The neutral Higgs self-couplings in the (h)MSSM
We consider the Minimal Supersymmetric extension of the Standard Model in the regime where the supersymmetric breaking scale is extermely large. In this MSSM, not only the Higgs masses will be affected by large radiative corrections, the dominant part of which is provided by the third generation quark/squark sector, but also the various self-couplings among the Higgs states. In this note, assuming that squarks are extremely heavy, we evaluate the next-to-leading order radiative corrections to the two neutral CP-even Higgs self-couplings $λ_{H h h}$ and $λ_{h h h}$ and to the partial decay width $Γ(H \to h h)$ that are most relevant at the LHC. The calculation is performed using an effective field theory approach that resums the large logarithmic squark contributions and allows to keep under control the perturbative expansion. Since the direct loop vertex corrections are generally missing in this effective approach, we have properly renormalised the effective theory to take them into account. Finally, we perform a comparison of the results in this effective MSSM with those obtained in a much simpler way in the so-called hMSSM approach in which the mass value for the lightest Higgs boson $M_{h}=125$ GeV is used as an input. We show that the hMSSM provides a reasonably good approximation of the corrected self-couplings and $H \to hh$ decay rate and, hence, it can be used also in these cases.