A study of general properties of the narrow-width approximation (NWA) with polarization/spin decorrelation is presented. We prove for total rates of arbitrary resonant decay or scattering processes with an on-shell intermediate state decaying via a c
ubic or quartic vertex that decorrelation effects vanish and the NWA is of order Gamma. Its accuracy is then determined numerically for all resonant 3-body decays involving scalars, spin-1/2 fermions or vector bosons. We specialize the general results to MSSM benchmark scenarios. Significant off-shell corrections can occur - similar in size to QCD corrections. We qualify the configurations in which a combined consideration is advisable. For this purpose, we also investigate process-independent methods to improve the NWA.
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l anti-l is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribut
ion is important for Higgs masses below the Z-pair threshold and that the gg-induced process yields a correction of about 15% relative to the NLO QCD prediction for the q anti-q-induced process when only a M(l anti-l), M(l anti-l) > 5 GeV cut is applied.
A modified narrow-width approximation that allows for O(Gamma/M)-accurate predictions for resonant particle decay with similar intermediate masses is proposed and applied to MSSM processes to demonstrate its importance for searches for particle physics beyond the Standard Model.
Corrections of 10-30% for backgrounds to the H --> WW --> l^+l^-sla{p}_T search in vector boson and gluon fusion at the LHC are reviewed to make the case for precise and accurate theoretical background predictions.
