[Vincia] Gluon splitting to quarks

[Peter Skands]

Hi Pythia and Vincia'ers

In the context of a workshop on charm at LHC this week at CERN, I took a look at 
how much g->ccbar Pythia actually produces at LEP, and compared it with Vincia.

Of course, the absolute rates are low, of order one in a thousand events, but 
there seems to be a clear factor 2 difference between the two, see the attached 
plot, which shows the spectrum of xCharm = 2|p|/eCM on a log scale, for Z -> 
uu/dd events. The case for bottom is included as well, and exhibits the same 
behaviour.

I realize that g->QQ is quite an uncertain process from a shower perspective, 
but a factor of 2 nonetheless seems disturbing, and makes me wonder eg whether 
all factors of TR = 1/2 (or TRhat = 1, in Vincia's case, with 1/2 coming from 
summing over either side of the gluon) have been implemented consistently in the 
two codes. Have any of you looked at similar issues? There is also a question 
about whether to use mQQ as the renormalization scale (Vincia) or pT (Pythia), 
but I doubt that could lead to this big a difference.

The background is also that an experimentalist at the workshop raised that 
Pythia seems to be overpopulating the g->cc component of W+charm events, 
according to a study performed for CMS SMP 12 002. He said the plots were not in 
the paper but that they could be seen on public plots on the analysis web page 
but I was not able to find them on there :( I did not take down his name.

Cheers,
Peter
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