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Physics Letters B 762 (2016) 334–352

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for new resonances in events with one lepton and missing

transverse momentum in pp collisions at

√

s = 13 TeV with the ATLAS

detector

.The ATLAS Collaboration



a r t i c l e i n f o a b s t r a c t

Article history:

Received

14 June 2016

Received

in revised form 16 August 2016

Accepted

20 September 2016

Available

online 28 September 2016

Editor:

W.-D. Schlatter

A search for W



bosons in events with one lepton (electron or muon) and missing transverse momentum

is presented. The search uses 3.2 fb

−1

of pp collision data collected at

√

s = 13 TeV by the ATLAS

experiment at the LHC in 2015. The transverse mass distribution is examined and no signiﬁcant excess

of events above the level expected from Standard Model processes is observed. Upper limits on the W



boson cross-section times branching ratio to leptons are set as a function of the W



mass. Within the

Sequential Standard Model W



masses below 4.07 TeV are excluded at the 95% conﬁdence level. This

extends the limit set using LHC data at

√

s = 8TeVby around 800 GeV.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

Many models of physics beyond the Standard Model (SM) pre-

dict

the existence of new spin-1 gauge bosons that could be dis-

covered

at the Large Hadron Collider (LHC). While the details of

the models vary, conceptually these particles are heavier versions

of the SM W and Z bosons and are generically called W



and Z



bosons.

this letter, a search for a W



boson is presented using

3.2 fb

−1

of pp collision data collected with the ATLAS detector

in 2015 at a centre-of-mass energy of 13 TeV. The results are

interpreted in the context of the benchmark Sequential Standard

Model (SSM), i.e. the extended gauge model described in Ref. [1],

in which the couplings of the W



SSM

to fermions are assumed to

be identical to those of the SM W boson. The decay of the SSM



to SM bosons is not allowed and interference between the

SSM W



and the SM W boson is neglected. The search is con-

ducted

in the W



→ ν channel, where  is an electron or a muon.

The signature is a charged lepton with high transverse momentum

) and substantial missing transverse momentum (E

miss

) due to

the undetected neutrino. The discriminant to distinguish signal and

background is the transverse mass



miss

(1 − cos φ

ν

), (1)



E-mail address: atlas.publications@cern.ch.

where φ

ν

is the angle between the lepton and E

miss

in the trans-

verse

plane.

The dominant background for the W



→ ν search is

the high-m

tail of the charged-current Drell–Yan (q



→ W →ν)

process.

Previous searches for W



SSM

bosons in the W



→ eν and W



→

μν channels were carried out by both the ATLAS and CMS col-

laborations

using the Run-1 data. The previous ATLAS analysis

is based on data corresponding to an integrated luminosity of

20.3 fb

−1

taken at a centre-of-mass energy of

√

s = 8 TeV and

sets a 95% conﬁdence level (CL) lower limit on the W



SSM

mass

of 3.24 TeV [2]. The CMS Collaboration published a search using

19.7 fb

−1

√

s = 8TeVdata from 2012 which excludes W



SSM

masses below 3.28 TeV at 95% CL [3].

2. ATLAS detector

The ATLAS experiment [4] at the LHC is a multi-purpose par-

ticle

detector with a forward–backward symmetric cylindrical ge-

ometry

and a near 4π coverage in solid angle. It consists of an

inner tracking detector (ID) surrounded by a thin superconducting

solenoid providing a 2T axial magnetic ﬁeld, electromagnetic (EM)

and hadronic calorimeters, and a muon spectrometer (MS). The in-

ner

tracking detector covers the pseudorapidity range |η| < 2.5. It

ATLAS uses a right-handed coordinate system with its origin at the nominal

interaction point (IP) in the centre of the detector and the z-axis along the beam

pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis

points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ

being the azimuthal angle around the beam pipe. The pseudorapidity is deﬁned in

terms of the polar angle θ as η =− lntan(θ/2).

http://dx.doi.org/10.1016/j.physletb.2016.09.040

0370-2693/

SCOAP

The ATLAS Collaboration / Physics Letters B 762 (2016) 334–352 335

consists of a silicon pixel detector including the newly installed

insertable B-layer [5,6], followed by silicon microstrip, and transi-

tion

radiation tracking detectors. Lead/liquid-argon (LAr) sampling

calorimeters provide EM energy measurements with high granular-

ity.

A hadronic (steel/scintillator-tile) calorimeter covers the cen-

tral

pseudorapidity range (|η| < 1.7). The endcap and forward re-

gions

are instrumented with LAr calorimeters for both the EM and

hadronic energy measurements up to |η| = 4.9. The muon spec-

trometer

surrounds the calorimeters and is based on three large

air-core toroid superconducting magnets with eight coils each. The

ﬁeld integral of the toroids ranges between 2.0 and 6.0 Tm for

most of the detector. It includes a system of precision tracking

chambers, over |η| < 2.7, and fast detectors for triggering, over

|η| < 2.4. A two-level trigger system is used to select events. The

ﬁrst-level trigger is implemented in hardware and uses a subset

of the detector information. This is followed by a software-based

trigger system that reduces the accepted event rate to about 1kHz.

3. Background and signal simulation

Monte Carlo (MC) simulation samples are used to model the

expected signal and background processes, with the exception of

data-driven background estimates for events in which one ﬁnal-

state

jet or photon satisﬁes the electron or muon selection criteria.

The

main background is due to the charged-current Drell–Yan

(DY) process, generated at next-to-leading order (NLO) in QCD us-

ing Powheg-Box v2

[7] and the CT10 parton distribution functions

(PDF) [8], with Pythia 8.186 [9] to model parton showering and

hadronisation. The same setup is used for the neutral-current DY

q → Z/γ

∗

→ ) process. In both cases, samples for all three

lepton ﬂavours are generated, and the ﬁnal-state photon radiation

(QED FSR) is handled by Photos [10]. The DY samples are nor-

malised

as a function of mass to a next-to-next-to-leading order

(NNLO) perturbative QCD (pQCD) calculation using VRAP [11] and

the CT14NNLO PDF set [12]. In addition, NLO electroweak (EW)

corrections beyond QED FSR are calculated with Mcsanc [13,14] at

LO in pQCD as a function of mass. In order to combine the QCD

and EW terms, the so-called additive approach is used where the

EW corrections are added to the NNLO QCD cross-section predic-

tion.

Backgrounds

from t

t and single top-quark production are esti-

mated

at NLO using Powheg-Box. These processes use the CT10

PDF set and are interfaced to Pythia 6.428 [15] for parton show-

ering

and hadronisation. Further backgrounds are due to diboson

(WW, WZ and ZZ) production. These processes are generated

with Sherpa 2.1.1 [16] using the CT10 PDF set.

Signal

samples for the W



→ eν and W



→ μν processes are

produced at leading order (LO) in QCD using Pythia 8.183 and the

NNPDF2.3 LO PDF set. The W



SSM

boson has the same couplings to

fermions as the Standard Model W boson and is assumed not to

couple to the SM W and Z bosons. Interference effects between

the W



and the SM W boson are neglected. In this model the

branching ratio to a charged lepton and a neutrino is 8.2% in the

entire mass range considered in this search. The decay W



→ τν,

where the τ lepton subsequently decays leptonically is not treated

as part of the signal. If included, this decay would constitute a very

small contribution. The signal samples are normalised to the same

mass-dependent NNLO pQCD calculation as used for the DY pro-

cess.

The EW corrections beyond QED FSR are not applied to the

signal samples because they depend on the couplings of the new

particle to W and Z bosons, and are therefore model-dependent.

The resulting cross-section times branching ratio for W



SSM

masses

of 2, 3 and 4TeVare 153, 15.3 and 2.25 fb, respectively.

For

all samples used in this analysis, the effects of multiple

interactions per bunch crossing (“pile-up”) are accounted for by

overlaying simulated minimum-bias events. The interaction of par-

ticles

with the detector and its response are modelled using a full

ATLAS detector simulation [17] performed by Geant4 [18]. Differ-

ences

between data and simulation are accounted for in the lepton

trigger, reconstruction, identiﬁcation [19,20], and isolation eﬃcien-

cies

as well as the lepton energy/momentum resolution and scale

[21,20].

4. Object reconstruction and event selection

Events in the muon channel are selected by a trigger requiring

that at least one muon with p

> 50 GeV is found. These muons

must be reconstructed in both the MS and the ID. In the elec-

tron

channel, events are selected by a trigger requiring at least one

electron candidate with p

> 24 GeV that satisﬁes the medium

identiﬁcation criteria or a trigger requiring at least one electron

with p

> 120 GeV that satisﬁes the loose identiﬁcation criteria.

The selection cuts used to select electron candidates at trigger level

are very similar to the ones used in the oﬄine reconstruction and

were optimised using a likelihood approach [19].

The

selected events must have a reconstructed primary ver-

tex,

which is the interaction vertex with the highest sum of p

tracks found in the event. Each vertex reconstructed in the event

consists of at least two associated tracks with p

> 0.4GeV. Only

data taken during periods when all detector components and the

trigger readout are functioning well are considered.

Muons are reconstructed from MS tracks and matching ID

tracks within |η| < 2.5, requiring that the MS tracks have at least

three hits in each of the three separate layers of MS chambers to

ensure optimal resolution for high-momentum muons [20]. In ad-

dition,

these combined muons are required to pass a track quality

selection based on the number of hits in the ID. To reduce sensitiv-

ity

to the relative barrel–endcap alignment in the MS, the region

1.01 < |η| < 1.10 is vetoed. Muons are rejected if the difference

between the muon charge-to-momentum ratios measured in the

ID and MS exceeds seven times the sum in quadrature of the cor-

responding

uncertainties, or if the track crosses poorly aligned MS

chambers. To ensure that the muons originate from the primary

vertex, the transverse impact parameter signiﬁcance, which is the

ratio of the absolute value of the transverse impact parameter (d

)

to its uncertainty, has to be below three. The distance between the

z-position of the point of closest approach of the muon track in

the ID to the beamline and the z-coordinate of the primary ver-

tex

is required to be less than 10 mm. Furthermore, only isolated

muons are considered. The scalar sum over the track p

in an iso-

lation

cone around the muon (excluding the muon itself) divided

by the muon p

is required to be below a p

-dependent cut tuned

for a 99% eﬃciency. The isolation cone size R =



(η)

+(φ)

is deﬁned as 10 GeV divided by the muon p

and has a maximum

size of R =0.3.

Electrons are formed from clusters of cells in the electromag-

netic

calorimeter associated with a track in the ID. The electron p

is obtained from the calorimeter energy measurement and the di-

rection

of the associated track. The electron must be within the

range |η| < 2.47 and outside the transition region between the

barrel and endcap calorimeters (1.37 < |η| < 1.52). In addition,

tight identiﬁcation criteria [19] need to be satisﬁed. The identi-

ﬁcation

uses a likelihood discriminant based on measurements of

calorimeter shower shapes and measurements of track properties

from the ID. To ensure that the electrons originate from the pri-

mary

vertex, the transverse impact parameter signiﬁcance must be

below ﬁve. Furthermore, calorimeter- and track-based isolation cri-

teria,

tuned for an overall eﬃciency of 98%, independent of p

, are

applied. The sum of the calorimeter transverse energy deposits in

the isolation cone of size R = 0.2(excluding the electron itself)

336 The ATLAS Collaboration / Physics Letters B 762 (2016) 334–352

divided by the electron p

is used in the discrimination criterion.

The track-based isolation is determined similarly to that for muons.

The scalar sum of the p

of all tracks in a cone around the elec-

tron,

divided by the electron p

has to be below a given value.

The cone has a size R = 10 GeV/p

(e) with a maximum value of

R =0.2.

The

calculation of the missing transverse momentum is based

on the selected electrons, photons, tau leptons, muons and jets

found in the event. The value of E

miss

is evaluated by the vec-

tor

sum of the p

of the physics objects selected in the analysis

and the tracks not belonging to any of these physics objects [22].

Jets used in the E

miss

calculation are reconstructed from clusters of

calorimeter cells with |η| < 5using the anti-k

algorithm [23] with

a radius parameter of 0.4. They are calibrated using the method

described in Ref. [24] and are required to have p

> 20 GeV.

Events

are selected if they have exactly one electron or muon

with p

> 55 GeV. The E

miss

value found in the event is required

to exceed 55 GeV and the transverse mass has to satisfy m

110 GeV. For these selection cuts the acceptance times eﬃciency,

deﬁned as the fraction of simulated candidate events that pass the

event selection, amounts to 81% (75%) for the electron channel and

53% (50%) for the muon channel at a W



mass of 2TeV(4TeV).

5. Background estimate and comparison to data

The background from processes with at least one prompt ﬁnal-

state

lepton is estimated with simulated events. The processes

with non-negligible contributions are charged-current DY (W pro-

duction),

t and single top-quark production, in the following re-

ferred

to as “top-quark” background, as well as neutral-current DY

(Z/γ

∗

production) and diboson production.

Background

contributions from events where one ﬁnal-state jet

or photon passes the lepton selection criteria are determined using

a data-driven “matrix” method. This includes contributions from

multijet, heavy-ﬂavour quark and γ + jet production, referred to

hereafter as the multijet background. The ﬁrst step of the ma-

trix

method is to calculate the factor f , the fraction of lepton

candidates that pass the nominal lepton identiﬁcation and isola-

tion

requirements in a background-enriched data sample contain-

ing

“loose” lepton candidates. These loose candidates satisfy only a

subset of the nominal criteria, which are stricter than the trigger

requirements imposed. Potential contamination of prompt ﬁnal-

state

leptons in the background-enriched sample is accounted for

using MC simulation. In addition to the factor f , the fraction of

real leptons r in the sample of loose objects satisfying the nominal

requirements is used in evaluating this background. This probabil-

ity

is computed from MC simulation.

The

contribution to the background from events with a fake

lepton is determined in the following way. The relation between

the number of real prompt leptons (N

) or fake leptons (N

) and

the number of measured objects found in the events containing

the loose lepton candidates (N

, N

) can be written as







(1 −r)(1 − f )





(2)

where the subscript T refers to leptons that pass the nominal se-

lection.

The subscript L corresponds to leptons that pass the loose

requirements described above but fail the nominal requirements.

The number of jets and photons misidentiﬁed as leptons (N

Multijet

)

in the total number of objects passing the signal selection (N

) is

given as

Multijet

= fN

r − f



r (N

+ N

) − N



(3)

The right-hand side of Eq. (3) is obtained by solving Eq. (2).

The

simulated top-quark and diboson samples as well as the

data-driven background estimate are statistically limited at large

. Therefore, the expected number of events is extrapolated into

the high-m

region using parameterisations of the m

shape ﬁt-

ted

to the expected background in the low-m

region. Several ﬁts

are carried out based on the functions f (m

) = a m

c log m

and

f (m

) =a/(m

+b)

. These ﬁts explore various ﬁt ranges typically

starting between 140 and 200 GeV and extending up to 600 to

900 GeV. The ﬁt with the best χ

per degree of freedom is used

as the extrapolated background contribution, with an uncertainty

evaluated using the envelope of all performed ﬁts.

Finally,

the expected number of background events is calculated

as the sum of the data-driven and simulated background estimates.

The background is dominated by the charged-current DY produc-

tion

for all values of m

, as can be seen in the upper panel of

Fig. 1. For example, the contribution from charged-current DY is

about 90% for both channels at m

> 1TeV. In both channels, the

number of observed events agrees with the background estimate,

as shown in the upper two panels of Fig. 1 and in Table 1. As can

be seen in the middle panels, the data are systematically above

the predicted background at low m

but are within the ±1σ un-

certainty

band, which is dominated by the E

miss

related systematic

uncertainties in this region. The lower panels of Fig. 1 show the

ratio of the data to the adjusted background that results from the

statistical analysis described in Section 7. The data agree well with

the adjusted background prediction.

6. Systematic uncertainties

Experimental systematic uncertainties arise from the back-

ground

and luminosity estimates, the trigger selection, the lepton

reconstruction, identiﬁcation and isolation criteria [19,20], as well

as effects of the energy/momentum scale and resolution [21,20].

The systematic uncertainties for the two channels are summarised

in Table 2. At large m

, the dominant source of uncertainty is due

to the background extrapolations in the electron and muon chan-

nels,

described in Section 5, and to the momentum resolution in

the muon channel. The extrapolation uncertainties are shown in

Table 2 for the data-driven multijet background and the combined

top-quark and diboson backgrounds. The multijet background un-

certainty

in the electron channel includes a 25% contribution from

the data-driven estimate, which is due to the dependence of the

factor f (see Section 5) on the speciﬁc selection used to derive the

background-enriched sample. No additional uncertainty is assigned

in the muon case as the multijet background is small.

The

electron and muon reconstruction, identiﬁcation and isola-

tion

eﬃciencies as well as their corresponding uncertainties were

evaluated from data using tag-and-probe methods in Z boson de-

cays

up to a p

of O(100 GeV). The ratio of the eﬃciency mea-

sured

in data to that of the MC simulation is then used to correct

the MC prediction. For electrons, these ratios are measured follow-

ing

the prescriptions of Ref. [25], with adjustments for the 2015

running conditions. For higher-p

electrons, an additional system-

atic

uncertainty of 2.5% is assigned to the identiﬁcation eﬃciency.

This is based on differences observed between data and simulation,

and their propagation to the simulated electrons. For the isolation

eﬃciency, an additional uncertainty of 2% is attributed to high-p

electrons from the variation of the mean values of the ratio of the

isolation eﬃciencies between data and simulation in various p

and η bins. For muons, no signiﬁcant dependence of the ratio of

the eﬃciencies measured in data over the ones measured in MC

simulation as a function of p

is observed [20]. For high-p

muons

an upper limit on the uncertainty of 2–3% per TeV is extracted

from simulation. For the isolation criterion an extrapolation of the

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