mode
VinciaShower:evolutionType
(default = 1
)option
1 :
Transverse Momentum. This evolution variable is roughly equal to the
inverse of the antenna function for gluon emission, and hence is in
some sense the most natural evolution variable. We define it as in
Ariadne, but with a normalization that makes it equal to SIK
at the upper edge of phase space,
option
2 :
Dipole Virtuality. This mass-like variable
represents a fairly moderate variation on the transverse
momentum. It will give slightly more priority to soft branchings
over collinear branchings, as compared to transverse
momentum. We define it as
option
3 :
Energy (of emitted parton, in dipole-antenna CM).
This option gives the highest possible prioritization of
collinear branchings over soft ones, and is in that sense the
asymptotic extreme of type 1 above.
We define it as
option
4 : V, an artificial measure constructed so as to
give the highest possible prioritization to soft branchings over collinear
ones, and is in that sense an more extreme variant of type 2
above. It is defined as
The normalizations are chosen such that the maximum value of the evolution variable is equal to the mass of the parent dipole-antenna, Qmax = mIK = mijk.
The contours below illustrate the progression of each evolution variable over the dipole-antenna phase space for three fixed values of yE = QE2/sIK:
Types 1 and 2: moderate variation | |
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Types 3 and 4: extreme variation | |
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Note that energy-ordering (type 3) is not infrared safe, since contours of finite value of that evolution variable intersect the collinear region along the axes. This would nominally lead to infinitely many collinear branchings being generated during a finite evolution interval, rendering our shower formalism inapplicable. It is therefore not possible to choose energy ordering with a hadronization cutoff in the evolution variable. Instead, energy ordering must be used with a cutoff either in pT or in mass, which is sufficient to regulate the divergence. Note that even with this regularization this ordering should still result in a logarithmically enhanced preponderance of near-collinear branchings.
In addition to the evolution variable, a main choice which decides the ordering of emissions in the shower is whether the evolution is performed as a so-called "global" or "sector" shower. Both options are available in VINCIA, with the default option being "global". In global showers, radiation from different dipole-antennae is allowed to overlap in each phase space point, such that the total result for n partons is obtained as a sum over all clusterings to (n-1) partons. The antenna functions used in this case are defined such that their sums reproduce the full QCD singular structure. In sector showers, only allow a single dipole-antenna is allowed to contribute in each phase space point. In this case, additional terms are added to each antenna function such that every single antenna contains the full collinear and soft singularities for the sector it is responsible for.
For global showers, the user has to select what kind of ordering will
be imposed, using the evolutionMode
parameter:
mode
VinciaShower:evolutionMode
(default = 3
; minimum = 0
)option
0 : No Ordering. Not recommended for physics runs.
Newly created dipole-antennae are
allowed to fill their full phase spaces, regardless of the ordering
variable. Since
energy and momentum are still conserved, a minimal amount of
ordering will still occur, due to the post-branching dipole-antennae
being smaller than the pre-branching one. This option could
therefore also be called "phase-space ordering". The
2→4 approximation is given by products of
nested 2→3 functions, without any further
modification. This leads to a large amount of overcounting at the 2→4
level and should give answers similar to standard showers with
virtuality-ordering with angular ordering switched off.
option
1 : Strong Ordering. This is identical to ordinary
strongly ordered showers. Newly created antennae are restarted at
the current evolution scale. Since the ordering condition acts like
a step function in phase space, this choice generally implies
that the shower may have some dead zones (points that are not
reached by any strongly ordered path) starting
from 2→4. For sensible evolution
variables and maps (i.e., ones that have the appropriate LL singular
limits), these dead zones only arise in non-LL-enhanced corners of
the full 2→4 space, in which zero may not
be such a terrible approximation, so they are not a priori
problematic. However, their presence does preclude the use of strongly ordered
showers as phase space generators for other purposes (e.g., for
matching). The size of these zones depend on
the evolution variable and kinematics maps and typically
covers a few percent of phase space beyond 2→4 for the standard VINCIA variables
(pT and mD).
option
2 : Smooth Ordering with QE-dampening. This option
smoothes out the ordinary strong ordering in QE by applying a
smooth dampening instead of a sharp cutoff at the ordering
scale. Nominally unordered branchings are thus allowed, but with a
suppressed probability,
0
)
while simultaneously giving a better approximation to
2→4 over all of phase space, with no dead regions.
Technically, this option is implemented in the following way:
After each branching, all
dipole-antennae are restarted at their full phase
space, but subsequent branchings are subjected to a veto
proportional to the Pimp factor above.
option
3 : Smooth Ordering with pT-dampening. As for
option =2
but with pT scales used instead of QE for
computing the suppression factor Pimp, regardless of which
evolution variable is used. For QE = pT
(see evolutionType
)
this option is obviously identical to =2
, but for other evolution
types this choice makes the
effective antenna functions independent of the evolution variable
and gives an extremely good approximation all the way through 2→6,
which is the highest order we have checked explicitly (comparing
tree-level expansions of VINCIA with Leading-Color matrix
elements from MadGraph).
Sector showers are implemented as a non-default option and can be switched on using the flag
flag
VinciaSector
(default = off
)option
off : No sector ordering is imposed. All antennae are
allowed to contribute freely, independently of overlapping
radiation.
option
on : Only one antenna is allowed to contribute to each
phase space point. Note: when this option is on, a set of additional
terms will automatically be added to VINCIA's
antenna functions, such that the entire collinear singularity of
each gluon is present in each antenna.
When the sector option is on, the following switch determines whether the additional gluon-collinear terms are added to the global antenna functions in order to create proper sector antennae from them,
flag
VinciaSector:useSectorTerms
(default = on
)on
for normal runs. Setting it to off
will
set the additional gluon-collinear sector antenna term coefficients to
zero.
For the time being, only one option for how to distinguish between sectors has been implemented, as follows. A given trial emission will only be accepted if, after the branching, it has the lowest pT (as defined for Type 1 evolution above) of all possible color-ordered 3→2 clusterings after the branching.
flag
VinciaShower:useCreationScales
(default = on
)parm
VinciaShower:pTmaxFudge
(default = 1.0
)TimeShower:pTmaxFudge
allowing the one used for VINCIA showers to be changed independently
of the PYTHIA 8 one. See the PYTHIA 8 documentation for more info.