<chapter name="PDF Selection"> 
 
<h2>PDF Selection</h2> 
 
This page contains several subsections. The first deals with how to 
pick the parton distribution set for protons, including from LHAPDF, 
to be used for all proton and antiproton beams. The second is a special 
option that allows a separate PDF set to be used for the hard process 
only, while the first choice would still apply to everything else. 
The third introduces the possibility of nuclear modifications. 
Further sections give access to pion, Pomeron and photon PDF's, 
respectively, the second being used to describe diffractive systems. 
Towards the end comes the possibility to switch off the lepton 
"parton density", and photons from lepton beams. More information 
on PDF classes is found <aloc href="PartonDistributions">here</aloc>. 
 
<flag name="PDF:extrapolate" default="off"> 
Allow PDF sets to be extrapolated to small <ei>x</ei> values, instead of 
being frozen at <ei>x_min</ei>. This is a global flag that affects all 
PDF sets used, whenever extrapolation has been implemented. Among 
internal PDFs, all Pomeron sets are affected by this flag, as are 
the CTEQ6/CT09 proton ones, the NNPDF 3.1 ones and others accessed 
by the <code>LHAGrid1</code> approach. For the rest some by default 
extrapolate to small <ei>x</ei> (GRV 94 L, MRST/MSTW) while others are 
frozen at the border (CTEQ 5 L, NNPDF 2.3). When in doubt, check whether 
and how the behaviour depends on the choice made for your region of 
interest. When LHAPDF (5 or 6) is used, the extrapolation switch is set 
according to the choice here, and the behaviour is according to the 
rules of the respective program. 
<br/>To put the issue in context, parton densities have a guaranteed 
range of validity in <ei>x</ei> and <ei>Q^2</ei>, and what should be done 
beyond that range usually is not explained by the authors of PDF sets. 
Nevertheless these boundaries very often are exceeded, e.g. minimum-bias 
studies at LHC may sample <ei>x</ei> values down to <ei>10^-8</ei>, while 
many PDF sets stop already at <ei>10^-5</ei>. The default behaviour is 
then that the PDF's are frozen at the boundary, i.e. <ei>xf(x,Q^2)</ei> is 
fixed at its value at <ei>x_min</ei> for all values <ei>x &lt; 
x_min</ei>, and so on. This is a conservative approach. Alternatively, 
if you switch on extrapolation, then parametrizations will be extended 
beyond the boundaries, by some prescription. In some cases this will 
provide a more realistic answer, in others complete rubbish. Another 
problem is that some of the PDF-set codes will write a warning message 
anytime the limits are exceeded, thus swamping your output 
file. Therefore you should study a set seriously before you run it 
with this switch on. 
<note>Warning:</note>It has been found out that the LHAPDF program by 
default uses a damping of PDFs at low <ei>Q</ei> scales, below 
<ei>Q_min</ei>, based on an anomalous dimension ansatz. This overlaps 
with the damping imposed in the MPI framework by the <ei>p_T0</ei> 
parameter, and to have both would probably imply doublecounting of 
effects. Therefore, as of version 8.227, PDFs are frozen below 
<ei>Q_min</ei>. This change affects the LHAPDF 5 interface. The native 
LHAPDF 6 interface already contained this restriction, as does the PDFs 
that come with PYTHIA. Also limits at <ei>Q_max</ei> and <ei>x_max</ei> 
are checked and PDFs frozen outside them, so the extrapolate option now 
is strictly a choice of low-<ei>x</ei> behaviour. 
</flag> 
 
<h3>Parton densities for protons</h3> 
 
PYTHIA comes with a reasonably complete list of recent LO fits built-in, 
both ones within the normal LO context and ones with modifications for 
better matching to event generators. In addition two older sets are 
included for backwards reference (most studies to date are based on 
CTEQ 5L). Therefore there is no real need to link any external PDF sets. 
 
<p/> 
If the internal PDF sets are not sufficient, the 
<a href="http://projects.hepforge.org/lhapdf/" target="page">LHAPDF 
library</a> <ref>Wha05,Buc15</ref> gives you access to a much wider 
selection. 
<note>Warning 1:</note> owing to previous problems with the behaviour 
of PDF's beyond the <ei>x</ei> and <ei>Q^2</ei> boundaries of a set, 
you should only use LHAPDF <b>version 5.3.0 or later</b>. 
<note>Warning 2:</note> the behaviour of the LHAPDF sets need not be 
identical with the implementation found in PYTHIA. Specifically we 
are aware of the following points that may influence a comparison. 
<br/>(a) CTEQ 5L in PYTHIA is the parametrization, in LHAPDF the grid 
interpolation. 
<br/>(b) MRST LO* and LO** in PYTHIA is based on an updated edition, 
where one makes use of the expanded MSTW grid format, while LHAPDF 
is based on the original smaller grid. 
<br/>(c) The CTEQ 6 and CT09MC sets in PYTHIA are frozen at the 
boundaries of the grid, by recommendation of the authors, while 
LHAPDF also offers an option with a smooth extrapolation outside 
the grid boundaries. 
 
<p/> 
If you do not want to install LHAPDF, it is possible to use LHAPDF6 
data grids natively in PYTHIA. This is based on a simplified 
implementation of interpolation in a <code>.dat</code> "lhagrid1" 
file, and so does not give fully identical results, and also is not 
foolproof. 
 
<p/> 
The selection of parton densities is made once and then is propagated 
through the program. It is essential to make an informed choice, 
for several reasons <ref>Kas10</ref>: 
<note>Warning 1:</note> the choice of PDF set affects a number of 
properties of events. A change of PDF therefore requires a complete 
retuning e.g.  of the multiparton-interactions model for minimum-bias and 
underlying events. Conversely, the 
<aloc href="Tunes">pp physics tunes</aloc> are all made for a specific 
PDF tune, and the chosen (or default) tune will therefore overwrite 
the <code>PDF:pSet</code> default value described below. If you want 
to set <code>PDF:pSet</code> differently it should be done <i>after</i> 
the <code>Tune:pp</code> value, if any, has been set. 
<note>Warning 2:</note> People often underestimate the differences 
between different sets on the market. The sets for the same order are 
constructed to behave more or less similarly at large <ei>x</ei> and 
<ei>Q^2</ei>, while the multiparton interactions are dominated by the 
behaviour in the region of small <ei>x</ei> and <ei>Q^2</ei>. A good 
PDF parametrization ought to be sensible down to <ei>x = 10^-6</ei> 
(<ei>x = 10^-7</ei>) and <ei>Q^2 = 1</ei> GeV^2 for Tevatron (LHC) 
applications. Unfortunately there are distributions on the market that 
completely derail in that region. The <code>main51.cc</code> and 
<code>main52.cc</code> programs in the <code>examples</code> 
subdirectory provide some examples of absolutely minimal sanity checks 
before a new PDF set is put in production. 
<note>Warning 3:</note> NLO and LO sets tend to have quite different 
behaviours, e.g. NLO ones have less gluons at small x, which then is 
compensated by positive corrections in the NLO matrix elements. 
Therefore do not blindly assume that an NLO tune has to be better than 
an LO one when combined with the LO matrix elements in PYTHIA. There are 
explicit examples where such thinking can lead you down the wrong alley, 
especially if you study low-<ei>pT</ei> physics. A longer discussion on 
this point can be found in <a href="../pdfdoc/pdfwarning.pdf">this note</a>. 
In the list below you should therefore be extra cautious when using 
set 6 or set 9. 
 
<word name="PDF:pSet" default="13"> 
Parton densities to be used for proton beams (and, by implication, 
antiproton ones). Note that the choice of a string input (rather than 
e.g. an integer) allows to pick either an internal, LHAPDF5 or LHAPDF6 
set in one single setting, by some behind-the-scenes machinations. 
<option value="1">GRV 94L, LO <ei>alpha_s(M_Z) = 0.128</ei> 
(this set is out of date, but retained for historical comparisons).</option> 
<option value="2">CTEQ 5L, LO <ei>alpha_s(M_Z) = 0.127</ei> 
(this set is also out of date, but not badly so, and many tunes 
are based on it).</option> 
<option value="3">MRST LO* (2007), 
NLO <ei>alpha_s(M_Z) = 0.12032</ei>.</option> 
<option value="4">MRST LO** (2008), 
NLO <ei>alpha_s(M_Z) = 0.11517</ei>.</option> 
<option value="5">MSTW 2008 LO (central member), 
LO <ei>alpha_s(M_Z) = 0.13939</ei>.</option> 
<option value="6">MSTW 2008 NLO (central member), 
NLO <ei>alpha_s(M_Z) = 0.12018</ei> (NLO, see Warning 3 above).</option> 
<option value="7">CTEQ6L, NLO <ei>alpha_s(M_Z) = 0.1180</ei>.</option> 
<option value="8">CTEQ6L1, LO <ei>alpha_s(M_Z) = 0.1298</ei>.</option> 
<option value="9">CTEQ66.00 (NLO, central member), 
NLO <ei>alpha_s(M_Z) = 0.1180</ei> (NLO, see Warning 3 above).</option> 
<option value="10">CT09MC1, LO <ei>alpha_s(M_Z) = 0.1300</ei>.</option> 
<option value="11">CT09MC2, NLO <ei>alpha_s(M_Z) = 0.1180</ei>.</option> 
<option value="12">CT09MCS, NLO <ei>alpha_s(M_Z) = 0.1180</ei>.</option> 
<option value="13">NNPDF2.3 QCD+QED LO <ei>alpha_s(M_Z) = 0.130</ei>. 
</option> 
<option value="14">NNPDF2.3 QCD+QED LO <ei>alpha_s(M_Z) = 0.119</ei>. 
</option> 
<option value="15">NNPDF2.3 QCD+QED NLO <ei>alpha_s(M_Z) = 0.119</ei>. 
</option> 
<option value="16">NNPDF2.3 QCD+QED NNLO <ei>alpha_s(M_Z) = 0.119</ei>. 
</option> 
<note>Warning :</note>the following four NNPDF 3.1 sets are quite 
different from the NNPDF 2.3 ones, and cannot be used interchangeably, 
but need retuning of the MPI framework. Some also do not contain QED 
evolution. 
<option value="17">NNPDF3.1 QCD LO <ei>alpha_s(M_Z) = 0.130</ei>. 
</option> 
<option value="18">NNPDF3.1 QCD LO <ei>alpha_s(M_Z) = 0.118</ei>. 
</option> 
<option value="19">NNPDF3.1 QCD+LUXQED NLO <ei>alpha_s(M_Z) = 0.118</ei>. 
</option> 
<option value="20">NNPDF3.1 QCD+LUXQED NNLO <ei>alpha_s(M_Z) = 0.118</ei>. 
</option> 
<option value="21">NNPDF3.1sx+LHCb NLO+NLLx LUXQED 
<ei>alpha_s(M_Z) = 0.118</ei> <ref>Ber18</ref>. While at NLO, the 
additional small-<ei>x</ei> resummation, anchored by LHC-b data, offers 
a more reasonable small-<ei>x</ei> behaviour than most NLO PDFs, as 
required for the successful usage e.g. with traditional "improved LL" 
parton showers. The photon part is unchanged from the earlier NNPDF 3.1 
QED analysis <ref>Ber17</ref>. 
<note>Warning :</note>in version 8.235 the 21 identifier was used to 
denote and earlier attempt to obtain a more reasonable small-<ei>x</ei> 
behaviour. This PDF set is superseded by the new 21 and 22 sets, and has 
been removed, as was forewarned. 
</option> 
<option value="22">NNPDF3.1sx+LHCb NNLO+NLLx LUXQED 
<ei>alpha_s(M_Z) = 0.118</ei>. Comments as for 21, but this set is at 
NNLO rather than NLO. 
</option> 
<option value="LHAPDF5:set/member">Use an external LHAPDF set 
where <code>set</code> is the name of the set to use 
and <code>member</code> is the member of the set to use. The value 
for <code>set</code> is the name of the PDF set to use while the value 
for <code>member</code> must be an integer and is the member of the 
set to use. If member is not supplied, then <code>0</code> is assumed. 
</option> 
<option value="LHAPDF6:set/member">Same as 
for <code>LHAPDF5:set/member</code> but now the LHAPDF6 library is 
used instead. 
</option> 
<option value="LHAGrid1:filename">Use the internal implementation 
of interpolation in <code>.dat</code> files in the default "lhagrid1" 
LHAPDF6 format. This is a simplified implementation, with cubic 
interpolation in <ei>ln(x)</ei> and in <ei>ln(Q2)</ei>. If 
there are several <ei>Q^2</ei> subgrids they have to have the same 
<ei>x</ei> grid. (Linear interpolation in <ei>ln(Q2)</ei> is used, 
should a subgrid contain fewer than four <ei>Q2</ei> values.) 
Other restrictions may also apply, so use with caution. 
If the <code>filename</code> begins with a / it is supposed to contain 
the absolute path to the file, and if not the file is supposed to be 
located in the standard <code>share/Pythia8/xmldoc</code> directory. 
Note that, unlike LHAPDF, there is no explicit hierarchy of a set 
containing separate members; each <code>.dat</code> file can be used 
without any reference to the set it is a member of. 
</option> 
<note>Warning 1:</note> the <ei>alpha_s(M_Z)</ei> values and the order of the 
running in the description above is purely informative, and does not 
affect any other parts of the program. Instead you have the freedom to 
set <ei>alpha_s(M_Z)</ei> value and running separately for 
<aloc href="CouplingsAndScales">hard processes</aloc> 
(including resonance decays), 
<aloc href="MultipartonInteractions">multiparton interactions</aloc>, 
<aloc href="SpacelikeShowers">initial-state radiation</aloc>, and 
<aloc href="TimelikeShowers">final-state radiation</aloc>. 
<note>Warning 2:</note> in order for <code>LHAPDF</code> PDF sets to work 
you must have compiled the appropriate LHAPDF plugins for PYTHIA and 
have set the <code>LHAPATH</code> environment variable 
(or <code>LHAPDF_DATA_PATH</code>) to provide the data-files directory 
of your local LHAPDF installation. See the README file in 
the <code>examples</code> directory for further instructions. 
<note>Warning 3:</note> it is technically possible to simultaneously 
use <code>LHAPDF5</code> and <code>LHAPDF6</code> PDF sets at the same 
time for the two beams, but such a configuration is not officially 
supported and strongly discouraged. 
</word> 
 
<word name="PDF:pSetB" default="void"> 
Parton densities to be used by <ei>proton beam B</ei>, with the same 
options available as for <code>PDF:pSet</code>. If this option is set 
to <code>void</code> then the same PDF set as <code>PDF:pSet</code> is 
used. 
</word> 
 
<p/> 
If you want to use PDF's not found in LHAPDF, or you want to interface 
LHAPDF another way, you have full freedom to use the more generic 
<aloc href="PartonDistributions">interface options</aloc>. 
 
<h3>Parton densities for protons in the hard process</h3> 
 
The above options provides a PDF set that will be used everywhere: 
for the hard process, the parton showers and the multiparton interactions 
alike. As already mentioned, therefore a change of PDF should be 
accompanied by a <b>complete</b> retuning of the whole MPI framework, 
and maybe more. There are cases where one may want to explore 
different PDF options for the hard process, but would not want to touch 
the rest. If several different sets are to be compared, a simple 
reweighting based on the <aloc href="EventInformation">originally 
used</aloc> flavour, <ei>x</ei>, <ei>Q^2</ei> and PDF values may offer the 
best route. The options in this section allow a choice of the PDF set 
for the hard process alone, while the choice made in the previous section 
would still be used for everything else. The hardest interaction 
of the minimum-bias process is part of the multiparton-interactions 
framework and so does not count as a hard process here. 
 
<p/> 
Of course it is inconsistent to use different PDF's in different parts 
of an event, but if the <ei>x</ei> and <ei>Q^2</ei> ranges mainly accessed 
by the components are rather different then the contradiction would not be 
too glaring. Furthermore, since standard PDF's are one-particle-inclusive 
we anyway have to 'invent' our own PDF modifications to handle configurations 
where more than one parton is kicked out of the proton <ref>Sjo04</ref>. 
 
<p/> 
The PDF choices that can be made are the same as above, so we do not 
repeat the detailed discussion. 
 
<flag name="PDF:useHard" default="off"> 
If on then select a separate PDF set for the hard process, using the 
variables below. If off then use the same PDF set for everything, 
as already chosen above. 
</flag> 
 
<word name="PDF:pHardSet" default="void"> 
Parton densities to be used by the proton beams of the hard process, 
with the same options available as for <code>PDF:pSet</code>. If this 
option is set to <code>void</code> then the same PDF set 
as <code>PDF:pSet</code> is used. 
</word> 
 
<word name="PDF:pHardSetB" default="void"> 
Parton densities to be used by <ei>proton beam B</ei> of the hard 
process, with the same options available as 
for <code>PDF:pSet</code>. If this option is set to <code>void</code> 
then the same PDF set as <code>PDF:pHardSet</code> is used. 
</word> 
 
<h3>Nuclear modifications of parton densities</h3> 
 
<p/> 
Nuclear modifications of the PDFs are implemented for the hard-process 
generation only. The final PDF value is calculated for an average nucleon 
within given nucleus, i.e. 
<ei>f_i^A(x,Q^2) = (Z/A)*f_i^(p/A) + ((A-Z)/A)*f_i^(n/A)</ei>, where 
<ei>A</ei> is the nuclear mass number and <ei>Z</ei> the number of protons, 
set using the PDG code for nucleus. The neutron PDFs are obtained by 
applying isospin symmetry, e.g. <ei>f_u^(n/A)(x,Q^2) = f_d^(p/A)(x,Q^2)</ei>. 
The nuclear PDFs implemented provide only the nuclear modification so the 
full PDF is calculated by multiplying the selected free proton PDF with the 
modification. 
<p/> 
 
<flag name="PDF:useHardNPDFA" default="off"> 
If on, the hard processes are generated with nuclear modifications for 
beam A. 
</flag> 
 
<flag name="PDF:useHardNPDFB" default="off"> 
If on, the hard processes are generated with nuclear modifications for 
beam B. 
</flag> 
 
<modepick name="PDF:nPDFSetA" default="1" min="0" max="3"> 
The nuclear modication to be used for beam A if enabled with the switch above. 
<option value = "0"> Only Isospin effect.</option> 
<option value = "1"> EPS09, LO <ref>Esk09</ref>.</option> 
<option value = "2"> EPS09, NLO <ref>Esk09</ref>. The grid files can be 
found from 
<a href="https://www.jyu.fi/science/en/physics/research/highenergy/urhic/npdfs/eps09"> 
here</a> and are to be stored in the same folder as other PDF grid files 
(usually share/Pythia8/xmldoc/). 
</option> 
<option value = "3"> EPPS16, NLO <ref>Esk16</ref>. The grid files can be 
found from 
<a href="https://www.jyu.fi/science/en/physics/research/highenergy/urhic/npdfs/epps16-nuclear-pdfs"> 
here</a>. 
</option> 
</modepick> 
 
<modepick name="PDF:nPDFSetB" default="1" min="0" max="3"> 
The nuclear modication to be used for beam B. Same options as above. 
<option value = "0"> Only Isospin effect.</option> 
<option value = "1"> EPS09, LO. </option> 
<option value = "2"> EPS09, NLO.</option> 
<option value = "3"> EPPS16, NLO.</option> 
</modepick> 
 
<modeopen name="PDF:nPDFBeamA" default="100822080"> 
The PDG code for nuclear beam A, provides the number of protons and 
neutrons. Default code for Pb. 
</modeopen> 
 
<modeopen name="PDF:nPDFBeamB" default="100822080"> 
The PDG code for nucleus B. 
</modeopen> 
 
<h3>Parton densities for pions</h3> 
 
The parton densities of the pion are considerably less well known than 
those of the proton. There are only rather few sets on the market, 
and none particularly recent. Only one comes built-in, but others can 
be accessed from LHAPDF. Input parametrizations are for the <ei>pi+</ei>. 
>From this the <ei>pi-</ei> is obtained by charge conjugation and the 
<ei>pi0</ei> from averaging (half the pions have <ei>d dbar</ei> 
valence quark content, half <ei>u ubar</ei>. 
 
<p/> 
Much of the switches are taken over from the proton case, with obvious 
modifications; therefore the description is briefer. Currently we have 
not seen the need to allow separate parton densities for hard processes. 
When using LHAPDF the <code>PDF:extrapolateLHAPDF</code> switch of the 
proton also applies to pions. 
 
<word name="PDF:piSet" default="1"> 
Parton densities that can be used for pion beams, currently with 
only one internal choice. 
<option value="1">GRV 92 L.</option> 
<option value="LHAPDF5:set/member">Use an external LHAPDF set 
where <code>set</code> is the name of the set to use 
and <code>member</code> is the member of the set to use. The value 
for <code>set</code> can either be a relative path to the LHAPDF path, 
or an absolute path. The value for <code>member</code> must be an 
integer. 
</option> 
<option value="LHAPDF6:set/member">Same as 
for <code>LHAPDF5:set/member</code> but now the LHAPDF6 library is 
used instead. 
</option> 
<option value="LHAGrid1:filename">Use the internal implementation 
of interpolation in <code>.dat</code> files in the default "lhagrid1" 
LHAPDF6 format, cf. the corresponding proton option. 
</option> 
</word> 
 
<word name="PDF:piSetB" default="void"> 
Parton density for <ei>pion beam B</ei>. If this option is set 
to <code>void</code> then the same PDF set as <code>PDF:piSet</code> 
is used. 
</word> 
 
<h3>Parton densities for Pomerons</h3> 
 
The Pomeron is introduced in the description of diffractive events, 
i.e. a diffractive system is viewed as a Pomeron-proton collision at a 
reduced CM energy. Here the PDF's are even less well known. 
Most experimental parametrizations are NLO, which makes them less 
well suited for Monte Carlo applications. Furthermore note that 
the momentum sum is arbitrarily normalized to a non-unity value. 
 
<word name="PDF:PomSet" default="6"> 
Parton densities that can be used for Pomeron beams. 
<option value="1"><ei>Q^2</ei>-independent parametrizations 
<ei>xf(x) = N_ab x^a (1 - x)^b</ei>, where <ei>N_ab</ei> ensures 
unit momentum sum. The <ei>a</ei> and <ei>b</ei> parameters can be 
set separately for the gluon and the quark distributions. The 
momentum fraction of gluons and quarks can be freely mixed, and 
production of <ei>s</ei> quarks can be suppressed relative to 
that of <ei>d</ei> and <ei>u</ei> ones, with antiquarks as likely 
as quarks. See further below how to set the six parameters of this 
approach. 
</option> 
<option value="2"><ei>pi0</ei> distributions, as specified in the 
section above. 
</option> 
<option value="3">the H1 2006 Fit A NLO <ei>Q^2</ei>-dependent 
parametrization, based on a tune to their data <ref>H1P06</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="4">the H1 2006 Fit B NLO <ei>Q^2</ei>-dependent 
parametrization, based on a tune to their data <ref>H1P06</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="5">the H1 2007 Jets NLO <ei>Q^2</ei>-dependent 
parametrization, based on a tune to their data <ref>H1P07</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="6">the H1 2006 Fit B LO <ei>Q^2</ei>-dependent 
parametrization, based on a tune to their data <ref>H1P06</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="7">the ACTW B NLO <ei>Q^2</ei>-dependent 
parametrization with <ei>epsilon=0.14</ei>, 
based on a tune to H1 and ZEUS data <ref>Alv99</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="8">the ACTW D NLO <ei>Q^2</ei>-dependent 
parametrization with <ei>epsilon=0.14</ei>, 
based on a tune to H1 and ZEUS data <ref>Alv99</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="9">the ACTW SG NLO <ei>Q^2</ei>-dependent 
parametrization with <ei>epsilon=0.14</ei>, 
based on a tune to H1 and ZEUS data <ref>Alv99</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="10">the ACTW D NLO <ei>Q^2</ei>-dependent 
parametrization with <ei>epsilon=0.19</ei>, 
based on a tune to H1 and ZEUS data <ref>Alv99</ref>, 
rescaled by the factor <code>PomRescale</code> below. 
</option> 
<option value="11">a rescaling of the proton PDF, 
<ei>xf^pom(x)=xf^p(x x_pom)</ei>, used in the 
<code>Angantyr</code> model for Heavy Ion collisions. For high <ei>x</ei> 
there is an additional suppression by <ei>(1-x)^p</ei>, where the power is 
given by <code>PDF:PomHixSupp</code> below. 
</option> 
<option value="12">The GKG18-DPDF LO Fit A central member 
<ei>Q^2</ei>-dependent parametrization based on a tune to 
H1 and ZEUS data <ref>Goh18</ref>. 
</option> 
<option value="13">The GKG18-DPDF LO Fit B central member 
<ei>Q^2</ei>-dependent parametrization based on a tune to 
H1 and ZEUS data <ref>Goh18</ref>. 
</option> 
<option value="14">The GKG18-DPDF NLO Fit A central member 
<ei>Q^2</ei>-dependent parametrization based on a tune to 
H1 and ZEUS data <ref>Goh18</ref>. 
</option> 
<option value="15">The GKG18-DPDF NLO Fit B central member 
<ei>Q^2</ei>-dependent parametrization based on a tune to 
H1 and ZEUS data <ref>Goh18</ref>. 
</option> 
<option value="LHAPDF5:set/member">Use an external LHAPDF5 set, 
cf. the corresponding proton option. 
</option> 
<option value="LHAPDF6:set/member">Use an external LHAPDF6 set, 
cf. the corresponding proton option. 
</option> 
<option value="LHAGrid1:filename">Use the internal implementation 
for a LHAPDF6 set, cf. the corresponding proton option. 
</option> 
</word> 
 
<parm name="PDF:PomGluonA" default="0." min="-0.5" max="2."> 
the parameter <ei>a</ei> in the ansatz <ei>xg(x) = N_ab x^a (1 - x)^b</ei> 
for option 1 above. 
</parm> 
 
<parm name="PDF:PomGluonB" default="3." min="0." max="10."> 
the parameter <ei>b</ei> in the ansatz <ei>xg(x) = N_ab x^a (1 - x)^b</ei> 
for option 1 above. 
</parm> 
 
<parm name="PDF:PomQuarkA" default="0." min="-0.5" max="2."> 
the parameter <ei>a</ei> in the ansatz <ei>xq(x) = N_ab x^a (1 - x)^b</ei> 
for option 1 above. 
</parm> 
 
<parm name="PDF:PomQuarkB" default="3." min="0." max="10."> 
the parameter <ei>b</ei> in the ansatz <ei>xq(x) = N_ab x^a (1 - x)^b</ei> 
for option 1 above. 
</parm> 
 
<parm name="PDF:PomQuarkFrac" default="0.2" min="0." max="1."> 
the fraction of the Pomeron momentum carried by quarks 
for option 1 above, with the rest carried by gluons. 
</parm> 
 
<parm name="PDF:PomStrangeSupp" default="0.5" min="0." max="1."> 
the suppression of the <ei>s</ei> quark density relative to that of the 
<ei>d</ei> and <ei>u</ei> ones for option 1 above. 
</parm> 
 
<parm name="PDF:PomRescale" default="1.0" min="0.5" max="5.0"> 
Rescale several of the fits above by this uniform factor, e.g. to bring 
up their momentum sum to around unity. By default many of the sets have 
a momentum sum of order 0.5, suggesting that a factor around 2.0 
should be used. You can use <code>examples/main51.cc</code> to get 
a more precise value. Note that also other parameters in the 
<aloc href="Diffraction">diffraction</aloc> framework may need to 
be retuned when this parameter is changed. Specifically 
<code>Diffraction:PomFluxRescale</code> should be set to the inverse 
of <code>PDF:PomRescale</code> to preserve the cross section for hard 
diffractive processes. 
</parm> 
 
<parm name="PDF:PomHixSupp" default="0." min="0." max="10."> 
the power in the suppression of the high-x PDF for option 11 above. 
</parm> 
 
<h3>Parton densities for photons</h3> 
 
Photon PDFs describe the partonic content of the resolved photons and 
can be used to generate any process initiated by quarks and gluons. 
 
<p/> 
There are several PDF sets available for photons, although there have not 
been much activity recently. Currently one internal set is included, but 
more sets are available from LHAPDF5. The sets from LHAPDF5 can only be 
used as PDFs in the hard process (see <code>PDF:GammaHardSet</code> below). 
In case of photons the parton shower and beam remnant generation 
require additional methods that are provided only for internal sets. 
Currently no photon PDFs have been included in LHAPDF6. 
 
<modepick name="PDF:GammaSet" default="1" min="1" max="1"> 
Parton densities that can be used for resolved photon beams. 
<option value="1"> CJKL, based on <ref>Cor03</ref> but the rescaling 
for heavy quarks due to kinematic constraints in DIS is undone to obtain 
correct behaviour for photon-photon/hadron collisions.</option> 
</modepick> 
 
<word name="PDF:GammaHardSet" default="void"> 
Parton densities to be used by the beams of the hard process. For photons 
the other options are the ones provided by LHAPDF5. If this option is set 
to <code>void</code> then the same PDF set as <code>PDF:GammaSet</code> is 
used. 
</word> 
 
<h3>Parton densities for leptons</h3> 
 
For electrons/muons/taus there is no need to choose between different 
parametrizations, since only one implementation is available, and 
should be rather uncontroversial (apart from some technical details). 
However, insofar as e.g. <ei>e^+ e^-</ei> data often are corrected 
back to a world without any initial-state photon radiation, it is 
useful to have a corresponding option available here. 
 
<flag name="PDF:lepton" default="on"> 
Use parton densities for lepton beams or not. If off the colliding 
leptons carry the full beam energy, if on part of the energy is 
radiated away by initial-state photons. In the latter case the 
initial-state showers will generate the angles and energies of the 
set of photons that go with the collision. In addition one collinear 
photon per beam carries any leftover amount of energy not described 
by shower emissions. If the initial-state showers are switched off 
these collinear photons will carry the full radiated energy. 
</flag> 
 
<p/> 
Neutrinos are always taken pointlike. Do note that the phase space 
selection machinery currently does not allow one resolved and one 
unresolved lepton beam. For lepton-neutrino collisions to work you must 
therefore set <code>PDF:lepton = off</code>. 
 
<h3>Photon fluxes</h3> 
 
Charged beam particles may emit photons that can act as initiators for 
different processes. In addition to being in an unresolved state, 
the low virtuality 
(real) photons may also be resolved and have some partonic structure. 
Both, unresolved (direct) and resolved contributions are included, 
see <aloc href="Photoproduction">Photoproduction</aloc> for details. 
 
<p/> 
The PDFs describing the resolved state can be obtained by convoluting 
the photon flux with the selected photon PDFs. The photon flux 
is modelled according to equivalent photon approximation (EPA). 
The photon fluxes can be enabled with the following options. Currently 
fluxes are internally defined for charged leptons and protons, but other 
fluxes (e.g. photons from heavy ions) can be externally defined and 
passed to Pythia for event generation. The photon sub-beams are enabled 
with the following flags 
 
<flag name="PDF:beamA2gamma" default="off"> 
Enables photon sub-beam from beam particle <code>A</code>, 
</flag> 
<flag name="PDF:beamB2gamma" default="off"> 
Enables photon sub-beam from beam particle <code>B</code>. 
</flag> 
 
Further options specific for different beam types are described below. 
 
<h4>Lepton beams</h4> 
 
In case of leptons the photon fluxes can also be enabled with the 
following flag. 
 
<flag name="PDF:lepton2gamma" default="off"> 
Enables photon sub-beam from all lepton beams. 
</flag> 
 
This option, however, has been superseded by the new options 
<code>PDF:BeamA2gamma</code> and <code>PDF:BeamB2gamma</code> above, but is 
kept for backwards compability. The resulting behaviour is the same in both 
cases. The applied photon PDF set is selected with the 
<code>PDF:GammaSet</code> and <code>PDF:GammaHardSet</code> options above. 
Events with two unresolved photon initiators can be generated also with 
<code>PDF:lepton = on</code>, but then additional phase-space cuts 
(e.g. cut on the invariant mass of the photon-photon pair) are not applied. 
 
<modepick name="PDF:lepton2gammaSet" default="1" min="0" max="1"> 
The type of photon flux. 
<option value="0"> Uses an approximation of the photon flux to sample 
processes and corrects this later with an externally provided flux. For 
leptons a bit less efficient than option 1, but allows straightforward 
implementation of photon fluxes from different particles. To use this option 
the user has to provide the external photon flux using method 
<code>Pythia::setPhotonFluxPtr(pdfAPtr, pdfBPtr)</code> as demonstrated in the 
sample program <code>main70.cc</code>.</option> 
<option value="1"> Convolute the photon flux from EPA with the selected photon 
PDF set. Convolution integral is performed "on the fly", meaning that the 
actual integral is not computed but the <ei>x_gamma</ei> is sampled 
event-by-event. Since the final PDF value depends on the sampled value for 
<ei>x_gamma</ei>, the phase-space sampling is set up using an overestimate for 
the PDFs. This makes the process selection somewhat less efficient compared 
to the case where the PDFs are fixed (e.g. for protons).</option> 
</modepick> 
 
<h4>Proton beams</h4> 
 
There are currently two internally defined photon fluxes from a proton, 
and the user can also provide an externally defined flux. The applied flux is 
set with the following option. 
 
<modepick name="PDF:Proton2gammaSet" default="2" min="0" max="2"> 
<option value="0">Use an externally provided photon flux. This flux 
has to be passed with method <code>Pythia::setPhotonFluxPtr(pdfAPtr, 
pdfBPtr)</code>.</option> 
<option value="1"> Use virtuality integrated photon flux from Budnev 
et al.  As the flux is integrated over <ei>Q^2</ei>, the virtuality is 
always set to zero and the scattered beam proton does not have any 
transverse momentum.  When this option is applied the <ei>Q^2</ei> 
sampling will be turned off.</option> 
<option value="2"> Use a virtuality-dependent flux similar to the 
approximation by Drees and Zeppenfeld. Uses a dipole form factor to 
appropriately suppress high virtualities.</option> 
</modepick> 
 
<h4>Nuclear beams</h4> 
 
Currently no flux from heavy ions are internally defined. However, using 
the above options it is possible to define a suitable flux and pass this 
to Pythia for event generation, see <code>main70.cc</code> for example. 
To increase the sampling efficiency fo nuclear beams, the following options 
have been introduced. 
 
<modepick name="PDF:beam2gammaApprox" default="1" min="1" max="2"> 
Controls which type of overestimate is used for photon flux sampling. 
<option value="1"> Estimate optimized for photons from leptons. Works 
reasonable well also for photoproduction in p+p.</option> 
<option value="2"> Estimate optimized for ultraperipheral heavy-ion collisions 
as presented in <code>main70.cc</code>. Here the estimate is divided into two 
regions, a power-law in <ei>x_gamma</ei> below <ei>x_cut</ei> and an 
exponential fall-off above, see the related parameters below. Default values 
are optimized for p+Pb collisions where Pb-nucleus provide the photons, but 
they should work reasonably well also for other similar 
configurations.</option> 
<note>Note 1:</note> Parameters do not affect the flux itself, only the 
sampling efficiency. 
</modepick> 
<note>Note 2:</note> This option was formerly named as 
<code>PDF:lepton2gammaApprox</code> but is renamed for version 8.3. 
 
<parm name="PDF:gammaFluxApprox2bMin" default="7.336" min="0.0"> 
Minimal allowed impact parameter for which the flux is considered. Units in 
<code>fm</code>. Should match the flux provided by user. 
</parm> 
 
<parm name="PDF:gammaFluxApprox2mBeam" default="0.9314" min="0.0"> 
Per-nucleon mass used for the overestimate. Units in 
<code>GeV</code> and should again match to the user-provided flux. 
</parm> 
 
<parm name="PDF:gammaFluxApprox2xPow" default="1.15" min="0.0"> 
Value of the exponent of the power law. The default value should work well 
for the foreseen cases, so vary with caution. 
</parm> 
 
<parm name="PDF:gammaFluxApprox2xCut" default="0.01" min="0.0" max = "1.0"> 
Value that defines at which <ei>x_gamma</ei> different approximations are 
used. As above, vary with caution. 
</parm> 
 
<h3>Incoming parton selection</h3> 
 
There is one useful degree of freedom to restrict the set of incoming 
quark flavours for hard processes. It does not change the PDF's as such, 
only which quarks are allowed to contribute to the hard-process cross 
sections. Note that separate but similarly named modes are available 
for multiparton interactions and spacelike showers. 
 
<modeopen name="PDFinProcess:nQuarkIn" default="5" min="0" max="5"> 
Number of allowed incoming quark flavours in the beams; a change 
to 4 would thus exclude <ei>b</ei> and <ei>bbar</ei> as incoming 
partons, etc. 
</modeopen> 
 
</chapter> 
 
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