MCFM interface for WW production exits in example setup

[Frank Siegert]

The example setup LHC_WW_PH produces the following error message:

Warning in MCFM_qqb_vv_Getter::operator:
   Try to initialise process qqb->VV in MCFM.
   Inconsistent setting with Sherpa:
Yuk(b) = 4.8 (should be 0), model = SM(should be 'SM', and top on = 1(should be 1 for WW).
   Will exit the run

There seem to be two issues:

• The interface code requires ACTIVE[6]=0, contrary to what the error message says. Which one is correct?
• By asking for b.Yuk()==0 in the interface one requires MASSIVE[5]=0 and MASS[5]=0.0. Is the real intention here to require YUKAWA_B=0, i.e. disabling the corresponding bbH vertices?

[Frank Krauss]

The error message may be shit, but the gist is correct. The MCFM interface makes the following assumptions:

1. No Higgs in it - must disallow effective gg->H vertex and all qqH-vertices. In our SM setup, the only two relevant quarks are then b and t. So, yes, Yuk[b]=0 is a must - and we can set this directly.
2. Only massless quarks in t-channel, i.e. no top. Therefore we must switch to a 4-flavour PDF (not implemented yet), or disallow b's in the IS in another form. I've realised this by demanding that 5 is not in 93.

I'm going to check the settings, but I guess this in a minor nuisance only.

There is, however, a follow-up question: We must figure a way to enforce 4-flavour PDFs ... .

[Frank Siegert]

I have committed a fix (r17455) which makes it use MODEL::s_model->ScalarConstant("Yukawa_b") instead of Flavour(kf_b).Yuk(). This was the main issue I was raising in this bug report. The typo in the top error message was also fixed, together with a few other minor issues. I'm attaching it as a patch to this bug report, in case any users are interested.