Abstract
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.
Highlights
Be isolated from other particles in order to reduce a large background of decay photons coming from neutral mesons
The nuclear modification factors exhibit little or no modifications of isolated photons in all ETγ and centrality bins in PbPb collisions, considering the quoted statistical and systematic uncertainties. This indicates that the isolated photons are not modified by the strongly interacting medium produced in heavy ion collisions, which is in contrast to hadrons in
The differential cross sections of photons isolated from nearby particles are reported at pseudorapidity |ηγ | < 1.44 for transverse energy from 25 to 200 GeV in proton-proton
Summary
The central feature of the CMS detector system is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Each detector element consists of a barrel and two endcap sections. The long fibers run the entire depth of the HF calorimeter (165 cm, or approximately 10 interaction lengths), while the short fibers start at a depth of 22 cm from the front of the detector. By reading out the two sets of fibers separately, it is possible to distinguish showers generated by electrons and photons, which deposit a large fraction of their energy in the long-fiber calorimeter segment, from those generated by hadrons, which produce on average nearly equal signals in both calorimeter segments. The first level (L1), composed of custom hardware processors, uses information from the calorimeters and muon detectors to select events at a rate of around 100 kHz within a time interval of less than 4 μs. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [31]
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