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 proton--proton collision data corresponding to an integrated luminosity of 3.2 fb$^{-1}$ at $\sqrt{s} = 13$ 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.

Highlights

  • Background estimatesTwo different methods are used to estimate the background contributions to the mγγ distribution

  • Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described

  • In the case of the functional-form approach to describe the background, the parameters of the function are nuisance parameters without penalty terms, and the systematic uncertainty in the background description is implemented by the “spurious” signal term, which is constrained by a Gaussian penalty term and, for a given hypothesis, has the same invariant mass distribution as the signal

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Summary

Introduction

Background estimatesTwo different methods are used to estimate the background contributions to the mγγ distribution. In the case of the functional-form approach to describe the background, the parameters of the function are nuisance parameters without penalty terms, and the systematic uncertainty in the background description is implemented by the “spurious” signal term, which is constrained by a Gaussian penalty term and, for a given (mX , α) hypothesis, has the same invariant mass distribution as the signal. This “spurious” signal uncertainty is considered separately for each (mX , α) hypothesis without any correlation between the different investigated mass ranges

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