Search for resonances decaying to photon pairs with masses between 4.9 and 19.4 GeV

Abstract

A search is presented for axion-like particles (ALPs) with masses between 4.9 and 19.4 GeV decaying to a pair of photons, using proton-proton collisions collected with the LHCb detector during 2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 2.1 fb$^{-1}$. The same strategy and sample is used to search for the decays of the $B^0_s$, $B^0$ and $\eta_b$ mesons into photon pairs. No significant excess is found. Upper limits on the photon-pair branching fraction times the cross-section of ALP production are determined as a function of the ALP mass. Limits on the branching fractions of the beauty states are determined to be $\mathcal{B}(B^0_s\to\gamma\gamma)<2.7\times10^{-5}$, $\mathcal{B}(B^0\to\gamma\gamma)<0.83\times10^{-5}$, and $ \sigma(pp\to\eta_b X)\times\mathcal{B}(\eta_b\to\gamma\gamma)<765\,\text{pb}$ at 95 % confidence level.