Search for a dijet resonance produced in association with a leptonically decaying W or Z boson with the ATLAS detector at √

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ATLAS-CONF-2013-074 18July2013

ATLAS NOTE

ATLAS-CONF-2013-074

July 16, 2013

Search for a dijet resonance produced in association with a leptonically decaying W or Z boson with the ATLAS detector at √

s = 8 TeV

The ATLAS Collaboration

Abstract

A search for a dijet resonance with an invariant mass in the range between 130 and 300 GeV is performed in the processes pp

→

W j j+X with W

→ℓ^±ν

and pp

→

Z j j+X with Z

→ ℓ⁺ℓ⁻

(ℓ

=

e, µ). The data used correspond to 20.3 fb

⁻¹

of pp collision data recorded at

√

s

=8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The results

are interpreted in terms of constraints on the Low Scale Technicolor model. No significant deviation from the Standard Model background prediction is observed. Upper limits on the production cross section times branching fraction are set for a hypothetical technipion produced in association with a W or Z boson from the decay of a technirho particle.

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

Resonant production of two high transverse momentum (p

_T

) jets in association with a Standard Model (SM) electroweak gauge boson is an interesting signature to probe the existence of various new physics scenarios beyond the SM, for example extra dimensions, technicolor and little Higgs models. In many cases the V j j state (V denoting a W or Z boson and j a jet) could be produced from the decay of a new massive particle. Resonant diboson (VV) production, e.g. from the decay of a graviton in the scenario of extra dimensions with warped bulk geometry, results in the V j j final state when one of the gauge bosons decays hadronically. In view of the recent discovery of a Higgs particle, the V j j search is also expected to be sensitive to the V H final state, including additional Higgs-like particles in, e.g. two Higgs doublet models.

In this report, a search for resonant dijet production in W j j

→ℓν

j j and Z j j

→ℓℓ

j j (ℓ

=

e, µ) events collected with the ATLAS detector at the LHC using pp collision data at

√

s

=

8 TeV corresponding to an integrated luminosity of 20.3 fb

⁻¹

is described. The analysis requires the presence of a leptonically decaying W or Z boson to suppress multijet background.

The Low Scale Technicolor (LSTC) model is used as the benchmark to interpret the analysis result.

In the LSTC model a spin 1 vector particle called technirho (ρ

T

) can decay into a lighter technimeson called technipion (π

_T

) and a SM W or Z boson (depending on the electric charge of the

ρ_T

) if the

ρ_T

mass is larger than the sum of the

π_T

and gauge boson masses [1, 2]. The CDF collaboration reported [3] a significant excess of events with a dijet invariant mass of about 160 GeV in W

+

two jet events recorded in ¯pp collisions at the Fermilab Tevatron. Prompted by this result, similar searches were performed by the D0 collaboration at the Tevatron [4] and the CMS collaboration [5] in pp collisions at

√

s

=

7 TeV at the CERN Large Hadron Collider, neither of which confirmed the CDF observation. Recently, CDF updated the analysis with the full Tevatron Run II data and reported that the data agrees well with the SM background prediction [6].

2 ATLAS Detector

The ATLAS experiment [7] is a multipurpose particle detector with forward-backward symmetric cylin- drical geometry

^a

covering the pseudorapidity range

|η| <

2.5 for measurement of charged particles and

|η| <

4.9 for all particles except for neutrinos. The inner tracking detector (ID) consists of a silicon pixel detector, a silicon microstrip detector, and a straw-tube transition radiation tracker. The ID is surrounded by a thin superconducting solenoid, providing a 2 T magnetic field, and then by a high- granularity liquid-argon (LAr) sampling electromagnetic (EM) calorimeter. A steel and scintillator tile calorimeter is located outside the LAr EM calorimeter and provides hadronic coverage in the central rapidity range. The end-cap and forward regions are instrumented with LAr calorimetry with copper or tungsten as the absorber for electromagnetic and hadronic measurements. The muon spectrometer (MS) surrounds the calorimeters and consists of three large superconducting toroid systems (each with eight coils) together with multiple layers of trigger and tracking chambers, providing precision track measurement for

|η|<

2.7 and triggering for

|η|<

2.4.

aATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis coinciding with the axis of the beam pipe. The x-axis points from the IP to the center of the LHC ring, and they-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane,φbeing the azimuthal angle around the beam pipe. The pseudorapidityηis defined in terms of the polar angleθasη=−ln[tan(θ/2)].

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3 Data and Simulated Event Samples

The analysis uses 20.3 fb

⁻¹

of data collected with the ATLAS detector in pp collisions at

√

s

=

8 TeV.

Events are selected with single-lepton triggers with two different p

_T

thresholds, 24 and 36 GeV for muons, and 24 and 60 GeV for electrons. The lower threshold trigger requires a candidate lepton to be isolated to reduce trigger rates. Inefficiency associated with the isolation requirement is recovered by adding the higher threshold trigger at high p

_T

without isolation requirement. For the Z j j analysis, a logical OR of the single-lepton triggers and dilepton triggers that require the presence of at least two same-flavor leptons with p

_T

thresholds of 12 (13) GeV for electrons (muons) is taken to increase signal acceptance. Stringent data quality requirements are imposed.

Signal and most background processes are modelled using Monte Carlo (MC) simulations. The multijet background is estimated directly from data. The LSTC signal events of

ρ^±_T^,0→

W

^±π^0,_T^∓

and

ρ^±_T →

Zπ

^±_T

are generated using PYTHIA6 [8] with MSTW2008LO [9] for the parton distribution functions (PDFs), and processed with PYTHIA8 [10] by transferring events in Les Houches Event Format [11]

for parton showering and hadronization. This production procedure is used because the LSTC model is not implemented in PYTHIA8. These signal samples are produced for

ρ_T

with masses between 250 and 500 GeV and

π_T

between 130 and 300 GeV, maintaining the mass relation m

ρT <

2m

πT

, where m

ρT

(m

πT

) is the

ρ_T

(π

_T

) mass, and the branching ratio to W/Zπ

_T

is close to 100%. The width of the

ρ_T

is

<

1 GeV and therefore negligible compared to the mass resolution (from 20 GeV raising for higher masses).

Among background processes, vector boson production in association with jets (W/Z+jets) represents the dominant contribution to the background. The W/Z+jets events are generated using SHERPA 1.4 [12]

with CT10 [13] PDFs. The cross sections are scaled to the next-to-leading order (NLO) inclusive cross sections. Samples of t¯t events are produced using MC@NLO v3.41 [14, 15] with CT10 [13] PDFs interfaced to HERWIG [16] for parton showering and JIMMY [17] for the underlying event. The cross section is scaled to the approximate next-to-next-to-leading order (NNLO) [18]. Single top events are generated using MC@NLO [19, 20] or AcerMC [21, 22, 23, 24]. Diboson events are generated using HERWIG and normalized to the NLO cross sections computed by MCFM [25].

The response of the ATLAS detector is modelled [26] using either a GEANT4 simulation [27] for background samples or for signal samples, a faster simulation that combines the GEANT4 simulation for the ID tracking and muon reconstruction with a fast simulation of calorimeter shower development. For both detector simulations the MC samples are processed using the same reconstruction chain as used for collision data. The simulated samples include a realistic modelling of the pile-up conditions observed in the data.

4 Event Selection

Events selected by the triggers described in Sec. 3 are required to contain a primary pp collision vertex reconstructed from at least three charged-particle tracks, each with p

_T >

400 MeV. In events where multiple vertices are reconstructed, the vertex with the largest

Σp²_T

of the associated tracks is defined as the primary vertex.

Electron candidates are formed by matching electromagnetic calorimeter energy clusters to tracks

reconstructed in the inner detector [28]. Tighter matching criteria are used for electrons in the W j j

channel than the Z j j channel to reduce background from jets faking electrons. The transverse energy

(E

_T

), calculated from the cluster energy and the track direction, must be greater than certain thresholds

chosen separately for the V j j channels and the leading/sub-leading leptons in the Z j j channel as de-

scribed below. Candidates are accepted if they lie in the region

|η|<

2.47, excluding the transition region

(1.37

< |η| <

1.52) between the barrel and endcap electromagnetic calorimeters. The longitudinal and

transverse shapes of the energy cluster must be consistent with those of an electromagnetic shower. For

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the electron-candidate track, the significance of the transverse impact parameter (d

0

) measured relative to the primary vertex must be smaller than 6. The longitudinal impact parameter (z

₀

), measured along the z direction with respect to the primary vertex, is required not to exceed 1 mm. To ensure that the elec- trons are isolated from hadronic activity in events, both calorimetric and tracking isolation criteria are applied to the candidate electrons. The total calorimeter E

_T

in a cone of

∆R=

0.2

^b

around the electron candidate, excluding the E

T

of the candidate itself, must be less than 14% of the electron E

T

. The scalar sum of the p

T

of tracks (with p

T >

1 GeV) within

∆R =

0.2 around the electron candidate is required to be less than 13% of the electron p

_T

value. The E

_T

of the calorimeter isolation cone is corrected for contributions from electron energy leakage and additional energy deposited by pile-up interactions.

Muon candidates are identified [29] by matching tracks reconstructed in the muon spectrometer to tracks reconstructed in the inner detector. Muon candidates must satisfy p

_T >

p

^min_T

and

|η|<

2.4, where p

^min_T

takes the same threshold values as for the electrons. The muon candidates must be consistent with originating from the primary vertex in order to reject muons from cosmic ray interactions and background from heavy flavor decays. Specifically, the d

₀

significance and

|

z

₀|

of the muon track are required to be less than 3 and 1 mm, respectively. To reduce misidentification and improve the muon momentum resolution, requirements on the minimum number of hits in the ID detectors are applied to the muon tracks. The selected muons are ensured to be isolated by requiring the scalar sum of p

_T

of tracks (total calorimeter E

_T

) within a cone of

∆R=

0.2 around the muon candidate to be less than 15% (14%) of the muon p

T

.

Jets are reconstructed from three dimensional topological clusters [30], defined at the electromag- netic energy scale, using the anti-k

_t

algorithm [31] with a distance parameter R

=

0.4. The reconstructed jets are then calibrated by subtracting pile-up contributions to the jet and applying a p

T

and

η

dependent correction factor derived from simulation [32]. The calibrated energy scale corresponds to that for jets reconstructed from the final state particles (including muons from hadron decays) before detector simu- lation. As a last step of the calibration, the jet energy is further corrected based on in-situ measurement to reduce the uncertainty on the jet energy scale.

Jets likely to have been produced in pile-up interactions are identified using the Jet Vertex Fraction (JVF). This variable is defined as the ratio of the scalar sum of p

T

of tracks associated with the jet and originating from the primary vertex to the scalar sum of p

_T

of all tracks associated with the jet.

For jets with p

_T <

50 GeV and

|η| <

2.4, a jet is considered likely to have originated from pile-up interactions if the JVF value is less than 0.5. Such jets are rejected. Jets are further required to pass quality criteria to suppress contributions from detector noise and beam related backgrounds, and to lie at a distance

∆R >

0.3 from selected electrons to avoid double counting the electron as a jet. Only jets having p

T >

25 GeV within the pseudorapidity range

|η|<

2.8 are considered.

The missing transverse momentum, E

^miss_T

, is estimated from reconstructed electrons with

|η|<

2.47, muons with

|η| <

2.7, jets with

|η| <

4.9, and clusters of energy in the calorimeter not included in reconstructed objects with

|η| <

4.5 [33]. The clusters are calibrated to the electromagnetic or hadronic energy scale according to cluster classification. The expected energy deposit of identified muons in the calorimeter is subtracted from the cluster energies.

For the Z j j channel, events are required to contain two same-flavor leptons with E

T

(p

T

) above 20 and 10 GeV. The two leptons must have opposite charges in the case of muons, but no such requirement is made for electrons due to their higher charge misidentification. The dilepton invariant mass m

_ℓℓ

is required to be within 8 GeV of the Z boson mass:

|

m

ℓℓ−

m

Z| <

8 GeV. For the W j j channel, events containing a lepton with E

T

(p

T

) above 25 GeV, E

_T^miss>

30 GeV, and the candidate W boson transverse mass m

^W_T =

q

2p

^ℓ_T

E

^miss_T

(1

−

cos

∆φ) above 40 GeV are selected, where p^ℓ_T

is the lepton p

_T

and

∆φ

is the azimuthal opening angle between the lepton and E

^miss_T

.

bThe angular distance∆R between reconstructed objects is defined as∆R= p

∆φ²+ ∆η².

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Based on kinematic distributions of signal and backgrounds from MC simulations, the following criteria are chosen to define the signal region:

•

p

^{ℓν(ℓℓ)}_T >

50 GeV for the W j j (Z j j) channel, where the p

^{ℓν(ℓℓ)}_T

is the transverse momentum of the lepton and E

^miss_T

(dilepton) system,

•

at least two jets with p

T >

30 GeV,

• |∆η_jj| <

1.75 and

|∆φ_jj| >

1.6, where

∆η_jj

and

∆φ_jj

are the differences in pseudorapidity and azimuthal angle, respectively, between the two jets

^c

,

•

115

<

m

_{j j} <

300 GeV.

After having applied these selections to the data, the dijet resonance signal is looked for in the invariant mass m

_{j j}

of the two jets.

5 Background Estimation

The estimation of background contributions is performed using the combination of theoretical predictions and estimation from data. The simulated MC samples and theoretical calculations of the cross sections are used for backgrounds containing prompt leptons from the primary vertex, such as t¯t, single-top and diboson processes. A technique based on fits to kinematic distributions using templates derived from the data is used to estimate the multijet background, in which a jet is misidentified as a lepton. The W

+

jets (Z

+

jets) process is expected to dominate the data samples in the signal region for the W j j (Z j j) channel. The fits used to estimate the multijet background determine the normalization of W

+

jets and Z

+

jets contributions simultaneously, assuming that the shapes of kinematic distributions used in the fit are properly modelled by the W/Z

+

jets MC samples. This assumption is validated using control regions orthogonal to the signal region.

For the data-driven multijet background determination, the lepton criteria are modified. For the muon channel, the control sample is constructed by inverting the transverse impact parameter requirement to

|

d

0/σ(d0

)

| >

3. The muon track still must be within 1 mm of the primary vertex in z. This sample is composed of muons that do not originate from the primary vertex, as expected for muons produced from heavy-flavor decays in jets. For the electron channel, the electron identification criteria are loosened and the selected electrons are required to fail the standard, tighter criteria to form the control sample. The kinematic variables used in the fits are chosen to be E

^miss_T

in the range of E

^miss_T <

200 GeV for the W j j channel, and the dilepton invariant mass m

_ℓℓ

for the Z j j channel. For each kinematic region, a

χ²

fit is performed to the E

^miss_T

or m

ℓℓ

distribution in data using templates of multijet background constructed from the control sample and other backgrounds obtained from MC simulations, in order to determine the normalizations of multijet and W/Z

+

jets backgrounds.

5.1 Control Regions

For W/Z

+

jets background modelling, two control regions with different boson p

T

cuts are defined.

W/Z Inclusive Control Region

The W/Z inclusive control regions are defined requiring at least two jets with p

_T >

25 GeV and either a W or a Z boson candidate (see Sec. 4). Although they include the signal region, events in these control regions are completely dominated by background events since no p

^ℓν_T

or p

^ℓℓ_T

cut is applied. The

cIn the model we are considering the jets from theπTtend to be back-to-back.

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[GeV]

mll

40 60 80 100 120 140 160 180 200

Events / 4 GeV

103

104

105

106 Data

W/Z+jets t WW/WZ, t Multijet

L dt = 20.3 fb-1

∫

s = 8 TeV

ATLAS Preliminary

2 jets

≥ ee +

→ Z

(a) ee j j channel

[GeV]

mll

40 60 80 100 120 140 160 180 200

Events / 4 GeV

103

104

105

106 Data

W/Z+jets t WW/WZ, t Multijet

L dt = 20.3 fb-1

∫

_s_{= 8 TeV}

2 jets

≥ µ + µ

→ Z

(b)µµj j channel

Figure 1: (a) Dielectron and (b) dimuon invariant mass distributions from the data, fitted with the two components of non-multijet background templates obtained from MC simulations and the multijet back- ground template obtained from data using the criteria described in the text, in the Z j j channel. The red arrows show the Z-mass window cuts applied to the m

ℓℓ

spectra.

signal contamination is expected to be less than 0.1%. For both the W and Z inclusive control regions, the transverse momentum spectrum of gauge bosons reconstructed from the lepton

+

E

_T^miss

or dilepton system is harder in W/Z

+

jets MC simulation than in data, which would result in an overestimation of this background at high p

_T

. To correct for this effect, after subtracting non W/Z

+

jets contributions, the ratio of data to MC distributions (electron and muon channels combined) of boson p

T

is fit to a polynominal function to parametrize the p

T

dependence. The parametrized ratio is applied to W/Z

+

jets MC events as a function of boson p

_T

as an event weight. The systematic uncertainty associated with this event weighting is extracted using different functions to parametrize the ratio, but is found to be negligible in the signal region.

Figure 1 shows the result of a fit to the m

_ℓℓ

distributions in the inclusive Z

+

jets control region (prior to the m

ℓℓ

mass window requirement). The multijet background in the Z j j channel is negligible after the m

_ℓℓ

window cut, and the normalization scale factor for Z

+

jets, defined as the ratio of the yield of Z

+

jets background that provides a best fit to the nominal yield from theoretical calculation, is found to be 1.05

±

0.04 for both electron and muon channels. Figure 2 shows the fit to the E

^miss_T

spectra for the inclusive W

+

jets control regions (prior to the E

_T^miss

cut). The multijet background is estimated to be about 11% (7%) for the electron (muon) channel at E

_T^miss >

30 GeV. A W

+

jets normalization scale factor of 0.98

±

0.03 is obtained.

Figures 3 and 4 show the reconstructed boson p

T

, dijet system transverse momentum and dijet mass distributions in these control regions after the boson p

_T

weighting described above has been applied to the W/Z

+

jets MC samples.

W/Z High pTControl Regions

The W/Z high p

_T

control regions are defined requiring p

^{ℓν(ℓℓ)}_T >

50 GeV,

∆φ_jj>

1.6 radian and

∆η_jj<

1.5 but excluding the signal region of 115

<

m

_{j j} <

300 GeV. These control regions are kinematically closer

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[GeV]

miss

ET

0 20 40 60 80 100 120 140 160 180 200

Events / 4 GeV

0 50 100 150 200 250 300 350 400

103

×

Data W/Z+jets

t WW/WZ, t Multijet L dt = 20.3 fb-1

∫

s = 8 TeV ATLAS Preliminary

2 jets

≥ ν +

→ e W

(a) eνj j channel

[GeV]

miss

ET

0 20 40 60 80 100 120 140 160 180 200

Events / 4 GeV

0 50 100 150 200 250 300

103

×

Data W/Z+jets

t WW/WZ, t Multijet L dt = 20.3 fb-1

∫

s = 8 TeV ATLAS Preliminary

2 jets

≥ ν + µ

→ W

(b)µνj j channel

Figure 2: E

^miss_T

distributions from the data, fitted with the two components of non-multijet background templates obtained from MC simulations and the multijet background template obtained from data using the criteria described in the text, in the W j j channel for (a) W

→

eν and (b) W

→µν. The red arrows

show the E

_T^miss

cut applied to the spectra.

to the signal region and therefore more relevant to background modelling for a boosted W/Z boson with recoiling hadronic jets. The boson p

T

weighting derived above is applied to the W/Z

+

jets MC samples in these control regions and other control regions described below. Good agreement is found between data and the expectation in all the main kinematic distributions. Since the cut of p

^{ℓν(ℓℓ)}_T >

50 GeV is applied, it is checked that the m

j j

shape does not significantly change in p

T

regions around the cut and therefore any residual mismodelling in p

^{ℓν(ℓℓ)}_T

does not bias the m

_{j j}

distribution.

t¯t Control Region

The t¯t control sample is defined by events which contain a single lepton satisfying the criteria for the W j j channel and at least four jets with p

_T >

25 GeV. This control region is defined only for the W j j channel as the t¯t background is negligible in the Z j j channel. Good agreement is observed between data and the expectation for all the main kinematic variables.

Multijet Control Region

The multijet control region is defined as a sample of events selected with the criteria of

≥

2 jets with p

T >

25 GeV, E

^miss_T <

25 GeV and m

^W_T >

10 GeV. This control region is defined only for the W j j channel as the multijet background is negligible in the Z j j channel. This region is used to extract both normalization and shape uncertainties on the multijet template used in the final fit, as described in Sec. 6.

6 Systematic Uncertainties

The effect of systematic uncertainties on both the normalization and the shape of the dijet invariant mass

distributions is considered. For each signal or background systematic uncertainty source, a template is

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0 50 100 150 200 250 300

Entries / 10 GeV

100 200 300 400 500 600

103

×

Data W/Z+jets Multijet

+single-t t t

WW/WZ/ZZ L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ν +

→ l W

[GeV]

ν l

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(a) p^ℓν_T

0 50 100 150 200 250 300

Entries / 10 GeV

100 200 300 400 500 600

103

×

+single-t t t

∫s = 8 TeV

2 jets

≥ ν +

→ l W

[GeV]

jj

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(b) p_T^{j j}

0 50 100 150 200 250 300 350 400

Entries / 10 GeV

50 100 150 200 250 300 350 400

103

×

+single-t t t

∫s = 8 TeV

2 jets

≥ ν +

→ l W

[GeV]

mjj

0 50 100 150 200 250 300 350 400

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(c) mj j

Figure 3: (a) Reweighted p

^ℓν_T

, (b) p

_T^{j j}

and (c ) m

jj

distributions for the inclusive W

+

jets control region,

defined by requiring a W boson candidate and at least 2 jets with p

_T >

25 GeV. The uncertainty band

includes the statistical uncertainty on the MC and the JES uncertainty.

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0 50 100 150 200 250 300

Entries / 10 GeV

20 40 60 80 100 120

103

×

Data W/Z+jets

-single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

[GeV]

ll

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(a) p^ll_T

0 50 100 150 200 250 300

Entries / 10 GeV

20 40 60 80 100 120

103

×

Data W/Z+jets

-single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

[GeV]

jj

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(b) p_T^{j j}

0 50 100 150 200 250 300 350 400

Entries / 10 GeV

10000 20000 30000 40000 50000 60000 70000

Data W/Z+jets

-single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

[GeV]

mjj

0 50 100 150 200 250 300 350 400

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(c) mj j

Figure 4: (a) Reweighted p

^ll_T

, (b) p

_T^{j j}

and (c) m

jj

distributions for the Z

+

jets inclusive control region,

defined by requiring a Z boson and at least 2 jets with p

_T >

25 GeV. The uncertainty band includes the

statistical uncertainty on the MC and the JES uncertainty.

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obtained for the m

j j

distribution. These templates are used in the statistical analysis fit described in Sec. 7. Systematic effects from the following sources are taken into account: background normalization, MC generator dependence, background modelling. The latter includes PDF choice, luminosity and object modelling such as jet energy scale and resolution, lepton trigger and reconstruction efficiencies and energy scale. Table 1 shows the summary of the systematic uncertainties.

Background Normalization

The W/Z

+

jets cross section is assigned an uncertainty of 20% [34]. The uncertainty is larger than the uncertainty for the inclusive W/Z cross section since most of the background from this process is composed of W/Z boson plus two or three jets events that are subject to larger cross section uncertainties.

The combined t¯t and single top cross section is assigned an uncertainty of 10% [35]. The diboson processes WW, WZ and ZZ are assigned a theoretical uncertainty of 5%.

MC Generator

The systematic uncertainty due to modelling the production of W/Z

+

jets events is estimated by taking the difference between the SHERPA and ALPGEN predictions. Systematic uncertainties arising from the modelling of the t¯t process are accounted for by comparing the MC@NLO and POWHEG predictions.

PDF

The uncertainty on the t¯t background acceptance due to PDFs is computed using the CT10 eigenvectors and the difference between the CT10 and MSTW2008NLO [9] PDF sets and is 7%. Similarly, the signal acceptance uncertainty is considered by taking the difference between CTEQ6L1 and MSTW2008LO PDF sets and amounts to 5%. The PDF uncertainty on W/Z+jets is accounted for since the two generators used (Alpgen and Sherpa) also use different PDFs.

Object Modelling

The systematic uncertainty from the reconstruction and energy calibration of jets, electrons and muons is propagated through the analysis. The main uncertainty arises from the jet energy scale. The jet energy scale is determined from a combination of test beam, simulation, and in-situ measurements [32]. The jet energy scale uncertainty is split into several independent components: calorimeter response, presence of close-by jets, different quark/gluon composition and response, the b-jet energy scale, impact from in-time and out-of-time event pile-up, and forward jet response. The jet energy scale uncertainty for jets with p

T>

25 GeV and

|η|<

4.5 varies between 1% and 10% depending on jet p

T

and

η. The variations of jet

energy scale and resolution are also propagated to the missing transverse momentum. For electrons and muons, the efficiencies for reconstruction and for the isolation and impact-parameter requirements are studied with W, Z and J/ψ events in data. Differences between data and simulated samples are corrected for in the analysis as scaling factors applied to simulations. The uncertainty on the scaling factors is used to evaluate the systematic uncertainties.

Multijet Background

The systematic uncertainty on the shape and normalization of the multijet background in the W j j channel

is determined from the multijet control region. The differences in event yields and shape of distributions

between the data and MC prediction in the control region are taken as systematic uncertainties. The

uncertainty on the yield amounts to 30%.

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Source

∆σ/σ% for Wjj ∆σ/σ% for Zjj

W

/Z+jets normalization ±

5

±

16 W

/Z jets shape variation ±

2

±

4 Multijet shape and normalization

±

5 N/A

Top normalization

±

4

±

7 Top Modeling

±

3

±

4 Jet energy scale (all samples)

±

10

±

11 Jet energy resolution (all samples)

±

2

±

3 Lepton reconstruction (all samples)

±

1

±

3 PDF (signal)

±

5

±

6 PDF (top)

±

6

±

3 Table 1: Relative statistical and systematic uncertainty contributions (in %) to the total uncertainty on one signal mass point with

π_T=180 GeV. Some of the systematic have been combined.

Luminosity

The uncertainty on the integrated luminosity is 2.8%. It is derived, following the methodology of Ref. [36]. It uses a preliminary calibration of the luminosity scale derived from beam-separation scans performed in November 2012.

7 Results

The dijet resonance signal is searched for by performing binned maximum likelihood fits to the m

j j

distributions in the signal region. The fits use templates derived from MC simulations for the signal and backgrounds except the multijet background for which the data-driven estimate is used. For both the W j j and Z j j channels, the fit is performed simultaneously in the electron and muon channels. Four components are considered in the fit for the W j j channel : signal, W/Z

+

jets, top (t¯t

+

single top) and multijet backgrounds. For the Z j j channel the multijet background is negligible and therefore is not included in the fit.

The binned likelihood function of (µ, θ) is constructed as the product of Poisson probability terms for each category. A signal strength parameter,

µ, multiplies the expected LSTC production cross section

in each bin. The dependence of the signal and background predictions on the systematic uncertainties is described by nuisance parameters,

θ, which are parametrized by Gaussian or log-normal priors. The

expected number of signal and background events in each bin are functions of

θ. Some of systematic

uncertainties, such as the shapes of V+jets, t¯t and multijet backgrounds and the multijet rate, are not allowed to vary in the fit. The effect for those is evaluated generating pseudo experiments based on randomly drawn values of the systematic in question (between

+1σ

and -1σ).

The observed data and the background expectation are in good agreement. Figures 5 and 6 show the fit results in the W j j and Z j j channels, respectively. The electron and muon channel combined plots are shown in Fig. 7. All fitted nuisance parameters are found to have values within the 1σ interval defined by the original uncertainties assigned to the different systematic sources. The nuisance parameters corre- sponding to the normalizations of V

+jets and t¯t are significantly constrained by the fit as expected from

the use of the full m

_jj

spectra in the available data. The smallest p-values are 0.03 and 0.08 for m

_π_T =

240 GeV and m

πT =

300 GeV, for the

ℓν

j j and

ℓℓ

j j final state respectively.

In the absence of any significant data excess, limits are set on the LSTC technirho production

cross section times branching fraction into a vector boson and technipion using the CLs method. This

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0 100 200 300 400

Entries / 10 GeV

20000 40000

+single-t t t

(240) X10 πT

→ W (400) ρT

(160) X10 πT

→ W (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ν +

→ e W

[GeV]

mjj

0 100 200 300 400

Data/Bkg ^0.9

1.0 1.1

(a) eνj j channel

0 100 200 300 400

Entries / 10 GeV

10000 20000 30000 40000

+single-t t t

(240) X10 πT

→ W (400) ρT

(160) X10 πT

→ W (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ν + µ

→ W

[GeV]

mjj

0 100 200 300 400

Data/Bkg ^0.9

1.0 1.1

(b)µνj j channel

Figure 5: Fit to the dijet mass distribution of the expected background in the signal region for the W j j channel with (a) W

→

eν and (b) W

→µν

. The bottom part of the figure shows the ratio of data to fit result.

method employs a log-likelihood ratio LLR

=

2 ln(L

_s+b/L_b

) as test-statistic, where L

_s+b

(L

_b

) is a binned likelihood function formed as a product of Poisson probabilities to observe the data under the sig- nal+background (background-only) hypothesis. Pseudo-experiments are generated for both hypotheses, taking into account per-bin statistical fluctuations that follow Poisson statistics and systematic uncertain- ties sampled using Gaussian or log-normal distributions for the backgrounds and signal.

Figure 8 shows expected and observed 95% confidence level (CL) cross section upper limits as a function of the technipion mass. For the W j j channel, observed and expected lower mass limits for the charged technipion are 180 and 190 GeV respectively. These can be translated into limits on the cross section times branching ratio assuming the mass relation of m

_ρ_T =

3/2

∗

m

_π_T +

55 GeV. For the Z j j channel we see an observed lower mass limit for the neutral technipion of 170 GeV while we did not expect to exclude any mass point. The signal selection efficiencies for each of the technipion mass points are given in Table 2 for the W j j selections to

ρ_T →

Wπ

T

and in Table 3 for the Z j j selections for

ρ_T→

Zπ

_T

.

8 Conclusion

A data-set of 20.3 fb

⁻¹

of pp collision data, collected at

√

s

=

8 TeV using the ATLAS detector, has

been analysed to search for a dijet resonance produced in association with a W or Z boson decaying

leptonically. No significant deviation from the Standard Model background prediction is observed in the

m

_{j j}

spectra in W/Z

+≥

2 jets events. From the results, 95% confidence level upper limits are set on the

cross section times branching fractions for the technirho decaying into a SM gauge boson and a

technipion, as predicted in the Low Scale Technicolor model. This translates into an observed lower

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0 100 200 300 400

Entries / 10 GeV

0 2000 4000

Data W/Z + jets

+single-t t t

(240) X10 πT

→ Z (400) ρT

(160) X10 πT

→ Z (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ee +

→ Z

[GeV]

mjj

0 100 200 300 400

Data/Bkg ^0.9

1.0 1.1

(a) ee j j channel

0 100 200 300 400

Entries / 10 GeV

0 2000 4000 6000

Data W/Z + jets

+single-t t t

(240) X10 πT

→ Z (400) ρT

(160) X10 πT

→ Z (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ µ + µ

→ Z

[GeV]

mjj

0 100 200 300 400

Data/Bkg ^0.9

1.0 1.1

(b)µµj j channel

Figure 6: Fit to the dijet mass distribution of the expected background in the signal region for the Z j j channel for (a) Z

→

ee and (b) Z

→µµ. The bottom part of the figure shows the ratio of data to fit result

with no signal.

Table 2: W j j efficiency times acceptance for the LSTC

ρ_T →

Wπ

T

signal with the

π^±_T

mass ranging from 130 GeV to 300 GeV. Last two columns show expected and observed upper limit on cross section times branching ratio.

ρ_T

mass [GeV]

π_T

mass [GeV]

ǫ×

A Expected Upper limit [pb] Observed [pb]

250 130 0.063

±

0.002 2.89

±

0.28 2.41

270 145 0.085

±

0.002 1.93

±

0.28 1.92

290 160 0.100

±

0.002 1.56

±

0.30 1.40

330 180 0.125

±

0.002 1.07

±

0.32 1.15

400 240 0.162

±

0.00 3 0.63

±

0.45 1.16

500 300 0.192

±

0.003 0.44

±

0.51 0.47

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0 100 200 300 400

Entries / 10 GeV

0 50 100

103

×

Data W/Z + jets Multijet

+single-t t t

(240) X10 πT

→ W (400) ρT

(160) X10 πT

→ W (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ν + µ) l(e,

→ W

[GeV]

mjj

0 100 200 300 400

Data/Bkg 0.9

1.0 1.1

(a) W j j channel

0 100 200 300 400

Entries / 10 GeV

0 5000 10000

Data W/Z + jets

+single-t t t

(240) X10 πT

→ Z (400) ρT

(160) X10 πT

→ Z (290) ρT

L dt = 20.3 fb-1

∫s = 8 TeV

2 jets

≥ ν + µ) l(e,

→ Z

[GeV]

mjj

0 100 200 300 400

Data/Bkg 0.9

1.0 1.1

(b) Z j j channel

Figure 7: Dijet mass distribution combined for the electron and muon channel for the (a) W j j channel and (b) Z j j after fit of the expected background. The bottom part of the figure shows the ratio of data to fit result.

[GeV]

πT

M

150 200 250 300

BR [pb]×σ

0 2 4 6

L dt = 20.3 fb-1

∫ = 8 TeV s

+55 GeV

πT

=3/2*m

ρT

assuming m

± 0,

πT

→ W

±,0

ρT

LSTC

Observed 95% Upper Limit Expected 95% Upper Limit +1 Sigma Uncertainty +2 Sigma Uncertainty

(a) W j j channel

[GeV]

πT

M

150 200 250 300

BR [pb]×σ

0 0.2 0.4 0.6 0.8 1

L dt = 20.3 fb-1

∫s = 8 TeV

+55 GeV πT

=3/2*m ρT

assuming m

± πT

→ Z

± ρT LSTC

Observed 95% Upper Limit Expected 95% Upper Limit +1 Sigma Uncertainty +2 Sigma Uncertainty

(b) Z j j channel

Figure 8: Observed and expected 95% CL upper limits on the Vπ

_T

cross section as a function of the

mass of the technipion for (a) V

=

W in the W j j channel and (b) V

=

Z in the Z j j channel. The LSTC

predictions for the

ρ_T

cross section (assuming the mass relation of m

ρT =

3/2

∗

m

πT +

55 GeV) are

also shown in blue. The inner and outer bands on the expected limit represent

±

1σ and

±

2σ variations,

respectively.

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Table 3: Z j j efficiency times acceptance for the LSTC

ρ_T →

Z

π_T

signal with the

π⁰_T

mass ranging from 130 GeV to 300 GeV. Last two columns show expected and observed upper limit on cross section times branching ratio.

ρ_T

mass [GeV]

π_T

mass [GeV]

ǫ×

A Expected Upper limit [pb] Observed [pb]

250 130 0.119

±

0.002 0.33

±

0.32 0.23

270 145 0.164

±

0.003 0.25

±

0.33 0.19

290 160 0.197

±

0.003 0.19

±

0.44 0.14

330 180 0.233

±

0.003 0.16

±

0.50 0.13

400 240 0.279

±

0.004 0.10

±

0.68 0.10

500 300 0.321

±

0.004 0.06

±

0.81 0.09

mass limit of 180 GeV (170 GeV for Z j j) for the technipion at 95% confidence under the assumption of m

_ρ_T =

3/2

∗

m

_π_T +

55 GeV.

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A Supporting Material

0 50 100 150 200 250 300

Entries / 10 GeV

0 50 100 150 200 250 300

103

×

-single-t t t

∫s = 8 TeV

2 jets

≥ ν +

→ l W

[GeV]

ν l

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(a) p^ℓν_T

0 50 100 150 200 250 300

Entries / 10 GeV

0 20 40 60 80 100 120 140 160

103

×

-single-t t t

∫s = 8 TeV

2 jets

≥ ν +

→ l W

[GeV]

jj

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(b) p_T^{j j}

0 1 2 3 4 5 6

Entries / 0.1

0 20 40 60 80 100 120 140 160 180

103

×

-single-t t t

∫s = 8 TeV

2 jets

≥ ν +

→ l W

Rjj

0 1 2 3 4 5 ∆ 6

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(c)∆R^{j j}

Figure 9: (a) p

^ℓν_T

, (b) p

_T^{j j}

and (c)

∆R_jj

distributions in the W j j signal region. The uncertainty band

includes the statistical uncertainty on the MC and the JES uncertainty.

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0 50 100 150 200 250 300

Entries / 5 GeV

2000 4000 6000 8000 10000 12000 14000 16000

18000 Data

W/Z+jets -single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

[GeV]

ll

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(a) p^ℓℓ_T

0 50 100 150 200 250 300

Entries / 10 GeV

2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Data W/Z+jets

-single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

[GeV]

jj

pT

0 50 100 150 200 250 300

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(b) p_T^{j j}

0 1 2 3 4 5 6

Entries / 0.1

2000 4000 6000 8000 10000 12000 14000 16000

18000 Data

W/Z+jets -single-t t t

∫s = 8 TeV

2 jets

≥ ll +

→ Z

Rjj

0 1 2 3 4 5 ∆ 6

(Data-Bkg)/Bkg

-0.2 -0.1 0.0 0.1 0.2

(c)∆R^{j j}

Figure 10: (a) p

^ℓℓ_T

, (b) p

_T^{j j}

and (c)

∆Rjj