Abstract
The ratio of the cross sections for inclusive isolated-photon production in pp collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb−1 and 20.2 fb−1, respectively. The ratio is measured as a function of the photon transverse energy in different regions of the photon pseudorapidity. The predictions from next-to-leading-order perturbative QCD calculations are compared with the measured ratio. The experimental systematic uncertainties as well as the uncertainties affecting the predictions are evaluated taking into account the correlations between the two centre-of-mass energies, resulting in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic uncertainties. The predictions based on several parameterisations of the proton parton distribution functions agree with the data within the reduced experimental and theoretical uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson production at 13 and 8 TeV using the decay channels Z → e+e− and Z → μ+μ− is made and compared with the theoretical predictions. In this double ratio, a further reduction of the experimental uncertainty is obtained because the uncertainties arising from the luminosity measurement cancel out. The predictions describe the measurements of the double ratio within the theoretical and experimental uncertainties.
Highlights
At leading order (LO) in pQCD, two processes contribute to prompt-photon production: the direct-photon process, in which the photon originates directly from the hard interaction, and the fragmentation-photon process, in which the photon is emitted in the fragmentation of a high transverse momentum parton [3, 4]
The measurements of inclusive isolated-photon cross sections performed by ATLAS at 13 TeV [9] and 8 TeV [8] were compared with the predictions of pQCD at next-to-leading order (NLO) [12, 13]
This paper presents measur√ements of the ratio of cross sections for isolated-photon production in pp collisions at s = 13 TeV and 8 TeV with the ATLAS detector at the LHC
Summary
The ATLAS experiment [18] at the LHC uses a multipurpose particle detector with a forward-backward symmetric cylindrical geometry and a near 4π coverage in solid angle. It consists of an inner tracking detector, electromagnetic (EM) and hadronic calorimeters, and a muon spectrometer. The first-level trigger was implemented in hardware and used a subset of the detector information This was followed by two software-based trigger levels that together reduce the accepted event rate to approximately 400 Hz. For the data taken at 13 TeV, the trigger was changed [21] to a two-level system, using custom hardware. Followed by a software-based level which runs offline reconstruction software, reducing the event rate to approximately 1 kHz
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