I generated an s-channel process p p > W+ > e+ ve, and in some events the W doesn't show up in the event record. Why is this?

This is perfectly normal. Since you haven't used the decay chain syntax for the W,

p p > (W+ > e+ ve) (in MG4) or

p p > W+, W+ > e+ ve (in MG5)

the W is not forced to be onshell - so when the invariant mass of e+ ve goes beyond the specified 15 widths (given by bwcutoff in the run_card) it is not included in the event file.

Note that in general, diagrams with a forced s-channel propagator are no longer gauge invariant far from the BW peak, so the decay chain syntax is preferred.

-- Main.JohanAlwall - 2011-01-29