Rare top-quark decays to Higgs boson in MSSM

Abstract

In full one-loop generality and in next-to-leading order in QCD, we study rare top to Higgs boson flavour changing decay processes t → qh with q = u, c quarks, in the general MSSM with R-parity conservation. Our primary goal is to search for enhanced effects on $\mathrm{\mathcal{B}}\left(t\to qh\right)$ that could be visible at current and high luminosity LHC running. To this end, we perform an analytical expansion of the amplitude in terms of flavour changing squark mass insertions that treats both cases of hierarchical and degenerate squark masses in a unified way. We identify two enhanced effects allowed by various constraints: one from holomorphic trilinear soft SUSY breaking terms and/or right handed up squark mass insertions and another from non-holomorphic trilinear soft SUSY breaking terms and light Higgs boson masses. Interestingly, even with $\mathcal{O}(1)$ flavour violating effects in the, presently unconstrained, up-squark sector, SUSY effects on $\mathrm{\mathcal{B}}\left(t\to qh\right)$ come out to be unobservable at LHC mainly due to leading order cancellations between penguin and self energy diagrams and the constraints from charge- and colour-breaking minima (CCB) of the MSSM vacuum. An exception to this conclusion may be effects arising from non-holomorphic soft SUSY breaking terms in the region where the CP-odd Higgs mass is smaller than the top-quark mass but this scenario is disfavoured by recent LHC searches. Our calculations for t → qh decay are made available in SUSY FLAVOUR numerical library.