Measurement of isolated-photon pair production in pp collisions at $ \sqrt{s}=7\;TeV $ with the ATLAS detector

Abstract

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 √ 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 (|η| < 1.37 and 1.52 < |η| < 2.37) and with an angular separation ∆R > 0.4, is 44.0The 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. The ATLAS collaboration 26 IntroductionThe measurement at the LHC of the production cross section, in pp collisions, of two isolated photons not originating from hadronic decays, pp → γγ + X, provides a tool to probe perturbative Quantum Chromodynamics (QCD) predictions and to understand the irreducible background to new physics processes involving photons in the final state. These processes include Higgs boson decays to photon pairs (H → γγ) or graviton decays predicted in some Universal Extra-Dimension models [1, 2]. Recent cross section measurements for di-photon production at hadron colliders were performed by the DØ In this paper, the production cross section of two isolated photons with transverse energies (E T ) above 25 GeV and 22 GeV respectively, in the acceptance of the ATLAS electromagnetic calorimeter (|η| < 1.37 and 1.52 < |η| < 2.37) and with an angular separation -1 - JHEP01(2013)086∆R > 0.4, is measured. The results are obtained using the data collected by the ATLAS experiment in 2011, which corresponds to an integrated luminosity 1 of (4.9 ± 0.2) fb −1 , thus increasing the sample size by more than a factor of 100 compared to the previous measurement. The transverse energy thresholds for the two photons are higher than in the previous measurement (16 GeV).The integrated di-photon production cross section is measured, as well as the differential cross sections as a function of four kinematic variables: the di-photon invariant mass (m γγ ), the di-photon transverse momentum (p T,γγ ), the azimuthal 2 separation between the photons in the laboratory frame (∆φ γγ ), and the cosine of the polar angle of the highest E T photon in the Collins-Soper di-photon rest frame (cos θ * γγ ) [9]. The first distribution is of obvious interest for resonance searches; the second and the third provide important information in the study of higher-order QCD perturbative effects and fragmentation, especially in some specific regions such as the small ∆φ γγ limit; t...

Event generation with Sherpa 2.2

“…Figure 9: Results for the photon transversemomentum distribution in pp → e + e − γ production in association with jets at the LHC, comparing to data from [178]. Figure 10: Result for the diphoton transversemomentum distribution at the LHC, in comparison to data taken from [180]. .…”

Exotic decays of the 125 GeV Higgs boson

“…Potentially relevant searches, such as supersymmetry searches and isolated photon-pair searches [315,316], are generally insensitive to h → 2γ2j, since (a) they employ relatively hard cuts and (b) without a cut on the total invariant mass, the QCD background is overwhelming.…”

Measurement of Higgs boson production in the diphoton decay channel inppcollisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector

“…The method is cross-checked with alternative in situ techniques as described in Refs. [100,101]. The number of events for each component in the selected diphoton events sample, obtained independently in each bin of m γγ , is shown in Fig.…”