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).
flag
Vincia:uncertaintyBands
(default = off
)flag
Vincia:nonRes
(default = off
)flag
Vincia:hyperjet
(default = off
)on
, VINCIA will automatically
use modified gluon emission antenna functions, in which the Eikonal (soft)
terms have been subtracted out, for the showers off the hardest
interaction. The gluon splitting antennae and the gluon emission
antennae for radiation inside resonance decays (and for showers off
MPI) are left
unmodified. Since the modified gluon emission antennae no longer contain
a soft singularity, this will NOT generate the correct DGLAP evolution
if used together with normal matrix elements for the hard
process. This option is therefore only intended for use together with
matrix element events in which the soft Eikonal has already been
resummed, such as when VINCIA is used together with J. Andersen's
high-energy-limit matrix elements. Since these matrix element
expressions are correct in the high-y-per-jet limit,
we have dubbed this running mode HyperJet
.
The 2->3 (LL) VINCIA antennae have names such as
Vincia:qQ:emit
(for gluon emission off a qqbar
antenna), Vincia:qg:split
(for gluon splitting to a
quark-antiquark pair inside a qg antenna).Vincia:AB:x
, where A
and B
are
the "mothers" and x
is either emit
or
split
, depending on whether the process is gluon emission
or gluon splitting.
The radiating (parent) antenna is interpreted as
spanned between the Les Houches colour tag of
A
and the anti-colour tag of B
, see
illustration to the right.
The functional form of the antennae are specified by giving the
coefficients of a double Laurent series in the two branching
invariants (the third invariant, specifying a
rotation around the dipole axis, is chosen uniformly).
The Laurent expansion starts at power (-1,-1)
corresponding to the double singularity.
For so-called "global" antennae (the default in VINCIA),
each antenna is fully specified by giving the
coefficients LaurentC(i,j)
of the following expansion:
A(y_ar,y_rb;s_AB) = 1/s_AB 4 pi
alpha_s chargeFactor LaurentC(i,j) pow(y_ar,i) pow(y_rb,j);
where i
and
j
are implicitly summed over,
s_AB
is the mass of the mother antenna,
chargeFactor
should be normalized to tend to NC
raised to the number of new color lines created in
the splitting in
the large-NC limit (i.e., the limiting value should be 3 for gluon
emission and 1 for gluon splitting), and y_ar,y_rb
are the
branching invariants scaled by the mass of the mother antenna,
y_ij = s_ij/s_AB
.
The antennae implemented in VINCIA are described in detail in a separate file:
The coefficients with negative indices are universal and should not be changed by the user. Finite terms, i.e., with both indices greater than or equal to zero, are arbitrary and in general the best choice to make will depend on the specific process considered. They are therefore not to be regarded as "tunable" parameters, but rather as an independent way of estimating the uncertainty due do uncalculated higher orders, an uncertainty which can be explicitly reduced by matrix-element matching. The default is therefore to allow these terms to be nonzero, but for special applications it may be convenient to have one global switch that switches them on and off:flag
Vincia:useFiniteTerms
(default = on
)on
for normal runs. Setting it to off
will
set all finite term coefficients to zero.
word
Vincia:antennaFile
(default = none
)antennae/
(see below).
More user-defined antennae can be added and stored in the same
directory, if desired. To use them with a program that
runs in your main VINCIA directory (default), include the following
command in your main program or command file:
Vincia:antennaFile = antennae/antennae-GGG.cmnd
antennae/
directory, give the full path- and filename instead, as in
Vincia:antennaFile = /Users/skands/vincia/antennae/antennae-GGG.cmnd
The default antennae have been chosen to be close to the GGG ones
and have been verified to give a good average agreement with Z->n
matrix elements. The following two examples, included with the
VINCIA package in the antennae/
subdirectory, have been defined so as to try to span a
reasonable min-max uncertainty range:
The number of quark flavours allowed in gluon splittings, phase space permitting, is given by
mode
Vincia:nGluonToQuark
(default = 3
; minimum = 0
; maximum = 5
)mode
Vincia: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 S_AB
at the upper edge of phase space,
option
2 : Daughter Antenna Mass. 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 contours below illustrate the progression of each evolution variable over the dipole-antenna phase space for three fixed values of y_E = Q_E^2/s_AB:
Types 1 and 2: moderate variation | |
![]() |
![]() |
Types 3 and 4: extreme variation | |
![]() |
![]() |
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.
mode
Vincia:orderingMode
(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).
flag
Vincia:useCreationScales
(default = on
)parm
Vincia: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.
Important note: sector ordering should only be used together with dedicated antenna functions whose collinear singularities have been tailored explicitly to work in this mode. If used with the default kind of antenna functions, such as the GGG or ARIADNE ones, correct DGLAP evolution will not result.
flag
Vincia:sectorOrdering
(default = off
)option
off : No sector ordering is imposed. All antennae are
allowed to contribute freely, independently of overlapping
radiation. Warning: this option should only be used with so-called
"global" antenna functions (the default),
whose singular terms are such that the
collinear singularity of one gluon is obtained by summing two
neighbouring antennae over all of phase space.
option
on : Only one antenna is allowed to contribute to each
phase space point. Warning: this option should only be used with so-called
"sector" antenna functions, whose singular terms are such that the
collinear singularity of one gluon is represented entirely by the
antenna which has the smallest value of pT for the given phase
space point.
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.
In the case of sector-ordered antennae (as opposed to the "global"
ones described above), explicit singularities at y_ij->1
may also be present. In addition to the C_ij
coefficients
defined above, we therefore also allow for the following
additional terms in the antenna function parametrization when sector
ordering is switched on:
... + LaurentD(i) pow(y_ar,i)
/ (1-y_rb) +
LaurentE(i) pow(y_rb,i) / (1-y_ar) + K / (1-y_ar) / (1-y_rb);
While the CM momenta of a 2->3 branching are fixed by the generated invariants (and hence by the antenna function), the global orientation of the produced 3-parton system with respect to the rest of the event (or, equivalently, with respect to the original dipole-antenna axis) suffers from an ambiguity outside the LL limits, which can affect the tower of subleading logs generated and can be significant in regions where the leading logs are suppressed or absent.
To illustrate this ambiguity, consider the emissision of a gluon from a qqbar antenna with some finite amount of transverse momentum (meaning transverse to the original dipole-antenna axis, in the CM of the dipole-antenna). The transverse momenta of the qqbar pair after the branching must now add up to an equal, opposite amount, so that total momentum is conserved, i.e., the emission generates a recoil. By an overall rotation of the post-branching 3-parton system, it is possible to align either the q or the qbar with the original axis, such that it becomes the other one that absorbs the entire recoil (the default in showers based on 1->2 branchings such as old-fashioned parton showers and Catani-Seymour showers), or to align both of them slightly off-axis, so that they share the recoil (the default in VINCIA, see illustration below).
mode
Vincia:kineMapType
(default = 1
; minimum = 1
; maximum = 3
)option
1 : The ARIADNE angle (see illustration).
The recoiling mothers share the recoil in
proportion to their energy fractions in the CM of the
dipole-antenna. Tree-level expansions of the VINCIA shower compared
to tree-level matrix elements through third order in alphaS have
shown this strategy to give the best overall approximation,
followed closely by the KOSOWER map below.
option
2 : LONGITUDINAL. The parton which has the
smallest invariant
mass together with the radiated parton is taken to be the "radiator". The
remaining parton is taken to be the "recoiler". The recoiler remains oriented
along the dipole axis in the branching rest frame and recoils
longitudinally against the radiator + radiated partons which have
equal and opposite transverse momenta (transverse to the original
dipole-antenna axis in the dipole-antenna CM). Comparisons to
higher-order QCD matrix elements show this to be by far the worst
option of the ones so far implemented, hence it could be
useful as an extreme case for uncertainty estimates, but should
probably not be considered for central tunes. (Note: exploratory attempts at
improving the behaviour of this map, e.g., by selecting
probabilistically between the radiator and the recoiler according to
approximate collinear splitting kernels, only resulted in
marginal improvements. Since such variations would introduce
additional complications in the VINCIA matching formalism, they
have not been retained in the distributed version.)
option
3 : The KOSOWER map. Comparisons to higher-order QCD
matrix elements show only very small differences between this and
the ARIADNE map above, but since the KOSOWER map is sometimes used in
fixed-order contexts, we deem it interesting to include it as a
complementary possibility. (Note: the KOSOWER maps in fact represent a
whole family of kinematics maps. For experts, the specific choice
made here corresponds to using r=sij/(sij+sjk) in the
definition of the map.)
parm
Vincia:alphaSvalue
(default = 0.138
)mode
Vincia:alphaSorder
(default = 1
; minimum = 0
; maximum = 1
)option
0 : zeroth order, i.e. alpha_strong is kept
fixed.
option
1 : first order. This option is recommended for LO
matrix elements and LL showers.
Vincia:alphaSorder
is non-zero,
the actual value is then regulated by running to the scale
K*muR, at which the shower evaluates
alpha_strong. The functional form of muR is given by
Vincia:alphaSmode
and the scale factor
K is given by Vincia:alphaSscaleFactor
.
mode
Vincia:alphaSmode
(default = 3
; minimum = 1
; maximum = 3
)option
1 : The evolution variable (specified by
Vincia:evolutionType
) evaluated at the
current branching.
option
2 : The invariant mass of the mother antenna.
option
3 : Transverse momentum, specifically
the Type 1 Evolution variable, regardless of what
ordering variable is being used
in Vincia:evolutionType
. Note that, since the VINCIA normalization
of the transverse-momentum variable corresponds to 2pT, this should
normally be used with Vincia:alphaSscaleFactor = 0.5
,
see below.
parm
Vincia:alphaSscaleFactor
(default = 0.5
; minimum = 0.0
)Vincia:alphaSmode
times this
scale factor, i.e., it gives the value of Kmu in the argument to
alphaS(Kmu*muR). Thus, e.g., for transverse momentum, this scale
factor should be 0.5, since VINCIA uses 2pT as evolution variable.
parm
Vincia:alphaSmuMin
(default = 0.5
; minimum = 0.0
)parm
Vincia:alphaSMax
(default = 1.0
; minimum = 0.0
)mode
Vincia:NLLMatchingLevel
(default = 0
; minimum = -1
; maximum = 1
)option
-1 : Off. Scale variations in alphaS are not compensated
at all in the shower evolution.
option
0 : Scale Cancellation Only. Scale variations in alphaS
are compensated up to 1st order by matching to the explicit
b0-dependent terms in the second-order (NLL)
antenna functions.
option
1 : (Reserved for future use.) Full matching to
second-order antenna functions.
It is possible to pass the parton systems produced by VINCIA
through Pythia's string hadronization model. Normally, this should
happen automatically, according to the setting of the Pythia switch
HadronLevel:all
. The main parameter from the shower
side is then the phase-space contour defined by the hadronization
cutoff.
The hadronization cutoff, a.k.a. the infrared regularization scale, defines the resolution scale at which the perturbative shower evolution is stopped. Thus, perturbative emissions below this scale are treated as fundmanentally unresolvable and are inclusively summed over.
Important Note: when hadronization is switched on, there is a delicate interplay between the hadronization scale used in the shower and the parameters of the hadronization model. Ideally, the parameters of the hadronization model should scale as a function of the shower cutoff. This, however, is not the case for current hadronization models, such as the string model employed by Pythia and hence by VINCIA as well. Instead, the parameters of the hadronization model are tuned for one specific shower setting at a time. In order to be able to use Pythia's hadronization model together with VINCIA without major retuning efforts, it is therefore essential that VINCIA's cutoff be taken as close as possible to that used by Pythia in the Pythia tuning. What this means in practice is, firstly, that since Pythia's evolution variable is a pT-like variable, the default cutoff in VINCIA is likewise taken to be in a contour of pT. Secondly, there are a few different tunings of Pythia 8 to e+e- data available, each using a different numerical value of the cutoff. It is therefore important to match the value of the hadronization cutoff as well, depending on the PYTHIA tune.
mode
Vincia:cutoffType
(default = 1
; minimum = -1
; maximum = 2
)option
-1 :
The cutoff is taken in Pythia 8's pTevol
variable (times two, in order to use the same normalization
as in the rest of Vincia). The cutoff scale is then set by the
PYTHIA 8 parameter 2*TimeShower:pTmin
. This is intended as a
first crude way of using Vincia together with Pythia 8's
hadronization model without having to retune the latter. Ultimately,
dedicated tunes of the hadronization parameters using VINCIA should
be used instead.
option
0 :
Automatically set cutoffType
equal to
Vincia:evolutionType
(only applies to infrared safe
evolution variables)
option
1 : Cutoff in pT (defined as the type 1 evolution
variable, see Vincia:evolutionType
)
option
2 : Cutoff in daughter antenna mass (defined as the
type 2 evolution variable, see Vincia:evolutionType
)
= -1 |
= 1 |
= 2 |
(cutoff at 2*TimeShower:pTmin ) |
(cutoff at Vincia:cutoffScale ) |
(cutoff at Vincia:cutoffScale ) |
![]() |
![]() |
![]() |
parm
Vincia:cutoffScale
(default = 1.0
)Vincia:cutoffType >= 0
, this parameter sets the
value (in GeV) of the shower cutoff, interpreted according to
the functional form selected under
Vincia:cutoffType
.
Note that all evolution/cutoff
variables are normalized so that their maximum value is equal to the
parent dipole mass. For instance, a type 1 cutoff at Q_I = 1.0
GeV therefore corresponds to a transverse-momentum cutoff at
pT = 0.5 GeV. See Vincia:evolutionType
for
further information on the normalization.
VINCIA accepts input of tune presets in the form of a standard PYTHIA 8 command file whose name and location can be specified by the user. In this way, a particular set of user-defined parameters can easily be made into a tune set by simply copying the relevant parts of the user's normal command file (i.e., omitting the process-specific and program control parameters) into a new file that can then be shared and/or submitted to the VINCIA authors for possible inclusion in future distributions.
Although there are obviously parameters that it makes more sense to tune than others, there is no explicit restriction imposed on what parameters are allowed to be present in this file. This implies some responsibility on the part of the user. As a guideline, the main parameters that need to be properly tuned are the non-perturbative hadronization parameters used in PYTHIA's string fragmentation model. Since PYTHIA and VINCIA treat soft radiation somewhat differently, there can be important differences between the two in the soft region that the hadronization model will not re-absorb automatically and which therefore only a retuning can address. Apart from this, the value of the strong coupling used in the shower is also normally considered a tuning parameter.
In versions before 1.022, no systematic attempt had been made to tune VINCIA to describe, e.g., LEP data, beyond setting the default shower alphaS and cutoff parameters to be similar to those used by PYTHIA 8 and then relying on universality of the corrections below the cutoff. Starting from VINCIA version 1.022, however, dedicated tunes of VINCIA itself have been included with the standard distribution.
word
Vincia:tuneFile
(default = tunes/jeppsson2.cmnd
)option
none : No tune file will be read.
option
tunes/jeppsson.cmnd : A first tune of VINCIA+PYTHIA to
LEP data, by M. H. Jeppsson, April 2010.
option
tunes/jeppsson2.cmnd : A slight modification of the
original Jeppssson tune of VINCIA+PYTHIA to
LEP data, by M. H. Jeppsson and P. Skands, April 2010.
Note on user tunes: in
order to make the tunings more stable against possible changes in the program
defaults (be it PYTHIA or VINCIA), it is advisable to include all
relevant parameter values explicitly in the tune file, rather than
letting parameters that retain
their (version-specific) default values be defined implicitly.
mode
Vincia:verbose
(default = 1
; minimum = 0
; maximum = 9
)VINCIA
. Settings different from zero and one are intended
for debugging purposes and hence should not be used for normal runs.
option
0 : No runtime output.
option
1 : Normal runtime output.
option
2 : Enhanced runtime output. Also, internal VINCIA
diagnostics histograms are booked and filled, especially for
matching. These can be printed by the user at any time (e.g., after
a run) using the
printHistos()
method.
option
3 : As for =2. And: a consistency check is added
to each branching by reclustering the resulting momenta back using the
corresponding inverse kinematics map and checking that the original
momenta are recovered within the desired numerical
precision.
option
4 : As for =3. And: each prepare()
and pTnext()
call is explicitly announced, with system
number and restart scale printed out, respectively. .
option
5 : As for =4. And: momentum listings are printed for
each configuration that violates P < 1.
option
6 : As for =5. And: each main function call is
explicitly announced with begin and end printed to output.
option
7 : As for =6. And: all function calls are explicitly
announced with begin and end printed to output.
option
8 : As for =7. And: last semi-sensible level of output.
option
9 : As for =8. And: all possible output.
parm
Vincia:TINY
(default = 1e-5
; minimum = 1e-12
; maximum = 1e-2
)parm
Vincia:deadZoneAvoidanceFactor
(default = 0.10
; minimum = 0.0
; maximum = 1.0
)