Photon Production In PbPb Collisions Research Articles

The production of isolated photons in PbPb and pp collisions at sqrt{s_{mathrm{NN}}} = 5.02 TeV

The transverse energy ( {E}_{mathrm{T}}^{upgamma} ) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at sqrt{s_{mathrm{NN}}} = 5.02 TeV with integrated luminosities of 27.4 pb−1 and 404 μb−1 for pp and PbPb data, respectively. The results are presented for photons with 25 < {E}_{mathrm{T}}^{upgamma} < 200 GeV in the pseudorapidity range |η| < 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do not interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order jetphox generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits.

Direct photon yield in pp and in Pb–Pb collisions measured with the ALICE experiment

The measurement of direct photon production in Pb-Pb collisions at sNN=2.76 TeV and in pp collisions at s=7 TeV with the ALICE experiment is presented. In Pb-Pb collisions a clear direct photon signal below 3 GeV/c is observed only for the 0–20 % most central collisions. No excess is observed for semi-central and peripheral Pb-Pb collisions and in pp collisions.Furthermore, in pp collision the measurement of direct photon production in the range 10≤pT≤60 GeV/c is reported. The analysis is performed on the EMCal-triggered data taken in 2011. The two main sources of background, namely photons from fragmentation processes and decay photons, have been subtracted from the inclusive photon spectrum by means of an isolation technique combined with the study of the transverse dispersion of electromagnetic showers. The measurement is in agreement both with NLO pQCD calculations and with those performed by the ATLAS and the CMS collaborations, extending the isolated photon spectrum investigated at LHC towards lower values of pT.

Direct photon measurement in Pb–Pb collisions at [formula omitted] with ALICE

The ALICE experiment has measured the direct photon spectra in Pb-Pb collisions at sNN=2.76 TeV for three different centrality selections. The measurement was performed emplying a method utilizing conversion of photons into e+e− pairs in the detector material, and a method using the PHOS calorimeter. The two measurements were combined in order to measure direct photons over a broad transverse momentum range of 0.9<pT<14 GeV/c. A direct photon signal was observed in the thermal region 0.9<pT<2.1 GeV/c, with a significance of 2.6σ in the 20% most central collisions. The corresponding direct photon spectrum can be described by an exponential with an inverse slope parameter of (304±11stat±40syst) MeV, without subtracting the hard scattering component, which is somewhat larger than the inverse slope parameter of the direct photon spectrum at RHIC energies. Within uncertainties, the data can be described by different models for photon production in heavy-ion collisions. These proceedings provide a summary of the results published in [J. Adam, others (ALICE Collaboration), Direct photon production in Pb-Pb collisions at sNN=2.76 TeV, arXiv:1509.07324] and [J. Adam, others (ALICE Collaboration), Supplemental figures: Direct photon production in Pb-Pb collisions at sNN=2.76 TeV. URL https://cds.cern.ch/record/2102398].