What is bwcutoff, and why is it important for decay chains?

The run_card parameter bwcutoff defines what is considered to be on-shell s-channel resonances: The resonance if considered to be on-shell if the invariant mass of an s-channel resonance is within M +/- bwcutoff*Gamma, and will then be included as a comment particle in the LHE event file (with status code 2). The value of bwcutoff does not affect the cross section of any processes, except those defined using the decay chain formalism, ab>(c>de)(f>gh) (see ​Basic syntax rules). Decay chain processes are strictly valid only in the narrow width limit, and we have therefore chosen to allow only production of on-shell particles. This means that the cross section will get smaller as bwcutoff gets smaller, since more of the tails of the Breit Wigner distributions are cut off. The default value is bwcutoff=15, for MG/ME v. 4.4.17 and later.

-- Main.JohanAlwall - 02 Mar 2009