H->gg and similar

[kilian]

Effective loop-induced vertices like Hgg are unwanted in most applications (and slow down phase space setup and integration), but needed in some.

We should decide about the default behavior, and let the user change it (at runtime) if desired.

[Juergen Reuter]

This partially a double entry of #142. Should we close it as duplicate entry?

[kilian]

Only if you convince me that it doesn't work at all now. Then we have to leave it switched off until #142 is done. Otherwise I would allow the user to enable it in 2.0, if he needs it.

[Christian Speckner]

Well, I'm pretty sure that the O'Mega side is lacking the necessary color structures to handle the effective ggH vertex properly (color flow gluons of the type gl/1/1 would be required for proper support). So,unless WHIZARD 2 contains code similar to JR's perl hack in 1.9x to work around this, the vertex is broken,and cross sections calculated this way will be wrong.

[ohl]

I just discussed this with Christian and we've found the solution: introduce a set of "ghost" particles that couple only to color singlets and are counted just like the normal ghosts. Each closed loop of these ghosts receives a (-1) so that we get 9 - 1 = 8. This can be derived formally by treating the Higgs as an external field and using the gluon propagator in this external field (i.e. with Higgs insertions resummed).

This can be implemented directly in O'Mega, once the color summation is moved to O'Mega.

[ohl]

Oops. I meant ticket #142.

[Juergen Reuter]

Milestone v2.0.0final deleted