Comment 5 for bug 1805107

Hi,

In the narrow-width approximation, you expect an error on the cross-section to be proportional to
Gamma/M so in your case ~25%. So indeed finding a 35% difference when doing a proper integration sounds reasonable. Now note that such computation is still neglecting the interference term with all the other diagram leading to the same final state (and the associated amplitude square).
With such large width, you also have to test if neglecting such diagram makes sense or not..

Cheers,

Olivier