Abstract
The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at a centre-of-momentum energy of sqrt{s}= 7 TeV. The measurement is performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of |eta |<0.27 in pseudorapidity and a transverse momentum range of 10< p_mathrm {T}^{gamma }< 60~mathrm {GeV}/c. The result extends the p_mathrm {T} coverage of previously published results of the ATLAS and CMS experiments at the same collision energy to smaller p_mathrm {T}. The measurement is compared to next-to-leading order perturbative QCD calculations and to the results from the ATLAS and CMS experiments. All measurements and theory predictions are in agreement with each other.
Highlights
In high-energy particle collisions, direct photons are those photons which are directly produced in elementary processes, and as such are not products from hadronic decays
The isolated direct photon production differential cross section can be obtained from the following equation: d2σ γ d pTγ dη d2 Nniso d pTγ dη where all the terms were described in the previous sections
The central values of the predictions were obtained by choosing factorisation, normalisation and fragmentation scales equal to the photon transverse momentum (μ f = μR = μF = pTγ )
Summary
In high-energy particle collisions, direct photons are those photons which are directly produced in elementary processes, and as such are not products from hadronic decays. The collinear part of such processes has been shown to contribute effectively at LO These measurements allow one to study a wide range of transverse momentum ( pT) of prompt photon production from 15 to 1000 GeV/c, the lowest limit being partially defined by the use of a high-energy photon trigger. PYTHIA does not contain all effects of higher orders in QCD systematically, but has some important enhancements beyond pure LO via initial- and final-state radiation In this calculation, we have not implemented an isolation cut, but we assume that for the purpose of this study, it is equivalent to simulating only the LO processes for partonic photon production. At lower pT, other sources of direct photons than prompt photons exist, in particular in high-energy nuclear collisions, where e.g. thermal photons are produced from the thermalised system These are important probes of the quarkgluon plasma.
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