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

0.37 MB·English

by  ATLAS Collaboration

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP-2012-344
Submitted to: Phys. Lett. B
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Search for single b -quark production with the ATLAS detector
n ∗
a at √s = 7 TeV
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p The ATLAS Collaboration
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Abstract
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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.
R
ThecrucialcomputingsupportfromallWLCGpart-
(b)
ners is acknowledged gratefully, in particular from
κbL/R 0.91 mb* [G 3e0V0] C(CEaRnNadaa)n,dNtDhGeFAT(DLAenSmTarike,r-1Nofarwcialiyt,ieSswaetdTenR)I,UCMCF-
400 IN2P3(France),KIT/GridKA(Germany),INFN-CNAF
0.8 500
600 (Italy),NL-T1(Netherlands),PIC(Spain),ASGC(Tai-
0.7 700
800 wan),RAL(UK)andBNL(USA)andintheTier-2fa-
0.6
900 cilitiesworldwide.
0.5 1000
0.4 ATLAS
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