Two-loop analysis of classically scale-invariant models with extended Higgs sectors

We present the first explicit calculation of leading two-loop corrections to the Higgs trilinear coupling λhhh in models with classical scale invariance (CSI), using the effective-potential approximation. Furthermore, we also study — for the first time at two loops — the relation that appears between the masses of all states in CSI theories, due to the requirement of reproducing correctly the 125-GeV Higgs-boson mass. In addition to obtaining analytic results for general CSI models, we consider two particular examples of Beyond-the-Standard-Model theories with extended Higgs sectors, namely an N-scalar model (endowed with a global O(N) symmetry) and a CSI version of the Two-Higgs-Doublet Model, and we perform detailed numerical studies of these scenarios. While at one loop the value of the Higgs trilinear coupling is identical in all CSI models, and deviates by approximately 82% from the (one-loop) SM prediction, we find that the inclusion of two- loop corrections lifts this universality and allows distinguishing different BSM scenarios with CSI. Taking into account constraints from perturbative unitarity and the relation among masses, we find for both types of scenarios we consider that at two loops λhhh deviates from its SM prediction by 100 ± 10% — i.e. a quite significant further deviation with respect to the one-loop result of ∼ 82%.

TESTING TIMES FOR SUPERSYMMETRY: LOOKING UNDER THE LAMP POST

We make a critical study of two highly-constrained models of supersymmetry — the constrained minimal supersymmetric standard model (cMSSM), and the nonuniversal Higgs mass (NUHM) model — in the light of the 125–126 GeV Higgs boson, the first observation of Bs→μμ at the LHCb, and the updated B → τν branching ratio at BELLE. It turns out that these models are still allowed by the experimental data, even if we demand that there be a light stop with mass less than 1.5 TeV. The only significant effects of all these constraints are to push the mass of the light stop above ~500 GeV, and to prefer the universal trilinear coupling A0to be large and negative. We calculate the Higgs boson branching ratios to WW, ZZ, ττ and γγ in these models and show that improved experimental limits on these could put them to the most stringent experimental tests yet.