Measurement of the inclusive isolated-photon cross section in pp collisions at $$ \sqrt{s} $$ = 13 TeV using 36 fb−1 of ATLAS data

Abstract

Measurement of the inclusive isolated-photon cross section in pp collisions at √ s = 13 TeV using 36 fb −1 of ATLAS dataThe ATLAS CollaborationThe differential cross section for isolated-photon production in pp collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb −1 . The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from Jetphox and Sherpa as well as next-to-next-to-leading-order QCD calculations from Nnlojet are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties. c 2019 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license. γ T > 125 GeV and |η γ | < 2.37, excluding the region 1.37 < |η γ | < 1.56. In addition, the double-differential cross section as a function of |η γ | in different regions of E γ T is also presented. The results are based on a data sample with a more than ten-fold increase in statistics relative to the previous study [5]. The measurement presented here is found to be consistent with the previous one in the overlapping kinematic regions. This increase in statistics allows improvements in the calibration of the photon energy and reductions in the experimental systematic uncertainties affecting the cross-section measurement, as well as an extension of the coverage in E γ T to higher values than previously measured. In this analysis, the region where the measurement is limited by systematic uncertainties is extended to E γ T ∼ 1 TeV, beyond what was achieved in the previous measurement. The NLO QCD predictions of Jetphox [19,20] and Sherpa [21] based on several parameterisations of the PDFs are compared with the measurement. The NNLO QCD prediction of Nnlojet [16], which has significantly reduced uncertainties due to fewer missing higher-order terms, is also confronted with the data. ATLAS detectorThe ATLAS detector [22][23][24] is a multipurpose detector with a forward-backward symmetric cylindrical geometry. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting toroid magnets. The inner-detector system is immersed in a 2 T axial magnetic field and provides charged-particle tracking in the range |η| < 2.5. The high-granularity silicon pixel detector is closest to the interaction region and provides four measurements per track. The pixel detector is followed by the silicon microstrip tracker, which typically provides four three-dimension...

Single Photon Production at Hadron Colliders at NNLO QCD with Realistic Photon Isolation

“…Earlier measurements of isolated photon [1-5, 8, 11] and photon-plus-jet production [9,10] in hadronic collisions were followed by measurements at the LHC. These measurements for isolated photon production [12,13,15,17] as well as for the production of a photon in association with a jet [14][15][16] are covering a large kinematic range and are reaching percent level accuracy demanding for theory predictions meeting this level of precision.…”

“…In the following we will compare our NNLO predictions for isolated photon production to 13 TeV ATLAS [13] and 7 TeV ALICE data [17] and our predictions for the photonplus-jet cross section to 8 TeV CMS [16] and 13 TeV ATLAS data [14]. Throughout this comparison we set the value for the electromagnetic coupling constant to α = α(0) = 1/137.036, as is appropriate for the coupling of an external resolved photon.…”

“…Their production cross sections have been studied at early hadron colliders [1][2][3][4][5] as well as in fixed-target experiments [6,7], followed by first precision measurements at the Fermilab Tevatron [8][9][10][11]. Production cross sections for photons at high transverse momentum have been measured at the LHC by the ATLAS [12][13][14], CMS [15,16] and ALICE [17] collaborations, either fully inclusively or in association with hadronic jets. Besides their importance for precision QCD phenomenology, these cross sections are also key ingredients in data-driven background estimations in new physics searches [18].…”

Measurement of the production cross section of pairs of isolated photons in pp collisions at 13 TeV with the ATLAS detector

“…This extrapolation is done performing a Poisson likelihood fit. The method is an extension of the 'twodimensional sideband' technique used in the single-photon analyses [43][44][45][46][47][48][49][50][51][52] to the twophoton case. It is similar to the method used in previous diphoton analyses [5,6].…”

Photon Fragmentation in the Antenna Subtraction Formalism

“…These observables probe fundamental QCD and QED dynamics, help to constrain the parton content of the colliding hadrons, and yield final states that are also of interest in new particle searches. At the LHC, measurements of single-photon [1][2][3][4] and di-photon [5][6][7] observables are now reaching an experimental accuracy of a few per cent, thereby demanding a comparable level of precision for the corresponding theory predictions.…”