Pythia 6.416 vs. 6.22

Previously, we found that the energy distribution in the forward region in Pythia 6.416 is grossly different from our FPD data. Event record suggests that most of the excessive high energy events were coming from beam fragmentation, although we are still not sure why beam remnants in 6.416 pick up pT greater than ~1GeV to end up hitting the FPD. 

Jingguo studied different versions of Pythia, and found that version 6.22, which was used for the pion cross section paper in 2003, indeed generated energy spectrum similar to the data. Link to Jingguo's page

Here we compare Pythia version 6.416 and 6.22  for the energy and mass spectrum after full GSTAR pass is made.

Both simulation sets were generated with MSEL set equal 1. This results in a small cross-section of ~28mb. (since it doesn't include any diffractive interaction)

Pythia 6.416: 11.2B Pythia events, ~0.4/pb.

Pythia 6.22: 25.2B Pythia events, ~0.9/pb.

Fig. 1. East South Detector Summed Energy distribution for Run6T data (Black), Pythia 6.416+GSTAR(Blue), and Pythia 6.22+GSTAR(Dark Green), normalized to 1.

The data has exponential slope of ~2.2 around 50GeV, which is what you would expect from (1-xF)^5/pT^6 hard scattering cross-section.  (Link to Steve's document on this) Pythia 6.22 shows a very good agreement with the data, in stark contrast to 6.416. However, we haven't done a thorough study on the origin of the high energy processes in Pythia 6.22 yet. The slope suggests that they are coming from  hard scattering, but it couls still be beam remnants that were tuned to give this slope. More on this will come shortly.

Fig. 2. Di-photon pT distribution for 35GeV<Esum<45GeV. Run6T data (Black), Pythia 6.416+GSTAR(Blue), and Pythia 6.22+GSTAR(Dark Green), normalized to 1.

Fig. 3a. Mgg distribution in log scale. Run6T data (Black), Pythia 6.416+GSTAR(Blue), and Pythia 6.22+GSTAR(Dark Green), normalized to 1.

Fig. 3b. Mgg distribution in linear scale. Run6T data (Black), Pythia 6.416+GSTAR(Blue), and Pythia 6.22+GSTAR(Dark Green), normalized to 1.

 Overall Pythia tend to underestimate the valley region between Pi0 and Eta, Ver. 6.22 more so than 6.416. Both versions do reasonable job in reproducing Pi0-Eta ratio.