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Search for single b*-quark production with the ATLAS detector at sqrt(s)=7 TeV PDF

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Preview Search for single b*-quark production with the ATLAS detector at sqrt(s)=7 TeV

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP-2012-344 Submitted to: Phys. Lett. B 3 1 0 2 Search for single b -quark production with the ATLAS detector n ∗ a at √s = 7 TeV J 8 ] x e - p The ATLAS Collaboration e h [ 1 v 3 Abstract 8 5 1 Theresultsofasearchforanexcitedbottom-quarkb in ppcollisionsat √s= 7 TeV,using4.7fb 1 . ∗ − 1 of data collected by the ATLAS detector at the LHC are presented. In the model studied, a single b - 0 ∗ 3 quarkisproducedthroughachromomagneticinteractionandsubsequentlydecaysto aW bosonand 1 atop quark. Thesearchisperformed in the dileptonandlepton+jets finalstates,which arecombined : v to set limits on b -quark couplings for a range of b -quark masses. For a benchmark with unit size i ∗ ∗ X chromomagneticandStandard Model-like electroweakb couplings,b quarks with massesless than ∗ ∗ r 870GeV are excludedat the 95%credibility level. a Search for single b -quark production with the ATLAS detector at √s = 7 TeV ∗ TheATLASCollaboration Abstract The results of a search for an excited bottom-quark b in pp collisions at √s = 7TeV, using 4.7 fb 1 of data ∗ − collectedbytheATLASdetectorattheLHCarepresented.Inthemodelstudied,asingleb -quarkisproducedthrough ∗ a chromomagneticinteraction and subsequentlydecays to a W boson and a top quark. The search is performedin the dilepton and lepton+jetsfinal states, which are combinedto set limits on b -quarkcouplingsfora range of b - ∗ ∗ quarkmasses. Fora benchmarkwith unitsizechromomagneticandStandardModel-likeelectroweakb couplings, ∗ b quarkswithmasseslessthan870GeVareexcludedatthe95%credibilitylevel. ∗ Keywords: ATLAS,b ,singletop-quark,excitedquark ∗ 1. Introduction t g b∗ The single top-quark signature is sensitive to many models of new physics [1]. Single top-quark produc- tionintheStandardModel(SM)hasbeenmeasuredat b W theLHCinthet-channel[2,3]andinassociationwith a W boson (Wt-channel) [4, 5]. Searches for resonant productionofanewparticlewhichdecayswithasingle top-quarkhavebeencarriedoutinthes-channelproduc- Figure1: Leading-order Feynmandiagram forsingle-b -quarkpro- tionofatopquarktogetherwitha bquark[6,7]. This ∗ ductionanddecaytoWt. Letter presents the first search for a resonance decay- ing to a single top-quarkand a W boson[8]. Here we consider the production of an excited quark b which ∗ [18,19] decays to a single top-quark and a W boson. This is thefirst searchforexcited-quarkscouplingto the third = gs G bσµν κbP +κbP b +h.c., (1) generationoffermions. L 2Λ µν L L R R! ∗ Previoussearchesforexcitedquarkshavefocusedon where g is the strong coupling, G the gauge field s µν theirstronginteractions[9,10],aswellastheirelectro- tensor of the gluon and Λ = m the scale of the new b magneticinteractions[11, 12] with SM quarks. These physics. P and P are the left-∗and right-handedpro- L R searchesexploitthecouplingbetweentheexcitedquark jection operatorsand κb and κb are the respectivecou- L R andupordownquarksintheproton. Heretheproduc- plingstrengths. Thisanalysisisthuscomplementaryto tion of excited-quarks coupling primarily to the third excited-quarksearchesfocusingon the couplingto the generation of SM quarks is investigated. This cou- firstgeneration[9,20,21]. Singleb -quarkproduction ∗ pling occurs for example in Randall–Sundrum models canalso revealthe chiralnatureofthe excitedbottom- thataddressthe stronginteractionsector[13, 14] orin quark[8]. modelswith a heavy gluon partner, such as composite In addition to the chromomagnetic coupling, the Higgsmodels[15–17]. Theb∗quarkisproducedsingly b∗ quarkinvestigatedhere also has weak couplings, as throughitscouplingtoabquarkandagluon,asshown in a general class of new physics models where new inFig.1. heavy particles stabilise the Higgs-boson mass at the TheLagrangiandescribingthisinteractionisgivenby electroweak scale [22–26]. In such models, the heavy PreprintsubmittedtoPhys.Lett.B January9,2013 quarkscan have left-handedorright-handedcouplings discriminantdistributions. totheWbosonorcanbevector-likewithequalstrength forbothcouplings.TheLagrangiandescribingtheelec- 2. TheATLASdetector troweakdecayoftheb quark,showninFig.1,is ∗ g = 2 W+tγµ g P +g P b +h.c., (2) The ATLAS detector [30] is a general purpose de- L √2 µ (cid:16) L L R R(cid:17) ∗ tectorwith a precise trackingsystem, calorimetersand an outer muon spectrometer. The inner tracking sys- whereg istheSU(2) weakcouplingandg andg are 2 L L R tem consists of a silicon pixel detector, a silicon mi- thecouplingstrengthsforleft-handedandright-handed crostrip tracker, and a straw-tube transition radiation couplings,respectively. tracker.Thissystemisimmersedina2Taxialmagnetic While the search is general and considers any res- fieldproducedbyasolenoidandprovideschargedparti- onance decaying into the Wt signature, three specific cletrackingandidentificationinthepseudorapidity1re- b -quarkcoupling scenarios are consideredin orderto ∗ gion η < 2.5. Thecentralcalorimetersystemconsists extractb -quarkcouplingandmasslimits: left-handed ∗ | | (κb, g non-zero and κb = g = 0), right-handed of a liquid-argon electromagnetic sampling calorime- (κbL =Lg = 0 and κb,Rg nonR-zero) and vector-like ter with high granularity and an iron/scintillator tile (κbL = κbL= κb and gR =R g = g non-zero) pro- calorimeterprovidinghadronicenergymeasurementsin L R L/R L R L/R thecentralpseudorapidityrange(η <1.7).Theendcap duction and decay. Limits are derived as a function | | of the b -quarkmass as well as the couplings κb and andforwardregionsareinstrumentedwithliquid-argon ∗ L,R calorimetersforbothelectromagneticandhadronicen- g . Theselimitstakeintoaccountboththechangeof L,R ergymeasurementsupto η =4.9.Themuonspectrom- the production cross-section and the b Wt decay ∗ | | → eterisoperatedinatoroidalmagneticfieldprovidedby branchingratio,whichdependonthecouplingsandthe air-coresuperconductingmagnetsandincludestracking b -quark mass. The branching ratio to Wt varies be- ∗ chambers for precise muon momentum measurements tween 20%at m = 300 GeV and 40%at higherval- b ∗ upto η = 2.7andtriggerchamberscoveringtherange ues, with decaysto bg, bZ andbH also allowed. Con- | | η <2.4. tributionsfromnon-Wtdecaymodesthatmayincrease | | the b -quark acceptance of this analysis are not con- ∗ sidered, resulting in conservative limits. Signal event 3. Dataandsimulatedsamples yields presented in the following tables are calculated withκb =g =1andκb =g =0. L L R R ThisanalysisusesdatacollectedwiththeATLASde- Foraleft-handedb at √s=7TeVwithκb =g =1 ∗ L L tectorin2011,correspondingtoanintegratedluminos- andκb = g = 0,theleading-ordercrosssectiontimes brancRhingrRatiotoWtis0.80pbform =900GeV[8]. ityof4.7±0.2fb−1[31,32]of7TeVproton–proton(pp) b∗ collisions deliveredby the LHC. The data are selected The uncertainties due to the choice of factorisation usingsingle-electronorsingle-muontriggerswhoseef- andrenormalisationscalesareevaluatedbyvaryingthe ficienciesreachtheirplateauat25GeVand20GeV,re- scales between m /2 and 2 m , and those due to b∗ × b∗ spectively [33, 34]. The data must also pass stringent the choice of PDF by comparing results obtained us- quality requirements [35]. Events are selected if they ing the CT10 [27], MRST [28] and NNPDF [29] sets. contain at least one primary vertex candidate with at These uncertainties are added in quadrature to yield leastfiveassociatedtracks. cross-sectionuncertaintiesrangingfrom12%at m = b∗ The signal is modelled using MadGraph5 [36] 300GeVto25%atm =1200GeV. b∗ and the CTEQ6L1 parton distribution functions This channel proceeds via two W bosons from b - ∗ (PDFs) [37]. Events with single top-quarks in the t- quark and top-quark decays. At least one W boson is channel are generated with the AcerMC [38] genera- required to decay to a lepton (electron or muon). The tor, usingthe MRST LO**PDFset [39]. MadGraph5 analysisisperformedseparatelyinthedileptonandlep- ton+jets final states. The lepton+jets channel has the advantage that the invariant mass of the b quark can ∗ 1ATLASusesaright-handedcoordinatesystemwithitsoriginat bereconstructed,whereasthedileptonchannelbenefits the nominal interaction point (IP)in the centre ofthe detector and from smaller backgrounds. A discriminant that sepa- thez-axisalongthebeampipe. Thex-axispointsfromtheIPtothe rates the b -quark signal from the backgrounds is de- centreoftheLHCring, andthe y-axispoints upwards. Cylindrical ∗ coordinates(r,φ)areusedinthetransverseplane,φistheazimuthal finedineachfinalstate. Limitsonb -quarkproduction ∗ anglearoundthebeampipe.Thepseudorapidityηisdefinedinterms areobtainedfromacombinedBayesiananalysisofboth ofthepolarangleθasη= ln(tanθ/2). − 2 and AcerMC are interfaced to Pythia [40] for par- p > 25GeVand η < 2.5andfulfiltightqualitycrite- T | | ton showering and modelling of the underlying event. ria [55]. Muonsmust be isolated from close-by tracks Other processes producing single top-quarks and top- ina coneof∆R < 0.3andfromenergydepositsin the quark pairs (tt¯) are modelled with the next-to-leading- calorimeterin a cone of ∆R < 0.2. The sum of trans- order(NLO)generatorMC@NLO[41]usingtheCT10 verse momenta of tracks in the cone must not exceed PDF set [27], interfaced to Herwig [42] for parton 2.5GeVandthesumofenergydepositsinthecalorime- showering and Jimmy [43] for the underlying event. terintheconemustbebelow4GeV. Alpgen [44] is used to model vector boson (W and Inorderto rejecteventsin whicha muonemittinga Z) productionin association with jets as well as dibo- hardphotonisalsoreconstructedasanelectron,events sonprocesses(WW, WZ andZZ) usingthe CTEQ6L1 are vetoed if a selected electron–muon pair shares the PDF set. It is interfaced to Herwig for parton shower sametrack. modelling. Inthelepton+jetsanalysisthedibosonpro- Jets are reconstructed from clusters of energy de- cessesaremodelledwithHerwigonly.Decaysofτlep- posits in the calorimeter [56] using the anti-k algo- t tonsare handledby Tauola[45]. A top-quarkmass of rithm [57] with a radius parameter R = 0.4. These 172.5GeV[46]isassumed. Approximatenext-to-next- jetsarecalibratedtothehadronicenergyscalethrough to-leading-order(NNLO)cross-section calculations are p - and η-dependent scale factors, which are derived T used to normalise the tt¯[47] (Hathor) and single top- from simulation. An additional uncertainty due to quarksamples[48–50], whilethevectorbosonanddi- residualdifferencesbetweensimulationanddataisap- boson samples are normalised using calculations with plied in the analysis [58]. Jets are required to have MCFM[51]atNNLOandNLO,respectively. p > 30 (25)GeV and η < 2.5 in the dilepton (lep- T | | Avariablenumberofadditionalppinteractions(pile- ton+jets) channel. The ratio of the scalar sum of the up) are overlaid on simulated events, which are then p of tracks associated with the jet and the primary T weighted to reproduce the distribution of the number vertex to the scalar sum of the p of all tracks asso- T of collisions per bunch crossing observed in data. All ciated with the jet must be at least 0.75 to reject jets samples are passed through a GEANT4-based simula- frompile-upinteractions. Muonsoverlappingwithjets tion [52] of the ATLAS detector [53] and are then re- within ∆R < 0.4 are removedand the jet is kept. The constructed using the same procedure as for collision closest jet overlappingwith electronswithin ∆R < 0.2 data. is removed and the electron is kept. If electrons sub- sequently still overlap with any remaining jet within ∆R < 0.4, they are removed. Information about jets 4. Physicsobjectselection containingbquarks[59]isalsousedinthelepton+jets Electroncandidatesarereconstructedfromclustersof channel.Aneuralnetworkcombineslifetime-relatedin- energydepositsinthecalorimeter[54]. Thetransverse formationreconstructedfromthetracksassociatedwith energyETofelectroncandidatesisrequiredtobelarger eachjet. Atthechosenworkingpoint,theb-taggingal- than25GeV andtheir pseudorapidityis requiredto be gorithm has an efficiency of 70% (20%/0.7%) for jets η < 2.47. Electronsinthebarrel–endcaptransitionre- containingbquarks(cquarks/lightquarksorgluons)in | | gion of the calorimeter, correspondingto 1.37 < η < asimulatedtt¯sample. | | 1.52,arenotconsidered.Selectedelectronsmustpassa The missing transverse momentum Emiss is calcu- T setof“tight”qualitycriteria[54]andtheelectonsmust lated using topological clusters of energy deposits bematchedtoatrackreconstructedintheinnertracking in the calorimeter and corrected for the presence of system. Electrons must also be isolated from close-by muons[60]. tracks in a cone of ∆R = (∆η)2+(∆φ)2 < 0.3 and from calorimeter energy depposits not belonging to the electron candidate in a cone of ∆R < 0.2. The isola- 5. Eventselectioninthedileptonchannel tionrequirementsonthesumoftransversemomentaof tracksintheconeandonthesumofenergydepositsin Theeventselectionandbackgroundmodellinginthe the calorimeterin the coneare chosen as a functionof dilepton channel is the same as in the ATLAS mea- p andηsuchthatanefficiencyof90%forelectronsin surementofthesingletop-quarkproductioninthe Wt- T thesimulationisachieved. channel[4]. Candidateeventsmustcontainexactlytwo Muon candidates are reconstructed from matching leptons(ee,µµoreµ)withoppositeelectricchargeand tracksinthemuonspectrometerandinnertrackingsys- exactlyonejet. Atleastoneoftheleptonsineachevent tem. Muonsarerequiredtohavetransversemomentum mustmatch the correspondingtrigger-levelobject. No 3 b-taggingrequirementismadesincethedominantback- groundfrom tt¯productionalso containsb quarks. The Emiss is required to be greater than 50GeV. In the ee V T e ATLAS and µµ channels, the invariantmass of the lepton pair, G 350 Data 0 ∫ Wt mℓℓ, is required to be outside the Z boson mass win- s / 1 300 L dt = 4.7 fb-1 tt dow: m < 81GeV or m > 101GeV. In all three nt Diboson ℓℓ ℓℓ ve 250 s = 7 TeV Z(e+e-/µ+µ-)+jets channels,theZ ττbackgroundisreducedbyadedi- E catedveto,whic→hrequiresthesumoftheazimuthalan- 200 FZa(τk+eτ- )d+ijleetpstons gledifferencesbetweeneachleptonandtheEmissvector 150 Dilepton channel T tobegreaterthan2.5rad. Afterallcuts,theacceptance 100 for signal events with m = 800GeV in which both b W bosonsdecayleptonical∗ly(toeithereorµ)is26%. 50 Themainbackground,accountingfor63%oftheto- 0 0 50 100 150 200 250 300 tal, comes from tt¯events in which one of the two jets plep1 [GeV] originatingfrom b quarksis not detected. The second T largest background is from SM Wt production, which (a) hasthe same finalstate as the b -quarksignal, andac- ∗ V countsfor13%ofthetotalbackground.Dibosonevents Ge 800 ATLAS Data producedinassociationwithjetsaccountfor12%ofthe 0 ∫ Wt totalbackground. Withtheexceptionofsingle-anddi- s / 1 700 L dt = 4.7 fb-1 tt nt 600 Diboson bosonsamples,thesebackgroundsaretakenfromNLO ve s = 7 TeV Z(e+e-/µ+µ-)+jets simulation and are normalised to their NNLO theoret- E 500 Z(τ+τ-)+jets ical predictions. Drell–Yan (DY) events contribute a 400 Fake dileptons Dilepton channel smallbackgroundof7.3%tothesumofeeandµµchan- 300 nel events. The events are taken from the simulation 200 and normalised to data using a two-dimensional side- 100 band region with low Emiss and/or m outside of the T ℓℓ 0 Z boson mass window [4]. The contribution from ττ 0 20 40 60 80 100 120 140 160 180 200 finalstates, whereboth τ leptonsdecayleptonically,is plep2 [GeV] T estimatedfromsimulatedsamples,with thenormalisa- (b) tioncheckedin anorthogonaldatasampleobtainedby reversingtheZ τ+τ vetocutdescribedabove. Z τ+τ events acc→ountfo−r 0.7%of the total backgroun→d. GeV 500 ATLAS Data − 0 ∫ Wt Tasheprsimmaarllybleapcktognrsouannddffrroommjneotsn-tphraotmarpetlmepistiodnesnt(iffiaekde nts / 1 400 L dt = 4.7 fb-1 tDtiboson dileptons)is modelledand normalisedusing data [61]. Eve 300 s = 7 TeV ZZ((eτ++τe--)/+µje+µts-)+jets Itaccountsfor4%ofthebackground. Fake dileptons The predicted event yields for the backgrounds and 200 Dilepton channel signalatafewmasspointsarecomparedtodatainTa- ble1.Thep distributionsofthetwoleptonsandthejet T 100 areshowninFig.2. A discriminating variable that separates the signal 0 0 50 100 150 200 250 300 fromthebackgroundsisH ,thescalarsumofthetrans- T pjet [GeV] verse momenta of the leptons, jet and Emiss. The H T T T distributionisshowninFig.3. (c) Figure2:Kinematicdistributionscomparingdatatopredictionsinthe 6. Eventselectioninthelepton+jetschannel dileptonchannelfor(a)theleadinglepton plep1,(b)thesub-leading T lepton plep2 and(c)thejet pjet. Thehatchedbandshowstheuncer- T T The analysis in the lepton+jets channel follows the tainty due to the background normalisation. The last bin includes overflows. same background modelling strategy as the cross- sectionmeasurementforsingletop-quarkproductionin 4 thet-channel[2]. Eventsarerequiredtohaveeitherex- actlyonemuonandEmiss >25GeVorexactlyoneelec- T tron and Emiss > 30GeV, as well as exactly three jets T with p > 25GeV. Exactly one of the jets is required T tobeb-taggedtoreducebackgrounds.Theleptonmust Table1:Observedandpredictedeventyieldsinthedileptonchannel withnormalisationuncertainties.Thesignalyieldsarecalculatedwith alsomatchthecorrespondingtriggerobject. Additional κbL=gL=1andκRb =gR=0. requirementsare made to reject multijet events, which Process Eventyield tendtohavelowEmissandalowtransversemass2ofthe T b (400GeV) 1250 170 lepton–Emiss system, mW. In the muon channelevents ∗ ± T T b (600GeV) 211 32 are required to have mW + Emiss > 60GeV, while in ∗ ± T T b (800GeV) 41 8 the electron channel a requirement of mW > 30GeV ∗ ± T b (1000GeV) 8.9 1.9 is made. The acceptance for signal events with m = ∗ b ± ∗ b (1200GeV) 2.1 0.5 800GeVinwhichoneoftheW bosonsdecaysleptoni- ∗ ± Wt 293 21 cally(eorµ)andtheotherhadronicallyis9%. ± tt¯ 1380 140 In this channel, one of the largest backgrounds is ± Diboson 255 63 W+jets production for which the normalisation and ± Z e+e 41 4 flavour composition (the heavy-flavour fraction, HF, − Z →µ+µ 118±12 includes b quarks and c quarks) are derived from − Z →τ+τ 14±9 data[62]. Theoverallnormalisationisdeterminedfrom − Fa→kedileptons 90±90 thechargeasymmetrybetweenW+ andW production − ± Totalexpectedbkg. 2190 180 in three-jet events without the b-tag requirement. The ± Totalobserved 2259 flavourcompositionis determinedin two-jeteventsby comparingthepredictedW+jetsyieldstodatawithand without a b-tag requirement. The resulting normali- sation and flavour scale factors are then applied to b- taggedW+3-jetsevents. About37%of the totalback- groundcomesfromW+jetsevents,including28%from eventswithheavyflavour. Backgrounds from tt¯ yield 41% of the total back- groundandsingletop-quarkproductioninthet-, s-and Wt-channel 9%. The multijet background is obtained using a data-based approach by comparing the num- V Ge ATLAS Data bers of events passing loose and tight lepton identifi- vents / 10 ∫ L dt = 4.7 fb-1 bWtt*t (800 GeV) cgarotiuonndc.rSitmeraiall[e6r3b]a.cIktgacrocuonudnstsffroomr9%Z+ojefttsheantodtadlibboascokn- E s = 7 TeV Diboson processesarenormalisedtotheirtheoreticalpredictions 1 Z(e+e-/µ+µ-)+jets Z(τ+τ-)+jets andcontribute4%. Fake dileptons The predicted event yields are compared to data in 10-1 Dilepton channel Table2.Thedistributionsofthe p ofthehighest-p jet T T andEmissareshowninFig.4. 10-2 T Inthelepton+jetschannelitispossibletoreconstruct the candidate b -quark mass from the decay products. 10-3 ∗ 0 500 1000 1500 Theonlymissinginformationistheneutrinolongitudi- H [GeV] nal momentum,which is set to zero. The resulting re- T constructedmassprovidesgooddiscriminationbetween Figure3:HTdistributionfordataandbackgroundexpectationforthe backgroundandsignal,asshowninFig.5. dileptonchannel. Thehatchedbandshowstheuncertaintyduetothe backgroundnormalisation.Thesignalforab -quarkmassof800GeV ∗ isalsoshown. 2The transverse mass, mW, is calculated from the lepton T transverse momentum plep and the difference of the azimuthal T angle, ∆φ, between the Emiss and plep vector as mW = T T T 2Emissplep(1 cos(∆φ(Emiss,plep))) q T T − T T 5 V Table2:Observedandexpectedeventyieldsinthelepton+jetschan- Ge5000 ATLAS Data nelwithnormalisationuncertainties. Thesignalyieldsarecalculated nts / 10 4000 ∫ L dt = 4.7 fb-1 WOWtt+hjeert st oHpF withκbL=PgLro=c1esasndκRb =gR=0. Eventyield e v W+light jets b (400GeV) 12100 1600 E3000 s = 7 TeV Z+jets ∗ ± b (600GeV) 1950 300 Diboson ∗ ± 2000 Multijet b∗(800GeV) 370±70 b (1000GeV) 79 17 lepton + jets channel ∗ ± b (1200GeV) 20 5 1000 ∗ ± Wt 1660 120 ± 0 singletops,t-channel 1960 140 0 50 100 150 200 250 300 ± pjet1 [GeV] tt¯ 15700 1600 T W+lightjets 3200±400 (a) W+jetsHF 10900±1400 ± Diboson 327 16 GeV12000 ATLAS Data Z+jets 1300±800 nts / 10 10000 ∫ L dt = 4.7 fb-1 WOWtt+hjeert st oHpF TMoutaltlijeextpectedbkg. 383550000±±21970000 ve8000 W+light jets Totalobserved 381±75 E s = 7 TeV Z+jets 6000 Diboson Multijet 4000 Lepton + jets channel 7. Systematicuncertainties 2000 Systematic uncertainties affecting the signal accep- 0 0 50 100 150 200 250 tance and the background normalisation are consid- Emiss [GeV] T ered, togetherwith uncertaintiesaffecting the shape of (b) the discriminant distributions. The main experimental source of systematic uncertainty comes from the lim- Figure4:Kinematicdistributionscomparingdatatopredictionsinthe itedknowledgeofthejetenergyscale[58], whichcar- lepton+jetschannelfor(a)thepjTet1ofthehighest-pTjetand(b)ETmiss. riesanuncertaintyof2–7%parameterisedasafunction “Othertop”includestt¯,s-andt-channelsingletop-quarkproduction. of jet p and η. The presence of a b quark in the jet Thehatchedbandshowstheuncertaintyduetothebackgroundnor- T malisation.Thelastbinincludesoverflows. adds an additional uncertainty of 2–5% to the jet en- ergy scale uncertainty, dependingon the jet p . Other T jet-related uncertainty sources are the jet energy reso- GeV ATLAS Data lution, jet reconstruction efficiency and b-tagging ef- s / 102 Lepton + jets channel b’ (800 GeV) ficiency [59]. Lepton-related uncertainties come from vent ∫ L dt = 4.7 fb-1 WOtther top triggerandidentificationefficienciesaswellasthelep- E W+jets HF ton energy scale and resolution. Event-related uncer- 10 s = 7 TeV W+light jets Z+jets tainties are due to the modelling of multiple proton- Diboson protoninteractionsandtheunderlyingeventaswellas Multijet 1 Emiss[60]. Theuncertaintyontheintegratedluminosity T is3.9%[31,32]. 10-1 Simulation uncertainties include modelling of the hard process, parton shower and hadronisation, and 0 400 800 1200 1600 2000 initial- and final-state radiation. These have been as- reconstructed mass [GeV] sessed for the tt¯backgroundevents by comparing dif- ferent generators (Powheg and MC@NLO), different Figure5: Reconstructedmassdistributionfordataandbackground showermodels(PythiaandHerwig),andfortt¯andsig- expectation forthelepton+jets channel. “Othertop”includes tt¯, s- andt-channelsingletop-quarkproduction. Thehatchedbandshows naleventsdifferentsettingsfortheamountofadditional theuncertaintyduetothebackgroundnormalisation.Thesignalfora radiation[64]. Othersourcesoftheoreticaluncertainty massof800GeVisalsoshown.Thelastbinincludesoverflows. includethenormalisationfortt¯(+7% )[47,65–67],sin- 10% − 6 gle top-quark ( 7%) [48–50] and diboson ( 5% with ± ± b] anadditional24%perextrajet)production[61],aswell p 102 ATLAS Expected limit asthechoiceofPDF.Thelatterwasassessedusingthe [Wt s = 7 TeV Expected ± 1σ CT10[27],MRST[28]andNNPDF[29]sets. → pp→ b* → Wt Expected ± 2σ Additionaluncertainties affect the data-drivenback- → b* 10 Ob*b (sκebr=vged= l1im;κibt=g =0) ground estimation. The uncertainty on the DY back- pp L L R R σ Theory uncertainty ground normalisation in the dilepton channel is 10% for ee and µµ final states and 60% for ττ final states. 1 The uncertaintyon the fake-dileptonsnormalisationin the dilepton channel is 100%. The uncertainty on the W+jetsnormalisationinthelepton+jetschannelis13%. 10-1 ∫ L dt = 4.7 fb-1 TheW+jetsflavourcompositionhastwoadditionalun- certainties: the HF contribution has a relative uncer- 400 600 800 1000 1200 taintyof6%,andtheWbb/WHF ratiohasanuncertainty mb* [GeV] of 17%. The multijet backgroundnormalisationin the lepton+jetschannelhasanuncertaintyof50%. Figure6:Expectedandobservedlimitsatthe95%CLasafunctionof theb -quarkmass. Alsoshownisthetheorypredictionforb -quark ∗ ∗ productionwithcouplingsκbL =gL =1andκRb = gR =0,including 8. Statisticalanalysis PDFandscaleuncertainties. Boththe H distributionin thedileptonchanneland T the reconstructed mass distribution in the lepton+jets Limits are also computed for models with right- channelshowgoodagreementbetweenthedataandthe handedandvector-likecouplingsof the b quark. Set- ∗ backgroundmodel.Thesetwodiscriminantsareusedto ting κb = g = 0 and κb = g = 1, the observed L L R R setlimitsontheb∗-quarksignalusingaBayesiananal- lowermass limit is 920GeV with an expectedlimit of ysistechnique[68]. The likelihoodfunctionisdefined 950GeV. Settingκb = κb = g = g = 1,theobserved L R L R as lowermasslimitis1030GeVwithanexpectedlimitof (dataσ )= µnkke−µk G , (3) 1030GeV. L | b∗ n ! i At each mass point, the correspondingcross section Yk k Yi isparameterisedasafunctionofthecouplingsκb and where k is the index of the discriminant template bin, L,R g in order to extract coupling limits in each of the L,R runningoverbothanalysischannels;µ = s +b isthe k k k three b -quark coupling scenarios. The resulting limit ∗ sumofpredictedsignalandbackgroundyields;n isthe k contours are shown in Fig. 7. The coupling limits in- observedyieldandG isaGaussianpriorfortheithsys- i crease as the theoretical cross-section decreases with tematicuncertainty. Aflatpriorisassumedforthesig- b mass, except for the region between 400GeV and ∗ nalcross-section. Upperlimitsontheb -quarkproduc- ∗ 500GeV wherethe backgroundsdecreaserapidlywith tioncross-sectiontimesbranchingratiotoWtaresetat increasingmass(seeFigs.3and5). the95%credibilitylevel(CL)foraseriesof b masses ∗ at100GeVintervals. The observed and expected cross-section limits as a functionof the b -quark mass for the left-handedcou- 9. Summary ∗ pling scenario (κb = g = 1 and κb = g = 0) are L L R R shown in Fig. 6, where the expected limit and its un- certainty are derived from ensembles of background- A search for a singly produced excited b -quark in ∗ only pseudo-datasets. The intersection of the theoret- 4.7 fb 1 of data collected with the ATLAS detector in − ical cross-section and the observed (expected) cross- ppcollisionsat √s=7TeVhasbeenpresented.Thisis section limit defines the observed (expected) b -quark the firstsearch forexcited-quarkscouplingto the third ∗ mass limit. The observed lower limit on the b -quark generation. Itconsidersthedileptonandlepton+jetsfi- ∗ massforthisleft-handedcouplingscenariois870GeV nalstates. Limitsarecomputedasafunctionoftheb gb ∗ withanexpectationof910GeV.Whenconsideringonly and b Wt couplings in three different scenarios. For ∗ the dilepton channel, the observed (expected) limit on purelyleft-handedcouplingsandunitstrengthchromo- the b -quark mass is 800GeV (820GeV); for the lep- magneticcoupling,b quarkswithmassbelow870GeV ∗ ∗ ton+jetschannel,thelimitsare800GeV(830GeV). areexcludedatthe95%credibilitylevel. 7 10. Acknowledgements bL 1 WethankCERNfortheverysuccessfuloperationof κ m [GeV] 0.9 b* 300 the LHC, as well as the support staff from our institu- 0.8 400 tionswithoutwhomATLAScouldnotbeoperatedeffi- 500 0.7 ciently. 600 0.6 700 WeacknowledgethesupportofANPCyT,Argentina; 0.5 800 YerPhI, Armenia; ARC, Australia; BMWF and FWF, 0.4 ATLAS Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq 95% CL limit and FAPESP, Brazil; NSERC, NRC and CFI, Canada; 0.3 ∫ s = 7 TeV; L dt = 4.7 fb-1 CERN; CONICYT, Chile; CAS, MOST and NSFC, 0.2 pp→ b* → Wt; Left-handed b*-quarks China; COLCIENCIAS, Colombia; MSMT CR, MPO 0.1 CR and VSC CR, Czech Republic; DNRF, DNSRC 0 -0.2 0 0.2 0.4 0.6 0.8 1 andLundbeckFoundation,Denmark;EPLANET,ERC g and NSRF, European Union; IN2P3-CNRS, CEA- L DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, (a) HGF,MPGandAvHFoundation,Germany;GSRTand κbR 1 m [GeV] NSRF,Greece;ISF,MINERVA,GIF,DIPandBenoziyo 0.9 b* 300 Center, Israel; INFN, Italy; MEXT and JSPS, Japan; 0.8 400 CNRST,Morocco;FOMandNWO,Netherlands;BRF 500 0.7 600 and RCN, Norway; MNiSW, Poland; GRICES and 700 FCT, Portugal; MERYS (MECTS), Romania; MES 0.6 800 0.5 900 of Russia and ROSATOM, Russian Federation; JINR; 0.4 ATLAS MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, 95% CL limit Slovenia; DST/NRF, South Africa; MICINN, Spain; 0.3 ∫ s = 7 TeV; L dt = 4.7 fb-1 SRCandWallenbergFoundation,Sweden;SER,SNSF 0.2 pp→ b* → Wt; Right-handed b*-quarks and Cantons of Bern and Geneva, Switzerland; NSC, 0.1 Taiwan; TAEK, Turkey; STFC, the Royal Society and 0-0.2 0 0.2 0.4 0.6 0.8 1 Leverhulme Trust, United Kingdom; DOE and NSF, g UnitedStatesofAmerica. 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