Tunes

Technically, PYTHIA parameters can be varied independently of each other, but the physical requirement of a sensible description of a set of data leads to correlations and anticorrelations between the parameters. Hence the need to produce tunes, not of one parameter at a time, but simultaneously for a group of them. A well-known such example is parton densities, where combined tunes to a wide range of data have been produced, that can then be obtained prepackaged.

Given the many PYTHIA parameters to be tuned, it is convenient to divide the task into subtasks. Firstly, if we assume jet universality, hadronization and final-state parton showers should be tuned to e^+e^- annihilation data, notably from LEP1, since this offers the cleanest environment. Secondly, with such parameters fixed, hadron collider data should be studied to pin down multiple interactions and other further aspects, such as initial-state radiation. (Thirdly would come anything else, such as physics with photon beams, which involve further parameters, but that is beyond the current scope.)

Sadly PYTHIA 8 did not yet take many steps along this long road. While the default values in PYTHIA 8 have been chosen "sensibly", so far there has not been a complete, consistent tuning of the program. For hadronization we can partly benefit from the LEP experience with PYTHIA 6, since the basic hadronization scheme did not change. However, there never was a combined LEP tune, since each of the four LEP collaborations produced their own tunes to their own data. The situation is worse for multiple interactions, where PYTHIA 8 is more different from PYTHIA 6. Nevertheless, the PYTHIA 6 tunes to CDF data, performed by R.D. Field, have been used as a rough guide in picking reasonable default values.

In the future we hope to see PYTHIA 8 tunes appear. Like with parton distributions, there is likely to be several tunes, because different sets of data will pull in different directions, by imperfections in the model or in the data, and by differences in the chosen tuning strategies. We therefore propose to collect some of these tunes here, in a prepackaged form. Of course, in all cases it is a matter of setting values for parameters already defined elsewhere, so the tunes offer no new functionality, only a more convenient setup.

If you set either the Tune:ee and Tune:pp modes below non-zero then all parameters used in the respective tune will be set accordingly when pythia.init(...) is called. You can check this by calling pythia.settings.listChanged() before and after initialization; before only the tune modes are nondefault, afterwards all the non-default-valued parameters in the tune appear. Therefore, for better or worse, you cannot combine a tune option with your own choices for some of the parameters used in the tune, since the values you set before pythia.init(...) would be overwritten at that point.

mode  Tune:ee   (default = 0)
Choice of tune to e^+e^- data, mainly for the hadronization and timelike-showering aspects of PYTHIA.
option 0 : no values are overwritten at initialization, so you can set the individual parameters as you wish.
option 101 : a tune by Marc Montull to the LEP 1 particle composition, as published in the RPP. The default PYTHIA parameter values tend to produce more vector mesons than observed in data. When more pseudoscalars are produced directly this also leads to a reduced s/u ratio and diquark fraction. Also other parameters related to the particle composition obtain changed values. No related (re)tune to event shapes has been performed so far, however.

mode  Tune:pp   (default = 0)
Choice of tune to pp / ppbar data, mainly for the multiple-interactions and initial-state-radiation aspects of PYTHIA.
Note: Currently this is only a placeholder, since no tunes alternative to the default values exist.
option 0 : no values are overwritten at initialization, so you can set the individual parameters as you wish.