Measurement of differential cross sections for the production of a pair of isolated photons in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS analysis measures differential cross sections for producing isolated photon pairs in pp collisions at 7 TeV, using a data-driven PF isolation template to separate prompt from non-prompt photons and unfolding to particle level. The study compares results to multiple QCD predictions, including NNLO and SHERPA, finding good agreement with NNLO in total yield and improved shape agreement with higher-order calculations. The approach leverages a two-dimensional template fit and rigorous control of systematics, providing precise tests of perturbative and non-perturbative QCD in a Higgs-background-dominated final state. The results enhance understanding of diphoton production mechanisms and constrain theoretical models used in Higgs analyses and beyond-standard-model searches.

Abstract

A measurement of differential cross sections for the production of a pair of isolated photons in proton-proton collisions at sqrt(s) = 7 TeV is presented. The data sample corresponds to an integrated luminosity of 5.0 inverse femtobarns 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 GeV respectively, in the pseudorapidity range abs(eta) < 2.5, abs(eta) not in [1.44,1.57] and with an angular separation Delta R > 0.45, is 17.2 +/- 0.2 (stat.) +/- 1.9 (syst.) +/- 0.4 (lum.) 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.

Figures (9)

• Figure 1: Comparison of prompt photon templates in data and simulation: prompt photons in the simulation (squares), prompt photon templates extracted with the random cone technique from simulation (triangles) and from data (dots); (top) candidates in the ECAL barrel, (bottom) candidates in the ECAL endcaps. All histograms are normalized to unit area.
• Figure 2: Comparison of non-prompt photon templates in data and simulation: non-prompt photons in the simulation (squares), non-prompt photon templates extracted with the sideband technique from simulation (triangles) and from data (dots); (top) candidates in the ECAL barrel, (bottom) candidates in the ECAL endcaps. All histograms are normalized to unit area.
• Figure 3: Result of the first step of the fitting procedure, for the $90\,\text{Ge\spaceV}\xspace<m_{\gamma\gamma}\xspace<95\,\text{Ge\spaceV}\xspace$ bin in the EB-EE category: isolation distribution for the photon reconstructed in the (left) ECAL barrel, (right) ECAL endcaps.
• Figure 4: Projections of the result of the final step of the fitting procedure, for the $90\,\text{Ge\spaceV}\xspace<m_{\gamma\gamma}\xspace<95\,\text{Ge\spaceV}\xspace$ bin in the EB-EE category: isolation distribution for the photon reconstructed in the (left) ECAL barrel, (right) ECAL endcaps.
• Figure 5: Fractions of prompt-prompt, prompt-non-prompt and non-prompt-non-prompt components as a function of $m_{\gamma\gamma}$, $p_{\mathrm{T}}\xspace^{\gamma\gamma}$, $\Delta \phi_{\gamma\gamma}$, $\lvert \cos\theta^* \rvert$ in the whole acceptance of the analysis. Uncertainties are statistical only.