Search for top-jet resonances in the lepton + jets channel of t ¯ t + jets events with the ATLAS detector in 4.7 fb

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ATLAS-CONF-2012-096 09July2012

ATLAS NOTE

ATLAS-CONF-2012-096

July 2, 2012

Search for top-jet resonances in the lepton + jets channel of t ¯ t + jets events with the ATLAS detector in 4.7 fb

⁻¹

of pp collisions at √

s = 7 TeV

The ATLAS Collaboration

Abstract

This note presents a search for a heavy new particle

X

produced singly in association with a

t(¯t) quark. The new particle decays to a ¯t(t) quark and a light flavor quark, leading

to a color singlet (triplet) resonance in the ¯

t(t)+ j

system of the

t¯t+

jets signature. The full 2011 ATLAS

pp

collisions dataset from the LHC (4.7 fb

⁻¹

) is used to select

t¯t

events produced in association with jets, in which one of the

W

bosons from the top quark decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation and a new particle with mass below 350 (430) GeV for

W⁰

(color triplet) models is excluded at 95% confidence level, assuming unit right-handed coupling.

c

Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.

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1 Introduction

In the past few decades, remarkable agreement has been shown between predictions of the Standard Model (SM) and experimental measurements in particle physics. The top quark sector is one important place to look for deviations from the SM, as the large top quark mass suggests that it may play a special role in electroweak symmetry breaking. The recent top quark forward-backward asymmetry measure- ments from the Tevatron experiments [1,

2] are among the few discrepancies between SM expectations

and observations. One possible explanation of these asymmetries is a top-flavor-violating process [3–5].

In these models, a heavy new particle

X

is produced in association with a top quark or an anti-top quark (

pp→ Xt/Xt). The massive particle then decays to ¯

¯

tq/tq, leading to a resonance in the ¯t

(or

t) plus jet

system of

t¯t+jets events. Figure1

shows example production diagrams for these new particles. When produced at a

pp

collider such as the LHC, the baryon asymmetry of the beam leads to a resonance pre- dominantly in either

tq

or ¯

tq. Thet(¯t) then decays toW⁺b

(W

⁻b), and this analysis considers the decay

¯ signature of events in which one

W

boson decays leptonically and the other

W

boson decays hadronically.

Two types of

X

resonances are considered: a color singlet

W⁰

model that would give a resonance pre- dominantly in the ¯

tq

system, and a di-quark color triplet (φ) model that would give a resonance largely in the

tq

system. The new resonances are assumed not to be self-conjugate, which then evades limits from searches for same-sign top quarks. The first direct search for such a particle was made at CDF [6], which excluded triplets with masses below 200 GeV and

W⁰

models with masses below 300 GeV, for particles with unit right-handed coupling (g

R

) to

tq. CMS recently performed a search that excluded a newW⁰

with a mass less than 840 GeV [7] for particles with

gR=

2

¹

.

g

t

q

t

W

⁰

t q

(a)

g

t

q

t

φ

t q

(b)

Figure 1: Example production and decay Feynman diagrams for the

W⁰

(a) and

φ

(b) models.

2 Data Selection

The analysis presented here uses the full 7 TeV ATLAS

pp

dataset collected in 2011, corresponding to 4.7

±

0.2 fb

⁻¹

of integrated luminosity [8,

9] delivered by the LHC. The data were collected by the ATLAS

experiment [10], a multi-purpose particle physics detector with cylindrical geometry.

²

The inner detector

1There are several differences between the models in Refs. [3] and [4]. The Lagrangian in the former (used in this note) includes a factor of 1/√

2, and the one in the latter (used by CMS) does not. In addition, Ref. [4] includes additional non- resonant diagrams with cross section that scale asg⁴_R. Such diagrams are not included in Ref. [3].

2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and thez-axis along the beam pipe. Thex-axis points from the IP to the center of the LHC ring, and theyaxis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane,φ

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(ID) system consists of a high-granularity silicon pixel detector and silicon micro-strip tracker, as well as a transition radiation straw tracker. The ID is immersed in a 2 T axial magnetic field, and provides charged particle tracking in the range

|η| <

2.5. Surrounding the ID, electromagnetic calorimetry is provided by barrel and endcap liquid-argon (LAr)/lead accordion calorimeters and LAr/copper sampling calorimeters in the forward regions. Hadronic calorimetry is provided in the barrel by a steel

/

scintillating tile sampling calorimeter, in the endcaps by LAr/copper, and in the forward region by LAr/tungsten sam- pling calorimeters. The muon spectrometer (MS) comprises separate trigger and high-precision tracking chambers measuring the deflection of muons in a magnetic field with a bending integral from 2 to 8 Tm, generated by three superconducting air-core toroid systems. A three-level trigger system is used to select interesting events. The level-1 trigger is implemented in hardware and uses a subset of detector informa- tion to reduce the event rate to a design value of at most 75 kHz. This is followed by two software-based trigger levels, level-2 and the event filter, which together reduce the event rate to a few hundred Hz.

Electrons reconstructed off-line are required to have a shower shape in the electromagnetic calorime- ter consistent with expectation, as well as a good quality track pointing to the cluster in the calorimeter.

Candidate electrons with transverse energy

E_T >

25 GeV are required to pass the “tight” ATLAS elec- tron quality cuts [11], to fall inside a well instrumented region of the detector (|η|

<

2.47, excluding 1.37

<|η|<

1.52), and to be well isolated from other objects in the event. Muons with transverse momentum

p_T >

20 GeV are required to pass standard ATLAS muon quality cuts [12], to be well measured in both the ID and the muon spectrometer, to fall within

|η| <

2.5, and to be isolated from other objects in the event. Events with an electron (muon) are required to have been triggered by an electron (muon) trigger with a threshold of 20 (18) GeV, ensuring that the trigger is fully e

ffi

cient.

Jets are reconstructed in the calorimeter using the anti-k

t

[13] algorithm with a distance parameter of 0.4. Jets are required to satisfy

E_T >

25 GeV and

|η|<

2.5. Events with jets arising from electronic noise bursts and beam backgrounds are rejected [14]. Jets are calibrated to the hadronic energy scale using

p_T

- and

η-dependent corrections derived from collision and test-beam data as well as simulation [15]. Jets

from the decay of heavy flavor hadrons are selected by the multivariate b-tagging algorithm [16] at an operating point with 70% e

ffi

ciency for

b-jets and a mis-tag rate of less than 1% in simulatedt¯t

events.

Neutrinos are inferred from the magnitude of the missing transverse momentum (E

^miss_T

) in the event [17].

The signal region for this analysis requires exactly one charged lepton and five or more jets, including at least one

b-tagged jet. To tag events with a leptonically decaying W

boson, events are required to have

E^miss_T >

30 GeV (E

^miss_T >

20 GeV) in the electron (muon) channel. Additionally, the event must have a transverse mass of the leptonic

W

boson

m^W_T >

30 GeV in the electron channel, or scalar sum

E^miss_T +m^W_T >

60 GeV in the muon channel [18].

3 Simulation Samples

A variety of Monte Carlo generators are used to study and estimate backgrounds. The generated events are processed through the full ATLAS detector simulation [19], based on GEANT4 [20], and include the effect of multiple

pp

interactions per bunch crossing. Events are reweighted so that the distribution of the number of

pp

collisions occurring in addition to the hard scatter process matches that in data.

The

t¯t

background is nominally modeled with MC@NLO v4.01 [21] interfaced to HERWIG v6.520 [22]

and JIMMY v4.31 [23]. An additional

t¯t

sample modeled with MC@NLO interfaced to PYTHIA v6.425 [24] is used to study potential systematic uncertainties. Other

t¯t

samples use POWHEG [25] in- terfaced either to PYTHIA or HERWIG, as well as ACERMC v3.8 [26] The background from production of single

W

bosons in association with extra jets is modeled by the ALPGEN v2.13 [27] generator inter- faced to HERWIG. The MLM matching scheme [28] is used to form inclusive

W

boson

+

jets samples.

Duplicate heavy flavor events in both the hard scatter and showering are removed, and the heavy flavor

contributions are reweighted based on data-driven estimates [29]. Diboson events are generated using

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HERWIG. Single-top quark events are modeled with MC@NLO, interfaced with JIMMY for the parton showering in the

s-channel andWt

channel, and ACERMC v3.8 in the

t-channel. The small background

in which multi-jet processes fake a prompt lepton, is modeled from a data-driven matrix method [30].

In determining the expected event yields, the

t¯t

cross section is normalized to approximate next-to-next- to-leading order QCD calculations of 167

⁺₋₁₈¹⁷

pb for a top quark mass of 172.5 GeV [31,

32], and the

total

W+jets background is normalized to inclusive next-to-next-to-leading-order predictions [33]. Sig-

nal events are produced with MADGRAPH v5.1.3.16 [34] and interfaced to PYTHIA v6.425. Leading order cross sections from Ref. [3] are used for the predicted signal normalization.

4 Fitting Method

Events are reconstructed with a kinematic fitting algorithm [35] that utilizes knowledge of the over- constrained

t¯t

system to assign jets to partons. The two top quark masses are each constrained to 172.5 GeV in the fit by a penalty in the likelihood computed from variations from this nominal value and the natural top quark width of 1.5 GeV. The two

W

boson masses are similarly constrained to 80.4 GeV within a width of 2.1 GeV. This also allows the z-component of the neutrino from the leptonically de- caying

W

boson to be computed. Both solutions from the quadratic ambiguity of this computation are tested. Charged lepton, neutrino and jet energies and momenta are constrained in the fit by resolution transfer functions derived from simulated

t¯t

events. The full shape of the transfer function is used. All assignments of any four jets to partons from the

t¯t

decay are tested, and the assignment with the largest likelihood output for the

t¯t

hypothesis is selected. After the assignment is selected, the originally mea- sured jet and lepton momenta and

E^miss_T

are used. The remaining jets not associated to the

t¯t

partons are included to form

mt j

and

mt j¯

masses, where the charge of the lepton is used to infer which is the top candidate and which is the anti-top candidate. All combinations of extra jets with the top and anti-top quark candidates are considered, and the pairings that give the largest

m_{t j}

and

m_¯_{t j}

masses are used. In this way, the same extra jet can (but does not necessarily have to) be used to form

mt j

and

mt j¯

. These two masses are used as observables for the search.

5 Control Regions

Several control regions are tested to ensure good modeling and understanding of the backgrounds before the signal region is examined. The dominant

t¯t

background is tested in a region with exactly four jets (including at least one

b-tagged jet) but no fifth jet. The rejection of events with more than four jets

ensures small signal contamination. A second

t¯t

control region is defined by events with exactly four jets with

p_T

above 25 GeV, one of which must be

b-tagged, and exactly one jet withp_T

between 20 and 25 GeV. Signal contamination is further reduced by requiring that the

∆R ≡ p

(∆

η)²+

(∆

φ)²

between the fifth jet and both the reconstructed top and anti-top quarks is greater than

π/2. Figure 2

shows distributions in the five-jet control region. Good agreement is observed between data and prediction.

The second major background, production of single

W

bosons in association with extra jets, is tested in

a region with five or more jets, vetoing events with

b-tags. The requirement of zerob-tagged jets reduces

both signal and

t¯t

contamination. The distributions shown in Figure

3

show good agreement between

data and prediction within uncertainties. Table

1

summarizes the expected and observed yields in the

control regions.

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500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104

L dt = 4.7 fb-1

∫

Data t t W + jets Other Singlet 300 GeV Triplet 400 GeV

ATLAS Preliminary

[GeV]

mtj

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(a)

500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104

L dt = 4.7 fb-1

∫

[GeV]

j

mt

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(b)

Figure 2: Expected and observed distributions of

mt j

(a) and

mt j¯

(b) in the five-jet

t¯t

control region. The

example signal distributions assume unit coupling for the new physics process. The total uncertainty

shown on the ratio includes both statistical and systematic effects.

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500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104 ^Data

∫

L dt = 4.7 fb^-1

t t W + jets Other Singlet 300 GeV Triplet 400 GeV

[GeV]

mtj

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(a)

500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104 ^Data

∫

L dt = 4.7 fb^-1

[GeV]

j

mt

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(b)

Figure 3: Expected and observed distribution of

mt j

(a) and

mt j¯

(b) in the

W+jets control region. The

example signal distributions assume unit coupling for the new physics process. The total uncertainty

shown on the ratio includes both statistical and systematic e

ff

ects.

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Table 1: Expected and observed yields in the four control regions (CR). The preselection control region requires at least four jets and no tag requirement. The

W

boson+jets control region requires more than four jets and zero

b-tags. The four-jet t¯t

region requires a

b-tagged jet and exactly four jets. The five

jet

t¯t

control region requires four jets, at least one

b-tagged jet, and exactly one soft jet between 20 and

25 GeV. The errors include all systematic uncertainties. The total background includes

t¯t, W

boson

+

jets and the other smaller backgrounds, single top production, diboson production and multi-jet events, which are not tabulated separately here.

Preselection CR

W+

jets CR four-jet

t¯t

CR five-jet

t¯t

CR

t¯t

50000

±4700

2000

±400

19000

±600

2100

±200

W+

jets 46000

±14000

7000

±2900

3800

±800

360

±170

Total Background 116000

±

21000 12000

±

3600 26000

±

1300 2900

±

440 Observed 110933 11858 26197 2736

300 GeV

W⁰

10400

±500

690

±80

2200

±300

300

±60

400 GeV

φ

6100

±200

430

±60

1100

±100

200

±20

6 Systematic Uncertainties

A variety of potential systematic e

ff

ects are evaluated on predictions for both the signal and background rates and shapes. The dominant systematic e

ff

ects of jet energy scale [15] and resolution [36] lead to uncertainties of up to 10% on the total background rate and up to 21% on the total signal expectation, depending on the signal mass. The other dominant systematic uncertainty, from di

ff

erences of the

b-

tagging e

ffi

ciency in simulation and data, leads to uncertainties of roughly 16% on both the signal and background rates. Effects due to lepton trigger uncertainties and ID efficiency as well as energy scale and resolution are assessed using

Z →ee

and

Z →µµ

data, which lead to systematic uncertainties of a few percent. Other potential systematic e

ff

ects considered are the size of the small multi-jet background (assigned 100% uncertainty),

t¯t

generator systematics (evaluated by comparing different results using the MC@NLO and POWHEG generators, 1-10%),

t¯t

showering and fragmentation systematics (evaluated by comparing samples using both PYTHIA and HERWIG, 1-6%), an uncertainty on the total integrated luminosity (3.9%) [8,

9], and the amount of QCD radiation for the signal and thet¯t

background (approx- imately 10%, evaluated using ACERMC). Total cross section uncertainties of 10% (55%) are used for the

t¯t

(W

+

jets) backgrounds.

7 Results

Figure

4

shows the expected and observed

m_{t j}

and

mt j¯

distributions in the signal region before any additional cuts. The data are found to be consistent with expectation. Expected and observed upper limits on the signal cross section are computed at discrete mass points as follows. For each signal mass point under consideration, a signal region is defined in the

m_{t j}

-m

t j¯

plane. When setting limits for the

W⁰

(φ) model, the

m_{t j}

(m

t j¯

) window is significantly wider than the

m_{t j}_¯

(m

t j

) window to account for the fact that the resonance is predominantly in the

mt j¯

(m

t j

) system. The windows are optimized to maximize sensitivity, accounting for the full e

ff

ect of systematic uncertainties. For each mass window, 95% C.L.

upper limits on the signal cross section are computed using a single bin frequentist CLs method [

37,38].

No further shape information is used. Table

2

shows the expected and observed event yields in several of the signal region windows. The data are consistent with the background, and no excess is observed.

Expected and observed 95% C.L. lower limits on the signal mass are derived, assuming a coupling of

gR =

1 and

gR =

2, and are shown in Figure

5. Assuming couplings ofg²_R

, the exclusion in the mass-

coupling plane is shown in Figure

6.

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500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104

L dt = 4.7 fb-1

∫

ATLAS Preliminary Preliminary

[GeV]

mtj

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(a)

500 1000 1500 2000 2500

Events / 175 GeV

1 10 102

103

104 ^Data

∫

L dt = 4.7 fb^-1

[GeV]

j

mt

500 1000 1500 2000 2500

(data-SM)/SM

-1 -0.5

0 0.5

1 Uncertainty

(b)

Figure 4: Expected and observed distributions of

mt j

(a) and

mt j¯

(b) in the signal region. The example signal distributions assume unit coupling for the new physics process. No mass window cuts are applied.

The uncertainty band includes both statistical and systematic uncertainties.

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Singlet Resonance Mass [GeV]

200 400 600 800 1000 1200 1400 1600 1800 2000

95% C.L. Cross-Section Limit [pb]

10-1

1 10 102

R = 1

Theory, g σ

R = 2

Theory, g σ

Observed limit Expected limit

σ

± 1 σ

± 2

=1

R

CDF excluded, g

ATLAS

= 7 TeV s

L dt = 4.7 fb-1

∫

Preliminary

(a)

Triplet Resonance Mass [GeV]

200 400 600 800 1000 1200 1400 1600 1800 2000

95% C.L. Cross-Section Limit [pb]

10-1

1 10 102

R = 1

Theory, g σ

R = 2

Theory, g σ

Observed limit Expected limit

σ

± 1 σ

± 2

R=1 CDF excluded, g

ATLAS

= 7 TeV s

L dt = 4.7 fb-1

∫

Preliminary

(b)

Figure 5: Expected and observed 95% C.L. upper limits on the

W⁰

(a) and

φ

(b) model cross sections.

The green and yellow bands show the one and two standard deviation variations on the expected limits.

The solid black line is the observed limit. The blue line shows the theoretical prediction for the cross section assuming unit coupling, and the dashed brown line shows the theoretical prediction at

gR =

2. The CDF result comes from [6].

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Singlet Resonance Mass [GeV]

500 1000 1500 2000 2500

RCoupling g

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

95% C.L. exclusion Expected exclusion CDF exclusion

t

σt

and Observed AFB

ATLAS

Preliminary

L dt = 4.7 fb-1

∫

= 7 TeV s

(a)

Triplet Resonance Mass [GeV]

500 1000 1500 2000 2500

RCoupling g

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

95% C.L. exclusion Expected exclusion CDF exclusion

t

σt

and Observed AFB

ATLAS

Preliminary

L dt = 4.7 fb-1

∫

= 7 TeV s

(b)

Figure 6: Expected and observed 95% C.L. upper limits on the

W⁰

(a) and

φ

(b) model cross sections

assuming coupling scales with

g²_R

. The blue hashed area shows regions of parameter space that improve

upon the goodness of fit to Tevatron over the SM expectation [39]. The CDF result comes from [6].

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Table 2: Expected and observed yields in different signal regions. The errors include all systematic uncertainties. Total refers to the total expected background, including

t¯t,W

boson

+

jets and the other smaller backgrounds, single top production, diboson production and multi-jet events. Signal window e

ff

refers to the efficiency for the signal to fall inside the optimized 2-d mass window. Signal region yield refers to the yield in the signal region after the mass window cuts at each signal point. Signal

σ

refers to the leading order signal cross section before any

t¯t

branching fraction or mass window cuts.

Entire signal region 300 GeVW⁰ 600 GeVW⁰ 400 GeVφ 800 GeVφ mt jwindow [GeV] 344<mt j<494 566<mt j<904 401<mt j<455 766<mt j<819 m¯t jwindow [GeV] 292<mt j¯ <339 549<mt j¯ <650 371<mt j¯ <608 628<mt j¯ <973

Signal window eff 7.5% 9.9% 11.9% 5.7%

t¯t 18000±3000 740±160 270±60 660±150 60±10

W+jets 1700±560 60±30 30±20 80±40 8±5

Total 22000±3700 820±190 320±80 780±180 70±20

Observed 22731 970 343 923 77

Signal region yield 416±89 70±17 413±103 20.1±6

Signalσ 14.0 pb 1.30 pb 7.9 pb 0.67 pb

8 Conclusions

In conclusion, a search for a heavy new particle

X

in the

t j

or ¯

t j

system of

t¯t

plus extra jet events with the

ATLAS detector has been presented. Such new particles could explain the anomalous forward-backward

asymmetry observed in top quark pair production at the Tevatron. The full 2011 ATLAS

pp

dataset

(4.7 fb

⁻¹

) was used in the search. Assuming a unit coupling, the expected 95% C.L. lower limit on the

mass of the new particle is 450 (700) GeV in the

W⁰

(φ) model. No significant excess of data above SM

expectation was observed, and 95% C.L. lower limits of 350 (430) GeV for the

W⁰

(φ) model were set. At

gR =

2, the limits are 1.10 (1.45) TeV for the

W⁰

(φ) model, with expected limits of 0.90 (1.30) TeV. Most

of the regions of phase space that explain the Tevatron forward background asymmetry were excluded.

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04 [36] ATLAS Collaboration, ATLAS-CONF-2010-054,

[37] W. Fisher, FERMILAB-TM-2386-E.

[38] T. Junk, Nucl. Instrum. Meth.

A434, 435-443, (1999), arXiv:hep-ex:9902006.

[39] This region simultaneously satisfies the observed high-m

tt¯AF B

, low-m

t¯tAF B

and

σt¯t

observed at

the Tevatron. Mathematically it is defined as the region with

χ² <

2.8, where

χ²

is defined in

Equation 22 in M. Gresham et al., Phys. Rev.

D85

(2012) 014022, arXiv:1107.4364. The

χ²