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 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.)
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 is default starting with
version 8.127.
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.
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; 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 = 2)
Choice of tune to pp / ppbar data, mainly for the
initial-state-radiation, multiple-interactions and beam-remnants
aspects of PYTHIA.
option 0 : no values are overwritten at initialization,
so you can set the individual parameters as you wish.
option 1 : old default used up to version 8.126, based on
some early and primitive comparisons with data.
option 2 : "Tune 1", new default from 8.127 onwards,
based on some data comparisions by Peter Skands.