This page contains five 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 and fourth give access to pion and Pomeron PDF's, respectively,
the latter being used to describe diffractive systems.
The fifth gives the possibility to switch off the lepton
"parton density". More information on PDF classes is found
here.
Parton densities for protons
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.
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. In the list below you
should therefore be extra cautious when using set 6 or set 9.
The simplest option is to pick one
of the distributions available internally:
Parton densities to be used for proton beams (and, by implication,
antiproton ones):
Note: 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.
This is a reasonably complete list of recent LO fits, 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). If you link to the
LHAPDF
libraryWha05 you get 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 off then the choice of proton PDF is based on PDF:pSet
above. If on then it is instead based on the choice of
PDF:LHAPDFset and PDF:LHAPDFmember below.
Note: in order for this option to work you must have
compiled PYTHIA appropriately and have set the LHAPATH
environment variable to provide the data-files directory of your local
LHAPDF installation. See the README file in the examples
directory for further instructions.
Name of proton PDF set from LHAPDF to be used. You have to choose
from the
list of available sets. Examples of some fairly recent ones
(but still less recent than found above) would be
cteq61.LHpdf, cteq61.LHgrid, cteq6l.LHpdf, cteq6ll.LHpdf,
MRST2004nlo.LHpdf, MRST2004nlo.LHgrid, MRST2004nnlo.LHgrid and
MRST2004FF3lo.LHgrid. If you pick a LHpdf set it will require some
calculation the first time it is called.
Technical note: if you provide a name beginning with a
slash (/) it is assumed you want to provide the full file path and then
initPDFsetM(name) is called, else the correct path is assumed
already set and initPDFsetByNameM(name) is called.
Further choice of a specific member from the set picked above. Member 0
should normally correspond to the central value, with higher values
corresponding to different error PDF's somewhat off in different
directions. You have to check from set to set which options are open.
Note: you can only use one member in a run, so if you
want to sweep over many members you either have to do many separate
runs or, as a simplification, save the
pdf weights at the hard scattering
and do an offline reweighting of events.
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.
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 for proton beams (and, by implication,
antiproton ones):
If off then the choice of proton PDF is based on hardpPDFset
above. If on then it is instead based on the choice of
hardLHAPDFset and hardLHAPDFmember below.
Note that if you want to use LHAPDF here, and you also use LHAPDF
for the "normal" PDF set, then LHAPDF must have been compiled so as to
handle (at least) two concurrent sets, with the configure statement
--with-max-num-pdfsets=2.
Name of proton PDF set from LHAPDF to be used.
Further choice of a specific member from the set picked above.
Note that there is no separate equivalent of the
PDF:extrapolateLHAPDF flag specifically for the hard
PDF. Since LHAPDF only has one global flag for extrapolation or not,
the choice for the normal PDF's also applies to the hard ones.
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.
Internal parton densities that can be used for pion beams, currently with
only one choice.
If off then the choice of proton PDF is based on PDF:piSet
above. If on then it is instead based on the choice of
PDF:piLHAPDFset and PDF:piLHAPDFmember below.
Name of pion PDF set from LHAPDF to be used. You have to choose from the
list of available sets.
Further choice of a specific member from the set picked above.
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. Furthemore 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.
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 beam. For lepton-neutrino collisions to work you must
therefore set PDF:lepton = off.
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.