=== How do I generate only a subset of diagrams? What are the decay chains? ===

   * Use " x x > <span style="color: #990099;">z</span> > y y y " form to require particle <span style="color: #990099;">z</span> to appear in a s-channel intermediate state.
   * Use " x x > y y y / <span style="color: #990099;">z</span>" form to exclude particle <span style="color: #990099;">z</span> to appear at all in any diagram.
   * Use " x x > y y y $ z" form to forbid on-shell s-channel z particles (note that this restriction set in the MadEvent run from MG5 v. 1.3.18).

Decay chains is a special recent syntax to allow only resonant diagrams to be generated. This is by definition an approximation that works well for small widths of the resonant particles and it is exact in the zero-width limit. The syntax is as follows
{{{
x x > Z W, Z > y y, W > k k 
}}}

means x x scattering into Z W, with Z then decays to yy and W to kk. Matrix elements obtained in this way keep the FULL spin correlation information and a Breit-Wigner width. (At variance with the DECAY approach where only diagonal terms of the spin-correlation matrix in the helicity basis are kept and the decayed particles virtuality is alwasys the mass (zero width).

(Note that the syntax for decay chains in MG4 was slightly different and not as powerful.)

-- Main.JohanAlwall - 2011-10-11