The following plot shows the Pi0 energy distribution in Pythia 6.410

We can see that the distribution from Pythia shows a two component shape, with dominant souce of production from quarks. Comparing to the data shown by Len, the Pythia distribution is higher by orders of magnitude at high energy. We still don't understand how Pythia produces so many high energy events.

Since the spectrum above is dominated by production from quark fragmentation. It might be useful to look at the quark distributions. The following plot is the rapidity and pt distribution for quarks. This plot is for Pi0 with energy greater than 30 GeV only. We see that the quarks can get pt as high as 5 GeV.

The plot below is the energy distribution for the same quarks as above.

For Pythia 5.707, it appears to be a different story. The following is the Pi0 energy distribution from this version. There is no high energy tail, and the production is dominatly from beam fragments.

From Pythia 6.222:

Clearly the spectrum is becoming harder and harder as the versions evolves. Pythia 6.222 is better in describing the FMS's shown by Len.