Measurement of the Production Cross Section for Pairs of Isolated Photons in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study measures the production cross section of isolated photon pairs in proton-proton collisions at 7 TeV using 2010 data, applying data-driven methods to separate signal from background and unfolding techniques to obtain differential cross sections. The analysis compares results to NLO pQCD predictions (DIPHOX and gamma2MC), finding overall agreement in the diphoton mass spectrum but a significant deficit in regions with small photon azimuthal separation (Δφγγ ≲ 2.8) and some angular distributions. The work validates the methodology for precision photon-pair measurements and highlights kinematic regimes where fixed-order calculations underestimate the data, informing future theoretical improvements and background modelling for Higgs and new-physics searches. The study provides detailed cross sections and uncertainties, strengthening the CMS diphoton program at the LHC.

Abstract

The integrated and differential cross sections for the production of pairs of isolated photons is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 36 inverse picobarns is analysed. A next-to-leading-order perturbative QCD calculation is compared to the measurements. A discrepancy is observed for regions of the phase space where the two photons have an azimuthal angle difference, Delta(phi), less than approximately 2.8.

Figures (10)

• Figure 1: Probability density functions of the ECAL isolation variable $\mathcal{I}$ for signal photons (solid blue) and background photons (dashed red) in the barrel (left) and in the endcap regions (right).
• Figure 2: Fit to the photon ECAL isolation $(\mathcal{I}_1,\mathcal{I}_2)$ in the bin $100 < m_{\gamma\gamma} < 140\,\text{Ge\spaceV}\xspace$ for photons with $|\eta| < 1.44$. The distribution of the isolation variable $\mathcal{I}_1$ of one photon candidate, arbitrarily chosen as the "first photon" and denoted with subscript "1", is displayed in the left figure, together with the fit result, integrated over $\mathcal{I}_2$; the shaded region shows the signal distribution, the dashed line represents the background contribution, while the solid line is the sum of the signal and background contributions. The same distributions for the second photon candidate are shown in the right figure. In this mass bin, the number of signal events is $72 \pm 14$, out of the total number of $161$ selected candidates.
• Figure 3: (Left) Diphoton differential cross section as a function of the photon pair invariant mass $m_{\gamma\gamma}$ from data (points) and from theory (solid line) for the photon pseudorapidity range $|\eta| < 2.5$. (Right) The difference between the measured and theoretically predicted diphoton cross sections, divided by the theory prediction, as a function of $m_{\gamma\gamma}$. In both plots, the inner and outer error bars on each point show the statistical and total experimental uncertainties. The 4% uncertainty on the integrated luminosity is not included in the error bars. The dotted line and shaded region represent the systematic uncertainties on the theoretical prediction from the theoretical scales and the PDFs, respectively.
• Figure 4: (Left) Diphoton differential cross section as a function of the photon pair invariant mass $m_{\gamma\gamma}$ from data (points) and from theory (solid line) for the photon pseudorapidity range $|\eta| < 1.44$. (Right) The difference between the measured and theoretically predicted diphoton cross sections, divided by the theory prediction, as a function of $m_{\gamma\gamma}$. In both plots, the inner and outer error bars on each point show the statistical and total experimental uncertainties. The 4% uncertainty on the integrated luminosity is not included in the error bars. The dotted line and shaded region represent the systematic uncertainties on the theoretical prediction from the theoretical scales and the PDFs, respectively.
• Figure 5: (Left) Diphoton differential cross section as a function of the photon pair transverse momentum $p_{T,\gamma\gamma}$ from data (points) and from theory (solid line) for the photon pseudorapidity range $|\eta| < 2.5$. (Right) The difference between the measured and theoretically predicted diphoton cross sections, divided by the theory prediction, as a function of $p_{T,\gamma\gamma}$. In both plots, the inner and outer error bars on each point show the statistical and total experimental uncertainties. The 4% uncertainty on the integrated luminosity is not included in the error bars. The dotted line and shaded region represent the systematic uncertainties on the theoretical prediction from the theoretical scales and the PDFs, respectively.