TimeShower
directly,
but only have it called from Pythia
. Nonetheless,
some of the parameters
below, in particular TimeShower:alphaSvalue
, would be of
interest for uncertainty estimates and tuning exercises. Note that
PYTHIA also incorporates an
automated framework
for shower uncertainty variations.
mode
TimeShower:pTmaxMatch
(default = 1
; minimum = 0
; maximum = 2
)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 (i.e. to half the dipole mass).
This option agrees with the corresponding one for
spacelike showers. There 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.
The argument is less compelling for timelike showers, but could
be a reasonable starting point.
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
(i.e. to half the dipole mass). This will simulate all possible event
topologies, but may lead to double-counting.
(Also known as power showers.)
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
TimeShower:pTdampMatch
.
UserHooks
. However, for Les Houches input the optional
Beams:strictLHEFscale = on
setting restricts all emissions, also in resonance decays, to be below
the input scale
value.
parm
TimeShower:pTmaxFudge
(default = 1.0
; minimum = 0.25
; maximum = 2.0
)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
TimeShower:pTmaxFudgeMPI
(default = 1.0
; minimum = 0.25
; maximum = 2.0
)mode
TimeShower:pTdampMatch
(default = 0
; minimum = 0
; maximum = 4
)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. This argument is more obvious and relevant
for ISR, where emissions could go the the kinematical limit, whereas they
are constrained by the respective dipole mass for FSR. Nevertheless this
matching option is offered for FSR to have a (semi-)symmetric description.
Note that a dampening factor is applied to all dipoles in the final state
of the hard process, which is somewhat different from the ISR implementation.
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.
parm
TimeShower:pTdampFudge
(default = 1.0
; minimum = 0.25
; maximum = 4.0
)parm
TimeShower:alphaSvalue
(default = 0.1365
; minimum = 0.06
; maximum = 0.25
)mode
TimeShower:alphaSorder
(default = 1
; minimum = 0
; maximum = 3
)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
timelike showers. Note that tunes using this option need lower values of
alpha_strong(m_Z^2) than tunes that do not.
flag
TimeShower:alphaSuseCMW
(default = off
)option
off : Do not apply the CMW rescaling.
option
on : Apply the CMW rescaling, increasing
Lambda_QCD for timelike showers by a factor roughly 1.6.
The following options for taming (regularising) the effective value of
the strong coupling at low (IR) scales are available:
parm
TimeShower:alphaSmax
(default = -1.
; minimum = -1.
; maximum = 10.
)alphaSmax > 0
, the effective value of the strong
coupling in timelike shower branchings is capped at this value,
effectively freezing it in the infrared. Any value alphaSmax
≤ 0
is interpreted to mean that no such constraint should be
imposed.
parm
TimeShower:alphaSrenormShift
(default = 0.
; minimum = 0.
; maximum = 4.
)renormShift
. The default value of x=0
corresponds to no shift. A value of x=1 corresponds to
shifting the pole to an input scale of zero. For larger values, the
effective coupling at zero input scale is finite and equal to 4 pi
/ (9 ln(x)). Since the scale argument is typically only shifted
by a few hundred MeV, changes to the value of the effective coupling
at perturbative input scales (well above Lambda_QCD(nF=3))
are generally small. The main effect is thus a suppression of the
value of the effective coupling near the shower pTmin
cutoff. For uncertainty estimates, variations of this parameter by a
factor 2-4 in either direction (for a given central choice) may be a
reasonable starting point. Detailed examinations of this assumption
would be welcome.
QED radiation is regulated by the alpha_electromagnetic
value at the pT^2 scale of a branching.
mode
TimeShower:alphaEMorder
(default = 1
; minimum = -1
; maximum = 1
)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 for dipoles stretching out
to the beam remnants, 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
TimeShower:renormMultFac
(default = 1.
; minimum = 0.1
; maximum = 10.
)parm
TimeShower:factorMultFac
(default = 1.
; minimum = 0.1
; maximum = 10.
)parm
TimeShower:pTmin
(default = 0.5
; minimum = 0.1
; maximum = 2.0
)parm
TimeShower:pTminChgQ
(default = 0.5
; minimum = 0.1
; maximum = 2.0
)parm
TimeShower:pTminChgL
(default = 1e-6
; minimum = 1e-10
; maximum = 2.0
)pTminChgQ
.
Shower branchings gamma → f fbar, where f is a
quark or lepton, in part compete with the hard processes involving
gamma^*/Z^0 production. In order to avoid overlap it makes
sense to correlate the maximum gamma mass allowed in showers
with the minimum gamma^*/Z^0 mass allowed in hard processes.
In addition, the shower contribution only contains the pure
gamma^* contribution, i.e. not the Z^0 part, so
the mass spectrum above 50 GeV or so would not be well described.
parm
TimeShower:mMaxGamma
(default = 10.0
; minimum = 0.001
; maximum = 5000.0
)HadronLevel:QED =
on
.
Traditionally, PYTHIA did not have a generic machinery for handling
QED radiation in hadron (and tau) decays. In order to include this, a
program like PHOTOS [Bar94, Dav10] could be used as an
afterburner. The options below can be used to enable PYTHIA's internal
shower machinery for QED radiation.
mode
TimeShower:gammaModeHad
(default = 1
; minimum = 1
; maximum = 2
)HadronLevel:QED = on
and the simple-shower model is
used, this switch determines how to handle hadron-level photon
radiation.
option
1 : Photon radiation only in two-body decays to a lepton
pair, see above. Matrix element corrections are only applied in the
case of V0 → ll and V± → l nu, based on the
equivalent corrections for Z and W decays.
option
2 : Photon radiation in all two-body hadron
decays. Note: this option is mainly intended for comparisons and not
for serious studies. Since the simple shower's splitting kernels do
not encode the correct Lorentz structures for hadron decays in many
cases, nor are the correct matrix-element corrections implemented,
this option is not expected to deliver a faithful description of QED
radiation in hadron decays. (Nor are hadronic form factors or photon
VMD effects taken into account.)
flag
TimeShower:interleave
(default = on
)flag
TimeShower:interleaveResDec
(default = off
)off
, the interleaved evolution
does not affect showering in resonance decays, such as a
Z^0. These decays are only introduced after the production
process has been considered in full, and the subsequent FSR is carried
out inside the resonance, with preserved resonance mass. When this
flag is set to on
, resonance decays are inserted in the
final-state shower evolution when it reaches the pT scale
defined by TimeShower:resDecScaleChoice
below.
When TimeShower:interleaveResDec
is set to
on
, the pT scale at which interleaved resonance
decays are inserted in the shower evolution is determined by the value
of the following switch:
mode
TimeShower:resDecScaleChoice
(default = 1
; minimum = 0
; maximum = 2
)option
0 : The on-shell width of the resonance.
option
1 : Off-shellness determined by |m2 -
m02|/m0. This implies, e.g., that the decay of a
resonance which has m = m0 ± Γ will be performed at
a scale pT ~ sqrt(2) Γ.
option
2 : Off-shellness determined by sqrt(|m2
- m02|). This implies, e.g., that the decay of a
resonance which has m = m0 ± Γ will be performed at
a scale pT ~ sqrt(2 Γ m0 ).
Technically, the following steps happen when an interleaved
resonance decay is inserted in the evolution:
TimeShower:resDecScaleChoice = 2
, however,
allow for potentially larger effects in the tails.
One aspect of FSR for a hard process in hadron collisions is that often
colour dipoles are formed between a scattered parton and a beam remnant,
or rather the hole left behind by an incoming partons. If such holes
are allowed as dipole ends and take the recoil when the scattered parton
undergoes a branching then this translates into the need to take some
amount of remnant energy also in the case of FSR, i.e. the roles of
ISR and FSR are not completely decoupled. The energy taken away is
bookkept by increasing the x value assigned to the incoming
scattering parton, and a reweighting factor
x_new f(x_new, pT^2) / x_old f(x_old, pT^2)
in the emission probability ensures that not unphysically large
x_new values are reached. Usually such x changes are
small, and they can be viewed as a higher-order effect beyond the
accuracy of the leading-log initial-state showers.
This choice is not unique, however. As an alternative, if nothing else
useful for cross-checks, one could imagine that the FSR is completely
decoupled from the ISR and beam remnants.
flag
TimeShower:allowBeamRecoil
(default = on
)flag
TimeShower:dampenBeamRecoil
(default = on
)flag
TimeShower:allowMPIdipole
(default = off
)flag
TimeShower:globalRecoil
(default = off
)TimeShower:nMaxGlobalRecoil
before the
branching.
mode
TimeShower:nMaxGlobalRecoil
(default = 2
; minimum = 1
)mode
TimeShower:globalRecoilMode
(default = 0
; minimum = 0
; maximum = 2
)option
0 : Global recoil mode as outlined above, i.e. using global
recoils until the number of final state particles exceeds
TimeShower:nMaxGlobalRecoil
.
option
1 : Global recoil only for the first branching of
final state legs that have an ancestor in the hard process, and
if the maximal number of branchings generated according to the global
recoil scheme (see TimeShower:nMaxGlobalBranch
below) has
not yet been reached.
option
2 : Global recoil only if the first branching in
the whole evolution is a timelike splitting of a parton in an
event with Born-like kinematics (i.e.\ an S-event).
The impact of global recoils should be minimal in this case.
This option is only sensible for interleaved evolution.
mode
TimeShower:nMaxGlobalBranch
(default = -1
)TimeShower:globalRecoilMode = 1
or TimeShower:globalRecoilMode = 2
mode
TimeShower:nPartonsInBorn
(default = -1
)TimeShower:globalRecoilMode = 2
.
flag
TimeShower:limitPTmaxGlobal
(default = off
) Info.QFac()
otherwise), r the
radiating parton, and s the recoiling particle that would have been
used in the local recoil scheme. This option is only used if wimpy showers are
enabled.
The global-recoil machinery does not work well with rescattering in the
MPI machinery, since then the recoiling system is not uniquely defined.
MultipartonInteractions:allowRescatter = off
by default,
so this is not a main issue. If both options are switched on,
rescattering will only be allowed to kick in after the global recoil
has ceased to be active, i.e. once the nMaxGlobalRecoil
limit has been exceeded. This should not be a major conflict,
since rescattering is mainly of interest at later stages of the
downwards pT evolution.
Further, it is strongly recommended to set
TimeShower:MEcorrections = off
(not default!), i.e. not
to correct the emission probability to the internal matrix elements.
The internal ME options do not cover any cases relevant for a multibody
recoiler anyway, so no guarantees are given what prescription would
come to be used. Instead, without ME corrections, a process-independent
emission rate is obtained, and user hooks
can provide the desired process-specific rejection factors.
flag
TimeShower:weakShower
(default = off
)mode
TimeShower:weakShowerMode
(default = 0
; minimum = 0
; maximum = 2
)option
0 : both W^+- and Z^0 branchings.
option
1 : only W^+- branchings.
option
2 : only Z^0 branchings.
parm
TimeShower:pTminWeak
(default = 1.0
; minimum = 0.1
; maximum = 2.0
)flag
TimeShower:MEcorrections
(default = on
)flag
TimeShower:MEextended
(default = on
)MEcorrections
above is switched on.
flag
TimeShower:MEafterFirst
(default = on
)MEcorrections
above is
switched on. Switching off this option currently does not take effect
for a few rare types of secondary branchings, where ME corrections
play a central role.
flag
TimeShower:skipFirstMECinHardProc
(default = off
)mvec
TimeShower:skipFirstMECinResDecIDs
(default = {}
)mode
TimeShower:weightGluonToQuark
(default = 4
; minimum = 1
; maximum = 8
)TimeShower:scaleGluonToQuark
.
option
1 : same splitting kernel (1/2) (z^2 + (1-z)^2) for
massive as massless quarks, only with an extra beta phase
space factor.
option
2 : a splitting kernel
(beta/2) (z^2 + (1-z)^2 + 8r_q z(1-z)).
option
3 : a splitting kernel z^2 + (1-z)^2 + 8r_q z(1-z),
normalized so that the z-integrated rate is
(beta/3) (1 + r/2).
option
4 : same as 3, but additionally a suppression factor
(1 - m_qq^2/m_dipole^2)^3, which reduces the rate of high-mass
q qbar pairs.
option
5 : same as 1, but reweighted to an alpha_s(k m_qq^2)
rather than the normal alpha_s(pT^2).
option
6 : same as 2, but reweighted to an alpha_s(k m_qq^2)
rather than the normal alpha_s(pT^2).
option
7 : same as 3, but reweighted to an alpha_s(k m_qq^2)
rather than the normal alpha_s(pT^2).
option
8 : same as 4, but reweighted to an alpha_s(k m_qq^2)
rather than the normal alpha_s(pT^2).
parm
TimeShower:scaleGluonToQuark
(default = 1.0
; minimum = 0.25
; maximum = 1.0
)TimeShower:weightGluonToQuark
options 5 - 8. Comes on top of
TimeShower:renormMultFac
, which affects alpha_s(pT^2)
alike.
flag
TimeShower:phiPolAsym
(default = on
)flag
TimeShower:phiPolAsymHard
(default = on
)flag
TimeShower:recoilDeadCone
(default = on
)mode
TimeShower:recoilStrategyRF
(default = 0
; minimum = 0
; maximum = 1
)option
0 : Let the W act as recoiler when the parton
with the top colour radiates. Before version 8.160 this was the only
possibility, which could give too much wide-angle radiation. From
version 8.314 onwards, the parameter TimeShower:weightRF
allows to suppress this wide-angle radiation by a factor proportional to
the RF eikonal, which should deliver a better description of RF
radiation patterns.
option
1 : Assign the b as recoiler for both colour
lines of the first emitted gluon, i.e. also the one with the top
colour. This assignment then is inherited in the subsequent shower
evolution. This option was the default between versions 8.160 and
8.313.
off/on
options of the
TimeShower:recoilToColoured
flag.
TimeShower:recoilStrategyRF = 0
, the following
parameter and flag allow to apply a correction to the emission
pattern for RF dipoles. The intention is that the radiation pattern should
attach to the one expected for a final-initial dipole,
while the recoil of this radiation is taken by the final particle that
best correlates with the original top momentum.
parm
TimeShower:weightRF
(default = 1.
; minimum = 0.
; maximum = 1.
)TimeShower:recoilStrategyRF =
0
the W is chosen as recoiler not only for the first emission
but also for subsequent emissions. The emission probability is
corrected by a factor 1. + weightRF
(eikTop/eikW -
1.), where eikTop is an eikonal factor in which the top
is considered to be the recoiler while eikW is an eikonal
factor in which the W is considered to be the recoiler. Thus,
for weightRF = 0
the radiation pattern is uncorrected
(and then corresponds to that for the W as recoiler), while
for weightRF = 1
it is corrected by a factor
eikTop/eikW. As a technical note, the denominator eikonal
factor depends on the TimeShower:recoilDeadCone
flag. If
on, the radiation pattern has already been corrected for the
W mass effects. Therefore the denominator eikonal will also
contain the W mass term. If off, the denominator will not
contain this term, such that the ratio introduces the mass correction.
For TimeShower:recoilStrategyRF = 0
, this parameter
allows to smoothly interpolate between applying the full RF eikonal
reweighting factor (obtained with the default value of weightRF
= 1.
) and no reweighting (for weightRF = 0.
).
flag
TimeShower:recoilRFUseParents
(default = off
)weightRF
factor above is normally evaluated with the
four-momenta of the three partons that have been produced by the
dipole emission. If this flag is set to on
, instead the
radiator and recoiler momenta before the emission are used. The
radiated parton obviously only exists afterwards, so there the choice
is unique. The eikonal is valid in the soft-emission limit, where the
two choices agree, but the off
option better attaches to
the singularity structure of matrix elements also for harder
emissions. The on
still represents a valid variation.
weightRF
and recoilRFuseParents
have no effect for TimeShower:recoilStrategyRF = 1.
flag
TimeShower:useFixedFacScale
(default = off
)parm
below. This option is unphysical and only intended
for toy-model and debug studies.
parm
TimeShower:fixedFacScale
(default = 100.
; minimum = 1.
)flag
above is on.
mode
TimeShower:pdfMode
(default = 0
; minimum = 0
; maximum = 2
)Merging:runtimeAMCATNLOInterface
described under
Merging.
option
0 : this default setting corresponds to the typical
shower treatment of including PDF ratios for dipole recoils in the
initial state, 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.
The parameters below are intended to explore uncertainties/variations arising from process-dependent non-singular terms and/or undetermined higher-order corrections to the effective branching kernels, beyond LL accuracy. This uses generalized forms of the shower splitting kernels, for gluon emissions and splittings respectively,
where μs is the standard renormalisation scale used for shower branchings and we take the scale for the second-order correction terms to be μh = mDip/4. The form used for the g→gg kernel is completely analogous to the one for q→qg above, with its own c coefficient but the same h coefficients.
For processes that do not have matrix-element corrections (MECs),
the parameters below allow to add a constant (non-singular) term to
the effective LO kernels for QCD branchings. For positive (negative)
values, this increases (decreases) the branching probabilities. These
terms are analogous to the ones called cNS
in the context
of uncertainty variations, and have the
effect of modifying the splitting kernels as P(z)/Q2 →
P(z)/Q2 + c.
parm
TimeShower:cEmitQ
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)parm
TimeShower:cEmitC
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)cEmitQ
+ cEmitC
.
parm
TimeShower:cEmitB
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)cEmitQ
+ cEmitB
.
parm
TimeShower:cEmitG
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)parm
TimeShower:cSplit
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)parm
TimeShower:cSplitC
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)cSplit
+ cSplitC
.
parm
TimeShower:cSplitB
(default = 0.0
; minimum = -5.0
; maximum = 5.0
)cSplit
+ cSplitB
.
The following parameters allow to add terms that are suppressed by an additional power of the strong coupling, representing variations of the effective branching kernels beyond LO and hence active also in the presence of MECs.
parm
TimeShower:hEmitHard
(default = 0.0
; minimum = -40.0
; maximum = 40.0
)parm
TimeShower:hEmitColl
(default = 0.0
; minimum = -20.0
; maximum = 20.0
)parm
TimeShower:hEmitSoft
(default = 0.0
; minimum = -20.0
; maximum = 20.0
)parm
TimeShower:hSplitHard
(default = 0.0
; minimum = -40.0
; maximum = 40.0
)parm
TimeShower:hSplitColl
(default = 0.0
; minimum = -20.0
; maximum = 20.0
)flag
TimeShower:QCDshower
(default = on
)mode
TimeShower:nGluonToQuark
(default = 5
; minimum = 0
; maximum = 5
)flag
TimeShower:QEDshowerByQ
(default = on
)flag
TimeShower:QEDshowerByL
(default = on
)flag
TimeShower:QEDshowerByOther
(default = on
)flag
TimeShower:QEDshowerByGamma
(default = on
)mode
TimeShower:nGammaToQuark
(default = 5
; minimum = 0
; maximum = 5
)mode
TimeShower:nGammaToLepton
(default = 3
; minimum = 0
; maximum = 3
)flag
TimeShower:recoilToColoured
(default = on
)off
), or else
let it recoil against the b also for this dipole
(on
). Before version 8.160 the former was the only
possibility, which could give unphysical radiation patterns. It is
kept as an option to check backwards compatibility. The same issue
exists for QED radiation, but obviously is less significant. Consider
the example W → e nu, where originally the nu
takes the recoil. In the old (off
) scheme the nu
would remain recoiler, while in the new (on
) instead
each newly emitted photon becomes the new recoiler.