Higher order and CP-violating effects in the neutralino and Higgs boson sectors of the MSSM

Abstract

Complete one-loop results are presented for neutralino and Higgs decay processes of the form chi^0_i->chi^0_j h_a and h_a->chi^0_i chi^0_j in the MSSM with CP-violating parameters. An on-shell renormalisation scheme is developed for the chargino–neutralino sector that consistently takes into account imaginary parts arising from complex parameters and absorptive parts of loop integrals. The genuine vertex contributions are combined with two-loop Higgs propagator-type corrections to obtain the most precise
prediction currently available for this class of processes. In the CP-violating CPX benchmark scenario, the corrections to the neutralino decay width are found to be particularly large – of order 45% for a Higgs mass of 40GeV. We find that in this unexcluded parameter region, which will be difficult to cover by standard Higgs search channels
at the LHC, the branching ratio for the decay chi^0_2->chi^0_1 h_1 is large. This may offer good prospects of detecting such a light Higgs boson in cascade decays of supersymmetric particles. We also study the full Higgs production and decay processes in scenarios where the intermediate Higgs bosons are nearly mass degenerate and interference effects can have a significant impact. We find that an on-shell approximation gives results in good numerical agreement with the full momentum-dependent Higgs propagator matrix calculation and we develop a generalised
narrow width approximation to be used in such a situation. We use these methods to study the asymmetry between the production of left-handed and right-handed neutralinos in Higgs decays at the LHC in the presence of CP-violating phases. Large asymmetries are found to be possible for large M_H^± > 500GeV and tan beta < 10, where the decay
into neutralinos may be the only possibility to detect the heavy Higgs bosons.