This page contains six 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, fourth and fifth give access to pion, Pomeron and photon PDF's,
respectively, the second being used to describe diffractive systems.
The sixth gives the possibility to switch off the lepton
"parton density". More information on PDF classes is found
here.
Allow PDF sets to be extrapolated, notably to small x values.
This is a global flag that affects all PDF sets used, whenever
extrapolation has been implemented. Currently the main use is for LHAPDF5,
while LHAPDF6 sets are not affected. Among internal PDFs, all Pomeron sets
are affected by this flag, as are the CTEQ6/CT09 proton ones. 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). When in doubt, check
whether and how the behaviour depends on the choice made for your region
of interest.
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.
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
libraryWha05,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.
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 main51.cc and
main52.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.
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.
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 approriate 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.
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.
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.
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.
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.
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.
Parton densities that can be used for pion beams, currently with
only one internal choice.
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 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.
Parton densities that can be used for Pomeron beams.
the parameter a in the ansatz xg(x) = N_ab x^a (1 - x)^b
for option 1 above.
the parameter b in the ansatz xg(x) = N_ab x^a (1 - x)^b
for option 1 above.
the parameter a in the ansatz xq(x) = N_ab x^a (1 - x)^b
for option 1 above.
the parameter b in the ansatz xq(x) = N_ab x^a (1 - x)^b
for option 1 above.
the fraction of the Pomeron momentum carried by quarks
for option 1 above, with the rest carried by gluons.
the suppression of the s quark density relative to that of the
d and u ones for option 1 above.
Rescale the four H1 fits above by this uniform factor, e.g. to bring
up their momentum sum to around unity. By default all three have
a momentum sum of order 0.5, suggesting that a factor around 2.0
should be used. You can use examples/main51.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.
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.
Parton densities that can be used for resolved photon beams.
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.
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.
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 PDF:lepton = off.
Photons from lepton beams
Lepton beams can emit photons and therefore may have partonic
content. The PDFs describing these can be obtained by convoluting
the photon flux with the selected photon PDFs. The photon flux
is modelled according to equivalent photon approximation (EPA) which
gives the flux of bremsstrahlung photons.
Gives photon beams from leptons. Both, unresolved (direct) and resolved
contributions are included, see
Photoproduction for details. Can be used only with charged leptons.
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 the PDF:lepton = on
but then additional phase-space cuts (e.g. cut on the invariant mass of the
photon-photon pair) are not applied.
The photon flux. Currently one option available.
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.
Number of allowed incoming quark flavours in the beams; a change
to 4 would thus exclude b and bbar as incoming
partons, etc.