Measurement of isolated-photon pair production in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

The ATLAS study presents a precision measurement of isolated-photon pair production in proton-proton collisions at 7 TeV, using 4.9 fb-1 of data. It reports the integrated and differential di-photon cross sections, obtained after data-driven background subtraction and unfolding to the particle level, and compares them to LO, NLO, and NNLO theoretical predictions. The NNLO 2γNNLO calculation matches the data well, while LO and NLO predictions underestimate in fragmentation-dominated or soft-gluon regions; SHERPA/Lund‑style MCs reproduce many features after normalization. The results validate perturbative QCD in di-photon production and provide a robust background benchmark for Higgs and other photon-involving signatures at the LHC.

Abstract

The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at sqrt(s) = 7 TeV. The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb-1, is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (|eta|<1.37 and 1.52<|eta|<2.37) and with an angular separation Delta R>0.4, is 44.0 (+3.2) (-4.2) pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins--Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.

Figures (5)

• Figure 1: Projections of the two-dimensional fit to the transverse isolation energies of the two photon candidates: leading photon (left) and sub-leading photon (right). The photon templates from Sherpa are shifted by +160 MeV (+120 MeV) for the leading (subleading) photon. Solid circles represent the observed data. The (black) solid line is the fit result, the (violet) dash-dotted curve shows the ${\gamma\gamma}$ component. The (red) dotted line shows in the left (right) figure the contribution from ${\gamma\mathrm{j}}$ (${\mathrm{j}\gamma}$) events. In both figures, the (blue) dashed line represents a broad background component in the photon candidates' sample: for the leading candidate this is due to ${\mathrm{j}\gamma}$ and ${\mathrm{jj}}$ final states, whereas for the sub-leading candidate it comes from ${\gamma\mathrm{j}}$ and ${\mathrm{jj}}$ final states.
• Figure 2: Differential spectra in data (solid circles) and from the two-dimensional fit, for the $\gamma\gamma$ (hollow histogram), $\gamma$j$+$j$\gamma$ (light solid histogram), and jj (dark solid histogram) contributions. The spectra are shown for the following di-photon variables: $m_{\gamma\gamma}$ (top left), $p_{{\rm T},{\gamma\gamma}}$ (top right), $\Delta\phi_{\gamma\gamma}$ (bottom left), $\cos\theta^*_{\gamma\gamma}$ (bottom right).
• Figure 3: Fraction of electron background (impurity) as a function of $m_{\gamma\gamma}$, $p_{{\rm T},{\gamma\gamma}}$, $\Delta\phi_{\gamma\gamma}$, and $\cos\theta^*_{\gamma\gamma}$.
• Figure 4: Comparison between the experimental cross sections and the predictions obtained with parton-shower LO simulations: $m_{\gamma\gamma}$ (top left), $p_{{\rm T},{\gamma\gamma}}$ (top right), $\Delta\phi_{\gamma\gamma}$ (bottom left), $\cos\theta^*_{\gamma\gamma}$ (bottom right). The LO cross sections have been scaled to the total data cross section, by a factor 1.2. Black dots correspond to data with error bars for their total uncertainties, which are dominated by the systematic component. The simulated cross sections include only statistical uncertainties.
• Figure 5: Comparison between the experimental cross sections and the predictions obtained with Diphox+gamma2mc (NLO) and 2$\gamma$NNLO (NNLO): $m_{\gamma\gamma}$ (top left), $p_{{\rm T},{\gamma\gamma}}$ (top right), $\Delta\phi_{\gamma\gamma}$ (bottom left), $\cos\theta^*_{\gamma\gamma}$ (bottom right). Black dots correspond to data with with error bars for their total uncertainties, which are dominated by the systematic component. The theoretical uncertainties include contributions from the limited size of the simulated sample, from the scale choice and from uncertainties on the parton distribution functions and on the hadronization and underlying event corrections.