EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)

arXiv:1301.1583v1 [hep-ex] 8 Jan 2013

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)

CERN-PH-EP-2012-344

Submitted to: Phys. Lett. B

Search for single

b

∗

_{-quark production with the ATLAS detector}

at

√

s

=

7

TeV

The ATLAS Collaboration

Abstract

The results of a search for an excited bottom-quark

b

in

pp

collisions at

√

s

=

7 TeV

, using

4

.

7

fb

of data collected by the ATLAS detector at the LHC are presented. In the model studied, a single

b

-quark is produced through a chromomagnetic interaction and subsequently decays to a

W

boson and

a top quark. The search is performed in the dilepton and lepton+jets final states, which are combined

to set limits on

b

-quark couplings for a range of

b

-quark masses. For a benchmark with unit size

chromomagnetic and Standard Model-like electroweak

b

_couplings,

_b

_{quarks with masses less than}

Search for single

b

-quark production with the ATLAS detector at

√

s

=

7 TeV

The ATLAS Collaboration

Abstract

The results of a search for an excited bottom-quarkb∗ in pp collisions at √s = 7 TeV, using 4.7 fb−1 of data collected by the ATLAS detector at the LHC are presented. In the model studied, a singleb∗_{-quark is produced through} a chromomagnetic interaction and subsequently decays to aW boson and a top quark. The search is performed in

the dilepton and lepton+jets final states, which are combined to set limits onb∗_{-quark couplings for a range ofb}∗ -quark masses. For a benchmark with unit size chromomagnetic and Standard Model-like electroweakb∗_couplings,

b∗_{quarks with masses less than 870 GeV are excluded at the 95% credibility level.}

Keywords: ATLAS,b∗, single top-quark, excited quark

1. Introduction

The single top-quark signature is sensitive to many models of new physics [1]. Single top-quark produc-tion in the Standard Model (SM) has been measured at the LHC in thet-channel [2, 3] and in association with

aW boson (Wt-channel) [4, 5]. Searches for resonant

production of a new particle which decays with a single top-quark have been carried out in thes-channel

produc-tion of a top quark together with abquark [6, 7]. This

Letter presents the first search for a resonance decay-ing to a sdecay-ingle top-quark and aW boson [8]. Here we

consider the production of an excited quarkb∗ which

decays to a single top-quark and aW boson. This is

the first search for excited-quarks coupling to the third generation of fermions.

Previous searches for excited quarks have focused on their strong interactions [9, 10], as well as their electro-magnetic interactions [11, 12] with SM quarks. These searches exploit the coupling between the excited quark and up or down quarks in the proton. Here the produc-tion of excited-quarks coupling primarily to the third generation of SM quarks is investigated. This cou-pling occurs for example in Randall–Sundrum models that address the strong interaction sector [13, 14] or in models with a heavy gluon partner, such as composite Higgs models [15–17]. Theb∗quark is produced singly

through its coupling to abquark and a gluon, as shown

in Fig. 1.

The Lagrangian describing this interaction is given by

b

g

b

t

W

Figure 1: Leading-order Feynman diagram for single-b∗-quark pro-duction and decay toWt.

[18, 19]

L= gs

2ΛGµνbσ

µν _κb

LPL+κbRPR

!

b∗+h.c. , (1) where gs is the strong coupling, Gµν the gauge field

tensor of the gluon and Λ = mb∗ the scale of the new physics. PLandPR are the left- and right-handed

pro-jection operators andκb_Landκb_R are the respective cou-pling strengths. This analysis is thus complementary to excited-quark searches focusing on the coupling to the first generation [9, 20, 21]. Singleb∗_{-quark production} can also reveal the chiral nature of the excited bottom-quark [8].

In addition to the chromomagnetic coupling, the

quarks can have left-handed or right-handed couplings to theWboson or can be vector-like with equal strength

for both couplings. The Lagrangian describing the elec-troweak decay of theb∗_{quark, shown in Fig. 1, is}

L= _√g2

2W

+

µtγ

µ

gLPL+gRPR

b∗+h.c. , (2) whereg2is the SU(2)Lweak coupling andgLandgRare

the coupling strengths for left-handed and right-handed couplings, respectively.

While the search is general and considers any res-onance decaying into the Wt signature, three specific b∗-quark coupling scenarios are considered in order to

extractb∗_{-quark coupling and mass limits: left-handed} (κb_L, gL non-zero and κbR = gR = 0), right-handed

(κb_L = gL = 0 and κbR, gR non-zero) and vector-like

(κb L = κ

b R = κ

b

L/R andgL = gR = gL/R non-zero)

pro-duction and decay. Limits are derived as a function of theb∗_{-quark mass as well as the couplingsκ}b

L,R and gL,R. These limits take into account both the change of

the production cross-section and the b∗ _{→ Wt decay} branching ratio, which depend on the couplings and the

b∗_{-quark mass. The branching ratio to Wtvaries} be-tween 20% atmb∗ = 300 GeV and 40% at higher val-ues, with decays tobg,bZandbHalso allowed.

Con-tributions from non-Wtdecay modes that may increase

the b∗-quark acceptance of this analysis are not

con-sidered, resulting in conservative limits. Signal event yields presented in the following tables are calculated withκb

L=gL=1 andκbR =gR=0.

For a left-handedb∗_at √_{s=7 TeV withκ}b

L=gL=1

andκb

R =gR =0, the leading-order cross section times

branching ratio toWtis 0.80 pb formb∗=900 GeV [8]. The uncertainties due to the choice of factorisation and renormalisation scales are evaluated by varying the scales between mb∗/2 and 2 ×mb∗, and those due to the choice of PDF by comparing results obtained us-ing the CT10 [27], MRST [28] and NNPDF [29] sets. These uncertainties are added in quadrature to yield cross-section uncertainties ranging from 12% atmb∗ = 300 GeV to 25% atmb∗ =1200 GeV.

This channel proceeds via twoW bosons fromb∗

-quark and top--quark decays. At least oneW boson is

required to decay to a lepton (electron or muon). The analysis is performed separately in the dilepton and lep-ton+jets final states. The lepton+jets channel has the

advantage that the invariant mass of theb∗ _{quark can} be reconstructed, whereas the dilepton channel benefits from smaller backgrounds. A discriminant that sepa-rates theb∗_{-quark signal from the backgrounds is} de-fined in each final state. Limits onb∗_{-quark production} are obtained from a combined Bayesian analysis of both

discriminant distributions.

2. The ATLAS detector

The ATLAS detector [30] is a general purpose de-tector with a precise tracking system, calorimeters and an outer muon spectrometer. The inner tracking sys-tem consists of a silicon pixel detector, a silicon mi-crostrip tracker, and a straw-tube transition radiation tracker. This system is immersed in a 2 T axial magnetic field produced by a solenoid and provides charged parti-cle tracking and identification in the pseudorapidity1 re-gion|η_|<2.5. The central calorimeter system consists of a liquid-argon electromagnetic sampling calorime-ter with high granularity and an iron/scintillator tile

calorimeter providing hadronic energy measurements in the central pseudorapidity range (_|η_|<1.7). The endcap and forward regions are instrumented with liquid-argon calorimeters for both electromagnetic and hadronic en-ergy measurements up to_|η_|=4.9. The muon spectrom-eter is operated in a toroidal magnetic field provided by air-core superconducting magnets and includes tracking chambers for precise muon momentum measurements up to|η_|=2.7 and trigger chambers covering the range

|η_|<2.4.

3. Data and simulated samples

This analysis uses data collected with the ATLAS de-tector in 2011, corresponding to an integrated luminos-ity of 4.7_±0.2 fb−1_{[31, 32] of 7 TeV proton–proton (pp)} collisions delivered by the LHC. The data are selected using single-electron or single-muon triggers whose ef-ficiencies reach their plateau at 25 GeV and 20 GeV, re-spectively [33, 34]. The data must also pass stringent quality requirements [35]. Events are selected if they contain at least one primary vertex candidate with at least five associated tracks.

The signal is modelled using MadGraph5 [36] and the CTEQ6L1 parton distribution functions (PDFs) [37]. Events with single top-quarks in the t

-channel are generated with the AcerMC [38] genera-tor, using the MRST LO** PDF set [39]. MadGraph5

1_{ATLAS uses a right-handed coordinate system with its origin at} the nominal interaction point (IP) in the centre of the detector and thez-axis along the beam pipe. Thex-axis points from the IP to the centre of the LHC ring, and they-axis points upwards. Cylindrical coordinates (r, φ) are used in the transverse plane,φis the azimuthal

angle around the beam pipe. The pseudorapidityηis defined in terms

and AcerMC are interfaced to Pythia [40] for par-ton showering and modelling of the underlying event. Other processes producing single quarks and top-quark pairs (tt¯) are modelled with the

next-to-leading-order(NLO) generator MC@NLO [41] using the CT10 PDF set [27], interfaced to Herwig [42] for parton showering and Jimmy [43] for the underlying event. Alpgen [44] is used to model vector boson (W and

Z) production in association with jets as well as

dibo-son processes (WW,WZandZZ) using the CTEQ6L1

PDF set. It is interfaced to Herwigfor parton shower modelling. In the lepton+jets analysis the diboson

pro-cesses are modelled with Herwigonly. Decays ofτ lep-tons are handled by Tauola[45]. A top-quark mass of 172.5 GeV [46] is assumed. Approximate next-to-next-to-leading-order(NNLO) cross-section calculations are used to normalise thett¯[47] (Hathor) and single top-quark samples [48–50], while the vector boson and di-boson samples are normalised using calculations with MCFM [51] at NNLO and NLO, respectively.

A variable number of additionalppinteractions

(pile-up) are overlaid on simulated events, which are then weighted to reproduce the distribution of the number of collisions per bunch crossing observed in data. All samples are passed through a GEANT4-based simula-tion [52] of the ATLAS detector [53] and are then re-constructed using the same procedure as for collision data.

4. Physics object selection

Electron candidates are reconstructed from clusters of energy deposits in the calorimeter [54]. The transverse energyETof electron candidates is required to be larger than 25 GeV and their pseudorapidity is required to be

|η|<2.47. Electrons in the barrel–endcap transition re-gion of the calorimeter, corresponding to 1.37 <_|η_| < 1.52, are not considered. Selected electrons must pass a set of “tight” quality criteria [54] and the electons must be matched to a track reconstructed in the inner tracking system. Electrons must also be isolated from close-by tracks in a cone of∆R = p(∆η)2+(∆φ)2 < 0.3 and from calorimeter energy deposits not belonging to the electron candidate in a cone of∆R < 0.2. The isola-tion requirements on the sum of transverse momenta of tracks in the cone and on the sum of energy deposits in the calorimeter in the cone are chosen as a function of

pTandηsuch that an efficiency of 90% for electrons in the simulation is achieved.

Muon candidates are reconstructed from matching tracks in the muon spectrometer and inner tracking sys-tem. Muons are required to have transverse momentum

pT >25 GeV and|η|<2.5 and fulfil tight quality crite-ria [55]. Muons must be isolated from close-by tracks in a cone of∆R <0.3 and from energy deposits in the calorimeter in a cone of ∆R <0.2. The sum of trans-verse momenta of tracks in the cone must not exceed 2.5 GeV and the sum of energy deposits in the calorime-ter in the cone must be below 4 GeV.

In order to reject events in which a muon emitting a hard photon is also reconstructed as an electron, events are vetoed if a selected electron–muon pair shares the same track.

Jets are reconstructed from clusters of energy de-posits in the calorimeter [56] using the anti-kt

algo-rithm [57] with a radius parameter R = 0.4. These jets are calibrated to the hadronic energy scale through

pT- and η-dependent scale factors, which are derived from simulation. An additional uncertainty due to residual differences between simulation and data is

ap-plied in the analysis [58]. Jets are required to have

pT > 30 (25) GeV and|η| < 2.5 in the dilepton (lep-ton+jets) channel. The ratio of the scalar sum of the

pT of tracks associated with the jet and the primary vertex to the scalar sum of the pT of all tracks asso-ciated with the jet must be at least 0.75 to reject jets from pile-up interactions. Muons overlapping with jets within∆R < 0.4 are removed and the jet is kept. The closest jet overlapping with electrons within∆R <0.2 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 containingbquarks [59] is also used in the lepton+jets

channel. A neural network combines lifetime-related in-formation reconstructed from the tracks associated with each jet. At the chosen working point, theb-tagging

al-gorithm has an efficiency of 70% (20%/0.7%) for jets

containingbquarks (cquarks/light quarks or gluons) in

a simulatedtt¯sample.

The missing transverse momentum E_Tmiss is

calcu-lated using topological clusters of energy deposits in the calorimeter and corrected for the presence of muons [60].

5. Event selection in the dilepton channel

The event selection and background modelling in the dilepton channel is the same as in the ATLAS mea-surement of the single top-quark production in theWt

b-tagging requirement is made since the dominant

back-ground fromtt¯production also containsbquarks. The Emiss_T is required to be greater than 50 GeV. In theee

andµµchannels, the invariant mass of the lepton pair,

mℓℓ, is required to be outside the Z boson mass

win-dow: mℓℓ < 81 GeV or mℓℓ > 101 GeV. In all three

channels, theZ_→ττbackground is reduced by a dedi-cated veto, which requires the sum of the azimuthal an-gle differences between each lepton and theEmiss_T vector

to be greater than 2.5 rad. After all cuts, the acceptance for signal events withmb∗ = 800 GeV in which both

Wbosons decay leptonically (to eithereorµ) is 26%. The main background, accounting for 63% of the to-tal, comes fromtt¯events in which one of the two jets

originating fromb quarks is not detected. The second

largest background is from SMWt production, which

has the same final state as theb∗_{-quark signal, and} ac-counts for 13% of the total background. Diboson events produced in association with jets account for 12% of the total background. With the exception of single- and di-boson samples, these backgrounds are taken from NLO simulation and are normalised to their NNLO theoret-ical predictions. Drell–Yan (DY) events contribute a small background of 7.3% to the sum ofeeandµµ chan-nel events. The events are taken from the simulation and normalised to data using a two-dimensional side-band region with low Emiss_T and/ormℓℓ outside of the

Z boson mass window [4]. The contribution fromττ final states, where bothτleptons decay leptonically, is estimated from simulated samples, with the normalisa-tion checked in an orthogonal data sample obtained by reversing theZ _→τ+_τ−_{veto cut described above.Z →} τ+_τ−_{events account for 0.7% of the total background.} The small background from jets that are misidentified as primary leptons and from non-prompt leptons (fake dileptons) is modelled and normalised using data [61]. It accounts for 4% of the background.

The predicted event yields for the backgrounds and signal at a few mass points are compared to data in Ta-ble 1. ThepTdistributions of the two leptons and the jet are shown in Fig. 2.

A discriminating variable that separates the signal from the backgrounds isHT, the scalar sum of the trans-verse momenta of the leptons, jet and E_Tmiss. The HT distribution is shown in Fig. 3.

6. Event selection in the lepton+jets channel

The analysis in the lepton+jets channel follows the

same background modelling strategy as the cross-section measurement for single top-quark production in

[GeV]

lep1 T

p

0 50 100 150 200 250 300

Events / 10 GeV

0 50 100 150 200 250 300 350 Data Wt t t Diboson )+jets -µ + µ / -e + Z(e )+jets -τ + τ Z( Fake dileptons -1

L dt = 4.7 fb

∫

= 7 TeV s Dilepton channel (a) [GeV] lep2 T p

0 20 40 60 80 100 120 140 160 180 200

Events / 10 GeV

0 100 200 300 400 500 600 700 800 Data Wt t t Diboson )+jets -µ + µ / -e + Z(e )+jets -τ + τ Z( Fake dileptons -1

L dt = 4.7 fb

∫

= 7 TeV s Dilepton channel (b) [GeV] jet T p 0 50 100 150 200 250 300

Events / 10 GeV

0 100 200 300 400 500 Data Wt t t Diboson )+jets -µ + µ / -e + Z(e )+jets -τ + τ Z( Fake dileptons -1

L dt = 4.7 fb

∫

= 7 TeV s

Dilepton channel

(c)

Table 1: Observed and predicted event yields in the dilepton channel with normalisation uncertainties. The signal yields are calculated with

κb_L=gL=1 andκb_R=gR=0.

Process Event yield

b∗_{(400 GeV) 1250±170}

b∗_{(600 GeV) 211±32}

b∗_{(800 GeV) 41±8}

b∗_{(1000 GeV) 8.9±1.9}

b∗_{(1200 GeV) 2.1±0.5}

Wt 293_±21 tt¯ 1380_±140

Diboson 255±63

Z_→e+_e− _41±4

Z_→µ+µ− 118±12

Z_→τ+_τ− _14±9 Fake dileptons 90±90

Total expected bkg. 2190_±180 Total observed 2259

[GeV]

T

H 0 500 1000 1500

Events / GeV

-3

10

-2

10

-1

10 1 10

Data b* (800 GeV) Wt

t t Diboson

)+jets

-µ + µ

/

-e

+

Z(e )+jets

-τ + τ

Z(

Fake dileptons

-1

L dt = 4.7 fb

∫

= 7 TeV s

Dilepton channel

Figure 3:HTdistribution for data and background expectation for the dilepton channel. The hatched band shows the uncertainty due to the background normalisation. The signal for ab∗_{-quark mass of 800 GeV} is also shown.

the t-channel [2]. Events are required to have either ex-actly one muon andEmiss_T >25 GeV or exactly one elec-tron andEmiss_T > 30 GeV, as well as exactly three jets with pT > 25 GeV. Exactly one of the jets is required to beb-tagged to reduce backgrounds. The lepton must

also match the corresponding trigger object. Additional requirements are made to reject multijet events, which tend to have lowEmiss_T and a low transverse mass2of the

lepton–E_Tmisssystem, mW_T. In the muon channel events

are required to have mW_T +Emiss_T > 60 GeV, while in the electron channel a requirement of mW_T > 30 GeV is made. The acceptance for signal events withmb∗ = 800 GeV in which one of theWbosons decays

leptoni-cally (eorµ) and the other hadronically is 9%.

In this channel, one of the largest backgrounds is

W+jets production for which the normalisation and

flavour composition (the heavy-flavour fraction, HF, includes b quarks and c quarks) are derived from

data [62]. The overall normalisation is determined from the charge asymmetry betweenW+_andW−_production in three-jet events without the b-tag requirement. The

flavour composition is determined in two-jet events by comparing the predictedW+jets yields to data with and

without a b-tag requirement. The resulting

normali-sation and flavour scale factors are then applied to b

-taggedW+3-jets events. About 37% of the total

back-ground comes fromW+jets events, including 28% from

events with heavy flavour.

Backgrounds from tt¯yield 41% of the total

back-ground and single top-quark production in thet-,s- and Wt-channel 9%. The multijet background is obtained

using a data-based approach by comparing the num-bers of events passing loose and tight lepton identifi-cation criteria [63]. It accounts for 9% of the total back-ground. Smaller backgrounds fromZ+jets and diboson

processes are normalised to their theoretical predictions and contribute 4%.

The predicted event yields are compared to data in Table 2. The distributions of thepTof the highest-pTjet andEmiss_T are shown in Fig. 4.

In the lepton+jets channel it is possible to reconstruct

the candidateb∗-quark mass from the decay products.

The only missing information is the neutrino longitudi-nal momentum, which is set to zero. The resulting re-constructed mass provides good discrimination between background and signal, as shown in Fig. 5.

2_{The transverse mass, m}W

T, is calculated from the lepton transverse momentum plep_T and the difference of the azimuthal angle, ∆φ, between the E_Tmiss and plep_T vector as mW

T =

q

[GeV]

jet1 T

p 0 50 100 150 200 250 300

Events / 10 GeV

0 1000 2000 3000 4000 5000 ATLAS

= 7 TeV s

-1

L dt = 4.7 fb

∫

lepton + jets channel

Data Wt Other top W+jets HF W+light jets Z+jets Diboson Multijet (a) [GeV] miss T E

0 50 100 150 200 250

Events / 10 GeV

0 2000 4000 6000 8000 10000 12000 _ATLAS

= 7 TeV s

-1

L dt = 4.7 fb

∫

Lepton + jets channel

Data Wt Other top W+jets HF W+light jets Z+jets Diboson Multijet (b)

Figure 4: Kinematic distributions comparing data to predictions in the lepton+jets channel for (a) thepjet1_T of the highest-pTjet and (b)Emiss_T . “Other top” includestt¯,s- andt-channel single top-quark production. The hatched band shows the uncertainty due to the background nor-malisation. The last bin includes overflows.

reconstructed mass [GeV] 0 400 800 1200 1600 2000

Events / GeV

-1 10 1 10 2 10 ATLAS -1

L dt = 4.7 fb

∫

Lepton + jets channel

= 7 TeV s

Data b’ (800 GeV) Wt Other top W+jets HF W+light jets Z+jets Diboson Multijet

Figure 5: Reconstructed mass distribution for data and background expectation for the lepton+jets channel. “Other top” includestt¯,s -andt-channel single top-quark production. The hatched band shows the uncertainty due to the background normalisation. The signal for a mass of 800 GeV is also shown. The last bin includes overflows.

Table 2: Observed and expected event yields in the lepton+jets chan-nel with normalisation uncertainties. The signal yields are calculated withκb

L=gL=1 andκbR=gR=0.

Process Event yield

b∗(400 GeV) 12100_±1600 b∗(600 GeV) 1950_±300 b∗(800 GeV) 370_±70 b∗(1000 GeV) 79_±17 b∗(1200 GeV) 20±5

Wt 1660_±120

single tops,t-channel 1960_±140

tt¯ 15700_±1600

W+light jets 3200_±400

W+jets HF 10900_±1400

Diboson 327_±16

Z+jets 1300_±800

Multijet 3500±1700

Total expected bkg. 38500±2900

Total observed 38175

7. Systematic uncertainties

Systematic uncertainties affecting the signal

accep-tance and the background normalisation are consid-ered, together with uncertainties affecting the shape of

the discriminant distributions. The main experimental source of systematic uncertainty comes from the lim-ited knowledge of the jet energy scale [58], which car-ries an uncertainty of 2–7% parameterised as a function of jet pT andη. The presence of a b quark in the jet adds an additional uncertainty of 2–5% to the jet en-ergy scale uncertainty, depending on the jet pT. Other jet-related uncertainty sources are the jet energy reso-lution, jet reconstruction efficiency and b-tagging

ef-ficiency [59]. Lepton-related uncertainties come from trigger and identification efficiencies as well as the

lep-ton energy scale and resolution. Event-related uncer-tainties are due to the modelling of multiple proton-proton interactions and the underlying event as well as

E_Tmiss[60]. The uncertainty on the integrated luminosity

is 3.9% [31, 32].

Simulation uncertainties include modelling of the hard process, parton shower and hadronisation, and initial- and final-state radiation. These have been as-sessed for thett¯background events by comparing

dif-ferent generators (Powheg and MC@NLO), different shower models (Pythiaand Herwig), and fortt¯and sig-nal events different settings for the amount of additional

radiation [64]. Other sources of theoretical uncertainty include the normalisation fortt¯(+7%

sin-gle top-quark (±7%) [48–50] and diboson (±5% with

an additional 24% per extra jet) production [61], as well as the choice of PDF. The latter was assessed using the CT10 [27], MRST [28] and NNPDF [29] sets.

Additional uncertainties affect the data-driven

ground estimation. The uncertainty on the DY back-ground normalisation in the dilepton channel is 10% foreeandµµ final states and 60% forττ final states. The uncertainty on the fake-dileptons normalisation in the dilepton channel is 100%. The uncertainty on the

W+jets normalisation in the lepton+jets channel is 13%.

TheW+jets flavour composition has two additional

un-certainties: the HF contribution has a relative uncer-tainty of 6%, and theWbb/WHFratio has an uncertainty

of 17%. The multijet background normalisation in the lepton+jets channel has an uncertainty of 50%.

8. Statistical analysis

Both theHT distribution in the dilepton channel and the reconstructed mass distribution in the lepton+jets

channel show good agreement between the data and the background model. These two discriminants are used to set limits on theb∗_{-quark signal using a Bayesian} anal-ysis technique [68]. The likelihood function is defined as

L(data|σb∗)= Y

k

µnk

k e−

µk

nk!

Y

i

Gi, (3)

wherekis the index of the discriminant template bin,

running over both analysis channels;µk=sk+bkis the

sum of predicted signal and background yields;nkis the

observed yield andGiis a Gaussian prior for theith

sys-tematic uncertainty. A flat prior is assumed for the sig-nal cross-section. Upper limits on theb∗_-quark produc-tion cross-secproduc-tion times branching ratio toWtare set at

the 95% credibility level (CL) for a series ofb∗_masses at 100 GeV intervals.

The observed and expected cross-section limits as a function of theb∗-quark mass for the left-handed

cou-pling scenario (κb_L = gL = 1 and κ_Rb = gR = 0) are

shown in Fig. 6, where the expected limit and its un-certainty are derived from ensembles of background-only pseudo-datasets. The intersection of the theoret-ical section and the observed (expected) cross-section limit defines the observed (expected)b∗_-quark mass limit. The observed lower limit on theb∗_-quark mass for this left-handed coupling scenario is 870 GeV with an expectation of 910 GeV. When considering only the dilepton channel, the observed (expected) limit on theb∗_{-quark mass is 800 GeV (820 GeV); for the} lep-ton+jets channel, the limits are 800 GeV (830 GeV).

[GeV]

b*

m

400 600 800 1000 1200

[pb] Wt → b* → pp σ -1 10 1 10 2

10 Expected limit

σ 1 ± Expected σ 2 ± Expected Observed limit =0) R =g R b κ =1; L =g L b κ b* ( Theory uncertainty ATLAS Wt → b* → pp = 7 TeV s

-1 L dt = 4.7 fb

∫

Figure 6: Expected and observed limits at the 95% CL as a function of theb∗_{-quark mass. Also shown is the theory prediction forb}∗_-quark production with couplingsκb_L=gL=1 andκb_R=gR =0, including

PDF and scale uncertainties.

Limits are also computed for models with right-handed and vector-like couplings of theb∗_quark. Set-ting κb

L = gL = 0 and κ b

R = gR = 1, the observed

lower mass limit is 920 GeV with an expected limit of 950 GeV. Settingκb_L=κb_R =gL=gR=1, the observed

lower mass limit is 1030 GeV with an expected limit of 1030 GeV.

At each mass point, the corresponding cross section is parameterised as a function of the couplingsκb_L,R and

gL,R in order to extract coupling limits in each of the

threeb∗-quark coupling scenarios. The resulting limit

contours are shown in Fig. 7. The coupling limits in-crease as the theoretical cross-section dein-creases with

b∗ _{mass, except for the region between 400 GeV and} 500 GeV where the backgrounds decrease rapidly with increasing mass (see Figs. 3 and 5).

9. Summary

A search for a singly produced excited b∗-quark in

4.7 fb−1 of data collected with the ATLAS detector in

ppcollisions at √s=7 TeV has been presented. This is

the first search for excited-quarks coupling to the third generation. It considers the dilepton and lepton+jets

L

g -0.2 0 0.2 0.4 0.6 0.8 1

L b κ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ATLAS

95% CL limit

-1

L dt = 4.7 fb

∫

= 7 TeV; s

Wt; Left-handed b*-quarks

→ b* → pp [GeV] b* m 300 400 500 600 700 800 (a) R g -0.2 0 0.2 0.4 0.6 0.8 1

R b_κ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ATLAS

95% CL limit

-1

L dt = 4.7 fb

∫

= 7 TeV; s

Wt; Right-handed b*-quarks

→ b* → pp [GeV] b* m 300 400 500 600 700 800 900 (b) L/R g -0.2 0 0.2 0.4 0.6 0.8 1

L/R b κ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ATLAS

95% CL limit

-1

L dt = 4.7 fb

∫

= 7 TeV; s

Wt; Vector-like b*-quarks

→ b* → pp [GeV] b* m 300 400 500 600 700 800 900 1000 (c)

Figure 7: Limit contours at the 95% CL as a function of the coupling parameters for several differentb∗_{-quark masses, for (a) left-handed}

b∗quarks, (b) right-handedb∗quarks and (c) vector-likeb∗quarks.

10. Acknowledgements

We thank CERN for the very successful operation of the LHC, as well as the support stafffrom our

institu-tions without whom ATLAS could not be operated effi

-ciently.

We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG,

HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain;

SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.

The crucial computing support from all WLCG part-ners is acknowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF

(Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Tai-wan), RAL (UK) and BNL (USA) and in the Tier-2 fa-cilities worldwide.

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G. Aad48_{, T. Abajyan}21_{, B. Abbott}111_{, J. Abdallah}12_{, S. Abdel Khalek}115_{, A.A. Abdelalim}49_{, O. Abdinov}11_, R. Aben105_{, B. Abi}112_{, M. Abolins}88_{, O.S. AbouZeid}158_{, H. Abramowicz}153_{, H. Abreu}136_{, M.I. Ochoa}77_,

B.S. Acharya164a,164b,a_{, L. Adamczyk}38_{, D.L. Adams}25_{, T.N. Addy}56_{, J. Adelman}176_{, S. Adomeit}98_{, P. Adragna}75_,

T. Adye129_{, S. Aefsky}23_{, J.A. Aguilar-Saavedra}124b,b_{, M. Agustoni}17_{, S.P. Ahlen}22_{, F. Ahles}48_{, A. Ahmad}148_,

M. Ahsan41_{, G. Aielli}133a,133b_{, T.P.A. Åkesson}79_{, G. Akimoto}155_{, A.V. Akimov}94_{, M.A. Alam}76_{, J. Albert}169_,

S. Albrand55_{, M. Aleksa}30_{, I.N. Aleksandrov}64_{, F. Alessandria}89a_{, C. Alexa}26a_{, G. Alexander}153_{, G. Alexandre}49_, T. Alexopoulos10_{, M. Alhroob}164a,164c_{, M. Aliev}16_{, G. Alimonti}89a_{, J. Alison}120_{, B.M.M. Allbrooke}18_{, L.J. Allison}71_,

P.P. Allport73_{, S.E. Allwood-Spiers}53_{, J. Almond}82_{, A. Aloisio}102a,102b_{, R. Alon}172_{, A. Alonso}36_{, F. Alonso}70_,

A. Altheimer35_{, B. Alvarez Gonzalez}88_{, M.G. Alviggi}102a,102b_{, K. Amako}65_{, C. Amelung}23_{, V.V. Ammosov}128,∗_, S.P. Amor Dos Santos124a_{, A. Amorim}124a,c_{, S. Amoroso}48_{, N. Amram}153_{, C. Anastopoulos}30_{, L.S. Ancu}17_, N. Andari115_{, T. Andeen}35_{, C.F. Anders}58b_{, G. Anders}58a_{, K.J. Anderson}31_{, A. Andreazza}89a,89b_{, V. Andrei}58a_, M-L. Andrieux55_{, X.S. Anduaga}70_{, S. Angelidakis}9_{, P. Anger}44_{, A. Angerami}35_{, F. Anghinolfi}30_{, A. Anisenkov}107_, N. Anjos124a_{, A. Annovi}47_{, A. Antonaki}9_{, M. Antonelli}47_{, A. Antonov}96_{, J. Antos}144b_{, F. Anulli}132a_{, M. Aoki}101_, S. Aoun83_{, L. Aperio Bella}5_{, R. Apolle}118,d_{, G. Arabidze}88_{, I. Aracena}143_{, Y. Arai}65_{, A.T.H. Arce}45_{, S. Arfaoui}148_, J-F. Arguin93_{, S. Argyropoulos}42_{, E. Arik}19a,∗_{, M. Arik}19a_{, A.J. Armbruster}87_{, O. Arnaez}81_{, V. Arnal}80_,

A. Artamonov95_{, G. Artoni}132a,132b_{, D. Arutinov}21_{, S. Asai}155_{, S. Ask}28_{, B. Åsman}146a,146b_{, D. Asner}29_{, L. Asquith}6_, K. Assamagan25,e_{, A. Astbury}169_{, M. Atkinson}165_{, B. Aubert}5_{, B. Auerbach}6_{, E. Auge}115_{, K. Augsten}126_,

M. Aurousseau145a_{, G. Avolio}30_{, D. Axen}168_{, G. Azuelos}93,f_{, Y. Azuma}155_{, M.A. Baak}30_{, G. Baccaglioni}89a_,

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F. Berghaus169_{, E. Berglund}105_{, J. Beringer}15_{, P. Bernat}77_{, R. Bernhard}48_{, C. Bernius}25_{, T. Berry}76_{, C. Bertella}83_, A. Bertin20a,20b_{, F. Bertolucci}122a,122b_{, M.I. Besana}89a,89b_{, G.J. Besjes}104_{, N. Besson}136_{, S. Bethke}99_{, W. Bhimji}46_,

R.M. Bianchi30_{, L. Bianchini}23_{, M. Bianco}72a,72b_{, O. Biebel}98_{, S.P. Bieniek}77_{, K. Bierwagen}54_{, J. Biesiada}15_,

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L. Bugge117_{, O. Bulekov}96_{, A.C. Bundock}73_{, M. Bunse}43_{, T. Buran}117_{, H. Burckhart}30_{, S. Burdin}73_{, T. Burgess}14_, S. Burke129_{, E. Busato}34_{, P. Bussey}53_{, C.P. Buszello}166_{, B. Butler}143_{, J.M. Butler}22_{, C.M. Buttar}53_,

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R. Camacho Toro34_{, P. Camarri}133a,133b_{, D. Cameron}117_{, L.M. Caminada}15_{, R. Caminal Armadans}12_{, S. Campana}30_,

M. Campanelli77_{, V. Canale}102a,102b_{, F. Canelli}31_{, A. Canepa}159a_{, J. Cantero}80_{, R. Cantrill}76_,

M.D.M. Capeans Garrido30_{, I. Caprini}26a_{, M. Caprini}26a_{, D. Capriotti}99_{, M. Capua}37a,37b_{, R. Caputo}81_,

R. Cardarelli133a_{, T. Carli}30_{, G. Carlino}102a_{, L. Carminati}89a,89b_{, S. Caron}104_{, E. Carquin}32b_,

G.D. Carrillo-Montoya145b_{, A.A. Carter}75_{, J.R. Carter}28_{, J. Carvalho}124a,i_{, D. Casadei}108_{, M.P. Casado}12_, M. Cascella122a,122b_{, C. Caso}50a,50b,∗_{, A.M. Castaneda Hernandez}173,j_{, E. Castaneda-Miranda}173_,

V. Castillo Gimenez167_{, N.F. Castro}124a_{, G. Cataldi}72a_{, P. Catastini}57_{, A. Catinaccio}30_{, J.R. Catmore}30_{, A. Cattai}30_, G. Cattani133a,133b_{, S. Caughron}88_{, V. Cavaliere}165_{, P. Cavalleri}78_{, D. Cavalli}89a_{, M. Cavalli-Sforza}12_,

V. Cavasinni122a,122b_{, F. Ceradini}134a,134b_{, A.S. Cerqueira}24b_{, A. Cerri}15_{, L. Cerrito}75_{, F. Cerutti}15_{, S.A. Cetin}19b_, A. Chafaq135a_{, D. Chakraborty}106_{, I. Chalupkova}127_{, K. Chan}3_{, P. Chang}165_{, B. Chapleau}85_{, J.D. Chapman}28_, J.W. Chapman87_{, D.G. Charlton}18_{, V. Chavda}82_{, C.A. Chavez Barajas}30_{, S. Cheatham}85_{, S. Chekanov}6_,

S.V. Chekulaev159a_{, G.A. Chelkov}64_{, M.A. Chelstowska}104_{, C. Chen}63_{, H. Chen}25_{, S. Chen}33c_{, X. Chen}173_{, Y. Chen}35_, Y. Cheng31_{, A. Cheplakov}64_{, R. Cherkaoui El Moursli}135e_{, V. Chernyatin}25_{, E. Cheu}7_{, S.L. Cheung}158_,

L. Chevalier136_{, G. Chiefari}102a,102b_{, L. Chikovani}51a,∗_{, J.T. Childers}30_{, A. Chilingarov}71_{, G. Chiodini}72a_,

A.S. Chisholm18_{, R.T. Chislett}77_{, A. Chitan}26a_{, M.V. Chizhov}64_{, G. Choudalakis}31_{, S. Chouridou}9_{, I.A. Christidi}77_, A. Christov48_{, D. Chromek-Burckhart}30_{, M.L. Chu}151_{, J. Chudoba}125_{, G. Ciapetti}132a,132b_{, A.K. Ciftci}4a_{, R. Ciftci}4a_,

D. Cinca34_{, V. Cindro}74_{, A. Ciocio}15_{, M. Cirilli}87_{, P. Cirkovic}13b_{, Z.H. Citron}172_{, M. Citterio}89a_{, M. Ciubancan}26a_, A. Clark49_{, P.J. Clark}46_{, R.N. Clarke}15_{, W. Cleland}123_{, J.C. Clemens}83_{, B. Clement}55_{, C. Clement}146a,146b_,

Y. Coadou83_{, M. Cobal}164a,164c_{, A. Coccaro}138_{, J. Cochran}63_{, L. Coffey}23_{, J.G. Cogan}143_{, J. Coggeshall}165_{, J. Colas}5_,

S. Cole106_{, A.P. Colijn}105_{, N.J. Collins}18_{, C. Collins-Tooth}53_{, J. Collot}55_{, T. Colombo}119a,119b_{, G. Colon}84_,

G. Compostella99_{, P. Conde Mui˜no}124a_{, E. Coniavitis}166_{, M.C. Conidi}12_{, S.M. Consonni}89a,89b_{, V. Consorti}48_, S. Constantinescu26a_{, C. Conta}119a,119b_{, G. Conti}57_{, F. Conventi}102a,k_{, M. Cooke}15_{, B.D. Cooper}77_,

A.M. Cooper-Sarkar118_{, K. Copic}15_{, T. Cornelissen}175_{, M. Corradi}20a_{, F. Corriveau}85,l_{, A. Cortes-Gonzalez}165_, G. Cortiana99_{, G. Costa}89a_{, M.J. Costa}167_{, D. Costanzo}139_{, D. Cˆot´e}30_{, G. Cottin}32a_{, L. Courneyea}169_{, G. Cowan}76_, B.E. Cox82_{, K. Cranmer}108_{, F. Crescioli}78_{, M. Cristinziani}21_{, G. Crosetti}37a,37b_{, S. Cr´ep´e-Renaudin}55_,

C.-M. Cuciuc26a_{, C. Cuenca Almenar}176_{, T. Cuhadar Donszelmann}139_{, J. Cummings}176_{, M. Curatolo}47_{, C.J. Curtis}18_, C. Cuthbert150_{, P. Cwetanski}60_{, H. Czirr}141_{, P. Czodrowski}44_{, Z. Czyczula}176_{, S. D’Auria}53_{, M. D’Onofrio}73_, A. D’Orazio132a,132b_{, M.J. Da Cunha Sargedas De Sousa}124a_{, C. Da Via}82_{, W. Dabrowski}38_{, A. Dafinca}118_{, T. Dai}87_, F. Dallaire93_{, C. Dallapiccola}84_{, M. Dam}36_{, D.S. Damiani}137_{, H.O. Danielsson}30_{, V. Dao}104_{, G. Darbo}50a_,

G.L. Darlea26b_{, J.A. Dassoulas}42_{, W. Davey}21_{, T. Davidek}127_{, N. Davidson}86_{, R. Davidson}71_{, E. Davies}118,d_,

M. Davies93_{, O. Davignon}78_{, A.R. Davison}77_{, Y. Davygora}58a_{, E. Dawe}142_{, I. Dawson}139_,

R.K. Daya-Ishmukhametova23_{, K. De}8_{, R. de Asmundis}102a_{, S. De Castro}20a,20b_{, S. De Cecco}78_{, J. de Graat}98_,

N. De Groot104_{, P. de Jong}105_{, C. De La Taille}115_{, H. De la Torre}80_{, F. De Lorenzi}63_{, L. De Nooij}105_{, D. De Pedis}132a_, A. De Salvo132a_{, U. De Sanctis}164a,164c_{, A. De Santo}149_{, J.B. De Vivie De Regie}115_{, G. De Zorzi}132a,132b_,

W.J. Dearnaley71_{, R. Debbe}25_{, C. Debenedetti}46_{, B. Dechenaux}55_{, D.V. Dedovich}64_{, J. Degenhardt}120_{, J. Del Peso}80_, T. Del Prete122a,122b_{, T. Delemontex}55_{, M. Deliyergiyev}74_{, A. Dell’Acqua}30_{, L. Dell’Asta}22_{, M. Della Pietra}102a,k_,

D. della Volpe102a,102b_{, M. Delmastro}5_{, P.A. Delsart}55_{, C. Deluca}105_{, S. Demers}176_{, M. Demichev}64_{, B. Demirkoz}12,m_,

S.P. Denisov128_{, D. Derendarz}39_{, J.E. Derkaoui}135d_{, F. Derue}78_{, P. Dervan}73_{, K. Desch}21_{, E. Devetak}148_, P.O. Deviveiros105_{, A. Dewhurst}129_{, B. DeWilde}148_{, S. Dhaliwal}158_{, R. Dhullipudi}25,n_{, A. Di Ciaccio}133a,133b_, L. Di Ciaccio5_{, C. Di Donato}102a,102b_{, A. Di Girolamo}30_{, B. Di Girolamo}30_{, S. Di Luise}134a,134b_{, A. Di Mattia}152_, B. Di Micco30_{, R. Di Nardo}47_{, A. Di Simone}133a,133b_{, R. Di Sipio}20a,20b_{, M.A. Diaz}32a_{, E.B. Diehl}87_{, J. Dietrich}42_, T.A. Dietzsch58a_{, S. Diglio}86_{, K. Dindar Yagci}40_{, J. Dingfelder}21_{, F. Dinut}26a_{, C. Dionisi}132a,132b_{, P. Dita}26a_{, S. Dita}26a_, F. Dittus30_{, F. Djama}83_{, T. Djobava}51b_{, M.A.B. do Vale}24c_{, A. Do Valle Wemans}124a,o_{, T.K.O. Doan}5_{, M. Dobbs}85_, D. Dobos30_{, E. Dobson}30,p_{, J. Dodd}35_{, C. Doglioni}49_{, T. Doherty}53_{, Y. Doi}65,∗_{, J. Dolejsi}127_{, Z. Dolezal}127_, B.A. Dolgoshein96,∗_{, T. Dohmae}155_{, M. Donadelli}24d_{, J. Donini}34_{, J. Dopke}30_{, A. Doria}102a_{, A. Dos Anjos}173_, A. Dotti122a,122b_{, M.T. Dova}70_{, A.D. Doxiadis}105_{, A.T. Doyle}53_{, N. Dressnandt}120_{, M. Dris}10_{, J. Dubbert}99_{, S. Dube}15_,