger efficiency is constant. he reconstructe electron was require to match an electron reconstructe above the p threshol in the trigger system with a separation R less than.2. he EJ caniates were built from electromagnetic (EM) clusters. At least two tracks from the primary vertex (PV) with p > GeV were require to have R <.1 of the cluster position in the secon sampling layer of the calorimeter. Aitional requirements were mae on the number of hits along the track in the silicon pixel an microstrip etectors to suppress backgrouns from photon conversions. he analysis require two LJ caniates in each event, with one cluster matching the electron reconstructe in the trigger system. he invariant mass of the two highest-p tracks associate with each EJ ha to be less than 2 GeV. o reuce the backgroun coming from multi-jet events, the electron cluster concentration (R η2 ), efine as the ratio of total energy in 3 7 cells to the total energy in 7 7 cells in η, φ in the secon sampling layer of the EM calorimeter (ECAL) was use, together with the electron cluster lateral shower with an number of R high threshol hits. A scale his is an Open Access article istribute uner the terms of the Creative Commons Attribution License 2., which permits unrestricte use, istribution, an reprouction in any meium, provie the original work is properly cite. Article available at or

2 EPJ Web of Conferences isolation variable, efine as the transverse energy within.1 < R<.4 aroun the cluster ivie by cluster E, was require below 3%. he fraction of the LJ energy foun in the ECAL ( f EM ) >.98 was use to reject activity from harons. Single muon-jet (MJ) events were selecte from events satisfying a trigger with a single muon having p > 18 GeV. Caniates for ouble MJs were taken with either a single-muon trigger with p > 18 GeV or a three-muon trigger with p > 6 GeV. Muon caniates must have been reconstructe in both the ID an the MS. Aitional requirements were mae on the number of associate hits in the silicon pixel an microstrip etectors, as well as on the number of track segments in the MS. he muons were require to come from the PV. he MJs were reconstructe in an iterative proceure using all caniate muons, by seeing the jet caniate with the highest-p muon, an aing all muons within R <.1: aitional jets were forme using the remaining muons. For the ouble MJ analysis, two muons with p > 11 GeV were require per jet with the leaing muon p be greater than 23 GeV for the single muon trigger events. For the single MJ analysis, four muons were require per jet with p > 19, 16, 14 GeV, respectively, for the three highest-p muons, an p > 4 GeV for all aitional muons. Within a MJ, the two muons closest in p were require to have an invariant mass < 2 GeV. o suppress muons from haronic jets, a scale isolation variable was forme by summing the E of all calorimeter cells within R =.3 of any of the MJ s muons while excluing cells foun within R =.5, an iviing by the MJ p. he scale isolation was require to be less than.3 (.15) per MJ for the ouble (single) MJ analyses. An ABCD metho [5] was use to estimate backgroun. For the EJ analysis, R η2 an f EM on the secon highest- E LJ were use to efine the four regions: in photon+jet events, the photon will typically eposit more energy in the EM calorimeter than the haronic jet. he ouble (single) MJ analysis use the scale isolation variable an the p cut on the fourth (thir) muon in the event, associate with a MJ. In the absence of signal, the numbers of events preicte in the signal region for the single MJ channel, the ouble MJ channel, an the ouble EJ channel are 3.±.,.5±.3, 15.2±2.7 respectively, to be compare with events observe in nearly 5 fb 1 of ata collecte in 211 7, 3 an 15 for the three analyses. he 95% confience level (CL) upper limits on the number of expecte events from new phenomena proucing collimate pairs of prompt leptons were calculate using the CLs [6] metho an reporte in able 1. he observe limits in the EJ channel are in goo agreement with the expecte limits. he limits in the MJ channels are slightly higher than expecte as a result of the slight excesses, but are within 2σ of the SM expectation for both channels. 3 W + prompt electron jets he associate prouction pp WH (assuming SM couplings between the Higgs boson an the W boson) is stum γ EJ 1 MJ 2 MJ (MeV) Obs (Exp) pb Obs (Exp) pb Obs (Exp) pb (.82) (.11).6 (.35).17 (.11) 5.2 (.21).15 (.9).19 (.12) (.) (.37).64 (.36).18 (.11) 5.39 (.39).53 (.35).18 (.11) (.11) (.4).99 (.55).2 (.12) (1.2).66 (.43).22 (.15) able 1: Observe an expecte limits for the prompt LJ analysis [5]. ie in [7]. he Higgs boson ecays, either via a threestep or a two-step cascae to hien sector particles whose masses are taken to be lower than the Higgs mass ( - 14 GeV), thus the Higgs ecay has a two-lj topology. he moels feature a γ that kinetically mixes with the SM photon, a neutral weakly interacting stable scalar an two hien scalars. A value of the kinetic mixing parameter ɛ > 5 implies γ with very short lifetimes; the chosen value of ɛ= 4 ensures that the ecay proucts are prompt. he γ mass must be less than 2 GeV to provie a viable explanation of the results of cosmic-ray an ark matter irect etection experiments [8], which observe an unexpecte excess of cosmic electrons an/or positrons, while there is no observe proton excess. For this analysis, m γ = an 2 MeV are consiere, proviing ecays in e + e pairs only. he signal has a istinct two-jet topology with each EJ having a multiplicity of > 4 electrons per jet, where the electrons are highly collimate. he W boson prouce in association with the Higgs boson is reconstructe in the W eν an W µν ecay moes in orer to achieve a high efficiency for online event selection an a high signal-to-backgroun ratio. he signal topology is an isolate large p lepton accompanie by missing transverse momentum (E miss ) an two or more EJs. For the W eν channel, at least one reconstructe electron trigger object with p > 22 GeV is require. For the W µν channel, a muon caniate trigger object with p > 18 GeV, reconstructe in both the ID an MS, is require. he muon trigger object must be consistent with having originate from the interaction region. In both cases, the leptons are require to match the trigger objects in R <.1. Signal events are require to have exactly one reconstructe W boson caniate in the e or µ ecay channel an at least two jets ientifie as EJs an E miss 25 GeV. he electron from the W ecay is require to pass the tight electron selection p > 25 GeV, while the µ caniate is require to be ientifie in both the inner etector (ID) an the muon spectrometer (MS) subsystems an to have p > 2 GeV. o increase the robustness against track mis-reconstruction, the ifference between the ID an MS p measurements is require to be < 15 GeV. o reuce backgroun from multi-jet events, electron an muon caniates are require to satisfy an isolation criterion: the sum of p of all tracks in a R =.4 cone aroun the elec- 177-p.2

3 LHCP 213 tron (muon) ivie by the electron (muon) p is require to be less than.3 (.2). Events with two or more isolate same-flavor leptons are rejecte, substantially reucing the backgroun from Drell-Yan prouction. o reuce the backgroun from cosmic rays, heavy-flavor prouction an photon conversions, the W caniate is require to originate from the PV. he electrons in an EJ are too closely collimate to be ientifie efficiently with the algorithm use for electrons from W boson ecays an thus are ientifie with the jet EM fraction ( f EM ).99 an the jet charge particle fraction ( f CH ).66, efine as the fraction of jet energy eposite in calorimeter cells that are associate with tracks within the jet. racks associate to the jet in a R =.4 cone (N track ) must have p 5 GeV, total number of pixel an SC hits (N PIX +N S C ) > 7, the fraction of high threshol R hits ( f H ).8 an be N track 2. Haronic jets with large f EM are expecte to contain mostly neutral pions ecaying to photons an, therefore, fewer charge tracks an low f CH. Photons that convert to e + e pairs in the material before entering the calorimeter increase the value of f CH. he ominant backgroun in this search is ue to the associate prouction of a W boson with haronic jets which mimic the EJ signature. he backgroun contamination in the signal region is estimate from the ata using a simplifie matrix metho which is completely ata-riven, which has been foun to be consistent with the ata-riven ABCD estimation [7]. Events are assigne to one of the four regions accoring to whether or not the jets meet the f EM an the track-quality conitions. In the signal region, two jets with f EM.99 are require an for both jets N track has to be > 2. he estimate backgroun is.41±.29±.12 events, while only one event has been observe in 2.4 fb 1 of 211 ata in the W µν channel. he expecte signal yiels are reporte in able 2. Assuming SM cross section for the WH prouction, Higgs branching ratios to EJs between 24 an 45% are exclue at 95% CL (Figure 1). the etector unnotice, resulting in two MJs from the γ ecays in the final state. Parameters for MC simulations are given in able 3. Since signal events are characterize by a four-muon final state with relatively low p, a low p multi-muon trigger with muons reconstructe only in the MS is neee. In orer to have an acceptably low trigger rate, at least three muons are require. Caniate events are collecte using an unprescale high level trigger with three reconstructe muons of p 6 GeV, seee by a Level 1 trigger with three ifferent muon RoIs. hese muons are reconstructe using the tracking at the trigger level only in the MS, since muons originating from a neutral particle ecaying outsie the pixel etector will not have a matching track in the ID. he trigger efficiency for the MC signal samples, efine as the fraction of events passing the trigger requirement with respect to the events satisfying the analysis selection criteria is.32 ±.1 stat for m H = GeV an.31 ±.1 stat for m H = 14 GeV. he main reason for the relatively low trigger efficiency is the small opening angle ( R) between the two muons of the γ ecay, which is often smaller than the Level 1 trigger granularity. MJs are ientifie by a clustering algorithm that associates all the muons in R=.2 cones, starting with the muon with highest p. he MJ irection an momentum are obtaine from the vector sum over all muons in the MJ. Only events with two MJs separate with φ 2 an each containing two reconstructe muons of opposite charge, are kept for the analysis. he main backgroun contribution is expecte from processes giving a high prouction rate of seconary muons which o not point to the primary vertex, such as ecays in flight of K/π an heavy flavour ecays in multi-jet processes, or muons ue to cosmic rays. he calorimetric isolation variable E isol has been efine as the ifference between the E in a R=.4 cone aroun the highest p muon an the E in a.2 cone; a cut E isol 5 GeV keeps almost all the signal an significantly reuce the backgroun. he isolation moelling is valiate 4 Displace muon-jet search he benchmark moel for this analysis [9] is a simplifie scenario where the Higgs boson ecays to a pair of neutral hien fermions ( f 2 ) each ecaying to one long-live γ an one stable neutral hien fermion ( f 1 ) that escapes m H (GeV) m γ = MeV m γ = 2 MeV 2-step moel 14.3±1.7± ±1.6± ±1.±.6.7±1.1± ±.8±.5 9.±.8±.4 3-step moel 22.6±2.1± ±2.1± ±1.2± ±1.4± ±.9± ±.9±.8 able 2: Expecte events in signal MC for W+prompt LJ analysis [7]. (WH) 95% CL limit on!(wh) BR(H" e-jets)/! SM 1 ALAS 211 Data, # s=7 ev Lt = 2.4 fb Signal moel: 3-step, m = MeV $ Observe Expecte ±1! ±2! [GeV] m H Figure 1: Observe an expecte 95% CL upper limits on the σ(wh) BR(H EJs)/σ S M (WH) as function of the Higgs mass for the 3-step moel for the W+prompt LJ analysis [7]. 177-p.3

4 EPJ Web of Conferences m H m f 1 m f 2 m γ cτ (GeV) (GeV) (GeV) (GeV) (mm) able 3: MC simulation for the isplace MJ analysis [9]. with a sample of Z µµ ecays. he scalar sum of the p of the tracks measure in the ID (Σ ID p ), insie a R=.4 cone aroun the irection of the MJ, is requeste to be < 3 GeV. he muon tracks of the MJ in the ID, if any, are not remove from the isolation sum; as a consequence, the Σ ID p cut will remove MJs with very short ecay length. For the cosmic-ray muon backgroun, we require the transverse an longituinal impact parameters of the muons with respect to the primary vertex to be < 2 mm an z < 27 mm. he γ reconstruction efficiency for the lifetimes use in this simulation, efine as the number of γ passing the offline selection ivie by the number of γ in the MS acceptance ( η < 2.4) with both muons having p 6 GeV, is aroun 35%. o estimate the multi-jet backgroun contamination in the signal region we use a ata-riven ABCD metho [9], slightly moifie in orer to cope with the low statistics: the two relatively uncorrelate variables use to separate signal an backgroun are the MJ E isol an φ. No events in the ata sample pass the selection with an expecte total +.66 backgroun of.6 ±.2 stat -.6 syst events. he efficiency of the selection criteria is evaluate for the simulate signal samples as a function of the mean lifetime of the γ, so the expecte number of signal events is preicte in a cτ up to 7 mm. hese numbers, together with the expecte number of multi-jet an cosmic ray events, no ata events surviving the selection criteria in 1.9 fb 1 an all the systematic uncertainties, are use as input to obtain limits at the 95% CL on the σ BR for the process H γ γ +X through the CLs metho (Figure 2). +X) [pb] γ 95% CL Limit on σ BR(H γ (b) expecte ± 2σ (stat.+syst.) expecte ± 1σ (stat.+syst.) 1 expecte limit observe limit σ BR(H γ γ +X) = σ SM ALAS s=7 ev, Lt = 1.9 fb = 14 GeV m H 2 3 Dark photon cτ [mm] Figure 2: 95% upper limits on the σ BR as a function of γ cτ for the isplace MJ analysis for Higgs boson mass = 14 GeV [9]. 5 Displace π v ecays in the MS his analysis escribes the first ALAS search for the Higgs ecay to two ientical neutral particles (π v ) that have isplace ecay b b, c c, τ + τ in the ratio 85:5:8 []. Four atasets have been simulate with Higgs masses 12 an 14 GeV an π v masses 2 an 4 GeV. Both π v ecays are require to occur near the outer raius of the haronic calorimeter (HCAL) ( 4 m) or in the MS. Such ecays give a (η,φ) cluster of charge an neutral harons in the MS. Requiring both π vs to have this topology improves backgroun rejection. A eicate signature-riven trigger, the muon Region Of Interests (RoI) cluster trigger [11], was evelope to trigger on events with a π v ecaying in the MS. It selects events with a cluster of three or more muon RoIs in a R=.4 cone in the MS barrel trigger chambers: one π v must ecay in the barrel, while the secon π v may ecay also in the forwar MS. Punch-through jets are suppresse by requiring no calorimeter jets with E > 3 GeV in a R=.7 cone an no ID tracks with p > 5 GeV within a region of η φ=.2.2 aroun the RoI cluster center. Monte Carlo (MC) stuies show the RoI cluster trigger is 3-5% efficient in the region from 4 m to 7 m. he π vs that ecay beyon 7 m o not leave hits in the trigger chambers locate at 7 m, while the π v ecays that occur before 4 m are locate in the calorimeter an o not prouce sufficient activity in the MS. A specialize tracking an vertex reconstruction algorithm was evelope to ientify π vs that ecay insie the MS. Such ecays prouce a high multiplicity of low p particles clustere in a small R region, containing charge particles an 5 π, resulting in large EM showers, which confuses stanar muon reconstruction. he π vs that ecay before the last sampling layer of the HCAL o not prouce a significant number of tracks in the MS. hus, etectable ecay vertices must be locate in the region between the outer raius of the HCAL an the mile station of the MS. he separation of the two multilayers insie a single muon chamber provies a powerful tool for track pattern recognition in this busy environment an a momentum measurement with resolution for tracks up to GeV in the barrel. he MS vertex algorithm begins by grouping the track segments forme out of hits in single muon chambers using a simple cone algorithm with R=.6. In the barrel the vertex is reconstructe as the point in (r,z) that uses the largest number of track segments to reconstruct a vertex with a χ 2 probability greater than 5%, while in the forwar MS, the vertex is foun using a least squares regression, that assumes the track segments are straight lines. A vertex is reconstructe using at least three track segments. After requiring the vertex to be separate from ID tracks with p 5 GeV an jets with E 15 GeV by R=.4 an.7, respectively, the algorithm has an efficiency of 4% in signal MC events throughout the barrel region (4 r 7.5 m) (Figure 3 (left)) an a resolution of 2 cm in z, 32 cm in r an 5 mra in φ. In the forwar MS, the algorithm is 4% efficient in the region 8 z 14 m. he MC escription of harons an photons in the MS was valiate on a sample of events 177-p.4

5 LHCP 213 Vertex Reconstruction Efficiency ALAS Simulation.2 m h =12 GeV, m =2 GeV m h =12 GeV, m =4 GeV.1 m h =14 GeV, m =2 GeV m h =14 GeV, m =4 GeV r [m] 95% CL Limit on "/" SM % CL Limit: m h =12 GeV, m!v =2 GeV 95% CL Limit: m h =12 GeV, m!v =4 GeV 95% CL Limit: m h =14 GeV, m!v =2 GeV 95% CL Limit: m h =14 GeV, m!v =4 GeV # ALAS Lt = 1.94 fb s= 7 ev proper ecay length [m] Figure 3: Vertex reconstruction efficiency as a function of the raial ecay position of the π v an observe 95% upper limits on H π v π v as a function of the π v proper ecay length for the isplace π v search []. containing a punch-through jet, which are similar to signal events as they contain both low energy photons an charge harons in a localize region of the MS. he final event selection requires two isolate MS vertices separate by R > 2. he backgroun of.3 ±.2 events is calculate using a fully ata-riven metho by measuring the probability for a ranom event to contain an MS vertex (P vertex ) an the probability of reconstructing a vertex given the event passe the trigger (P reco ). P vertex was measure using events selecte by a ranom generator in coincience with the bunch crossing an P reco was measure on collision ata from events that pass the trigger. No events in the ata have two isolate, back-to-back vertices in the MS. Since no significant excess over the backgroun preiction is foun, exclusion limits for σ H BR(H π v π v ) as a function of the π v proper ecay length (cτ) are set by rejecting the signal hypothesis at the 95% CL, as shown in Figure 3 (right). 6 Conclusions In 211 pp collisions at 7 ev recore with the ALAS etector at the LHC, there is no evience of an excess of events for any of the analyses escribe in this paper. A search for collimate pairs of leptons has been performe on nearly 5 fb 1 : no significant excess of ata compare to the SM expectation was observe in any of the three channels, an 95% CL upper limits have been set on the σ BR for several parameters of a Hien Valley moel. A search is also presente for a light Higgs boson ecaying to highly collimate jets of electrons, which are expecte to be seen in the etector as istinct prompt electron-jets. he analysis has been performe using 2.4 fb 1 in the WH prouction moe. he observe ata are consistent with the SM backgroun hypothesis. Limits at 95% CL are set on the WH prouction σ BR into electron-jets. No excess has been observe for the process H γ γ +X: assuming the SM prouction rate for a 14 GeV Higgs, its BR to two γ s is foun to be below % in the cτ range from 7 to 82 mm. For events with two isolate back-toback vertices in the ALAS MS, a wie range of π v proper ecay length from.5 to 25 m can be exclue, assuming % BR for H π v π v. References [1] ALAS Collaboration, Phys. Lett. B (212) [2] CMS Collaboration, Phys. Lett. B (212) [3] M. J. Strassler, Phys. Lett. B 661, (28) [4] ALAS Collaboration, JINS 3 S83 (28) [5] ALAS Collaboration, Phys. Lett. B 719, 299 (213) [6] A. Rea, J. Phys. G 28, 2693 (22) [7] ALAS Collaboration, NJP 15 (213) 439 [8] HEA Collaboration, Astr. J. Lett. 482 L191 (1997) Fermi LA Collaboration, PRL (29) PAMELA Collaboration, Nature Chang J et al, Nature [9] ALAS Collaboration, arxiv: [] ALAS Collaboration, PRL (212) [11] ALAS Collaboration, AL-PHYS-PUB [12] A. Falkowski et al., JHEP 5 (2) 77 A. Falkowski et al., PRL (2) 177-p.5