Search for Physics beyond the Standard Model in Events with Overlapping Photons and Jets

Results are reported from a search for new particles that decay into a photon and two gluons, in events with jets. Novel jet substructure techniques are developed that allow photons to be identified in an environment densely populated with hadrons. The analyzed proton-proton collision data were collected by the CMS experiment at the LHC, in 2016 at $\sqrt{s}=13\mathrm{TeV}$, and correspond to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The spectra of total transverse hadronic energy of candidate events are examined for deviations from the standard model predictions. No statistically significant excess is observed over the expected background. The first cross section limits on new physics processes resulting in such events are set. The results are interpreted as upper limits on the rate of gluino pair production, utilizing a simplified stealth supersymmetry model. The excluded gluino masses extend up to 1.7 TeV, for a neutralino mass of 200 GeV and exceed previous mass constraints set by analyses targeting events with isolated photons.

Figure 1

The decay diagram for a single gluino as predicted by stealth SUSY. This analysis searches for pair produced gluinos and thus two such decay chains are expected in each signal event.

Figure 2

Distribution of the photon subjet energy fraction ($f_{\gamma }$) for jets that satisfy the loose photon jet requirements. Simulated distributions for signal are denoted by the broken lines, each depicting a different mass of ${\tilde{\chi }}_1^0$ and $\tilde{g}$. The shaded area represents the QCD jets distribution.

Figure 3

The $H_{\mathrm{T}}$ distributions in the signal regions for the three-jet AK8 (upper row) and the $\ge 4$ AK8 jets categories (lower row). Events with zero, one, and two tight photon jets are presented from left to right. The magenta line with the gray band corresponds to the background expectation obtained from data while the blue and red colored lines present two signal benchmarks. The lower panels present the data-to-background ratio with their respective uncertainties.

Figure 4

The upper limit at 95% confidence level on the $\tilde{g}$ pair production cross section as a function of $\tilde{g}$ and ${\tilde{\chi }}_1^0$ masses. The region enclosed by the red (lighter) solid line is excluded. The black (darker) solid line presents the expected excluded area. The uncertainty in the observed limit corresponds to the theoretical uncertainties in the signal cross section. Exclusion in the low ${\tilde{\chi }}_1^0$ and high $\tilde{g}$ mass region is a result of the implementation of the substructure techniques.