Scaling Properties of Pythia Tunes

 Recently introduced tunes to Pythia, including the Perugia and Pro-pT0 tunes, promise improved modeling of non-perturbative physics at many energy scales.  In particular, precision studies of minimum bias data show that the newer tunes better describe the physics than the older CDF Tune A, the currently tune implemented in STAR pp simulations.  Unlike CDF Tune A, which used only CDF underlying event data, the Perugia tunes incorporate 200 GeV data explicitly in the form of UA5 minimum bias data, and should consequently offer better modeling of the scaling before from Tevatron energies down to RHIC energies.


Past STAR simulations made with CDF Tune A, however, have well described jet observables within the available statistics.  Before abandoning CDF Tune A in favor of a newer tune, then, let's first validate that the newer tunes are not at risk of ruining the agreement seen with CDF Tune A.


The following study was based on six Pythia data sets, each of 10^{6} events, produced with CDF Tune A, Perugia 0, and Pro-pT0 at both 200 GeV and 900 GeV center of mass energies.  Otherwise all six samples were the same, sampling from hard QCD interactions (Pythia subprocesses 11, 12, 13, 28, 53, and 68) with partonic pT between 5 GeV and infinity (as set by the CKIN variables).


The observable under consideration is jet energy as modeled by PYCELL.  In particular, a seed energy of 1 GeV was required with a total energy threshold of 7 GeV within a R = 0.4 cone.


As expected, the three tunes are in agreement at 900 GeV (where each receives most if not all of its tuning data) and the two newer tunes are indistinguishable all the way down to 200 GeV.  The agreement between CDF Tune A and the newer tunes, however, is not as pleasant, with the newer tunes growing much faster at low energies.



Note, however, that the shapes of the three distributions are in fair agreement.  In fact, past simulations did not consider the absolute normalization of Pythia, instead floating the normalization to best fit the data.  It is the shape of CDF Tune A, then, that has been validated by earlier STAR data.


Comparing the shapes of the new tunes to CDF Tune A shows reasonable agreement, the ratio mostly linear across the relevant energies.




As the absolute normalization will continue to be allowed to float, this agreement between shapes confirms that the newer tunes do not induce significant changes to the physics of 200 GeV jets.