Interactions involving one or two photons, either in photon-photon or
photon-hadron collision or photons emitted from lepton beams.
Includes both direct and resolved contributions and also soft QCD and MPIs
for events with resolved photons. Only (quasi-)real photons are considered
so virtuality of the photons is restricted. The PDF set for resolved photons
is selected in the PDF selection.
This page describes some of the special features related to these collisions
and introduces the relevant parameters.
Types of photon processes
Photons can be either resolved or act as point-like particles (direct).
Therefore for a photon-photon interaction there are four different
contributions, resolved-resolved, resolved-direct, direct-resolved and
direct-direct. In case of photon-hadron collisions there are two
contributions. With the default value of the parameter below, a mix of
relevant contributions is generated but each process type can also be
generated individually. Note that for photon-hadron collisions the code
for direct contribution depends on which of the beams is photon.
The sample main program main69.cc demonstrates different
possibilities.
Sets desired contribution for interactions with one or two photons.
The type of the generated process can be obtained from
Info class with method
int Info::photonMode() which follows the conventions above.
Resolved photon
Photons can either interact directly as an unresolved particle or as a
hadronic state ("Vector Meson Dominance"). In the latter case the hard
process can be simulated using PDFs to describe the partonic structure
of the resolved photon. The evolution equations for photons include an
additional term that corresponds to gamma → q qbar splittings.
Due to this, the PDFs are somewhat different for photons than for hadrons
and some parts of event generation need special attention.
Process-level generation
Due to the additional term in the evolution equations the quarks in a
resolved photon may carry a very large fraction (x~1) of the photon
momentum. In these cases it may happen that, after the hard process, there is
no energy left to construct the beam remnants. This is true especially if
a heavy quark is taken out from the beam and a corresponding massive
antiquark needs to be added to the remnant system to conserve flavour. Even
though these events are allowed based on the PDFs alone, they are not physical
and should be rejected. Therefore some amount of errors can be expected when
generating events close to the edge of phase space, e.g. when collision
energy is low.
Spacelike showers
The parton showers are generated according to the DGLAP evolution equations.
Due to the gamma → q qbar splitting in the photon evolution,
a few modifications are needed for the ISR algorithm.
The additional term corresponds to a possibility to find the original beam
photon during the backwards evolution, which is added to the QED part of the
spacelike shower evolution. If this splitting happens there is no need to
construct the beam remnants for the given beam.
The heavy quark production threshold with photon beams is handled in a
similar manner as for hadrons, but now the splitting that is forced
to happen is gamma → Q Qbar.
As the splittings in backwards evolution increases the x of the
parton taken from the beam, the splittings can lead to a situation where
there is no room left for massive beam remnants. To make sure that the
required remnants can be constructed, splittings that would not leave
room for the beam remnants are not allowed.
MPIs with photon beams
Multiparton interactions with resolved photon beams are generated as with
hadron beams. The only difference follows again from the additional
gamma → q qbar splittings where the beam photon becomes
unresolved. If this splitting happens during the interleaved evolution
for either of the photon beams no further MPIs below the branching scale
pT are allowed since the photon is not resolved anymore.
If there have been multiple interactions and a gamma → q qbar
splitting occur, the kinematics of this branching are not constructed
in the spacelike shower. Instead the pT scale of the branching is
stored and the relevant momenta are then fixed in the beam remnant handling.
Therefore the status codes for the partons related to this splitting
actually refer to beam remnants.
If there are no MPIs before the gamma → q qbar splitting,
this splitting is constructed in the spacelike shower in the usual way,
but the mother beam photon is not added to the event record, since a copy
of it already exists at the top of the event record. This is unlike the
documentation of other ISR splittings, where the mother of the branching
is shown, but consistent with the photon not being added (a second time)
for events that contain several MPIs. Optionally the photon can be shown,
using the following flag.
Show the evolution steps of the beam photon in the event record, if on.
Currently the default values for the parameters related to multiparton
interactions are the same as in hadronic collision so no tuning for the
MPIs in photon-photon or photon-hadron has been done. This holds also for
the parameters related to the impact-parameter dependence. Preliminary
studies indicate that a larger value of pT0Ref would be preferred
for photon-photon case: The inclusive hadron pT spectra in LEP
is well reproduced with pT0Ref = 3.30 GeV.
The total cross section for photon-photon collisions is paramerized as in
Sch97. Since the total cross section includes contribution also
from elastic and diffractive events, a multiplicative factor is introduced
to control the non-diffractive component.
Fraction of non-diffractive cross section of the total cross section.
Default value is motivated by earlier Pythia 6 studies.
Beam Remnants
To construct the beam remnants, one should know whether the parton
taken from the beam is a valence parton or not. The valence partons of a
photon includes the partons that originate from gamma → q qbar
splittings of the original beam photon and the valence partons from the
hadron-like part of the PDF. In either case, the flavour of the valence
quarks can fluctuate. Unfortunately the decomposition to the different
components are typically not provided in the PDF sets and some further
assumptions are needed to decide the valence content.
When ISR is applied for photon beams it is possible to end up to the original
beam photon during the evolution. Therefore there are three possibilities for
the remnants:
Remnants need to be constructed for both beams.
Remnants are constructed only for one side.
No need for remnants on either side.
The last case is the simplest as all the partons in the event are already
generated by the parton showers. In the first case the remnants and
primordial kT are constructed similarly as for normal hadronic
interactions Sjo04. For the second case the momenta of the
remnant partons can not be balanced between the two beams as the kinematics
of the other side are already fixed. In these cases the momenta are balanced
between the scattered system and the remnants.
Since the primordial kT increases the invariant mass of the remnants
and the scattered system, it may again happen that there is no room for the
remnant partons after kT is added, so the kinematics can not be
constructed. In this case new values for kT are sampled. If this
does not work, a new shower is generated and in some rare cases the
parton-level generation fails and the hard process is rejected. The inclusion
of additional MPIs increases the invariant mass of the remnants and takes
more momentum from the beam particles. Even though the MPIs that would
not leave enough room for the remnants are rejected, these can still lead
to a situation where the kinematics cannot be constructed due to the added
primordial kT. This may cause some amount of errors especially when
the invariant mass of gamma-gamma system is small.
Photons from lepton beams
Interaction of photons from leptons including photon-photon interactions in
lepton-lepton collisions and photon-hadron lepton-hadron collisions can be
set up as described in
PDF selection. Since the current
framework can handle only (quasi-)real photons, a upper limit for the
photon virtuality needs to be set. This can be done with the parameter
Photon:Q2max. The upper limit for virtuality will set also
the upper limit for the k_T of the photon, which in turn will
be the same as the k_T of the scattered lepton. Also some other
cuts can be imposed.
Upper limit for (quasi-)real photon virtuality in GeV^2.
Lower limit for invariant mass of gamma-gamma system in GeV.
In lepton-hadron collisions W corresponds to invariant mass of
photon-hadron system.
Upper limit for invariant mass of gamma-gamma
(gamma-hadron) system in GeV.
A value below Photon:Wmin means that the invariant mass of
the original l+l- (lepton-hadron) system is used as an
upper limit.
Upper limit for scattering angle of lepton A in rad. A negative
value means that no cut is applied. Since k_T depends on virtuality
of the emitted photon, the Photon:Q2max cut is usually more
restrictive unless a very small angle is used.
As above but for lepton B.
MPIs with lepton beams
The invariant mass of gamma-gamma or gamma-hadron system
from lepton beams will vary. Therefore, to generate MPIs and non-diffractive
events in gamma-gamma and gamma-hadron
collisions from lepton beams, the MPI framework is initialized with five
values of W from Photon:Wmin to
Photon:Wmax. The parameter values are then interpolated
for the sampled W.