These switches allow to quickly switch major parts of VINCIA on and off.
flag
Vincia
(default = on
)on
and the
VINCIA plug-in is linked correctly to PYTHIA 8
(see section on installation and
linking and/or the example program vincia01.cc
included with the VINCIA plug-in),
PYTHIA will use the VINCIA showers instead of its
internal ones. If set to off
instead, PYTHIA will use its
own internal showers, regardless of whether the VINCIA plug-in is
linked (useful for quick comparisons).
For global showers, the user has to select what kind of ordering will
be imposed, using the orderingMode
parameter (note that
this parameter will be used in the hard system only):
mode
Vincia:orderingMode
(default = 2
; minimum = 0
; maximum = 3
)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. 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, defined as
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 up to Vincia:matchingLO
,
i.e. option 2 for the matrix-element corrected branchings and
option 1 fo all further branchings.
A parameter with the same options controls the ordering of the shower in all other systems (MPI, decays that are not part of the hard system, ...):
mode
Vincia:orderingModeMPI
(default = 1
; minimum = 0
; maximum = 2
)To fill all of phase space the shower has to populate regions where the evolution variable is larger than the factorisation of the hard process. Based on the process, QCD (all hard processes with coloured objects in the final state, unless they arise from a decay, i.e. gg → H → gg is NOT a QCD process) or non-QCD, we allow the hard jets to be generated differently, with the following switches. Note that they only apply to the hard process.
For QCD processes VINCIA allows the first emission to be larger than the
factorisation scale and treats it with smooth ordering (independent of
what has be chosen for Vincia:orderingMode
), where the
factorisation scale acts as QEhat. By switching
flag
Vincia:hardJetsQCD
(default = on
)
to off, the shower acts as an ordinary shower, starting at the factorisation
scale.
Note 1:
With smooth ordering and Vincia:hardJetsQCD
switched off,
the shower can still populate regions of phase space
where the evolution variable is larger than the factorisation scale, starting
from the second branching.
Note 2: Matrix element corrections are not applied if swiched off.
For non-QCD processes VINCIA uses two event samples (not visible for the user): The first sample includes the inclusive Born events at the factorisation scale of the hard process. The second event sample generates Born + jets events, where the scale of the first jet is larger than the factorisation scale. This procedure slightly modifies the input cross section and comes with weighted events, mostly peaked around 2. By switching
flag
Vincia:hardJets
(default = on
)
to off, the shower acts as an ordinary shower, starting at the factorisation
scale, with one event sample.
Note 1: With smooth ordering the shower can still populate regions of
phase space where the evolution variable is larger
than the factorisation scale, starting from the second branching.
Note 2: Matrix element corrections cannot be applied beyond first order
if Vincia:hardJets
is switched off, as it would require
clustering all the way to the Born kinematics.
Note 3: The weights can of course be divided by 2, to be peaked around
unity. If so the cross section needs to be multiplied by 2 to compensate for
the change.
mode
Vincia:hardJetsAlphaS
(default = 0
)option
0 : Use the same scale and running
of αs as the shower does.
option
1 : Use the same scale, but no CMW improvement.
option
2 : Use Q2fac+Q2branch
as the scale.
option
3 : Use Q2fac+Q2branch
as the scale and no CMW improvement.
VINCIA will ignore any externally provided scale information, such as SCALUP. For externally provided multi-parton systems, VINCIA will construct its own unique scales directly from the parton system.
The reasoning behind this is that any externally provided scales are not guaranteed to be compatible with those used by VINCIA, neither in functional form nor in normalisation. The exception to this is if the event was generated by VINCIA itself, in which case the correct scales will be reconstructed from the momenta.
Any more complete implementation of external scales should be implemented via PYTHIA's UserHooks interface, using the proper definition of the scale defined by the external program.