The normal user is not expected to call SpaceShower
directly, but only have it called from Pythia,
via PartonLevel. Nonetheless, some of the parameters below,
in particular SpaceShower:alphaSvalue,
would be of interest for uncertainty estimates and tuning exercises.
Note that
PYTHIA also incorporates an
automated framework for shower
uncertainty variations.
mode SpaceShower:pTmaxMatch
(default = 0; minimum = 0; maximum = 2)
Way in which the maximum shower evolution scale is set to match the
scale of the hard process itself.
option 0 : (i) if the final state of the hard process
(not counting subsequent resonance decays) contains at least one quark
(u, d, s, c ,b), gluon or photon then pT_max
is chosen to be the factorization scale for internal processes
and the scale value for Les Houches input;
(ii) if not, emissions are allowed to go all the way up to
the kinematical limit.
The reasoning is that in the former set of processes the ISR
emission of yet another quark, gluon or photon could lead to
double-counting, while no such danger exists in the latter case.
option 1 : always use the factorization scale for an internal
process and the scale value for Les Houches input,
i.e. the lower value. This should avoid double-counting, but
may leave out some emissions that ought to have been simulated.
(Also known as wimpy showers.)
option 2 : always allow emissions up to the kinematical limit.
This will simulate all possible event topologies, but may lead to
double-counting.
(Also known as power showers.)
Note 1: Some processes contain matrix-element matching
to the first emission; this is the case notably for single
gamma^*/Z^0, W^+- and H^0 production. Then default
and option 2 give the correct result, while option 1 should never
be used.
Note 2: as enumerated in the text, these options take effect
both for internal and external processes. Whether a particular option
makes sense depends on the context. For instance, if events for the same
basic process to different orders are to be matched, then option 1 would
be a reasonable first guess. Note, however, that a program like the
POWHEG BOX uses a pT definition for ISR and FSR that does not
quite agree with the PYTHIA evolution scale, and thus there will be some
amount of mismatch. In more sophisticated descriptions, therefore,
option 2 could be combined with UserHooks vetoes on emissions
that would lead to double-counting, using more flexible phase space
boundaries. Further details are found in the
Matching and Merging description,
with an example in examples/main152.
Option 0, finally, may be most realistic when only Born-level
processes are involved, possibly in combination with a nonzero
SpaceShower:pTdampMatch. The rules used for avoiding
double-counting are not foolproof, however. As an example, for the
t-channel process gamma gamma → e^+ e^- its pT
scale is the plausible upper shower limit, with only dampened emissions
above it. But the initial state is not checked and, had only incoming
quarks and gluons been taken into account, only the s-channel
process q qbar → gamma^*/Z^0 → e^+ e^- would have
been possible, where indeed the whole phase space should be populated.
So this is erroneously used, giving too much emissions.
Note 3: These options only apply to the hard interaction.
If a "second hard" process is present, the two are analyzed and
set separately for the default 0 option, while both are affected
the same way for non-default options 1 and 2.
Emissions off subsequent multiparton interactions are always constrained
to be below the factorization scale of each process itself.
parm SpaceShower:pTmaxFudge
(default = 1.0; minimum = 0.25; maximum = 2.0)
In cases where the above pTmaxMatch rules would imply
that pT_max = pT_factorization, pTmaxFudge
introduces a multiplicative factor f such that instead
pT_max = f * pT_factorization. Only applies to the hardest
interaction in an event, and a "second hard" if there is such a one,
cf. below. It is strongly suggested that f = 1, but variations
around this default can be useful to test this assumption.
parm SpaceShower:pTmaxFudgeMPI
(default = 1.0; minimum = 0.25; maximum = 2.0)
A multiplicative factor f such that
pT_max = f * pT_factorization, as above, but here for the
non-hardest interactions (when multiparton interactions are allowed).
mode SpaceShower:pTdampMatch
(default = 3; minimum = 0; maximum = 4)
These options only take effect when a process is allowed to radiate up
to the kinematical limit by the above pTmaxMatch choice,
and no matrix-element corrections are available. Then, in many processes,
the fall-off in pT will be too slow by one factor of pT^2.
That is, while showers have an approximate dpT^2/pT^2 shape, often
it should become more like dpT^2/pT^4 at pT values above
the scale of the hard process. Whether this actually is the case
depends on the particular process studied, e.g. if t-channel
gluon exchange is likely to dominate. If so, the options below could
provide a reasonable high-pT behaviour without requiring
higher-order calculations.
option 0 : emissions go up to the kinematical limit,
with no special dampening.
option 1 : emissions go up to the kinematical limit,
but dampened by a factor k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac),
where Q_fac is the factorization scale and k is a
multiplicative fudge factor stored in pTdampFudge below.
option 2 : emissions go up to the kinematical limit,
but dampened by a factor k^2 Q^2_ren/(pT^2 + k^2 Q^2_ren),
where Q_ren is the renormalization scale and k is a
multiplicative fudge factor stored in pTdampFudge below.
option 3 : as option 1, but in addition to the standard requirements
for dampening it is further necessary to have at least two top or
beyond-the-Standard-Model coloured particles in the final state.
Examples include t tbar and squark gluino production.
option 4 : as option 2, but in addition to the standard requirements
for dampening it is further necessary to have at least two top or
beyond-the-Standard-Model coloured particles in the final state.
Examples include t tbar and squark gluino production.
Note: These options only apply to the hard interaction.
Specifically, a "second hard" interaction would not be affected.
Emissions off subsequent multiparton interactions are always constrained
to be below the factorization scale of the process itself.
parm SpaceShower:pTdampFudge
(default = 1.0; minimum = 0.25; maximum = 4.0)
In cases 1 and 2 above, where a dampening is imposed at around the
factorization or renormalization scale, respectively, this allows the
pT scale of dampening of radiation by a half to be shifted
by this factor relative to the default Q_fac or Q_ren.
This number ought to be in the neighbourhood of unity, but variations
away from this value could do better in some processes.
The amount of QCD radiation in the shower is determined by
parm SpaceShower:alphaSvalue
(default = 0.1365; minimum = 0.06; maximum = 0.25)
The alpha_strong value at scale M_Z^2.
The actual value is then regulated by the running to the scale pT^2, at which it is evaluated
mode SpaceShower:alphaSorder
(default = 1; minimum = 0; maximum = 3)
Order at which alpha_strong runs,
option 0 : zeroth order, i.e. alpha_strong is kept
fixed.
option 1 : first order, which is the normal value.
option 2 : second order. Since other parts of the code do
not go to second order there is no strong reason to use this option,
but there is also nothing wrong with it.
option 3 : third order, with the same comment as for second
order. The expression in the 2006 RPP is used here.
The CMW rescaling of Lambda_QCD (see the section on StandardModelParameters) can be applied to the alpha_strong values used for spacelike showers. Note that tunes using this option need lower values of alpha_strong(m_Z^2) than tunes that do not.
flag SpaceShower:alphaSuseCMW
(default = off)
option off : Do not apply the CMW rescaling.
option on : Apply the CMW rescaling, increasing
Lambda_QCD for spacelike showers by a factor roughly 1.6.
QED radiation is regulated by the alpha_electromagnetic value at the pT^2 scale of a branching.
mode SpaceShower:alphaEMorder
(default = 1; minimum = -1; maximum = 1)
The running of alpha_em.
option 1 : first-order running, constrained to agree with
StandardModel:alphaEMmZ at the Z^0 mass.
option 0 : zeroth order, i.e. alpha_em is kept
fixed at its value at vanishing momentum transfer.
option -1 : zeroth order, i.e. alpha_em is kept
fixed, but at StandardModel:alphaEMmZ, i.e. its value
at the Z^0 mass.
The natural scale for couplings and PDFs is pT^2. To explore uncertainties it is possibly to vary around this value, however, in analogy with what can be done for hard processes. (Note that there is also an automated framework for shower uncertainties.)
parm SpaceShower:renormMultFac
(default = 1.; minimum = 0.1; maximum = 10.)
The default pT^2 renormalization scale is multiplied by
this prefactor. For QCD this is equivalent to a change of
Lambda^2 in the opposite direction, i.e. to a change of
alpha_strong(M_Z^2) (except that flavour thresholds
remain at fixed scales). Below, when pT^2 + pT_0^2 is used
as scale, it is this whole expression that is multiplied by the prefactor.
parm SpaceShower:factorMultFac
(default = 1.; minimum = 0.1; maximum = 10.)
The default pT^2 factorization scale is multiplied by
this prefactor.
There are two complementary ways of regularizing the small-pT divergence, a sharp cutoff and a smooth dampening. These can be combined as desired but it makes sense to coordinate with how the same issue is handled in multiparton interactions.
flag SpaceShower:samePTasMPI
(default = off)
Regularize the pT → 0 divergence using the same sharp cutoff
and smooth dampening parameters as used to describe multiparton interactions.
That is, the MultipartonInteractions:pT0Ref,
MultipartonInteractions:ecmRef,
MultipartonInteractions:ecmPow and
MultipartonInteractions:pTmin parameters are used to regularize
all ISR QCD radiation, rather than the corresponding parameters below.
This is a sensible physics ansatz, based on the assumption that colour
screening effects influence both MPI and ISR in the same way. Photon
radiation is regularized separately in either case.
Note: For photon-photon collisions these parameters are set
as in Photoproduction.
Warning: if a large pT0 is picked for multiparton
interactions, such that the integrated interaction cross section is
below the nondiffractive inelastic one, this pT0 will
automatically be scaled down to cope. Information on such a rescaling
does NOT propagate to SpaceShower, however.
The actual pT0 parameter used at a given CM energy scale, ecmNow, is obtained from a power law or a logarithmic parametrization. The former is default with hadron beams and the latter for photon-photon collisions.
mode SpaceShower:pT0parametrization
(default = 0; minimum = 0; maximum = 1)
Choice of pT0 parametrization.
option 0 : Power law dependence on ecmNow:
pT0 = pT0(ecmNow) = pT0Ref * (ecmNow / ecmRef)^ecmPow
option 1 : Logarithmic dependence on ecmNow:
pT0 = pT0(ecmNow) = pT0Ref + ecmPow * log (ecmNow / ecmRef)
where pT0Ref, ecmRef and ecmPow are the
three parameters below.
parm SpaceShower:pT0Ref
(default = 2.0; minimum = 0.5; maximum = 10.0)
Regularization of the divergence of the QCD emission probability for
pT → 0 is obtained by a factor pT^2 / (pT0^2 + pT^2),
and by using an alpha_s(pT0^2 + pT^2). An energy dependence
of the pT0 choice is introduced by the next two parameters,
so that pT0Ref is the pT0 value for the reference
cm energy, pT0Ref = pT0(ecmRef).
parm SpaceShower:ecmRef
(default = 7000.0; minimum = 1.)
The ecmRef reference energy scale introduced above.
parm SpaceShower:ecmPow
(default = 0.0; minimum = 0.; maximum = 0.5)
The ecmPow energy rescaling pace introduced above.
parm SpaceShower:pTmin
(default = 0.2; minimum = 0.1; maximum = 10.0)
Lower cutoff in pT, below which no further ISR branchings
are allowed. Normally the pT0 above would be used to
provide the main regularization of the branching rate for
pT → 0, in which case pTmin is used mainly for
technical reasons. It is possible, however, to set pT0Ref = 0
and use pTmin to provide a step-function regularization,
or to combine them in intermediate approaches. Currently pTmin
is taken to be energy-independent.
parm SpaceShower:pTminChgQ
(default = 0.5; minimum = 0.01)
Parton shower cut-off pT for photon coupling to a coloured
particle.
parm SpaceShower:pTminChgL
(default = 0.0005; minimum = 0.0001)
Parton shower cut-off mass for pure QED branchings.
Assumed smaller than (or equal to) pTminChgQ.
flag SpaceShower:rapidityOrder
(default = on)
Force emissions, after the first, to be ordered in rapidity,
i.e. in terms of decreasing angles in a backwards-evolution sense.
Could be used to probe sensitivity to unordered emissions.
Only affects QCD emissions, and only the hard subcollision of an event.
(For the case "soft QCD" processes the first MPI counts as the hard
subcollision.)
flag SpaceShower:rapidityOrderMPI
(default = on)
Same as the last switch, but this time only emissions in secondary
scattering systems from MPIs are forced to be ordered in rapidity.
Each MPI is ordered separately from the others.
The existing initial-initial global recoil scheme is maintained for an emission off a colour line that stretches through the hard process, so it is the handling of initial-final dipole ends that is changed. Here the single recoiler is picked based on the colour flow of the hard process. Additionally the description unifies the emission of a gluon from the initial-final and final-initial dipole ends, and handles both as part of the ISR framework. Therefore the separation into ISR and FSR is not a meaningful classification, and either both should be simulated or none.
Note that this option should not be combined with the global option
for FSR, TimeShower:globalRecoil. Furthermore some settings
are neglected internally to ensure the same behaviour as obtained for
TimeShower:allowBeamRecoil = on,
TimeShower:dampenBeamRecoil = off, and
SpaceShower:phiIntAsym = off.
The dipole recoil option for the first time allows the simulation
of Deeply Inelastic Scattering processes in PYTHIA 8, see the
main341.cc example. Note that the simultaneous emission of
photons off the lepton leg has not yet been implemented, so you need to
set PDF:lepton = off and
TimeShower:QEDshowerByL = off. You are further recommended
to set SpaceShower:pTmaxMatch = 2 to fill the whole phase
space with parton showers. This is allowed since the shower and
matrix-element behaviours match well over the whole phase space
(at least for the first emission).
flag SpaceShower:dipoleRecoil
(default = off)
Option to switch on the dipole-recoil scheme as described above.
flag SpaceShower:weakShower
(default = off)
Allow a weak shower, yes or no.
mode SpaceShower:weakShowerMode
(default = 0; minimum = 0; maximum = 2)
Determine which branchings are allowed.
option 0 : both W^+- and Z^0 branchings.
option 1 : only W^+- branchings.
option 2 : only Z^0 branchings.
parm SpaceShower:pTminWeak
(default = 1.0; minimum = 0.1; maximum = 2.0)
Parton shower cut-off pT for weak branchings.
There are three flags you can use to switch on or off selected branchings in the shower:
flag SpaceShower:QCDshower
(default = on)
Allow a QCD shower; on/off = true/false.
flag SpaceShower:QEDshowerByQ
(default = on)
Allow quarks to radiate photons; on/off = true/false.
flag SpaceShower:QEDshowerByL
(default = on)
Allow leptons to radiate photons; on/off = true/false.
There are some further possibilities to modify the shower:
flag SpaceShower:MEcorrections
(default = on)
Use of matrix element corrections; on/off = true/false.
flag SpaceShower:MEafterFirst
(default = on)
Use of matrix element corrections also after the first emission,
for dipole ends of the same system that did not yet radiate.
Only has a meaning if MEcorrections above is
switched on.
flag SpaceShower:phiPolAsym
(default = on)
Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
flag SpaceShower:phiPolAsymHard
(default = on)
Extend the above azimuthal asymmetry (if on) also back to gluons produced
in the hard process itself, where feasible; on/off = true/false.
flag SpaceShower:phiIntAsym
(default = on)
Azimuthal asymmetry induced by interference; on/off = true/false.
parm SpaceShower:strengthIntAsym
(default = 0.7; minimum = 0.; maximum = 0.9)
Size of asymmetry induced by interference. Natural value of order 0.5;
expression would blow up for a value of 1.
mode SpaceShower:nQuarkIn
(default = 5; minimum = 0; maximum = 5)
Number of allowed quark flavours in g → q qbar branchings,
when kinematically allowed, and thereby also in incoming beams.
Changing it to 4 would forbid g → b bbar, etc.
flag SpaceShower:useFixedFacScale
(default = off)
Allow the possibility to use a fixed factorization scale, set by
the parm below. This option is unphysical and only
intended for toy-model and debug studies.
parm SpaceShower:fixedFacScale
(default = 100.; minimum = 1.)
The fixed factorization scale, in GeV, that would be used in the
evaluation of parton densities if the flag above is on.
mode SpaceShower:pdfMode
(default = 0; minimum = 0; maximum = 2)
This setting should not be touched by non-experts. Deviating
from the default setting will only lead to consistent results
after explicit external intervention. This setting can be useful
in the context of interfaces to external code as done when using
the flag Merging:runtimeAMCATNLOInterface described under
Merging.
option 0 : this default setting corresponds to the typical
shower treatment of including PDF ratios in the backwards-evolution
branching rates, leading to the generation of normal no-emission
probabilities.
option 1 : disable the PDF dependence, which leads to the
generation of Sudakov factors according to the momentum sum rule.
option 2 : disable the PDF dependence, which leads to the
generation of Sudakov factors like option 1, but with a lower cut-off
zMin = 0.5 on the energy-fraction integral.