[Vincia] [Pythia8] Gluon splitting to quarks

[Stephen Mrenna]

Hi, Peter,

Quite a while ago I sent around the results of a study where I counted the
number of ccbar and bbbar pairs coming from Pythia6 with q2 and pT2 
ordering,
pythia8, and Vincia.   I also found differences of about 2 between them.
I will try to find that email and recirculate it.

--S


On 01/16/2014 06:20 AM, 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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