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A generalisation of the narrow-width approximation (NWA) is formulated whichallows for a consistent treatment of interference effects between nearlymass-degenerate particles in the factorisation of a more complicated processinto production and decay parts. It is demonstrated that interference effectsof this kind arising in BSM models can be very large, leading to drasticmodifications of predictions based on the standard NWA. The application of thegeneralised NWA is demonstrated both at tree level and at one-loop order for anexample process where the neutral Higgs bosons $h$ and $H$ of the MSSM areproduced in the decay of a heavy neutralino and subsequently decay into afermion pair. The generalised NWA, based on on-shell matrix elements or theirapproximations leading to simple weight factors, is shown to produce UV- andIR-finite results which are numerically close to the result of the full processat tree level and at one-loop order, where an agreement of better than $1\%$ isfound for the considered process. The most accurate prediction for this processbased on the generalised NWA, taking into account also corrections that areformally of higher orders, is briefly discussed.

Interference effects in BSM processes with a generalised narrow-width approximation