Abstract
A measurement of differential cross sections for the production of a pair of isolated photons in proton–proton collisions at sqrt{s}=7,text {TeV} is presented. The data sample corresponds to an integrated luminosity of 5.0,text {fb}^{-1} collected with the CMS detector. A data-driven isolation template method is used to extract the prompt diphoton yield. The measured cross section for two isolated photons, with transverse energy above 40 and 25,text {GeV} respectively, in the pseudorapidity range |eta |<2.5, |eta |notin [1.44,1.57] and with an angular separation Delta R > 0.45, is 17.2 pm 0.2,text {(stat)} pm 1.9,text {(syst)} pm 0.4,text {(lumi)} text {,pb}. Differential cross sections are measured as a function of the diphoton invariant mass, the diphoton transverse momentum, the azimuthal angle difference between the two photons, and the cosine of the polar angle in the Collins–Soper reference frame of the diphoton system. The results are compared to theoretical predictions at leading, next-to-leading, and next-to-next-to-leading order in quantum chromodynamics.
Highlights
The measurement of differential diphoton production cross sections offers an important test of both perturbative and non-perturbative quantum chromodynamics (QCD)
In the (η, φ) plane, and for |η| < 1.48, the hadron calorimeter (HCAL) cells map onto 5 × 5 electromagnetic calorimeter (ECAL) crystal arrays to form calorimeter towers projecting radially outwards from points slightly offset from the nominal interaction point
The trigger efficiency for photons selected in this analysis is measured to be between 98.8 and 100 % depending on the pseudorapidity and the interaction with the material in front of the ECAL
Summary
The measurement of differential diphoton production cross sections offers an important test of both perturbative and non-perturbative quantum chromodynamics (QCD). Diphoton production is sensitive to the emission of soft gluons in the initial state and to the non-perturbative fragmentation of quarks and gluons to photons in the final state Due to this rich phenomenology, theoretical predictions are challenging especially in restricted regions of phase space. The main features used to discriminate a prompt photon from a non-prompt one are the shape of the shower measured by the electromagnetic calorimeter (ECAL) [12] and the isolation energy in a cone around the photon direction [11,13] This information can be used to trigger on diphoton candidate events and, at the analysis level, to statistically evaluate the fraction of prompt diphoton candidates.
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