Search for resonances in diphoton events at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on protonproton collision data corresponding to an integrated luminosity of 3.2 fb −1 at √ s = 13 TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall-Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported. Conclusion 26The ATLAS collaboration 33 IntroductionNew high-mass states decaying into two photons are predicted in many extensions of the Standard Model (SM). The diphoton final state provides a clean experimental signature with excellent invariant mass resolution and moderate backgrounds. Searches for new high-mass resonances decaying into two photons are described, using CERN Large Hadron Collider (LHC) [1] proton-proton (pp) collision data at √ s= 13 TeV recorded in 2015 by the ATLAS detector. The data correspond to an integrated luminosity of 3.2 fb −1 .The decay photons would have different kinematic properties depending on whether the hypothetical particle has spin-0 or spin-2. These are exploited by applying two different -1 - JHEP09(2016)001selections, with looser kinematic selection requirements for the spin-2 resonance search. The photon identification criteria and the event pre-selection are common to both searches.The search for a spin-2 γγ resonance uses the Randall-Sundrum (RS) model [2] graviton as a benchmark. This entails a lightest spin-2 graviton excitation (G * ) with a dimensionless coupling k/M Pl , where M Pl = M Pl / √ 8π is the reduced Planck scale and k the curvature scale of the extra dimension. The lightest graviton excitation is expected to be a fairly narrow resonance for k/M Pl < 0.3 [4], with the width given by 1.44(k/M Pl ) 2 m G * , where m G * is the mass of the lightest graviton state. For k/M Pl = 0.1, the natural width increases from 11 GeV at m G * = 800 GeV to 30 GeV at m G * = 2200 GeV. For m G * = 800 GeV, the contributions of the natural width and of the experimental mass resolution to the width of the resonance are comparable. The shape of the invariant mass distribution of the main background from the production of prompt photon pairs is estimated from theoretical computations, and the contribution from the reducible background of events where at least one jet is misidentified as a photon is added f...

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