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 multiparton 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 multiparton 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 was taken, with a tune to LEP1 data by Hendrik Hoeth,
using the Rivet + Professor framework. Starting with version 8.125 it
defined the default values for hadronization parameters and timelike
showers.
The situation is more complicated for hadronic interactions in general
and multiparton interactions in particular, where PYTHIA 8 is more
different from PYTHIA 6, and therefore more work is needed. Specifically,
it is not possible to "port" a PYTHIA 6 tune to PYTHIA 8.
A first simple tune, appropriately called "Tune 1", became default
starting with version 8.127. It was noted, in particular by Hendrik
Hoeth, that this tune had a tension between parameters needed to
describe minimum-bias and underlying-event activity. Therefore some
further physics features were introduced in the code itself
[Cor10a], which were made default as of 8.140. This version
also included 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 showed 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 newer 4C tune, that is based on a more serious study of some early
LHC data, see [Cor10a]. Following the comparative studies in
[Buc11], which independently confirmed a reasonable agreement
with LHC data, tune 4C was made the default from 8.150. A variant is
tune 4Cx, where the Gaussian matter profile has an x-dependent
width [Cor11].
Tune 4C was the basis for many subsequent LHC tunes. Several ATLAS tunes
have been included here, obtained with different PDFs and with different
emphasis on minimum-bias and underlying-event data [ATL12].
These typically require LHAPDF to be linked, but this can be avoided
in cases where the same PDF set is implemented internally. Also two CMS
underlying-event tunes are implemented [CMS14]. The ATLAS AZ tune
[ATL14] instead puts emphasis on the pT spectrum of
the Z^/gamma^*0 boson.
The Monash 2013 tune [Ska14] is based on a larger set of LHC
distributions. It starts out from a more careful comparison with and
tuning to LEP data, and so involves several parameter changes. The PDF
used is the NNPDF2.3 QCD+QED LO one with alpha_s(M_Z) = 0.130,
which includes more recent data than used in the previous default, and
opens up for processes with incoming photons to the hard process.
It is the default starting from 8.200.
Recent tunes by the LHC collaborations are based on the Monash 2013 one.
This includes the CMS tune MonashStar, or formally CUETP8M1-NNPDF2.3LO
(currently unpublished). More significantly, ATLAS has produced and
published a whole family for underlying-event tunes based on a major
effort, including simultaneous fits of ten parameters [ATL14a].
This includes four central tunes, with four different PDF sets, and
ten variations around the NNPDF2.3 QCD+QED LO central tune, grouped in
five pairs of variations up and down. The publication [ATL14a]
should be consulted for further details, like with what additional
settings various processes have been generated, which should be respected
to reap full benefit of the tunes.
Central diffraction is a recent addition to the "soft QCD" process palette,
and is thus not yet included in tunes; indeed its cross section is actively
zeroed. You can switch it back on after you have selected your tune,
with SigmaTotal:zeroAXB = off
. But note that, since the
total cross section is assumed unchanged, the nondiffractive cross section
is reduced and thus also the MPI machinery affected, even if effects
should not be big (for a small central diffractive cross section).
Note that comparisons with data also require that other aspects agree,
such as that decay chains are stopped at an agreed-on level. For instance,
in the ATLAS tunes all particles with a lifetime above 10 mm
are considered stable, ParticleDecays:limitTau0 = on
,
ParticleDecays:tau0Max = 10
. We have chosen not to
include this as part of the tune settings itself, since the tune as
such could still be used with any other choice of stable and
unstable particles.
Further comparisons have been posted on the
MCPLOTS pages.
They have been produced with help of the
Rivet package
[Buc10].
To simplify comparisons for the user, we propose to collect some of
the 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 setup of the tunes is special, in that the choice of a tune forces
the change of several different flags, modes and parameters. Furthermore
a design principle has been that it should be possible to start out
from a tune and then change a few of its settings. This gives power
and flexibility at the expense of requiring a more careful ordering
of commands. We therefore here sketch the order in which operations
are carried out.
- The constructor of a
Pythia
instance will read in
all settings, and initialize them with their default values.
- At the end of this operation, the
Tune:ee
and
Tune:pp
modes (see further below) are checked. If either
of them are positive the methods Settings::initTuneEE(...)
and Settings::initTunePP(...)
, respectively, are called
to overwrite the whole collection of settings in the relevant tune.
Zero (or negative) means that nothing will be done.
Since most pp/ppbar tunes have been made in the context
of an e^+e^- one, initTunePP(...)
usually
calls initTuneEE(...)
to provide this synchronization.
- After the
Pythia
constructor all the relevant values
for the default tune(s) have thus been set up.
- You as a user can now start to overwrite the values at will,
using
Pythia::readFile(...)
to read a configuration file,
or a list of Pythia::readString(...)
commands,
or the lower-level Settings
methods. All changes
are made in the order in which the commands are encountered during
the execution. A given variable can be changed multiple times,
but it is the latest change that sets the current value.
- The two
Tune:ee
and Tune:pp
modes can also
be changed in exactly the same way as described for all other settings
above. Unique for them, however, is that when one of them is encountered
it also initiates a call to the initTuneEE(...)
or
initTunePP(...)
method, respectively. In such cases all
settings affected by the e^+e^- or pp/ppbar tune
are first reset to the default values (the -1
options)
and thereafter the relevant tune is set up.
Recall that initTunePP(...)
in its turn is allowed to call
initTuneEE(...)
.
- It is possible to mix commands of type 4 and 5 in any order; it
is always the last change that counts. That is, any changes you have
made to variables of a tune before a
Tune:ee
or
Tune:pp
command are overwritten by it, while variables
you set after will overwrite the tune values. Further,
the Tune:pp
command usually implies an e^+e^-
tune as well. Therefore Tune:ee
would rarely be used for
LHC applications. As a rule, instead, you want to begin with the
Tune:pp
choice, and thereafter modify only a small part
of its settings.
- Needless to say, the flexibility 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.
mode
Tune:ee
(default = 7
; minimum = -1
; maximum = 7
)
Choice of tune to e^+e^- data, mainly for the hadronization
and timelike-showering aspects of PYTHIA. You should study the
Settings::initTuneEE(...)
method to find exactly which
are the settings for the respective tune.
option
-1 : reset all values that are affected by any of the
e^+e^- tunes to the default values. This option can be used
on its own, but is also automatically used as a first step for either
of the positive tune values below, to undo the effect of previous tune
settings.
option
0 : no values are overwritten during the initial setup,
step 2 above. Note that changing to 0
in the user code
has no effect; if you want to restore the individual settings you
should instead use -1
.
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 were the default values
starting from version 8.125.
option
4 : a tune to LEP data by Peter Skands, by hand, both
to hadronization and timelike-shower parameters (September 2013).
Note the use of the CMW convention for the shower alpha_s
scale.
option
5 : first tune to LEP data by Nadine Fischer
(September 2013), based on the default flavour-composition
parameters. Input is event shapes (ALEPH and DELPHI),
identified particle spectra (ALEPH), multiplicities (PDG),
and B hadron fragmentation functions (ALEPH).
option
6 : second tune to LEP data by Nadine Fischer
(September 2013). Similar to the first one, but event shapes
are weighted up significantly, and multiplicites not included.
option
7 : the Monash 2013 tune by Peter Skands at al.
[Ska14], to both e^+e^- and pp/pbarp data.
mode
Tune:preferLHAPDF
(default = 1
; minimum = 0
; maximum = 2
)
Tunes made by experimental collaborations typically use the LHAPDF
package to obtain their PDF values, and so PYTHIA must be built
accordingly. See the PDF
documentation for more information. For PDFs implemented
natively in PYTHIA it is possible to use the respective tunes, without
having to use LHAPDF, if you set Tune:preferLHAPDF =
0
before the Tune:pp
choice.
option
0 : Use the internal PYTHIA PDFs.
option
1 : Use LHAPDF5 PDFs.
option
2 : Use LHAPDF6 PDFs.
mode
Tune:pp
(default = 14
; minimum = -1
; maximum = 34
)
Choice of tune to pp/ppbar data, mainly for the
initial-state-radiation, multiparton-interactions and beam-remnants
aspects of PYTHIA. You should study the
Settings::initTunePP(...)
method to find exactly which
are the settings for the respective tune. Note that all early tunes,
including those done by the LHC collaborations based on tune 4C,
imply the settings of Tune:ee = 3
, while the Monash 2013
tune and the further tunes based on it imply Tune:ee = 7
.
This is set automatically, and has to be overridden afterwards if not
the wanted behaviour.
option
-1 : reset all values that are affected by any of the
pp/ppbar tunes to the default values. This option can be used
on its own, but is also automatically used as a first step for either
of the positive tune values below, to undo the effect of previous tune
settings.
option
0 : no values are overwritten during the initial setup,
step 2 above. Note that changing to 0
in the user code
has no effect; if you want to restore the individual settings you
should instead use -1
.
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", newer tune, introduced with 8.145
[Cor10a]. Starts out from tune 2C, but with a reduced cross
section for diffraction, plus modified multiparton interactions parameters
to give a higher and more rapidly increasing charged pseudorapidity
plateau, for better agreement with some early key LHC numbers.
See also the comparative study in [Buc11].
The starting point for many later tunes.
option
6 : "Tune 4Cx", based on tune 4C, but using the x-dependent
matter profile, MultipartonInteractions:bProfile = 4
and an
increased MultipartonInteractions:pT0Ref
[Cor11].
option
7 : "ATLAS MB Tune A2-CTEQ6L1", a minimum-bias tune based
on tune 4Cx, but without rapidity-ordered spacelike emissions
[ATL12]. Uses CTEQ 6L1, by default from LHAPDF.
option
8 : "ATLAS MB Tune A2-MSTW2008LO", as above,
but uses MSTW 2008 LO, by default from LHAPDF.
option
9 : "ATLAS UE Tune AU2-CTEQ6L1", an underlying-event tune
based on tune 4Cx, but without rapidity-ordered spacelike emissions
[ATL12]. Uses CTEQ 6L1, by default from LHAPDF.
option
10 : "ATLAS UE Tune AU2-MSTW2008LO", as above,
but uses MSTW 2008 LO, by default from LHAPDF.
option
11 : "ATLAS UE Tune AU2-CT10", as above,
but uses CT 10, which is not currently implemented in PYTHIA,
so you must link LHAPDF.
option
12 : "ATLAS UE Tune AU2-MRST2007LO*", as above,
but uses MRST 2007 LO*, by default from LHAPDF.
option
13 : "ATLAS UE Tune AU2-MRST2007LO**", as above,
but uses MRST 2007 LO**, by default from LHAPDF.
option
14 : the Monash 2013 tune by Peter Skands at al.
[Ska14], to both e^+e^- and pp/pbarp data.
The starting point for many later tunes.
option
15 : "CMS UE Tune CUETP8S1-CTEQ6L1", an underlying-event
tune based on tune 4C [CMS14]. Uses CTEQ 6L1, by default
from LHAPDF.
option
16 : "CMS UE Tune CUETP8S1-HERAPDF1.5LO", an underlying-event
tune based on tune 4C [CMS14]. Uses HERAPDF1.5LO, which is not
currently implemented in PYTHIA, so you must link LHAPDF.
option
17 : "ATLAS Tune AZ", is tuned to the pT spectrum
of the Z^/gamma^*0 boson in a set of rapidity bins [ATL14].
option
18 : "CMS Tune MonashStar", alias CUETP8M1-NNPDF2.3LO,
an underlying-event tune based on the Monash 2013 tune.
option
19 : "ATLAS A14 central tune with CTEQL1", a full-scale
tune to most ATLAS jet and underlying-event observables [ATL14a],
starting out from the Monash 2013 tune. The following tunes 20 - 32
belong to the same group.
option
20 : "ATLAS A14 central tune with MSTW2008LO",
see above tune 19.
option
21 : "ATLAS A14 central tune with NNPDF2.3LO",
see above tune 19. Defines the center of the 23 - 32 variations,
so would be a good choice if you only want to study one tune from
the A14 family.
option
22 : "ATLAS A14 central tune with HERAPDF1.5LO",
see above tune 19. Uses HERAPDF1.5LO, which is not currently implemented
in PYTHIA, so you must link LHAPDF.
option
23 : "ATLAS A14 variation 1+" of tune 21.
option
24 : "ATLAS A14 variation 1-" of tune 21.
option
25 : "ATLAS A14 variation 2+" of tune 21.
option
26 : "ATLAS A14 variation 2-" of tune 21.
option
27 : "ATLAS A14 variation 3a+" of tune 21.
option
28 : "ATLAS A14 variation 3a-" of tune 21.
option
29 : "ATLAS A14 variation 3b+" of tune 21.
option
30 : "ATLAS A14 variation 3b-" of tune 21.
option
31 : "ATLAS A14 variation 3c+" of tune 21.
option
32 : "ATLAS A14 variation 3c-" of tune 21.
option
33 : tune that includes close-packing of strings and
hadron rescattering, Gaussian model for pT and flavour selection
[Fis16]. Based on Monash tune, mainly tuned to pT spectra.
option
34 : tune that includes close-packing of strings and
hadron rescattering, thermodynamical model for pT and flavour selection
[Fis16]. Based on Monash tune, mainly tuned to pT spectra.