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.