Tunes

Since some physics aspects cannot be derived from first principles, this program contains many parameters that represent a true uncertainty in our understanding of nature. Particularly afflicted are the areas of hadronization and multiple interactions, which both involve nonperturbative QCD physics.

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 (separate) 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. Ideally this would be done separately for diffractive and non-diffractive events, although it is not possible to have a clean separation. (Thirdly would come anything else, such as physics with photon beams, which involve further parameters, but that is beyond the current scope.)

The first step in this program has now been taken, with a tune to LEP1 data by Hendrik Hoeth, using the Rivet + Professor framework. Starting with version 8.125 it defines the default values for hadronization parameters and timelike showers.

The situation is worse for multiple interactions, where PYTHIA 8 is more different from PYTHIA 6. Nevertheless, a first simple tune is now available, appropriately called "Tune 1", and became default starting with version 8.127.

It was noted, in particular by Hendrik Hoeth, that the program had a tension between parameters needed to describe minimum-bias and underlying-event activity. Therefore some further physics features have been introduced in the code itself [Cor10a], which are made default as of 8.140. This version also includes two new tunes, 2C and 2M, based on the CTEQ 6L1 and the MRST LO** PDF sets, respectively. These have been made by hand, as a prequel to complete Professor-style tunings.

The very first data to come out of the LHC shows a higher rapidity plateau than predicted for current PYTHIA 6 tunes, also for the lower energies. This may suggest some tension in the data. Two alternatives, 3C and 3M, were produced by a few brute-force changes of 2C and 2M. These were introduced in 8.140, but discontinued in 8.145 in favour of the new 4C tune, that is based on a more serious study of some early LHC data.

Some comparisons between these tunes and data are published in [Cor10a], and further comparisons have been posted on http://home.thep.lu.se/~richard/pythia81/. Most of the plots have been produced with the Rivet package [Buc10].

In the future we hope to see further 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.

You should be aware that the evolution of the program will not guarantee complete backwards compatibility between versions. Most obviously this concerns bug fixes. But also for some other major changes, like the introduction of the new diffractive machinery, the default behaviour of old tunes has been changed retroactively. (Which should be fine for diffraction, since previous tunes were not based on data strongly influenced by diffraction.)

The constructor of a Pythia instance will check if Tune:ee or Tune:pp (see further below) are nonvanishing by default, and if so set the corresponding tune variables accordingly, before any user changes are possible. For now both tune switches are vanishing by default, however, so that the default values of other variables remain intact during the setup.

Thereafter, if you set Tune:ee and/or Tune:pp non-zero, then all variables used in the respective tune will be set accordingly. This is done as soon as either command is encountered, in the Pythia::readFile(...)configuration file in the list of Pythia::readString(...) commands, or in the lower-level Settings::readString(...), Settings::mode(...), Settings::forceMode(...) or Settings::resetMode(...)methods. That is, any changes you made to variables of the tune before the respective Tune:ee or Tune:pp command will be overwritten at that point, while variables you set after will overwrite the tune values. Needless to say, this can lead to unwanted setups if you do not exercise some discipline. It is therefore recommended that you always check the listing obtained with Pythia::settings.listChanged() to confirm that the final set of changes is the intended one. Also note that variables not set by the tune options are assumed to remain at their default values, to the extent that they would affect the tunes if not.

mode  Tune:ee   (default = 0; minimum = 0; maximum = 3)
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 1 : the original PYTHIA 8 parameter set, based on some very old flavour studies (with JETSET around 1990) and a simple tune of alpha_strong to three-jet shapes to the new pT-ordered shower. These were the default values before version 8.125.
option 2 : a tune by Marc Montull to the LEP 1 particle composition, as published in the RPP (August 2007). No related (re)tune to event shapes has been performed, however.
option 3 : a tune to a wide selection of LEP1 data by Hendrik Hoeth within the Rivet + Professor framework, both to hadronization and timelike-shower parameters (June 2009). These are the default values starting from version 8.125, so currently there is no need for this option.

mode  Tune:pp   (default = 0; minimum = 0; maximum = 5)
Choice of tune to pp / ppbar data, mainly for the initial-state-radiation, multiple-interactions and beam-remnants aspects of PYTHIA. Note that the previous crude (non-)tunes 3C and 3M are removed as of 8.145, superseded by the 4C tune.
option 0 : no values are overwritten at initialization, so you can set the individual parameters as you wish. Most default values are based on "Tune 1", option 2 below, but some new options introduced in 8.140 means that the two no longer agree.
option 1 : default used up to version 8.126, based on some early and primitive comparisons with data.
option 2 : "Tune 1", default in 8.127 - 8.139, based on some data comparisons by Peter Skands. Largely but not wholly overlaps with the default option 0.
option 3 : "Tune 2C", introduced with 8.140 [Cor10a]. It uses the CTEQ 6L1 PDF, and is intended to give good agreement with much of the published CDF data.
option 4 : "Tune 2M", introduced with 8.140 [Cor10a]. It is uses the MRST LO** PDF, which has a momentum sum somewhat above unity, which is compensated by a smaller alpha_s than in the previous tune. Again it is intended to give good agreement with much of the published CDF data.
option 5 : "Tune 4C", new tune, introduced with 8.145 [Cor10a]. Starts out from tune 2C, but with a reduced cross section for diffraction, plus modified multiple interactions parameters to give a higher and more rapidly increasing charged pseudorapidity plateau, for better agreement with some early key LHC numbers.