Natural NMSSM Higgs Bosons

We study the phenomenology of Higgs bosons close to 126 GeV within the scale invariant unconstrained next-to-minimal supersymmetric Standard Model (NMSSM), focusing on the regions of parameter space favoured by low fine-tuning considerations, namely stop masses of order 400 GeV to 1 TeV and an effective µ parameter between 100–200 GeV, with large (but perturbative) λ and low tanβ =2–4. We perform scans over the above parameter space, focusing on the observable Higgs cross sections into γγ, WW, ZZ, bb, ττ final states, and study the correlations between these observables. We show that the γγ signal strength may be enhanced up to a factor of about two not only due to the effect of singlet-doublet mixing, which occurs more often when the 126 GeV Higgs boson is the next-to-lightest CP-even one, but also due to light stops (and to a lesser extent light chargino and charged Higgs loops). There may be also smaller enhancements in the Higgs decay channels into WW, ZZ, correlated with the γγ enhancement. However there is no such correlation observed involving the Higgs decay channels into bb, ττ. The requirement of having perturbative couplings up to the GUT scale favours the interpretation of the 126 GeV Higgs boson as being the second lightest NMSSM CP-even state, which can decay into pairs of lighter neutralinos, CP-even or CP-odd Higgs bosons, leading to characteristic signatures of the NMSSM. In a non-negligible part of the parameter range the increase in the γγ rate is due to the superposition of rates from nearly degenerate Higgs bosons. Resolving these Higgs bosons would rule out the Standard Model, and provide evidence for the NMSSM