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.
Highlights
Many models of new physics predict additional particles beyond the standard model
We introduce a generalisation of the narrowwidth approximation (NWA) which allows for a consistent treatment of interference effects between nearly mass-degenerate particles
We introduced a generalisation of the NWA to include the interference term in an on-shell approximation that maintains the convenient factorisation into a production cross section and branching ratio as in the standard NWA
Summary
Depending on the specific model and its underlying parameters, some of the new states might be nearly mass-degenerate, for example two of the neutral Higgs bosons or some sfermions of the MSSM. In such cases with a small mass gap between two intermediate particles and simultaneously a sizeable mixing among them, the interference term between contributions involving either of the nearly degenerate particles may become large. The wellknown narrow-width approximation is useful to calculate the production and decay of an intermediate particle. This can be iterated until a complicated process is decomposed into sufficiently short sub-processes. The gNWA can be used for the CP-violating interference between H and A in case of complex parameters and in the context of other new physics models
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