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JHEP03(2016)041

Published for SISSA by Springer

Received: December 21, 2015

Revised: January 29, 2016

Accepted: February 17, 2016

Published: March 8, 2016

Search for new phenomena with photon+jet events in

proton-proton collisions at

√

s = 13 TeV with the

ATLAS detector

The ATLAS collaboration

E-mail: atlas.publications@cern.ch

Abstract: A search is performed for the production of high-mass resonances decaying

into a photon and a jet in 3.2 fb

−1

of proton-proton collisions at a centre-of-mass energy

√

s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. Selected

events have an isolated photon and a jet, each with transverse momentum above 150 GeV.

No signiﬁcant deviation of the γ+jet invariant mass distribution from the background-

only hypothesis is found. Limits are set at 95% conﬁdence level on the cross sections of

generic Gaussian-shaped signals and of a few benchmark phenomena beyond the Standard

Model: excited quarks with vector-like couplings to the Standard Model particles, and non-

thermal quantum black holes in two models of extra spatial dimensions. The minimum

excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios

of 2% decrease from about 6 fb for a mass of 1.5 TeV to about 0.8 fb for a mass of 5 TeV.

The minimum excluded visible cross sections for Gaussian-shaped resonances with width-

to-mass ratios of 15% decrease from about 50 fb for a mass of 1.5 TeV to about 1.0 fb for

a mass of 5 TeV. Excited quarks are excluded below masses of 4.4 TeV, and non-thermal

quantum black holes are excluded below masses of 3.8 (6.2) TeV for Randall-Sundrum

(Arkani-Hamed-Dimopoulous-Dvali) models with one (six) extra dimensions.

Keywords: Hadron-Hadron scattering

ArXiv ePrint: 1512.05910

Open Access, Copyright CERN,

for the beneﬁt of the ATLAS Collaboration.

Article funded by SCOAP

doi:10.1007/JHEP03(2016)041

JHEP03(2016)041

(

M in the RS1 model and M

in the ADD model) to a few TeV. As a consequence, the

LHC could produce quantum black holes (QBH) with masses near M

∗

[7, 8], which would

then decay before thermalizing, producing low-multiplicity ﬁnal states [9, 10]. The RS1

model studied in this article has n = 1 extra dimensions. For the ADD model, the same

benchmark scenario (n = 6) is investigated as in the previous ATLAS publication [11]. In

this article it is also assumed that the mass threshold for black hole production is equal to

the Planck scale, M

= M

∗

. The maximum mass for black hole production, which in any

case cannot exceed the pp centre-of-mass energy, is set to 3M

∗

(when M

∗

√

s/3), to avoid

the high-mass regime in which a classical description of the black hole should replace the

quantum one. A continuum of black holes with invariant masses between the threshold mass

and the maximum mass can therefore be produced, with a probability rapidly decreasing

with the mass. The total expected production cross sections times decay branching ratios

for pp → QBH + X → γ + q/g + X as a function of the threshold mass, assuming that all

QBHs decay to two-body ﬁnal states and summing over all parton types in the initial and

ﬁnal state, are shown in ﬁgure 1. These cross sections were obtained with the QBH 2.02

event generator [12]. At

√

s = 13 TeV the predicted total production cross section times

branching ratio is 1.4 fb (390 fb) for RS1 (ADD) black holes with M

= 4 TeV.

Both the q

∗

→ qγ and QBH → qγ, gγ decays (regardless of the number of extra dimen-

sions) would yield ﬁnal states with a photon and a jet having large transverse momenta and

large invariant mass m

γj

. Such events would manifest themselves in the m

γj

distribution as

a broad peak above the steeply falling background from SM prompt γ + jet events [13, 14],

typically produced by QCD Compton scattering (qg → qγ).

In this article, a search for a localized, high-mass excess in the γ + jet invariant mass

distribution is presented. The excess would arise from s−channel production of a resonant

signal. The measurement uses 3.2 fb

−1

of pp collisions collected at a centre-of-mass energy

√

s = 13 TeV by the ATLAS detector in 2015.

The results are interpreted in terms of the visible cross section (i.e. the product of the

production cross section, the branching ratio, the detector acceptance and the selection

eﬃciency) of a generic Gaussian-shaped signal with mass M

and width σ

. The results

are also interpreted in terms of the cross section times branching ratio to a photon and a

quark or a gluon in three benchmark models: a q

∗

state, a non-thermal RS1 QBH, and a

non-thermal ADD QBH (for n = 6).

For the case of a Gaussian-shaped signal, the width σ

is assumed to be proportional

to M

; three possible values of σ

are considered: 2%, 7% and 15%. The experimental

photon+jet invariant mass resolution improves from 2.4% at 1 TeV to 1.5% at 6 TeV. The

smallest value of σ

(2%) thus corresponds to the typical photon+jet invariant mass

resolution and hence represents the case of an intrinsically narrow resonance.

The RMS width of the q

∗

lineshape is expected to increase from about 250 GeV at

∗

= 1 TeV to more than 1 TeV at m

∗

= 6 TeV and beyond. Quantum black holes are

expected to produce even broader signals, due to the production of a continuum of QBHs

with masses between the threshold mass M

and the maximum mass.

Previous searches for generic Gaussian-shaped resonances, excited quarks, and ADD

quantum black holes in the γ + jet ﬁnal state have been performed by the ATLAS and

– 2 –

JHEP03(2016)041

[TeV]

or m

1 2 3 4 5 6 7 8 9

BR [fb]× σ

3−

2−

1−

ADD QBH (n=6)

RS1 QBH (n=1)

= 13 TeVs

Figure 1. Production cross section times γ + jet branching ratio for an excited quark q

∗

and two

diﬀerent non-thermal quantum black hole models (RS1, ADD) as a function of the q

∗

mass or the

mass threshold for black hole production M

, in pp collisions at

√

s = 13 TeV. The q

∗

cross section

is computed at leading order in α

with the Pythia 8.186 event generator [3]. The excited-quark

model assumes that the compositeness scale is equal to the excited-quark mass m

∗

, and that gauge

interactions of excited quarks are like those in the SM, with the SU (3), SU(2), and U(1) coupling

multipliers ﬁxed to f

= f = f

= 1. The quantum black hole cross sections are obtained with

the QBH 2.02 event generator [12]. In the RS1 and ADD quantum black hole models the number

of extra spatial dimensions is n = 1 and n = 6, respectively. The maximum mass for black hole

production is set to the pp centre-of-mass energy or to 3M

if M

√

s/3. The cross sections are

calculated in 0.5 GeV mass steps (dots) and interpolated with a continuos function (solid lines).

CMS collaborations using pp collisions at either

√

s = 7 TeV [15] or 8 TeV [11, 16]. No

signiﬁcant excess of events over the background was found, leading to lower limits on the

mass of excited quarks at 3.5 TeV from both the ATLAS and CMS experiments [11, 16]

and on the ADD QBH mass at 4.6 TeV by ATLAS (for n = 6) [11]. No limits on RS1

quantum black holes with the photon+jet ﬁnal state have been set so far. Using the dijet

ﬁnal state, ATLAS has set a lower limit on the q

∗

mass at 4.1 TeV and on the ADD QBH

mass at 5.8 TeV for n = 6 [17], while CMS has excluded q

∗

masses below 3.3 TeV and ADD

QBH masses below 4.0-5.3 TeV depending on n [18, 19]. ATLAS has also searched for

quantum black hole production by looking for high-mass dilepton [20] and lepton+jet [21]

resonances in

√

s = 8 TeV data.

Recently, using data collected at

√

s = 13 TeV, the ATLAS and CMS collaborations

performed searches for excited quarks in dijet ﬁnal states [22, 23], and ATLAS searched

for quantum black holes in dijet ﬁnal states [22] and for thermal black holes in multijet

ﬁnal states [24]. Excited quarks with masses below 5.2 TeV and RS1 or ADD (n = 6)

quantum black holes with masses below 5.3 TeV and 8.3 TeV respectively, decaying to

dijets, are excluded.

– 3 –

JHEP03(2016)041

The searches presented in this article exploit analysis techniques and a selection strat-

egy similar to those in a previous search using 20.3 fb

−1

of pp collisions at

√

s = 8 TeV [11].

Despite the six times smaller integrated luminosity at

√

s = 13 TeV, the sensitivity of the

present search to the q

∗

and QBH signals exceeds the exclusion limits obtained with 8 TeV

data. This is due to the signiﬁcant growth of the q

∗

and QBH production cross section

with the increase of the pp centre-of-mass energy from 8 TeV to 13 TeV. For instance, for

a mass of 5 TeV, the production cross sections rise by more than two orders of magnitude

for both the q

∗

and QBHs, while the background cross section increases by less than an

order of magnitude.

The article is organized as follows. In section 2 a brief description of the ATLAS

detector is given. Section 3 summarizes the data and simulation samples used in this

study. The event selection is discussed in section 4. The signal and background modelling

are presented in section 5. The systematic uncertainties are described in section 6. In

section 7, the signal search and limit-setting strategies are discussed, and ﬁnally the results

are presented in section 8.

2 The ATLAS detector

The ATLAS detector [25] is a multi-purpose particle detector with approximately forward-

backward symmetric cylindrical geometry.

The inner tracking detector (ID) covers |η| <

2.5 and consists of a silicon pixel detector (including the newly installed innermost pixel

layer [26]), a silicon microstrip detector, and a straw-tube transition radiation tracker.

The ID is surrounded by a thin superconducting solenoid providing a 2 T axial magnetic

ﬁeld and by a high-granularity lead/liquid-argon (LAr) sampling electromagnetic (EM)

calorimeter. The EM calorimeter measures the energy and the position of electromagnetic

showers with |η| < 3.2. It includes a presampler (for |η| < 1.8) and three sampling layers,

longitudinal in shower depth, up to |η| = 2.5. The hadronic calorimeter, surrounding

the electromagnetic one and covering |η| < 4.9, is a sampling calorimeter which uses either

scintillator tiles or LAr as the active medium, and steel, copper or tungsten as the absorber

material. The muon spectrometer (MS) surrounds the calorimeters and consists of three

large superconducting air-core toroid magnets, each with eight coils, a system of precision

tracking chambers (|η| < 2.7), and fast tracking chambers (|η| < 2.4) for triggering.

Events containing photon candidates are selected by a two-level trigger system. The

ﬁrst-level trigger is hardware based; using a trigger cell granularity coarser than that of

the EM calorimeter, it searches for electromagnetic clusters within a ﬁxed window of size

0.2×0.2 in η×φ and retains only those whose total transverse energy in two adjacent trigger

cells is above a programmable threshold. The second, high-level trigger is implemented in

software and employs algorithms similar to those used oﬄine to identify jets and photon

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 z-axis. The pseudorapidity is deﬁned in terms of the polar

angle θ as η = − ln tan(θ/2).

– 4 –

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