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<h2>Tunes</h2>

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. 

<p/>
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.  

<p/>
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 
<i>e^+e^-</i> 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.)

<p/>
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. 

<p/>
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 <i>not</i> possible to "port" a PYTHIA 6 tune to PYTHIA 8. 

<p/>
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 
[<a href="Bibliography.html" target="page">Cor10a</a>], 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. 

<p/>
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 new 4C tune, that is based on a more serious study of some early 
LHC data, see [<a href="Bibliography.html" target="page">Cor10a</a>]. Following the comparative studies in 
[<a href="Bibliography.html" target="page">Buc11</a>], which independently confirmed a reasonable agreement 
with LHC data, tune 4C was made the default as of 8.150. A variant is 
tune 4Cx, where the Gaussian matter profile has an <i>x</i>-dependent
width [<a href="Bibliography.html" target="page">Cor11</a>].

<p/>
Several ATLAS tunes have now been included, obtained with different PDFs
and with different emphasis on minimum-bias and underlying-event data
[<a href="Bibliography.html" target="page">ATL12</a>]. These typically require LHAPDF to be linked, but this 
can be avoided in cases where the same PDF set is implemented internally. 

<p/>
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 <i>after</i> you have selected your tune, 
with <code>SigmaTotal:zeroAXB = off</code>. 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).

<p/>
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, <code>ParticleDecays:limitTau0 = on</code>,
<code>ParticleDecays:tau0Max = 10</code>. 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.

<p/>
Further comparisons have been posted on the
<a href="http://mcplots.cern.ch/">MCPLOTS</a> pages.
They have been produced with help of the 
<a href="http://projects.hepforge.org/rivet/">Rivet</a> package 
[<a href="Bibliography.html" target="page">Buc10</a>]. 

<p/>
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. 

<p/>
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.)  

<p/>
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.
<ol>
<li>The constructor of a <code>Pythia</code> instance will read in
all settings, and initialize them with their default values.
</li>
<li>At the end of this operation, the <code>Tune:ee</code> and 
<code>Tune:pp</code> modes (see further below) are checked. If either
of them are positive the methods <code>Settings::initTuneEE(...)</code>
and <code>Settings::initTunePP(...)</code>, respectively, are called
to overwrite the whole collection of settings in the relevant tune. 
Zero (or negative) means that nothing will be done.
</li> 
<li>After the <code>Pythia</code> constructor all the relevant values
for the default tune(s) have thus been set up.
</li>
<li>You as a user can now start to overwrite the values at will,
using <code>Pythia::readFile(...)</code> to read a configuration file, 
or a list of <code>Pythia::readString(...)</code> commands, 
or the lower-level <code>Settings</code> methods. All changes
are made in the order in which the commands are encountered during
the execution. A given variable can be changed multiparton times,
but it is the latest change that sets the current value.
</li>
<li>The two <code>Tune:ee</code> and <code>Tune:pp</code> 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 <code>initTuneEE(...)</code> or
<code>initTunePP(...)</code> method, respectively. In such cases all
settings affected by the <i>e^+e^-</i> or <i>pp/ppbar</i> tune 
are first reset to the default values (the <code>-1</code> options) 
and thereafter the relevant tune is set up.
</li>
<li>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 <i>before</i> a <code>Tune:ee</code> or 
<code>Tune:pp</code> command are overwritten by it, while variables 
you set <i>after</i> will overwrite the tune values. As a rule,
therefore, you want to begin with the tune choice, and thereafter
modify only a small part of its settings.
</li>
<li>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 
<code>Pythia::settings.listChanged()</code> to confirm that the 
final set of changes is the intended one. 
</li>
</ol>

<p/><code>mode&nbsp; </code><strong> Tune:ee &nbsp;</strong> 
 (<code>default = <strong>0</strong></code>; <code>minimum = -1</code>; <code>maximum = 3</code>)<br/>
Choice of tune to <i>e^+e^-</i> data, mainly for the hadronization
and timelike-showering aspects of PYTHIA. You should study the 
<code>Settings::initTuneEE(...)</code> method to find exactly which 
are the settings for the respective tune.
<br/><code>option </code><strong> -1</strong> : reset all values that are affected by any of the 
<i>e^+e^-</i> 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. 
  
<br/><code>option </code><strong> 0</strong> : no values are overwritten during the initial setup,
step 2 above. Note that changing to <code>0</code> in the user code
has no effect; if you want to restore the individual settings you 
should instead use <code>-1</code>. 
  
<br/><code>option </code><strong> 1</strong> : the original PYTHIA 8 parameter set, based on some
very old flavour studies (with JETSET around 1990) and a simple tune 
<i>of alpha_strong</i> to three-jet shapes to the new 
<i>pT</i>-ordered shower. These were the default values before
version 8.125. 
  
<br/><code>option </code><strong> 2</strong> : 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.  
  
<br/><code>option </code><strong> 3</strong> : 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.
  
  

<p/><code>flag&nbsp; </code><strong> Tune:preferLHAPDF &nbsp;</strong> 
 (<code>default = <strong>on</strong></code>)<br/>
Tunes made by experimental collaborations typically use the LHAPDF
package to obtain their PDF values, and so PYTHIA has to be linked 
accordingly. For PDFs implemented natively in PYTHIA it is possible 
to use the respective tunes, without having to link to LHAPDF, if you 
set <code>Tune:preferLHAPDF = off</code> <i>before</i> the 
<code>Tune:pp</code> choice. 
  

<p/><code>mode&nbsp; </code><strong> Tune:pp &nbsp;</strong> 
 (<code>default = <strong>5</strong></code>; <code>minimum = -1</code>; <code>maximum = 13</code>)<br/>
Choice of tune to <i>pp/ppbar</i> data, mainly for the 
initial-state-radiation, multiparton-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. 
You should study the <code>Settings::initTunePP(...)</code> method 
to find exactly which are the settings for the respective tune.
<br/><code>option </code><strong> -1</strong> : reset all values that are affected by any of the 
<i>pp/ppbar</i> 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. 
  
<br/><code>option </code><strong> 0</strong> : no values are overwritten during the initial setup,
step 2 above. Note that changing to <code>0</code> in the user code
has no effect; if you want to restore the individual settings you 
should instead use <code>-1</code>. 
  
<br/><code>option </code><strong> 1</strong> : default used up to version 8.126, based on 
some early and primitive comparisons with data.
  
<br/><code>option </code><strong> 2</strong> : "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.
  
<br/><code>option </code><strong> 3</strong> : "Tune 2C", introduced with 8.140 [<a href="Bibliography.html" target="page">Cor10a</a>]. 
It uses the CTEQ 6L1 PDF, and is intended to give good agreement with 
much of the published CDF data.
  
<br/><code>option </code><strong> 4</strong> : "Tune 2M", introduced with 8.140 [<a href="Bibliography.html" target="page">Cor10a</a>].
It is uses the MRST LO** PDF, which has a momentum sum somewhat above 
unity, which is compensated by a smaller <i>alpha_s</i> than in the
previous tune. Again it is intended to give good agreement with much of 
the published CDF data.
  
<br/><code>option </code><strong> 5</strong> : "Tune 4C", new tune, introduced with 8.145 
[<a href="Bibliography.html" target="page">Cor10a</a>]. 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 [<a href="Bibliography.html" target="page">Buc11</a>].
  
<br/><code>option </code><strong> 6</strong> : "Tune 4Cx", based on tune 4C, but using the x-dependent
matter profile, <code>MultipartonInteractions:bProfile = 4</code> and an
increased <code>MultipartonInteractions:pT0Ref</code> [<a href="Bibliography.html" target="page">Cor11</a>].
  
<br/><code>option </code><strong> 7</strong> : "ATLAS MB Tune A2-CTEQ6L1", a minimum-bias tune based 
on tune 4Cx, but without rapidity-ordered spacelike emissions
[<a href="Bibliography.html" target="page">ATL12</a>]. Uses CTEQ 6L1, by default  from LHAPDF. 
  
<br/><code>option </code><strong> 8</strong> : "ATLAS MB Tune A2-MSTW2008LO", as above,
but uses MSTW 2008 LO, by default from LHAPDF. 
  
<br/><code>option </code><strong> 9</strong> : "ATLAS UE Tune AU2-CTEQ6L1", an underlying-event tune
based on tune 4Cx, but without rapidity-ordered spacelike emissions
[<a href="Bibliography.html" target="page">ATL12</a>]. Uses CTEQ 6L1, by default from LHAPDF. 
  
<br/><code>option </code><strong> 10</strong> : "ATLAS UE Tune AU2-MSTW2008LO", as above, 
but uses MSTW 2008 LO, by default from LHAPDF. 
  
<br/><code>option </code><strong> 11</strong> : "ATLAS UE Tune AU2-CT10", as above, 
but uses CT 10, which is not currently implemented in PYTHIA, 
so you must link LHAPDF. 
  
<br/><code>option </code><strong> 12</strong> : "ATLAS UE Tune AU2-MRST2007LO*", as above, 
but uses MRST 2007 LO*, by default from LHAPDF. 
  
<br/><code>option </code><strong> 13</strong> : "ATLAS UE Tune AU2-MRST2007LO**", as above, 
but uses MRST 2007 LO**, by default from LHAPDF. 
  
  

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