Enhanced Rates for Diphoton Resonances in the MSSM

We propose a simple mechanism for copiously producing heavy Higgs bosons with enhanced decay rates to two photons at the LHC, within the context of the Minimal Supersymmetric extension of the Standard Model (MSSM). In the CP-conserving limit of the theory, such a diphoton resonance may be identified with the heavier CP-even $H$ boson, whose gluon-fusion production and decay into two photons are enhanced by loops of the lightest supersymmetric partner of the top quark $\tilde{t}_1$ when its mass $m_{\tilde{t}_1}$ happens to be near the $\tilde{t}^*_1\tilde{t}_1$ threshold, i.e.~for $m_{\tilde{t}_1}\!\simeq \!\frac12 M_H$. The scenario requires a relatively low supersymmetry-breaking scale $M_S\stackrel{<}{{}_\sim} 1$ TeV, but large values of the higgsino mass parameter, $\mu \stackrel{>}{{}_\sim} 1$ TeV, that lead to a strong $H \tilde{t}^*_1 \tilde{t}_1$ coupling. Such parameters can accommodate the observed mass and standard-like couplings of the 125 GeV $h$ boson in the MSSM, while satisfying all other constraints from the LHC and dark matter searches. Additional enhancement to the diphoton rate could be provided by Coulombic QCD corrections and, to a lesser extent, by resonant contributions due to $\tilde{t}_1^* \tilde{t}_1$ bound states. To discuss the characteristic features of such a scenario, we consider as an illustrative example the case of a diphoton resonance with a mass of approximately 750 GeV, for which an excess was observed in the early LHC 13 TeV data and which later turned out to be simply a statistical fluctuation.