Dynamics of isolated-photon plus jet production in pp collisions at with the ATLAS detector☆
Introduction
The production of prompt photons in association with a jet in proton–proton collisions, , 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 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 hard collinear scattering can be investigated using the variable , where and Δy is the difference between the rapidities1 of the two final-state particles. The variable 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 when . In contrast, processes dominated by t-channel quark exchange, such as production shown in Fig. 1(b), are expected to have an asymptotic 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 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 () parton [3], [4]. The direct-photon contribution, as shown in Fig. 1(c), is expected to exhibit a dependence when , whereas that of fragmentation processes, as shown in Fig. 1(d), is predicted to be the same as in dijet production, namely . For both processes, there are also s-channel contributions which are, however, non-singular when . As a result, a measurement of the cross section for prompt-photon plus jet production as a function of 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 (), the leading-jet transverse momentum () and rapidity (), the difference between the azimuthal angles of the photon and the jet (), the photon–jet invariant mass () and , where the variable is referred to as 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 , centred around the photon direction (defined by and ). The jets were defined using the anti- jet algorithm [10] with distance parameter . The measurements were performed in the phase-space region of , (excluding the region ), , and . The measurements of and were performed for , and ; these additional requirements select a region where the and 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 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 was also performed without the restrictions on or on . 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 and consists of a silicon pixel detector, a silicon microstrip detector and, for , 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 . This data set was chosen to study the dynamics of isolated-photon plus jet production down to .
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 decaying into two collimated photons, which carries most of the jet energy. The very small contributions expected from diphoton and plus jet events [5], [9]
Cross-section measurement procedure
Isolated-photon plus jet cross sections were measured for photons with , (excluding the region ) and . The jets were reconstructed using the anti- jet algorithm with and selected with , and . Bin-averaged cross sections were measured as functions of , , and . Bin-averaged cross sections as functions of and were measured in the kinematic region , 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 (with the requirements on and applied):
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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 (), factorisation () and fragmentation () scales were chosen to be . 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 and fall by three orders of magnitude in the measured range. The measured and display a maximum at and , respectively. The measured () decreases (increases) as () 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, , have been presented using an integrated luminosity of . The jets were reconstructed using the anti- jet algorithm with . Isolated-photon plus jet bin-averaged cross sections were measured as functions of , , , , and . The bin-averaged cross-sections and were measured with additional selection criteria on
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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- a
Also at Department of Physics, Kingʼs College London, London, United Kingdom.
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Also at Laboratorio de Instrumentacao e Fisica Experimental de Particulas – LIP, Lisboa, Portugal.
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Also at Faculdade de Ciencias and CFNUL, Universidade de Lisboa, Lisboa, Portugal.
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Also at Particle Physics Department, Rutherford Appleton Laboratory, Didcot, United Kingdom.
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Also at TRIUMF, Vancouver, BC, Canada.
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Also at Department of Physics, California State University, Fresno, CA, United States.
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Also at Novosibirsk State University, Novosibirsk, Russia.
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Also at Department of Physics, University of Coimbra, Coimbra, Portugal.
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Also at Università di Napoli Parthenope, Napoli, Italy.
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Also at Institute of Particle Physics (IPP), Canada.
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Also at Department of Physics, Middle East Technical University, Ankara, Turkey.
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Also at Louisiana Tech University, Ruston, LA, United States.
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Also at Departamento de Fisica and CEFITEC of Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal.
- n
Also at Department of Physics and Astronomy, Michigan State University, East Lansing, MI, United States.
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Also at Department of Financial and Management Engineering, University of the Aegean, Chios, Greece.
- p
Also at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain.
- q
Also at Department of Physics, University of Cape Town, Cape Town, South Africa.
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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.
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Also at School of Physics and Engineering, Sun Yat-sen University, Guanzhou, China.
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Also at Academia Sinica Grid Computing, Institute of Physics, Academia Sinica, Taipei, Taiwan.
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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.
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Also at Dipartimento di Fisica, Università La Sapienza, Roma, Italy.
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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.
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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.
- ae
Also at Department of Physics, The University of Texas at Austin, Austin, TX, United States.
- af
Also at Department of Physics and Astronomy, University of South Carolina, Columbia, SC, United States.
- ag
Also at Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest, Hungary.
- ah
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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Also at Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow, Russia.
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Also at Nevis Laboratory, Columbia University, Irvington, NY, United States.
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Also at Physics Department, Brookhaven National Laboratory, Upton, NY, United States.
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Also at Department of Physics, Oxford University, Oxford, United Kingdom.
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Also at Department of Physics, The University of Michigan, Ann Arbor, MI, United States.
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Also at Discipline of Physics, University of KwaZulu-Natal, Durban, South Africa.
- ⁎
Deceased.
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E-mail address: [email protected].