Search for exotic Higgs boson decays to merged photons employing a novel deep learning technique at CMS

A search for exotic Higgs boson decays, of the form H $\rightarrow$ aa, with a $\rightarrow$ $\gamma$ $\gamma$ is performed. The hypothetical particle a is a light, scalar or pseudoscalar particle decaying to two highly merged photons, reconstructed as a single photon object in the CMS detector. A novel, end-to-end deep learning-based technique is used to directly measure the invariant mass of merged a $\rightarrow$ $\gamma$ $\gamma$ candidates, additionally providing a first measurement of the mass spectrum of objects reconstructed as single photons. Analysis criteria similar to those used in the standard model H $\rightarrow$ $\gamma$ $\gamma$ search are applied, to probe the possibility that existing measurements in this decay mode may conceal a contribution from a low-mass particle a. The search is performed using the full CMS Run II data set, corresponding to a total integrated luminosity of 136 fb$^{-1}$, at a proton-proton center-of-mass collision energy of a $\sqrt{s}$ = 13 TeV. No significant excess over standard model expectations is found. Branching fractions for this process of $\beta$(H $\rightarrow$ aa $\rightarrow$ 4$\gamma$) $\geq$ 0.9-3.3 x 10$^{-3}$ are excluded at the 95% confidence level, for particle masses between 0.1 $\leq$ $_{a}$ 1.2 GeV, assuming negligible lifetime.