Elsevier

Nuclear Physics B

Volume 875, Issue 3, 21 October 2013, Pages 483-535
Nuclear Physics B

Dynamics of isolated-photon plus jet production in pp collisions at s=7 TeV with the ATLAS detector

https://doi.org/10.1016/j.nuclphysb.2013.07.025Get rights and content

Abstract

The dynamics of isolated-photon plus jet production in pp collisions at a centre-of-mass energy of 7 TeV has been studied with the ATLAS detector at the LHC using an integrated luminosity of 37 pb1. Measurements of isolated-photon plus jet bin-averaged cross sections are presented as functions of photon transverse energy, jet transverse momentum and jet rapidity. In addition, the bin-averaged cross sections as functions of the difference between the azimuthal angles of the photon and the jet, the photon–jet invariant mass and the scattering angle in the photon–jet centre-of-mass frame have been measured. Next-to-leading-order QCD calculations are compared to the measurements and provide a good description of the data, except for the case of the azimuthal opening angle.

Introduction

The production of prompt photons in association with a jet in proton–proton collisions, ppγ+jet+X, provides a testing ground for perturbative QCD (pQCD) in a cleaner environment than in jet production, since the photon originates directly from the hard interaction. The measurements of angular correlations between the photon and the jet can be used to probe the dynamics of the hard-scattering process. Since the dominant production mechanism in pp collisions at the LHC is through the qgqγ process, measurements of prompt-photon plus jet production have been used to constrain the gluon density in the proton [1], [2]. Furthermore, precise measurements of photon plus jet production are also useful for the tuning of the Monte Carlo (MC) models. In addition, these events constitute the main reducible background in the identification of Higgs bosons decaying to a photon pair.

The dynamics of the underlying processes in 22 hard collinear scattering can be investigated using the variable θ, where cosθtanh(Δy/2) and Δy is the difference between the rapidities1 of the two final-state particles. The variable θ coincides with the scattering angle in the centre-of-mass frame, and its distribution is sensitive to the spin of the exchanged particle. For processes dominated by t-channel gluon exchange, such as dijet production in pp collisions shown in Fig. 1(a), the differential cross section behaves as (1|cosθ|)2 when |cosθ|1. In contrast, processes dominated by t-channel quark exchange, such as W/Z+jet production shown in Fig. 1(b), are expected to have an asymptotic (1|cosθ|)1 behaviour. This fundamental prediction of QCD can be tested in photon plus jet production at the centre-of-mass energy of the LHC.

At leading order (LO) in pQCD, the process ppγ+jet+X proceeds via two production mechanisms: direct photons (DP), which originate from the hard process, and fragmentation photons (F), which arise from the fragmentation of a coloured high transverse momentum (pT) parton [3], [4]. The direct-photon contribution, as shown in Fig. 1(c), is expected to exhibit a (1|cosθ|)1 dependence when |cosθ|1, whereas that of fragmentation processes, as shown in Fig. 1(d), is predicted to be the same as in dijet production, namely (1|cosθ|)2. For both processes, there are also s-channel contributions which are, however, non-singular when |cosθ|1. As a result, a measurement of the cross section for prompt-photon plus jet production as a function of |cosθ| provides a handle on the relative contributions of the direct-photon and fragmentation components as well as the possibility to test the dominance of t-channel quark exchange, such as that shown in Fig. 1(c).

Measurements of prompt-photon production in a final state with accompanying hadrons necessitates of an isolation requirement on the photon to avoid the large contribution from neutral-hadron decays into photons. The production of inclusive isolated photons in pp collisions has been studied previously by ATLAS [5], [6] and CMS [7], [8]. Recently, the differential cross sections for isolated photons in association with jets as functions of the photon transverse energy in different regions of rapidity of the highest transverse momentum (leading) jet were measured by ATLAS [9]. The analysis presented in this paper is based on the same data sample and similar selection criteria as in the previous publication, but extends the study by measuring also cross sections in terms of the leading-jet and photon-plus-jet properties. The goal of the analysis presented here is to study the kinematics and dynamics of the isolated-photon plus jet system by measuring the bin-averaged cross sections as functions of the leading-photon transverse energy (ETγ), the leading-jet transverse momentum (pTjet) and rapidity (yjet), the difference between the azimuthal angles of the photon and the jet (Δϕγj), the photon–jet invariant mass (mγj) and cosθγj, where the variable θ is referred to as θγj here and henceforth. The photon was required to be isolated by using the same isolation criterion as in previous measurements [5], [6], [9] based on the amount of transverse energy inside the cone given by (ηηγ)2+(ϕϕγ)2ΔR=0.4, centred around the photon direction (defined by ηγ and ϕγ). The jets were defined using the anti-kt jet algorithm [10] with distance parameter R=0.6. The measurements were performed in the phase-space region of ETγ>45 GeV, |ηγ|<2.37 (excluding the region 1.37<|ηγ|<1.52), pTjet>40 GeV, |yjet|<2.37 and ΔRγj2=(ηγηjet)2+(ϕγϕjet)2>1. The measurements of dσ/dmγj and dσ/d|cosθγj| were performed for |ηγ+yjet|<2.37, |cosθγj|<0.83 and mγj>161 GeV; these additional requirements select a region where the mγj and |cosθγj| distributions are not distorted by the restrictions on the transverse momenta and rapidities of the photon and the jet. Next-to-leading-order (NLO) QCD calculations were compared to the measurements. Photon plus jet events constitute an important background in the identification of the Higgs decaying into diphotons; the |cosθ| distribution for the diphoton events has been used [11] to study the spin of the new “Higgs-like” particle observed by ATLAS [12] and CMS [13]. To understand the photon plus jet background in terms of pQCD and to aid in better constraining the contributions of direct-photon and fragmentation processes in the MC models, a measurement of the bin-averaged cross section as a function of |cosθγj| was also performed without the restrictions on mγj or on |ηγ+yjet|. Predictions from both leading-logarithm parton-shower MC models and NLO QCD calculations were compared to this measurement.

Section snippets

The ATLAS detector

The ATLAS experiment [14] uses a multi-purpose particle detector with a forward–backward symmetric cylindrical geometry and nearly 4π coverage in solid angle.

The inner detector covers the pseudorapidity range |η|<2.5 and consists of a silicon pixel detector, a silicon microstrip detector and, for |η|<2, a transition radiation tracker. The inner detector is surrounded by a thin superconducting solenoid providing a 2 T magnetic field and is used to measure the momentum of charged-particle tracks.

Data selection

The data used in this analysis were collected during the proton–proton collision running period of 2010, when the LHC operated at a centre-of-mass energy of s=7 TeV. This data set was chosen to study the dynamics of isolated-photon plus jet production down to ETγ=45 GeV.

Only events taken in stable beam conditions and passing detector and data-quality requirements were considered. Events were recorded using a single-photon trigger, with a nominal transverse energy threshold of 40 GeV; this

Monte Carlo simulations

Samples of simulated events were generated to study the characteristics of signal and background. These MC samples were also used to determine the response of the detector to jets of hadrons and the correction factors necessary to obtain the particle-level cross sections. In addition, they were used to estimate hadronisation corrections to the NLO QCD calculations.

The MC programs Pythia 6.423 [20] and Herwig 6.510 [21] were used to generate the simulated signal events. In both generators, the

Background subtraction and signal-yield estimation

A non-negligible background contribution remains in the selected sample, even after the application of the tight identification and isolation requirements on the photon. This background comes predominantly from multi-jet processes, in which a jet is misidentified as a photon. This jet usually contains a light neutral meson, mostly a π0 decaying into two collimated photons, which carries most of the jet energy. The very small contributions expected from diphoton and W/Z plus jet events [5], [9]

Cross-section measurement procedure

Isolated-photon plus jet cross sections were measured for photons with ETγ>45 GeV, |ηγ|<2.37 (excluding the region 1.37<|ηγ|<1.52) and ET,partiso<4 GeV. The jets were reconstructed using the anti-kt jet algorithm with R=0.6 and selected with pTjet>40 GeV, |yjet|<2.37 and ΔRγj>1. Bin-averaged cross sections were measured as functions of ETγ, pTjet, |yjet| and Δϕγj. Bin-averaged cross sections as functions of mγj and |cosθγj| were measured in the kinematic region |ηγ+yjet|<2.37, |cosθγj|<0.83 and

Systematic uncertainties

The following sources of systematic uncertainty were considered; average values, expressed in percent and shown in parentheses, quantify their effects on the cross section as a function of |cosθγj| (with the requirements on mγj and |ηγ+yjet| applied):

  • Simulation of the detector geometry. The systematic uncertainties originating from the limited knowledge of the material in the detector were evaluated by repeating the full analysis using a different detector simulation with increased material in

Next-to-leading-order QCD calculations

The NLO QCD calculations used in this analysis were computed using the program Jetphox [35]. This program includes a full NLO QCD calculation of both the direct-photon and fragmentation contributions to the cross section.

The number of flavours was set to five. The renormalisation (μR), factorisation (μF) and fragmentation (μf) scales were chosen to be μR=μF=μf=ETγ. The calculations were performed using the CTEQ6.6 [36] parameterisations of the proton PDFs and the NLO photon BFG set II photon

Results

The measured bin-averaged cross sections are presented in Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14 and Table 1, Table 2, Table 3, Table 4, Table 5, Table 6. The measured dσ/dETγ and dσ/dpTjet fall by three orders of magnitude in the measured range. The measured dσ/d|yjet| and dσ/dΔϕγj display a maximum at |yjet|0 and Δϕγjπ, respectively. The measured dσ/dmγj (dσ/d|cosθγj|) decreases (increases) as mγj (|cosθγj|) increases.

The predictions of the NLO QCD calculations from the Jetphox

Summary and conclusions

Bin-averaged cross sections for isolated photons in association with a jet in 7 TeV proton–proton collisions, ppγ+jet+X, have been presented using an integrated luminosity of 37.1 pb1. The jets were reconstructed using the anti-kt jet algorithm with R=0.6. Isolated-photon plus jet bin-averaged cross sections were measured as functions of ETγ, pTjet, |yjet|, Δϕγj, mγj and cosθγj. The bin-averaged cross-sections dσ/dmγj and dσ/d|cosθγj| were measured with additional selection criteria on |ηγ+y

Acknowledgements

We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently.

We acknowledge the support of NPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck

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    © CERN for the benefit of the ATLAS Collaboration.

    a

    Also at Department of Physics, Kingʼs College London, London, United Kingdom.

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    Also at Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan.

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    Also at Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany.

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    Also at Manhattan College, New York, NY, United States.

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    Also at Institute of Physics, Academia Sinica, Taipei, Taiwan.

    v

    Also at School of Physics and Engineering, Sun Yat-sen University, Guanzhou, China.

    w

    Also at Academia Sinica Grid Computing, Institute of Physics, Academia Sinica, Taipei, Taiwan.

    x

    Also at Laboratoire de Physique Nucléaire et de Hautes Energies, UPMC and Université Paris-Diderot and CNRS/IN2P3, Paris, France.

    y

    Also at School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar, India.

    z

    Also at Dipartimento di Fisica, Università La Sapienza, Roma, Italy.

    aa

    Also at DSM/IRFU (Institut de Recherches sur les Lois Fondamentales de lʼUnivers), CEA Saclay (Commissariat à lʼEnergie Atomique et aux Energies Alternatives), Gif-sur-Yvette, France.

    ab

    Also at Moscow Institute of Physics and Technology State University, Dolgoprudny, Russia.

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    Also at Section de Physique, Université de Genève, Geneva, Switzerland.

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    Also at Departamento de Fisica, Universidade de Minho, Braga, Portugal.

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    Also at Department of Physics, The University of Texas at Austin, Austin, TX, United States.

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    Also at Department of Physics and Astronomy, University of South Carolina, Columbia, SC, United States.

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    Also at Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest, Hungary.

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    Also at DESY, Hamburg and Zeuthen, Germany.

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    Also at International School for Advanced Studies (SISSA), Trieste, Italy.

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    Deceased.

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