kocolosk's blog

I'm concerned about an apparent deficit of g-g scattering events in our CDF Tune A Pythia samples. I first noticed this deficit when I was looking at MC asymmetries for pi-minus production, where the difference between asymmetries from g-g and q-g events is extreme. For reference, here is Vogelsang's prediction for the subprocess mixture for inclusive pi0 production:

Compare that to the following plot from Pythia for pi-plus production after combining partonic samples starting at 3_4:

If I add 2_3 GeV the gg contribution at low pT gets a boost, but I'm still seeing ~zero pions from g-g scattering at 8 GeV, where Vogelsang predicts 20% g-g.

Renee suggested skipping the observables and just plotting the event partonic pT. The turnover from gg to qg occurs at 3 GeV ... seems too low to me. On the other hand, the mixture is ~20% g-g at 16 GeV, which seems about right.

One other tidbit for any Pythia tuners out there. When I was running standalone Pythia to get the various partonic xsections for weighting purposes I saw a number of advisory warnings like

Advisory warning: maximum violated by  1.120D+00 in event     625
XSEC(28,1) increased to  5.982D-01

in this case, I was running the 5_7 sample. The initial max value for the q-g xsec was 5.3392D-01, and the final measured xsec was 1.515D-01. Not sure if it's significant.

Update 11-19-2007

I looked into the relationship between charged pion p_{T} and event partonic p_{T} in more detail. Here are plot of charged pion multiplicity and mean pion p_{T} versus event p_{T}:

so g-g events actually produce more charged pions per event, but these pions are all at low p_{T}. The second conclusion is reinforced by the PDF I've linked at the bottom of the page, which shows pion p_{T} spectra split by subprocess for a range of partonic p_{T} bins. The g-g events have a much steeper slope. Those plots are not normalized by the # of events per subprocess, so the conclusion about the number of pions per event is not immediately evident.

Update 11-27-2007

I tried manually rescaling the g-g scattering contribution to the PYTHIA asymmetries; it looks like increasing the g-g by a factor of 5 does a decent job of reproducing the NLO theoretical predictions:

no rescaling