PDF Selection
This page contains three 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 gives the possibility to switch off the lepton
"parton density".
Parton densities for protons
There is one main physics choice to be made with the Pythia
class, namely which parton densities to use, a choice that then is
propagated through the program.
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 multiple-interactions model for minimum-bias and
underlying events.
Warning 2: People often underestimate the differences
between different sets on the market. The sets are constructed to behave
more or less similarly at large x and Q2, while the
multiple interactions are dominated by the behaviour in the region of
small x and Q2. A good PDF parametrization ought to be
sensible down to x = 10^{-6} (x = 10^{-7}) and
Q2 = 1 GeV2 for Tevatron (LHC) applications. Unfortunately there
are distributions on the market that completely derail in that region.
The main41.cc
and main42.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: 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.
The simplest option is to pick one
of the few distributions available internally:
mode
PDF:pSet
(default = 2
; minimum = 1
; maximum = 2
)
Parton densities to be used for proton beams (and, by implication,
antiproton ones):
option
1 : GRV 94 L;
option
2 : CTEQ 5 L.
Obviously this choice is mainly intended to get going, and if you link to
the LHAPDF
library [Wha05] you get access to a much wider selection.
flag
PDF:useLHAPDF
(default = off
)
If off then the choice of proton PDF is based on pPDFset
above. If on then it is instead based on the choice of
LHAPDFset
and 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.
word
PDF:LHAPDFset
(default = MRST2004FF4lo.LHgrid
)
Name of proton PDF set from LHAPDF to be used. You have to choose
from the
list of available sets. Examples of some recent ones 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.
mode
PDF:LHAPDFmember
(default = 0
; minimum = 0
)
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.
The current LHAPDF code does not provide a way to find the x
and Q2 limits of a set, nor does it guarantee a sensible
behaviour outside of those limits. (The LHAGLUE interface fixes this
problem, but is not used here. The limits are reported in the
PDF set list, however.) Error messages may abound, or execution
may stop unexpectedly. In a near future it will become possible to
interrogate the limits. Meanwhile you can yourself require that
PDF's should only be invoked in a specified range, to avoid problems.
flag
PDF:limitLHAPDF
(default = on
)
If on then you can set x and Q2 limits below.
parm
PDF:xMinLHAPDF
(default = 1e-6
)
Freeze x f_i(x, Q2) below this x value.
parm
PDF:xMaxLHAPDF
(default = 0.9999
)
Set x f_i(x, Q2) = 0 above this x value.
parm
PDF:Q2MinLHAPDF
(default = 1.
)
Freeze x f_i(x, Q2) below this Q2 value.
parm
PDF:Q2MaxLHAPDF
(default = 1e8
)
Freeze x f_i(x, Q2) above this Q2 value.
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 multiple interactions
alike. As already mentioned, therefore a change of PDF should be
accompanied by a complete retuning of the whole MI 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, Q2 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 multiple-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 Q2 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.
mode
PDF:pHardSet
(default = 2
; minimum = 1
; maximum = 2
)
Parton densities to be used for proton beams (and, by implication,
antiproton ones):
option
1 : GRV 94 L;
option
2 : CTEQ 5 L.
flag
PDF:useHardLHAPDF
(default = off
)
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.
word
PDF:hardLHAPDFset
(default = MRST2004FF4lo.LHgrid
)
Name of proton PDF set from LHAPDF to be used.
mode
PDF:hardLHAPDFmember
(default = 0
; minimum = 0
)
Further choice of a specific member from the set picked above.
flag
PDF:limitHardLHAPDF
(default = on
)
If on then you can set x and Q2 limits below.
parm
PDF:xMinHardLHAPDF
(default = 1e-6
)
Freeze x f_i(x, Q2) below this x value.
parm
PDF:xMaxHardLHAPDF
(default = 0.9999
)
Set x f_i(x, Q2) = 0 above this x value.
parm
PDF:Q2MinHardLHAPDF
(default = 1.
)
Freeze x f_i(x, Q2) below this Q2 value.
parm
PDF:Q2MaxHardLHAPDF
(default = 1e8
)
Freeze x f_i(x, Q2) above this Q2 value.
Parton densities for leptons
For electrons/leptons 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.