Measurement of prompt photon production in sNN=8.16 TeV p + Pb collisions with ATLAS

Abstract

Measurement of prompt photon production in √ s NN = 8.16 TeV p+Pb collisions with ATLASThe ATLAS Collaboration The inclusive production rates of isolated, prompt photons in p+Pb collisions at √ s NN = 8.16 TeV are studied with the ATLAS detector at the Large Hadron Collider using a dataset with an integrated luminosity of 165 nb −1 recorded in 2016. The cross-section and nuclear modification factor R pPb are measured as a function of photon transverse energy from 20 GeV to 550 GeV and in three nucleon-nucleon centre-of-mass pseudorapidity regions, (−2.83, −2.02), (−1.84, 0.91), and (1.09, 1.90). The cross-section and R pPb values are compared with the results of a next-to-leading-order perturbative QCD calculation, with and without nuclear parton distribution function modifications, and with expectations based on a model of the energy loss of partons prior to the hard scattering. The data disfavour a large amount of energy loss and provide new constraints on the parton densities in nuclei.1 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) and the rapidity of the components of the beam, y, are defined in terms of their energy, E, and longitudinal momentum, p z , as y = 0.5 lncompartments. The first compartment is a copper/LAr electromagnetic calorimeter, while the remaining two tungsten/LAr calorimeter compartments collect the hadronic energy.During data-taking, events were initially selected using a level-1 trigger, implemented in custom electronics, based on energy deposition in the electromagnetic calorimeter. The high-level trigger [29] was then used to select events consistent with a high-E γ T photon candidate. The high level trigger was configured with five online E γ T thresholds from 15 GeV to 35 GeV. Each trigger is used for an exclusive region of the E γ T spectrum, starting 5 GeV above the trigger threshold because there the trigger is fully efficient. The highest-threshold trigger is used in the measurement over the whole E γ T range above 40 GeV and is unprescaled. The lower-threshold, prescaled, triggers are used to perform the measurement for E γ T in the range of 20 − 40 GeV.Data-taking was divided into two periods with different configurations of the LHC beams. In the first period, the lead ions circulated in beam 1 (clockwise) and protons circulated in beam 2, while in the second period the beams were reversed. These periods corresponded to integrated luminosities of 57 nb −1 and 108 nb −1 respectively. Photon reconstruction and identificationPhotons are reconstructed following a procedure used extensively in previous ATLAS measurements [10], of which only the main features are summarised here.Photon ca...

nNNPDF2.0: quark flavor separation in nuclei from LHC data

“…We have benchmarked the results of this MCFM-based calculation with the theory predictions presented in ref. [107] based on the JETPHOX program [108], finding a reasonable agreement but also some differences at large rapidities.…”

“…In order to disentangle hot from cold nuclear matter effects, photon production has been measured in proton-lead collisions at the LHC, providing a new channel to constrain the nuclear modifications of the gluon PDF. Here we focus on the recent ATLAS measurements from Run II at √ s = 8.16 TeV based on an integrated luminosity of L = 165 nb −1 [107]. This analysis provides the inclusive production rates of isolated prompt photons in three different rapidity regions as a function of E γ T , the photon transverse energy, in the range 20 GeV to 550 GeV.…”

“…Current measurements of direct photon production at the LHC, such as those discussed above from the ATLAS collaboration [107] as well as related measurements from CMS and ALICE [109], are restricted to the central rapidity region. The reason is that this is the only region instrumented with electromagnetic calorimeters and thus suitable to identify photons.…”

Prompt photon production in high-energy pA collisions at forward rapidity

“…There is a clear resemblance between the two pp and p(d )A cases. The published data from (dAu) PHENIX [45] and (pPb) ATLAS [34] are included. For the sake of illustration, we also included the preliminary data from PHENIX for pAu collisions ( √ s = 200 GeV) [55,56] as well as the preliminary ALICE pPb data ( √ s = 5.02 TeV) [57].…”

A QCD analysis of LHCb D-meson data in p+Pb collisions

“…low-mass Drell-Yan dilepton and isolated-photon production at low transverse momentum p T [29][30][31][32][33][34][35]. Isolated photons in p+Pb collisions have already been measured at central rapidities [36], and the large-y measurements appear to be within the capabilities of the LHCb collaboration [37]. In further future, measurements of isolatedphoton production would be a central goal of the ALICE FoCal upgrade [38].…”