PDF Selection
- Parton densities for protons
- Parton densities for protons in the hard process
- Nuclear modifications of parton densities
- Parton densities for pions
- Parton densities for other hadrons
- Parton densities for Pomerons
- Parton densities for photons
- Parton densities for leptons
- Parton densities for neutrinos
- Photon fluxes
- Incoming parton selection
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 here.
flag PDF:extrapolate
(default = off)
Allow PDF sets to be extrapolated to small x values, instead of
being frozen at x_min. 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 LHAGrid1 approach. For the rest some by default
extrapolate to small x (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.
To put the issue in context, parton densities have a guaranteed
range of validity in x and Q^2, 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 x values down to 10^-8, while
many PDF sets stop already at 10^-5. The default behaviour is
then that the PDF's are frozen at the boundary, i.e. xf(x,Q^2) is
fixed at its value at x_min for all values x <
x_min, 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.
Warning:It has been found out that the LHAPDF program by
default uses a damping of PDFs at low Q scales, below
Q_min, based on an anomalous dimension ansatz. This overlaps
with the damping imposed in the MPI framework by the p_T0
parameter, and to have both would probably imply doublecounting of
effects. Therefore, as of version 8.227, PDFs are frozen below
Q_min. 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 Q_max and x_max
are checked and PDFs frozen outside them, so the extrapolate option now
is strictly a choice of low-x behaviour.
Parton densities for protons
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.
If the internal PDF sets are not sufficient, the
LHAPDF
library [Wha05,Buc15] gives you access to a much wider
selection.
Warning 1: owing to previous problems with the behaviour
of PDF's beyond the x and Q^2 boundaries of a set,
you should only use LHAPDF version 5.3.0 or later.
Warning 2: 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.
(a) CTEQ 5L in PYTHIA is the parametrization, in LHAPDF the grid
interpolation.
(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.
(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.
Because the PDF sets are typically sampled heavily within PYTHIA,
small speed differences in sampling the PDFs can lead to large overall
changes in timing. In most PDF sets the c/cbar and
b/bbar PDF sets are symmetric. Consequently, when
updating the PDF for LHAPDF, by default the cbar PDF is not
queried, but instead set to the value of the c. The same is
done for the b and bbar. The following flags allow
this symmetrization to be turned off, with the explicit understanding
that there will be a reduction in performance. Additionally, the
s can also be symmtrezed when appropriate for further timing
gains.
flag LHAPDF:sSymmetric
(default = off)
Assume the s and sbar PDF content is symmetric when
using LHAPDF PDF sets.
flag LHAPDF:cSymmetric
(default = on)
Assume the c and cbar PDF content is symmetric when
using LHAPDF PDF sets.
flag LHAPDF:bSymmetric
(default = on)
Assume the b and bbar PDF content is symmetric when
using LHAPDF PDF sets.
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 .dat "lhagrid1"
file, and so does not give fully identical results, and also is not
foolproof.
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 [Kas10]:
Warning 1: 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
pp physics tunes are all made for a specific
PDF tune, and the chosen (or default) tune will therefore overwrite
the PDF:pSet default value described below. If you want
to set PDF:pSet differently it should be done after
the Tune:pp value, if any, has been set.
Warning 2: 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 x and
Q^2, while the multiparton interactions are dominated by the
behaviour in the region of small x and Q^2. A good
PDF parametrization ought to be sensible down to x = 10^-6
(x = 10^-7) and Q^2 = 1 GeV^2 for Tevatron (LHC)
applications. Unfortunately there are distributions on the market that
completely derail in that region. The main201.cc and
main202.cc programs in the examples
subdirectory provide some examples of absolutely minimal sanity checks
before a new PDF set is put in production.
Warning 3: 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-pT physics. A longer discussion on
this point can be found in this note.
In the list below you should therefore be extra cautious when using
set 6 or set 9.
word 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 1 : GRV 94L, LO alpha_s(M_Z) = 0.128
(this set is out of date, but retained for historical comparisons).
option 2 : CTEQ 5L, LO alpha_s(M_Z) = 0.127
(this set is also out of date, but not badly so, and many tunes
are based on it).
option 3 : MRST LO* (2007),
NLO alpha_s(M_Z) = 0.12032.
option 4 : MRST LO** (2008),
NLO alpha_s(M_Z) = 0.11517.
option 5 : MSTW 2008 LO (central member),
LO alpha_s(M_Z) = 0.13939.
option 6 : MSTW 2008 NLO (central member),
NLO alpha_s(M_Z) = 0.12018 (NLO, see Warning 3 above).
option 7 : CTEQ6L, NLO alpha_s(M_Z) = 0.1180.
option 8 : CTEQ6L1, LO alpha_s(M_Z) = 0.1298.
option 9 : CTEQ66.00 (NLO, central member),
NLO alpha_s(M_Z) = 0.1180 (NLO, see Warning 3 above).
option 10 : CT09MC1, LO alpha_s(M_Z) = 0.1300.
option 11 : CT09MC2, NLO alpha_s(M_Z) = 0.1180.
option 12 : CT09MCS, NLO alpha_s(M_Z) = 0.1180.
option 13 : NNPDF2.3 QCD+QED LO alpha_s(M_Z) = 0.130.
option 14 : NNPDF2.3 QCD+QED LO alpha_s(M_Z) = 0.119.
option 15 : NNPDF2.3 QCD+QED NLO alpha_s(M_Z) = 0.119.
option 16 : NNPDF2.3 QCD+QED NNLO alpha_s(M_Z) = 0.119.
Warning :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 17 : NNPDF3.1 QCD LO alpha_s(M_Z) = 0.130.
option 18 : NNPDF3.1 QCD LO alpha_s(M_Z) = 0.118.
option 19 : NNPDF3.1 QCD+LUXQED NLO alpha_s(M_Z) = 0.118.
option 20 : NNPDF3.1 QCD+LUXQED NNLO alpha_s(M_Z) = 0.118.
option 21 : NNPDF3.1sx+LHCb NLO+NLLx LUXQED
alpha_s(M_Z) = 0.118 [Ber18]. While at NLO, the
additional small-x resummation, anchored by LHC-b data, offers
a more reasonable small-x 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 [Ber17].
Warning :in version 8.235 the 21 identifier was used to
denote and earlier attempt to obtain a more reasonable small-x
behaviour. This PDF set is superseded by the new 21 and 22 sets, and has
been removed, as was forewarned.
option 22 : NNPDF3.1sx+LHCb NNLO+NLLx LUXQED
alpha_s(M_Z) = 0.118. Comments as for 21, but this set is at
NNLO rather than NLO.
option 23 : GJR LO (2007).
option 24 : SU (2021). This is a simple parameterization based on the
same approach as other SU21 sets. It is intended for comparison with those
sets, but is too inaccurate to use for practical applications.
option LHAPDF5:set/member : Use an external LHAPDF set
where set is the name of the set to use
and member is the member of the set to use. The value
for set is the name of the PDF set to use while the value
for member must be an integer and is the member of the
set to use. If member is not supplied, then 0 is assumed.
option LHAPDF6:set/member : Same as
for LHAPDF5:set/member but now the LHAPDF6 library is
used instead.
option LHAGrid1:filename : Use the internal implementation
of interpolation in .dat files in the default "lhagrid1"
LHAPDF6 format. This is a simplified implementation, with cubic
interpolation in ln(x) and in ln(Q2). If
there are several Q^2 subgrids they have to have the same
x grid. (Linear interpolation in ln(Q2) is used,
should a subgrid contain fewer than four Q2 values.)
Other restrictions may also apply, so use with caution.
If the filename 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 share/Pythia8/xmldoc directory.
Note that, unlike LHAPDF, there is no explicit hierarchy of a set
containing separate members; each .dat file can be used
without any reference to the set it is a member of.
Warning 1: the alpha_s(M_Z) 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 alpha_s(M_Z) value and running separately for
hard processes
(including resonance decays),
multiparton interactions,
initial-state radiation, and
final-state radiation.
Warning 2: in order for LHAPDF PDF sets to work
you must have compiled the appropriate LHAPDF plugins for PYTHIA and
have set the LHAPATH environment variable
(or LHAPDF_DATA_PATH) to provide the data-files directory
of your local LHAPDF installation. See the README file in
the examples directory for further instructions.
Warning 3: it is technically possible to simultaneously
use LHAPDF5 and LHAPDF6 PDF sets at the same
time for the two beams, but such a configuration is not officially
supported and strongly discouraged.
word PDF:pSetB
(default = void)
Parton densities to be used by proton beam B, with the same
options available as for PDF:pSet. If this option is set
to void then the same PDF set as PDF:pSet is
used.
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
interface options.
Parton densities for protons in the hard process
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 complete 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 originally
used flavour, x, Q^2 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.
Of course it is inconsistent to use different PDF's in different parts
of an event, but if the x and Q^2 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 [Sjo04].
The PDF choices that can be made are the same as above, so we do not
repeat the detailed discussion.
flag 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.
word PDF:pHardSet
(default = void)
Parton densities to be used by the proton beams of the hard process,
with the same options available as for PDF:pSet. If this
option is set to void then the same PDF set
as PDF:pSet is used.
word PDF:pHardSetB
(default = void)
Parton densities to be used by proton beam B of the hard
process, with the same options available as
for PDF:pSet. If this option is set to void
then the same PDF set as PDF:pHardSet is used.
Nuclear modifications of parton densities
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.
f_i^A(x,Q^2) = (Z/A)*f_i^(p/A) + ((A-Z)/A)*f_i^(n/A), where
A is the nuclear mass number and Z the number of protons,
set using the PDG code for nucleus. The neutron PDFs are obtained by
applying isospin symmetry, e.g. f_u^(n/A)(x,Q^2) = f_d^(p/A)(x,Q^2).
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.
flag PDF:useHardNPDFA
(default = off)
If on, the hard processes are generated with nuclear modifications for
beam A.
flag PDF:useHardNPDFB
(default = off)
If on, the hard processes are generated with nuclear modifications for
beam B.
mode PDF:nPDFSetA
(default = 1; minimum = 0; maximum = 3)
The nuclear modication to be used for beam A if enabled with the switch above.
option 0 : Only Isospin effect.
option 1 : EPS09, LO [Esk09].
option 2 : EPS09, NLO [Esk09]. The grid files can be
found from
here and are to be stored in the same folder as other PDF grid files
(usually share/Pythia8/xmldoc/).
option 3 : EPPS16, NLO [Esk16]. The grid files can be
found from
here.
mode PDF:nPDFSetB
(default = 1; minimum = 0; maximum = 3)
The nuclear modication to be used for beam B. Same options as above.
option 0 : Only Isospin effect.
option 1 : EPS09, LO.
option 2 : EPS09, NLO.
option 3 : EPPS16, NLO.
mode PDF:nPDFBeamA
(default = 100822080)
The PDG code for nuclear beam A, provides the number of protons and
neutrons. Default code for Pb.
mode PDF:nPDFBeamB
(default = 100822080)
The PDG code for nucleus B.
Parton densities for pions
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 pi+.
>From this the pi- is obtained by charge conjugation and the
pi0 from averaging (half the pions have d dbar
valence quark content, half u ubar.
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 PDF:extrapolateLHAPDF switch of the
proton also applies to pions.
word PDF:piSet
(default = 2)
Parton densities that can be used for pion beams, currently with
only one internal choice.
option 1 : GRV 92 L.
option 2 : GRS 99 L.
option 3 : SU 21.
option LHAPDF5:set/member : Use an external LHAPDF set
where set is the name of the set to use
and member is the member of the set to use. The value
for set can either be a relative path to the LHAPDF path,
or an absolute path. The value for member must be an
integer.
option LHAPDF6:set/member : Same as
for LHAPDF5:set/member but now the LHAPDF6 library is
used instead.
option LHAGrid1:filename : Use the internal implementation
of interpolation in .dat files in the default "lhagrid1"
LHAPDF6 format, cf. the corresponding proton option.
word PDF:piSetB
(default = void)
Parton density for pion beam B. If this option is set
to void then the same PDF set as PDF:piSet
is used.
Parton densities for other hadrons
The SU21 family of PDFs [Sjo21] has been constructed to provide
PDFs for essentially all hadrons. By strong isospin symmetry, and
assuming no dependence on ordinary spin, this is reduced to some twenty
different sets. They are based on the Glück-Reya philosophy of a
valence-like content at some very low Q scale, including
valence-like gluons and sea. At this scale also heavier valence quarks
are assumed to take a larger fraction of the momentum. They are evolved
to higher scales with the QCDNUM package, in the leading-order approach,
as is most relevent for applications to multiparton interactions. For
completeness also proton and pion PDFs are provided in the same simplified
ansatz, but especially the former obviously is a poor competitor to more
detailed fits.
Normally the additional sets would come into play when the option of
allowing variable incoming beam particles is used, and then automatically
be loaded without any choice, except for the proton and pion switches
above.
The PDFs are stored in the LHAGrid1 format,
stretching between 10^-9 and 1 in x and between
0.51 and 10^4 GeV in Q. The file names are chosen
to associate with the base particle, which then can be rearranged
to be used also for isospin partners. They are
piplus,eta,phi,
Kplus,Dzero,Dsplus,
Jpsi,Bplus,Bszero,
Bcplus,Upsilon,proton,
Sigmaplus,Xizero,Omega,
Sigmacplusplus,Xicplus,Omegac,
Sigmabplus,Xibzero andOmegab,
all with the .dat suffix.
Parton densities for Pomerons
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 PDF:PomSet
(default = 6)
Parton densities that can be used for Pomeron beams.
option 1 : Q^2-independent parametrizations
xf(x) = N_ab x^a (1 - x)^b, where N_ab ensures
unit momentum sum. The a and b 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 s quarks can be suppressed relative to
that of d and u ones, with antiquarks as likely
as quarks. See further below how to set the six parameters of this
approach.
option 2 : pi0 distributions, as specified in the
section above.
option 3 : the H1 2006 Fit A NLO Q^2-dependent
parametrization, based on a tune to their data [H1P06],
rescaled by the factor PomRescale below.
option 4 : the H1 2006 Fit B NLO Q^2-dependent
parametrization, based on a tune to their data [H1P06],
rescaled by the factor PomRescale below.
option 5 : the H1 2007 Jets NLO Q^2-dependent
parametrization, based on a tune to their data [H1P07],
rescaled by the factor PomRescale below.
option 6 : the H1 2006 Fit B LO Q^2-dependent
parametrization, based on a tune to their data [H1P06],
rescaled by the factor PomRescale below.
option 7 : the ACTW B NLO Q^2-dependent
parametrization with epsilon=0.14,
based on a tune to H1 and ZEUS data [Alv99],
rescaled by the factor PomRescale below.
option 8 : the ACTW D NLO Q^2-dependent
parametrization with epsilon=0.14,
based on a tune to H1 and ZEUS data [Alv99],
rescaled by the factor PomRescale below.
option 9 : the ACTW SG NLO Q^2-dependent
parametrization with epsilon=0.14,
based on a tune to H1 and ZEUS data [Alv99],
rescaled by the factor PomRescale below.
option 10 : the ACTW D NLO Q^2-dependent
parametrization with epsilon=0.19,
based on a tune to H1 and ZEUS data [Alv99],
rescaled by the factor PomRescale below.
option 11 : a rescaling of the proton PDF,
xf^pom(x)=xf^p(x x_pom), used in the
Angantyr model for Heavy Ion collisions. For high x
there is an additional suppression by (1-x)^p, where the power is
given by PDF:PomHixSupp below.
option 12 : The GKG18-DPDF LO Fit A central member
Q^2-dependent parametrization based on a tune to
H1 and ZEUS data [Goh18].
option 13 : The GKG18-DPDF LO Fit B central member
Q^2-dependent parametrization based on a tune to
H1 and ZEUS data [Goh18].
option 14 : The GKG18-DPDF NLO Fit A central member
Q^2-dependent parametrization based on a tune to
H1 and ZEUS data [Goh18].
option 15 : The GKG18-DPDF NLO Fit B central member
Q^2-dependent parametrization based on a tune to
H1 and ZEUS data [Goh18].
option LHAPDF5:set/member : Use an external LHAPDF5 set,
cf. the corresponding proton option.
option LHAPDF6:set/member : Use an external LHAPDF6 set,
cf. the corresponding proton option.
option LHAGrid1:filename : Use the internal implementation
for a LHAPDF6 set, cf. the corresponding proton option.
parm PDF:PomGluonA
(default = 0.; minimum = -0.5; maximum = 2.)
the parameter a in the ansatz xg(x) = N_ab x^a (1 - x)^b
for option 1 above.
parm PDF:PomGluonB
(default = 3.; minimum = 0.; maximum = 10.)
the parameter b in the ansatz xg(x) = N_ab x^a (1 - x)^b
for option 1 above.
parm PDF:PomQuarkA
(default = 0.; minimum = -0.5; maximum = 2.)
the parameter a in the ansatz xq(x) = N_ab x^a (1 - x)^b
for option 1 above.
parm PDF:PomQuarkB
(default = 3.; minimum = 0.; maximum = 10.)
the parameter b in the ansatz xq(x) = N_ab x^a (1 - x)^b
for option 1 above.
parm PDF:PomQuarkFrac
(default = 0.2; minimum = 0.; maximum = 1.)
the fraction of the Pomeron momentum carried by quarks
for option 1 above, with the rest carried by gluons.
parm PDF:PomStrangeSupp
(default = 0.5; minimum = 0.; maximum = 1.)
the suppression of the s quark density relative to that of the
d and u ones for option 1 above.
parm PDF:PomRescale
(default = 1.0; minimum = 0.5; maximum = 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 examples/main201.cc to get
a more precise value. Note that also other parameters in the
diffraction framework may need to
be retuned when this parameter is changed. Specifically
Diffraction:PomFluxRescale should be set to the inverse
of PDF:PomRescale to preserve the cross section for hard
diffractive processes.
parm PDF:PomHixSupp
(default = 0.; minimum = 0.; maximum = 10.)
the power in the suppression of the high-x PDF for option 11 above.
Parton densities for photons
Photon PDFs describe the partonic content of the resolved photons and
can be used to generate any process initiated by quarks and gluons.
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 PDF:GammaHardSet 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.
mode PDF:GammaSet
(default = 1; minimum = 1; maximum = 1)
Parton densities that can be used for resolved photon beams.
option 1 : CJKL, based on [Cor03] but the rescaling
for heavy quarks due to kinematic constraints in DIS is undone to obtain
correct behaviour for photon-photon/hadron collisions.
word 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 void then the same PDF set as PDF:GammaSet is
used.
Parton densities for leptons
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. e^+ e^- 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 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.
Parton densities for neutrinos
Neutrinos are always taken pointlike. A flag has been added to separate these
from charged-lepton PDFs above to have the correct default behaviour.
flag PDF:neutrino
(default = off)
Unresolved neutrino beams when set off. As no PDFs are currently implemented
for neutrinos this should not be changed.
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 also PDF:lepton = off.
Photon fluxes
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 Photoproduction for details.
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 PDF:beamA2gamma
(default = off)
Enables photon sub-beam from beam particle A,
flag PDF:beamB2gamma
(default = off)
Enables photon sub-beam from beam particle B.
Further options specific for different beam types are described below.
Lepton beams
In case of leptons the photon fluxes can also be enabled with the
following flag.
flag PDF:lepton2gamma
(default = off)
Enables photon sub-beam from all lepton beams.
This option, however, has been superseded by the new options
PDF:BeamA2gamma and PDF:BeamB2gamma 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
PDF:GammaSet and PDF:GammaHardSet options above.
Events with two unresolved photon initiators can be generated also with
PDF:lepton = on, but then additional phase-space cuts
(e.g. cut on the invariant mass of the photon-photon pair) are not applied.
mode PDF:lepton2gammaSet
(default = 1; minimum = 0; maximum = 1)
The type of photon flux.
option 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
Pythia::setPhotonFluxPtr(pdfAPtr, pdfBPtr) as demonstrated in the
sample program main344.cc.
option 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 x_gamma is sampled
event-by-event. Since the final PDF value depends on the sampled value for
x_gamma, 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).
Proton beams
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.
mode PDF:Proton2gammaSet
(default = 2; minimum = 0; maximum = 2)
option 0 : Use an externally provided photon flux. This flux
has to be passed with method Pythia::setPhotonFluxPtr(pdfAPtr,
pdfBPtr).
option 1 : Use virtuality integrated photon flux from Budnev
et al. As the flux is integrated over Q^2, the virtuality is
always set to zero and the scattered beam proton does not have any
transverse momentum. When this option is applied the Q^2
sampling will be turned off.
option 2 : Use a virtuality-dependent flux similar to the
approximation by Drees and Zeppenfeld. Uses a dipole form factor to
appropriately suppress high virtualities.
Nuclear beams
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 main344.cc for example.
To increase the sampling efficiency fo nuclear beams, the following options
have been introduced.
mode PDF:beam2gammaApprox
(default = 1; minimum = 1; maximum = 2)
Controls which type of overestimate is used for photon flux sampling.
option 1 : Estimate optimized for photons from leptons. Works
reasonable well also for photoproduction in p+p.
option 2 : Estimate optimized for ultraperipheral heavy-ion collisions
as presented in main344.cc. Here the estimate is divided into two
regions, a power-law in x_gamma below x_cut 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.
Note 1: Parameters do not affect the flux itself, only the
sampling efficiency.
Note 2: This option was formerly named as
PDF:lepton2gammaApprox but is renamed for version 8.3.
parm PDF:gammaFluxApprox2bMin
(default = 7.336; minimum = 0.0)
Minimal allowed impact parameter for which the flux is considered. Units in
fm. Should match the flux provided by user.
parm PDF:gammaFluxApprox2mBeam
(default = 0.9314; minimum = 0.0)
Per-nucleon mass used for the overestimate. Units in
GeV and should again match to the user-provided flux.
parm PDF:gammaFluxApprox2xPow
(default = 1.15; minimum = 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 PDF:gammaFluxApprox2xCut
(default = 0.01; minimum = 0.0)
Value that defines at which x_gamma different approximations are
used. As above, vary with caution.
Incoming parton selection
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.
mode PDFinProcess:nQuarkIn
(default = 5; minimum = 0; maximum = 5)
Number of allowed incoming quark flavours in the beams; a change
to 4 would thus exclude b and bbar as incoming
partons, etc.