Search for new phenomena in high-mass diphoton final states using 37 fb$^{-1}$ of proton-proton collisions collected at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

The ATLAS search for high-mass diphoton final states using $36.7\ \mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s}=13\ \mathrm{TeV}$ investigates spin-0 and spin-2 resonances (including RS1 gravitons) and non-resonant ADD scenarios via the diphoton channel. The study employs optimized photon reconstruction, a detailed signal–background modelling framework, and data-driven background estimates, analyzed through maximum-likelihood fits and CLs limits. No significant excess is found; the results set upper limits on $\sigma\times\mathcal{B}(\gamma\gamma)$ for resonances and lower limits on ADD $M_S$ across formalisms, with RS1 exclusions for chosen couplings. The results extend and supersede prior Run-1/Run-2 ATLAS results, constraining a broad range of extra-dimensional theories and informing future explorations of diphoton signatures in high-energy collisions.

Abstract

Searches for new phenomena in high-mass diphoton final states with the ATLAS experiment at the LHC are presented. The analysis is based on $pp$ collision data corresponding to an integrated luminosity of 36.7 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s}=13$ TeV recorded in 2015 and 2016. Searches are performed for resonances with spin 0, as predicted by theories with an extended Higgs sector, and for resonances with spin 2, using a warped extra-dimension model as a benchmark model, as well as for non-resonant signals, assuming a large extra-dimension scenario. No significant deviation from the Standard Model is observed. Upper limits are placed on the production cross section times branching ratio to two photons as a function of the resonance mass. In addition, lower limits are set on the ultraviolet cutoff scale in the large extra-dimensions model.

Figures (4)

• Figure 1: The diphoton invariant-mass distributions of the data are shown in the upper panels for (a) the spin-0 and (b) the spin-2 selections and their decomposition into contributions from genuine diphoton ($\gamma\gamma$), photon+jet ($\gamma j$ and $j \gamma$) and dijet ($jj$) events as determined using the 2x2D sideband method. The bottom panels show the purity of diphoton events as determined by the matrix method and the 2$\times$2D sideband method. Each point in the distributions is plotted in the centre of the corresponding bin. The total uncertainties, including statistical and systematic components added in quadrature, are shown as error bars.
• Figure 2: Distributions of the diphoton invariant mass for events passing (a) the spin-0 selection or (b) the spin-2 selection, with the background-only fits superimposed. The data points are plotted at the centre of each bin. The error bars indicate statistical uncertainties. The differences between the data and the fits are shown in the bottom panels. The arrows in the lower panels indicate values outside the range by more than one standard deviation. There is no data event with $m_{\gamma\gamma}>2700$.
• Figure 3: Compatibility, in terms of local $p_0$ quantified in standard deviations $\sigma$, with the background-only hypothesis (a) as a function of the assumed signal mass $m_X$ and relative width $\Gamma_X/m_X$ for the spin-0 resonance search and (b) as a function of the assumed signal mass $m_{G^*}$ and for the spin-2 resonance search. Only positive excesses are considered.
• Figure 4: (a) Upper limits on the fiducial cross section times branching ratio to two photons at $\sqrt{s} = 13$ of a narrow-width ($\Gamma_X$ = 4 ) spin-0 resonance as a function of its mass $m_X$. (b) Upper limits on the production cross section times branching ratio to two photons at $\sqrt{s} = 13$ of the lightest KK graviton as a function of its mass $m_{G^*}$ for $\coupling=0.1$. For $m_{G^*}>2500~\GeV$, the observed and expected limits are determined with pseudo-experiments shown by the blue solid and dashed lines, respectively. Predictions are shown for the RS1 model, where the grey shaded band represents the PDF uncertainty.