Search for pair production of a new quark that decays to a Z boson and a bottom quark with the ATLAS detector

Search for pair production of a new quark that decays to a Z boson and a bottom quark with the ATLAS detector
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    a  r   X   i  v  :   1   2   0   4 .   1   2   6   5  v   2   [   h  e  p  -  e  x   ]   2   2   A  u  g   2   0   1   2 EUROPEANORGANISATIONFORNUCLEARRESEARCH(CERN) CERN-PH-EP-2012-073 Submittedto:PhysicalReviewLetters Search for pair production of a new quark that decays to a  Z   boson anda bottom quark with the ATLAS detector TheATLASCollaboration Abstract Asearchisreportedforthepairproductionofanewquark,  b ′ ,withatleastone  b ′ decayingtoa  Z  bosonandabottomquark.Thedata,correspondingto2.0fb − 1 ofintegratedluminosity,werecollectedfrom  pp collisionsat √  s  =  7 TeVwiththeATLASdetectorattheCERNLargeHadronCollider.Usingeventswitha  b -taggedjetanda  Z  bosonreconstructedfromopposite-chargeelectrons,themassdistributionoflargetransversemomentum b ′ candidatesistestedforanenhancement.Noevidencefora  b ′ signalisdetectedintheobservedmassdistribution,resultingintheexclusionat95%confidencelevelof  b ′ quarkswithmasses  m b ′  <  400 GeVthatdecayentirelyvia  b ′ →  Z  + b .Inthecaseofavector-likesinglet  b ′ mixingsolelywiththethirdStandardModelgeneration,masses  m b ′  <  358 GeVareexcluded.  Search for pair production of a new quark that decays to a  Z   boson and a bottom quarkwith the ATLAS detector The ATLAS Collaboration A search is reported for the pair production of a new quark,  b ′ , with at least one  b ′  decaying to a  Z   boson and abottom quark. The data, corresponding to 2.0 fb − 1 of integrated luminosity, were collected from  pp  collisions at √  s =  7 TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a  b -tagged jet anda  Z   boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum  b ′ candidates istestedfor anenhancement. Noevidence for a b ′  signal isdetectedintheobserved massdistribution,resulting in the exclusion at 95% confidence level of   b ′  quarks with masses  m b ′  <  400 GeV that decay entirelyvia  b ′ →  Z  + b . In the case of a vector-like singlet  b ′  mixing solely with the third Standard Model generation,masses  m b ′  <  358 GeV are excluded. PACS numbers: 14.65.Fy, 14.65.Jk, 12.60.-i The matter sector of the Standard Model (SM) consistsof three generations of chiral fermions, with each generationcontaining a quark doublet and a lepton doublet. A naturalquestion is whether quarks and leptons exist beyond the thirdgeneration [1]. In this Letter we present a search for the pairproductionof a new quark with electric charge − 1 / 3,denoted b ′ , usingdata collectedbythe ATLASexperimentat theLargeHadron Collider. New quarks appear in a variety of modelsthat address shortcomings of the SM [1–5]. In addition to sig- naling a richer matter content at high energy, their existencewould impact lower-scale physics, such as altering Higgs bo-son (  H  )phenomenology[6], andprovidingnewsourcesof CPviolation potentially sufficient to generate the baryon asym-metry in the universe [7].Several collaborations have previously searched for a chi-ral  b ′ . A search by D0 [8] for the decay  b ′ → γ  + b  excludes b ′  quarks with masses below  m  Z   + m b  =  96 GeV. CDF [9]searches for the decay  b ′  →  Z   +  b  exclude masses below m W   + m t   =  256 GeV. These limits apply to prompt  b ′  decays.CDFandD0havealsosearchedfornon-prompt b ′ →  Z  + b de-cays [10], excluding, for example,  b ′  masses below 180 GeVfor  c τ = 20 cm [11]. More recently, CDF [12], CMS [13], and ATLAS[14]havesearchedforthepromptcharged-currentde-cay  b ′ → W   + t  . This decay mode is dominant for a chiral  b ′ with mass in excess of   m W   + m t  , as the neutral-current modesonly occur through loop diagrams [1]. The ATLAS result ex-cludes chiral  b ′  quarks with masses below 480 GeV.Extensions to the SM often propose new quarks transform-ing as vector-like representations of the electroweak gaugegroups [2–5]. The decay of a vector-like  b ′  to a  Z   boson anda bottom quark is a tree-level process with a branching ratiocomparable to that of the decay  b ′ → W   + t  . In particular, thebranchingratios Wt   :  Zb  :  Hb  approachthe proportion2:1: 1in the limit of large b ′  mass as a consequenceof the Goldstoneboson equivalence theorem [2, 5]. Furthermore, if a signal were observed in the WtWt   final state, a search for a resonant  Z   + b  signal would aid in establishing the charge of the newquark. In light of these observations, this search explores the  Z  + b -jet final state for the presence of a  b ′  quark.The ATLAS detector [15] consists of particle-tracking de-tectors, electromagnetic and hadronic calorimeters, and amuon spectrometer. At small radii transverse to the beam-line, the inner tracking system utilizes fine-granularity pixeland microstrip detectors designed to provide precision track impact parameter and secondary vertex measurements. Thesesilicon-based detectors cover the pseudorapidity [16] range | η | <  2 . 5. A gas-filled straw tube tracker complements thesilicon tracker at larger radii. The tracking detectors are im-mersed in a 2 T magnetic field produced by a thin super-conducting solenoid located in the same cryostat as the bar-rel electromagnetic (EM) calorimeter. The EM calorimetersemploy lead absorbers and utilize liquid argon as the activemedium. The barrel EM calorimeter covers | η | <  1 . 5, and theend-capEM calorimeters1 . 4 < | η | < 3 . 2. Hadroniccalorime-try in the region  | η | <  1 . 7 is achieved using steel absorbersand scintillating tiles as the active medium. Liquid argoncalorimetrywithcopperabsorbersisemployedinthehadronicend-cap calorimeters, which cover the region 1 . 5  < | η | <  3 . 2.The search for the decay  b ′  →  Z   + b  is performed in thefinal state with the  Z   boson decaying to an electron-positronpair ( e + e − ) using a dataset collected in 2011 correspondingto an integrated luminosity of 1 . 98 ± 0 . 07 fb − 1 [17]. Theselected events were recorded with a single-electron triggerthat is over 95% efficient for reconstructedelectrons [18] withmomentum transverse to the beam direction,  p T , exceeding25 GeV. At least two opposite-charge electron candidates arerequired, each satisfying  p T  >  25 GeV and reconstructed inthe pseudorapidity region | η | <  2 . 47, excluding the barrel toend-cap calorimeter transition region, 1 . 37  < | η | <  1 . 52. Inaddition, the electron candidates satisfy  medium  quality re-quirements [18] on the reconstructed track and properties of the electromagnetic shower. The two opposite-charge elec-tron candidates yielding an invariant mass,  m ee , that satisfies | m ee − m  Z  | <  15 GeV and is closest to the  Z   boson mass de-fine the  Z   candidate. Approximately 475,000 events pass the  Z  → e + e −  selection criteria.Jets are reconstructed using the anti- k  t   clustering algo-rithm [19] with a distance parameter of 0.4. The in-puts to the algorithm are three-dimensional clusters formedfrom calorimeter energy deposits. Jets are calibrated us-  2ing  p T - and  η -dependent factors determined from simu-lation and validated with data [20]. Jets are rejected if they do not satisfy quality criteria to suppress noise andnon-collision backgrounds, as are jets whose axis is within ∆  R  =   ( ∆η ) 2 +( ∆φ ) 2 =  0 . 5 of a reconstructed electron as-sociated with the  Z   candidate. A requirement is made toensure at least 75% of the total  p T  of all tracks associatedwith the jet be attributed to tracks also associated with the se-lected  pp  collision vertex [21]. Lastly, jets in this analysisare restricted to the region covered by the tracking detectors, | η | <  2 . 5, and satisfy  p T  >  25 GeV. Approximately 81,000events pass the  Z  → e + e −  candidate selection and contain atleast one selected jet.The SM production of   Z   bosons in association with jets ac-counts for most events passing the  Z  + ≥ 1 jet selection. Twoleading-order Monte Carlo (MC) generators,  ALPGEN  [22]and  SHERPA  [23], are used to assess the background arisingfrom this process, with  ALPGEN  providing the baseline pre-diction. A description of the generation of these samples,in particular in regard to differences between  ALPGEN  and SHERPA  in the modeling of   Z   boson production in associa-tion with  b -jets, is detailed in Ref. [24]. The predictions of both are normalized such that the inclusive  Z   boson cross sec-tion is equal to a next-to-next-to-leading-order (NNLO) cal-culation [25]. All MC samples fully simulate the ATLAS de-tector [26] and are reconstructed with the same algorithms asthose applied to data. The  Z  +bottom background categorycomprises simulated  Z   +  jet ( s )  events in which a generated  p T  >  5 GeV bottom quark is matched to a selected recon-structed jet. Similarly, events with a jet matched to a charmquark, but not a bottom quark, constitute the  Z  +charm cate-gory. In the  Z  +light category, none of the selected jets arematched to a bottom or charm quark.Additional SM backgrounds modeled with MC eventsinclude top quark pair production ( t  ¯ t  ), single top pro-duction, heavy vector boson pair (diboson) production,  Z  ( → ττ )+  jet(s) events, and  W  ( → e  ν )+  jet(s) events. Pro-cesses with a top quarkare simulated with  MC @ NLO  [27, 28]. The  t  ¯ t   cross section used is the  HATHOR  [29] approximateNNLO value, while  MC @ NLO  [28] values are used for thesingle top processes.  HERWIG  [30] models the contributionof diboson events, with the cross sections set by the  MCFM  [31]NLO predictions. The remaining  W  /  Z  +  jet(s) backgroundsare simulated with  ALPGEN , and normalizedusing single vec-tor boson production NNLO cross sections [25]. The multi- jet background is estimated using a data sample with bothelectron candidates passing  loose  criteria [18] but failing theslightly tighter  medium  criteria. This sample is normalizedto the difference in the inclusive  Z   sample between the dataand all other backgrounds in the region 50  <  m ee  <  65 GeV.The small single top, diboson,  Z  → ττ , W   → e  ν , and multi-jetcontributions are combined and denoted Other SM.Figure 1 presents the  e + e −  invariant mass distribution forevents passing the  Z  + ≥ 1 jet selection, before imposing the | m ee − m  Z  | <  15 GeV requirement, together with the SM pre-diction. The observed and predicted number of events are    E  v  e  n   t  s   /   5   G  e   V 10 2 10 3 10 4 10=7 TeV )sData 2011 ( Z+lightZ+charmZ+bottomttOther SM ATLAS  1  L dt = 2.0 fb ∫   [GeV] ee m50100150200250300350    D  a   t  a   /   P  r  e   d   i  c   t   i  o  n FIG. 1:  e + e −  invariant mass distribution for events passing the  Z  + ≥  1 jet selection, before imposing the  | m ee − m  Z  |  <  15 GeVrequirement. The predicted contributions of the SM backgroundsources are shown stacked. The lower panel shows the ratio of thedata to the SM prediction, and the solid yellow band denotes the sys-tematic uncertainty on the SM prediction.    E  v  e  n   t  s   /   5   G  e   V 110 2 10 3 10=7 TeV )sData 2011 ( Z+lightZ+charmZ+bottomttOther SM ATLAS  1  L dt = 2.0 fb ∫   [GeV] ee m50100150200250300350    D  a   t  a   /   P  r  e   d   i  c   t   i  o  n FIG. 2:  e + e −  invariant mass distribution for events passing the  Z  + ≥ 1  b -jet selection, before imposing the  | m ee − m  Z  | <  15 GeVrequirement. listedinTableIforthisandtwootherstagesoftheeventselec-tion. Most events passing the  Z  + ≥ 1 jet selection arise fromthe  Z  +light category. The appreciablelifetime of the  b -hadronsrcinating from the bottom quark in the decay  b ′ →  Z  + b provides a means to reduce this background source. A  b -jettagging algorithm referred to as IP3D+SV1 [32] is utilizedto select events with at least one  b -jet from the  Z  + ≥ 1 jetsample. The discriminant combines two likelihood variablesbasedon the tracks associated with a jet. The first employsthe  3 Source  Z  + ≥ 1 jet  Z  + ≥ 1  b -jet  p T (  Zb )  >  150 GeV  Z  +light 74400 ± 7300 590 ± 140 19 ± 7  Z  +charm 5340 ± 520 870 ± 210 18 ± 7  Z  +bottom 2540 ± 250 1710 ± 270 52 ± 17 t  ¯ t   320 ± 40 220 ± 40 20 ± 4Other SM 1010 ± 280 70 ± 20 1 . 6 ± 0 . 4Total SM 83600 ± 8100 3460 ± 580 110 ± 30 Data 80519 3466 100 m b ′  =  350 GeV 110 ± 12 93 ± 11 55 ± 7 m b ′  =  450 GeV 27 ± 3 20 ± 2 14 ± 2TABLE I: Number of predicted and observed events at three stages in the event selection. The contributions from SM backgrounds are shownindividually, as well as combined into the total SM prediction. The uncertainties on the predicted number of events combine all sources of uncertainty. The number of expected signal events is also listed for two representative  b ′  masses in the case where  BR ( b ′ →  Zb ) =  1. longitudinalandtransversetrackimpact parameters,while thesecondutilizes propertiesof a reconstructedsecondaryvertex.In a simulated  t  ¯ t   sample, the requirement on the discriminantdefining a  b -jet is 60% efficient for jets with a  b -hadron, andyields a light flavor jet rejection rate of 300 [32].A total of 3,466 events satisfy the  Z  +  ≥  1  b -jet selec-tion. Figure 2 presents the  e + e −  invariant mass distribu-tion in this sample and the SM prediction, before imposingthe  | m ee − m  Z  | <  15 GeV requirement. The accurate model-ing of the mass distribution for values beyond the  Z   bosonmass supports the prediction of   t  ¯ t   and Other SM backgroundevents. Within the windowaroundthe  Z   bosonmass,  ALPGEN and  SHERPA  agree to within 1% and 7% in the predictionof the number of   Z  +light and  Z  +charm events, respectively.However,  ALPGEN  and  SHERPA  disagree in the predictionof the  Z  +bottom contribution, a fact previously reported inan ATLAS cross section measurement of   Z   bosons producedin association with  b -jets using a smaller dataset [24]. The ALPGEN  and  SHERPA  Z  +bottom predictions are scaled to ac-count for the difference between data and all other predictedbackgrounds in a subsample of the  Z  + ≥ 1  b -jet sample thatcontainseventsfailingtherequirementdiscussedbelowonthetransverse momentum of the  b ′  candidate. The scale factorsare consistent with those measured in Ref. [24], and the in-variant mass distribution of secondary vertex tracks is used toconfirm the validity of the resulting prediction for the flavorcomposition in the  Z  + ≥ 1  b -jet sample [24].Simulated  b ′ ¯ b ′  events are generatedfora range of   b ′  massesusing  MADGRAPH  [33] with the G4LHC extension [6]. PYTHIA  [34] performs fragmentation and hadronization of the parton-level events. The signal cross sections are ob-tained with  HATHOR  [29], and vary from 80 pb to 30 fb overthe range  m b ′  =  200 − 700 GeV. In each sample, one  b ′  de-cays in the mode  b ′ →  Z   + b , with the  Z   boson decaying via  Z  → e + e − . Two separate samples are produced for each massvalue, with the other  b ′  decaying either via  b ′  →  Z   + b  or b ′ → W   + t  , and with all decay modes of the  Z   and  W   bosonsallowed. The factor  β  =  2 ×  BR ( b ′ →  Zb ) −  BR ( b ′ →  Zb ) 2 characterizes the fraction of signal events with at least one b ′  →  Z   + b  decay as a function of the branching ratio. The    E  v  e  n   t  s   /   5   0   G  e   V 110 2 10 3 10=7 TeV )sData 2011 ( Z+lightZ+charmZ+bottomttOther SM = 350 GeV (VLS) b’ m=1) β = 450 GeV ( b’ m ATLAS  1  L dt = 2.0 fb ∫  (Zb) [GeV] T p0100200300400500600700    D  a   t  a   /   P  r  e   d   i  c   t   i  o  n FIG. 3: Transverse momentum distribution of the  b ′  candidate inevents passing the  Z  + ≥ 1  b -jet selection. The predicted contribu-tions of the SM background sources are stacked, while the distribu-tions for the two signal scenarios described in the text are overlaid. case  β  =  1 is equivalent to previous measurements [9] whichassumed  BR ( b ′ →  Zb ) =  1. The case of a vector-like singlet(VLS) mixing solely with the third SM generationis also con-sidered by computing  β  as a function of   b ′  mass [5]. Overthe range  m b ′  =  200 − 700 GeV,  β  varies from 0 . 9 to 0 . 5. ASM Higgs of mass 125 GeV is assumed.The b ′  candidateis formedfromthe e + e −  pairandthe high-est  p T  b -jet. The mass of the  b ′  candidate,  m (  Zb ) , is thediscriminant distinguishing the background-only and signal-plus-background hypotheses. In  b ′  pair production, the newquarks are typically produced with large transverse momen-tum,  p T (  Zb ) . Therefore, a  p T (  Zb )  >  150 GeV requirementis applied to increase the signal sensitivity. Figure 3 presentsthe  p T (  Zb )  distribution for data and the predicted SM back-grounds. Additionally, the signal distribution is overlaid fora  b ′  mass of 350 GeV, assuming the VLS scenario value β  =  0 . 63, and for a mass of 450 GeV, assuming  β  =  1.  4    E  v  e  n   t  s   /   5   0   G  e   V 0510152025=7 TeV )sData 2011 ( Z+lightZ+charmZ+bottomttOther SM = 350 GeV (VLS) b’ m=1) β = 450 GeV ( b’ m ATLAS  1  L dt = 2.0 fb ∫  m(Zb) [GeV]1002003004005006007008009001000    D  a   t  a   /   P  r  e   d   i  c   t   i  o  n FIG. 4: Mass distribution of the  b ′  candidate in events passing the  Z  + ≥ 1 b -jetselectionand satisfying  p T (  Zb ) > 150GeV. Thehighestmass bin also includes the data and prediction for  m (  Zb )  >  1 TeV. Thefractionofsignaleventspassingall requirementsvariesfrom 7% to 43% between  m b ′  =  200 − 700 GeV, assuming β  =  1, with the efficiency to pass the minimum  p T (  Zb )  re-quirement contributing most to the degree of variation. Therequirement  p T (  Zb )  >  150 GeV was determined by assess-ing the signal sensitivity for different minimum  p T (  Zb )  val-ues, as quantified by the expected cross section exclusionlimit. The limit is computed using a binned Poisson likeli-hood ratio test [35] of the  m (  Zb )  distribution for different  m b ′ hypotheses. Pseudo-experiments are generated according tothe background-onlyand signal-plus-backgroundhypotheses,and incorporate the impact of systematic uncertainties. Thecross section limit is evaluated using the CL s  modified fre-quentist approach [35].The impact of each systematic uncertainty on the nor-malization and shape of the  m (  Zb )  distribution is assessedfor each SM background source and the expected  b ′  sig-nal. The fractional uncertainty on the total number of back-ground events passing the  p T (  Zb )  >  150 GeV requirement is27%. Significant contributions arise from uncertainties in the  p T (  Zb )  distribution shape in  Z   +  jet ( s )  events. Such sourcesof uncertainty include the renormalization and factorizationscale choice (14%, evaluated using  MCFM  [36]), shape dif-ferences observed between  ALPGEN  and  SHERPA  (12%), andvariations in the degree of initial and final state QCD radia-tion (9%). The uncertainty in the efficiency of the  b -taggingrequirement contributes an additional 12%. Other sources of uncertainty contributing at the level of 6% or less include the jet energyscale[20], partondistributionfunctions(PDF),MCsample sizes, electron identification efficiency,  Z   boson crosssection, luminosity,  b -jet mis-tag rate,  t  ¯ t   cross section, jet en-ergy resolution, trigger efficiency, and the Other SM eventyield. Most oftheaboveuncertainties,with thenotableexcep-  [GeV] b’ m 200300400500600700    Z   b   )   [  p   b   ]    →    (   b   ’     β   ×    ’   )   b   b   ’    →    (  p  p    σ 2 10 1 10110 2 10 (BR = 100%) β× HATHOR σ  (BR VLS) β× HATHOR σ expected limitobserved limit σ 1 ± expected limit σ 2 ± expected limit ATLAS   = 7 TeV )sData 2011 ( 1  L dt = 2.0 fb ∫  FIG. 5: The expected and observed 95% C.L. cross section limits asa function of   b ′  mass. The signal cross section is shown with un-certainties arising from PDFs and renormalization and factorizationscale choice. The prediction is also multiplied by the  β  factors de-scribed in the text. tion of the  p T (  Zb )  modelinguncertainties in  Z  +  jet ( s )  events,contribute to the total uncertainty on the signal normalization,whichvariesbetween11%and14%dependingonthe b ′  mass.Figure4 presentsthe  b ′  candidatemass distributionafterre-quiring  p T (  Zb ) > 150GeV andthe predictedSMbackground.Thedistributionsforthe signal scenariosdepictedin Fig. 3 areshown overlaid. The data are in agreement with the SM pre-diction over the full range of   m (  Zb )  values. In the absenceof evidence of an enhancement, 95% confidence level (C.L.)cross section exclusion limits are derived. Figure 5 presentsthe expected and observed cross section limits as a functionof   m b ′ , computed under the assumption  β  =  1. The expectedcross section limit was checked to be stable to within 15%over the full mass range considered using the signal samplesin which one  b ′  quark decays via  b ′  →  Z   + b  and the otherdecays via  b ′  → W   + t  . The approximate NNLO  b ′ ¯ b ′  crosssection prediction is shown multiplied by  β  =  1, as well as bythe VLS  β  value, with the shaded region representingthe totaluncertainty arising from PDF uncertainties and the factoriza-tion and renormalization scale choice. From the intersectionof the observed cross section limit and the theoretical predic-tion,  b ′  quarks with masses  m b ′  <  400 GeV decaying entirelyvia  b ′ →  Z  + b  are excluded at 95% C.L., representing a sig-nificant improvement with respect to the previous best limitof 268 GeV [9]. In the case of a vector-like singlet  b ′  mixingsolely with the third SM generation, masses  m b ′  <  358 GeVare excluded.In conclusion, a search with 2.0 fb − 1 of ATLAS data is pre-sented for  b ′  quark pair production, with at least one  b ′  de-caying to a  Z   boson and a bottom quark. This decay mode isparticularly relevant in the context of vector-like quarks andis an essential complement to searches in the mode with both b ′  decaying to a  W   boson and a top quark. No evidence for a b ′  is observed in the  Z   + b -jet final state, and new limits are
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