Interference effects of neutral MSSM Higgs bosons with a generalised narrow-width approximation

TL;DR

This paper introduces a generalized narrow-width approximation (gNWA) that accurately accounts for interference effects between nearly mass-degenerate neutral Higgs bosons in the MSSM, improving theoretical predictions.

Contribution

The paper develops and applies a gNWA that includes interference effects at tree and 1-loop levels, enhancing the precision of Higgs boson process calculations in the MSSM.

Findings

gNWA reproduces full interference within 1% accuracy

Interference effects are significant between nearly degenerate Higgs bosons

gNWA allows inclusion of higher-order contributions

Abstract

Mixing effects in the MSSM Higgs sector can give rise to a sizeable interference between the neutral Higgs bosons. On the other hand, factorising a more complicated process into production and decay parts by means of the narrow-width approximation (NWA) simplifies the calculation. The standard NWA, however, does not account for interference terms. Therefore, we introduce a generalisation of the NWA (gNWA) which allows for a consistent treatment of interference effects between nearly mass-degenerate particles. Furthermore, we apply the gNWA at the tree and 1-loop level to an example process where the neutral Higgs bosons $h$ and $H$ are produced in the decay of a heavy neutralino and subsequently decay into a fermion pair. The $h-H$ propagator mixing is found to agree well with the approximation of Breit-Wigner propagators times finite wave-function normalisation factors, both leading to a significant interference contribution. The factorisation of the interference term based on on-shell matrix elements reproduces the full interference result within a precision of better than 1% for the considered process. The gNWA also enables the inclusion of contributions beyond the 1-loop order into the most precise prediction.