[Vincia] [Pythia8] Gluon splitting to quarks

[Torbjorn Sjostrand]

Hello Peter and Steve,

Thanks for input, it is most welcome. I have a master's student who
should begin a 4 month project on Monday, and c/b(/t) was on my list
  to study in the shower. From a completely different perspective,
however, inspired by criticsm I have received that Pythia is producing
too few c/b. For instance, in this workshop you mention, study slide 4
in Michelangelo's presentation to see how Pythia undershoots
c production by about a factor of 2. Of course one will need to
understand what has been done, also also separate Pythia 6 vs. Pythia 8,
which is why I look forward to the promised input from Steve.
Peter, was this slide discussed in the same context of too much/little
in showers? Or any other ideas you have?

Anyway, we should stay in touch. However, so as not to spam too much,
maybe we should restrict to those expressing an interest.

Best, Torbjörn

On 16/01/2014 13:20, Peter Skands wrote:
> 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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