Within the dipole-antenna formalism, antenna functions are the analogs of the splitting functions used in traditional parton showers. The antenna functions are constructed so as to reproduce the Altarelli-Parisi splitting functions P(z) in collinear limits and the eikonal dipole factor in the soft limit.
flag
Vincia:ISR
(default = on
)The default is to leave all antenna functions switched on. For special theory studies, it may be convenient to check what happens when one or more antenna functions are switched off. The switches below give this functionality:
flag
Vincia:QQemitII
(default = on
)flag
Vincia:GQemitII
(default = on
)flag
Vincia:GGemitII
(default = on
)flag
Vincia:QXSplitII
(default = on
)flag
Vincia:GXConvII
(default = on
)
flag
Vincia:QQemitIF
(default = on
)flag
Vincia:GQemitIF
(default = on
)flag
Vincia:QGemitIF
(default = on
)flag
Vincia:GGemitIF
(default = on
)flag
Vincia:QXSplitIF
(default = on
)flag
Vincia:GXConvIF
(default = on
)flag
Vincia:XGSplitIF
(default = on
)The number of quark flavours allowed in initial-state gluon conversions, phase space permitting, is given by
mode
Vincia:nGluonToQuarkI
(default = 5
; minimum = 0
; maximum = 5
)
The initial state shower uses the same AlphaStrong
as the Final State Shower.
Any need for different effective values can be absorbed
in a scale factor choice below.
By default, the CMW correction is applied in VINCIA. The choice for the eikonal term are similar to the final state ones:
mode
Vincia:CMWtypeII
(default = 2
)option
0 :
No CMW rescaling.
option
1 :
CMW rescaling with the eikonal term defined as
2 sAB / ( saj sjb ).
option
2 :
CMW rescaling with the eikonal term defined as
2 sab / ( saj sjb ),
which is larger compared to the term in option 1.
mode
Vincia:CMWtypeIF
(default = 2
)option
0 :
No CMW rescaling.
option
1 :
CMW rescaling with the eikonal term defined as
2 sAK / ( saj sjk ).
option
2 :
CMW rescaling with the eikonal term defined as
2 sak / ( saj sjk ),
which can be both, larger or smaller, compared to the
term in option 1.
When Vincia:alphaSorder
is non-zero,
the actual value is regulated by running to the scale
kμ*μR, at which the shower evaluates
αs.
The functional form of μR is given by
the evolution variable
and the scale factor kμ is given by
parm
Vincia:alphaSkMuI
(default = 0.75
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:alphaSkMuSplitI
(default = 0.7
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:alphaSkMuConv
(default = 0.7
; minimum = 0.1
; maximum = 10.0
)
and Vincia:alphaSkMuSplitF
for gluon splitting in the final state.
The normalisation of colour factors in VINCIA is chosen such that the coupling factor for all antenna functions is αS/4π. With this normalisation choice, all gluon-emission colour factors tend to NC in the large-NC limit while all gluon-splitting colour factors tend to unity. (Thus, e.g., the default normalisation of the qqbar → qgqbar antenna function is 2CF.)
parm
Vincia:QQemitII:chargeFactor
(default = 2.66666667
)parm
Vincia:GQemitII:chargeFactor
(default = 2.83333333
)parm
Vincia:GGemitII:chargeFactor
(default = 3.0
)parm
Vincia:QXSplitII:chargeFactor
(default = 1.0
)parm
Vincia:GXConvII:chargeFactor
(default = 2.66666667
)parm
Vincia:QQemitIF:chargeFactor
(default = 2.66666667
)parm
Vincia:GQemitIF:chargeFactor
(default = 2.83333333
)parm
Vincia:QGemitIF:chargeFactor
(default = 2.83333333
)parm
Vincia:GGemitIF:chargeFactor
(default = 3.0
)parm
Vincia:QXSplitIF:chargeFactor
(default = 1.0
)parm
Vincia:GXConvIF:chargeFactor
(default = 2.66666667
)parm
Vincia:XGSplitIF:chargeFactor
(default = 1.0
)
For Vincia:hardJetsQCD
and/or Vincia:hardJets
being
switched off, the first branching is by default limited to the factorisation
scale used for the hard process. This scale (in GeV) can be enhanced by the factor
parm
Vincia:QmaxEnhanceI
(default = 1.0
; minimum = 1.0
; maximum = 10.0
)Note however, that for non-unity values the generated Sudakov factor does not have the factorisation scale as a starting scale, leading to leftover PDF ratios in the exclusive cross sections produced by the shower.
Choice of functional form of the shower evolution variable (a.k.a. ordering variable) for initial state radiation (see illustrations below).
Gluon emissions in initial-initial antennae are ordered in transverse momentum. This evolution variable is the physical (lightcone) transverse momentum for massless partons:
Gluon emissions in initial-final antennae are ordered in transverse momentum. This evolution variable is defined as:
Splittings and conversion in initial-initial and initial-final antennae are by
default ordered in the invariant mass of the gq, qq, or qqbar pair
respectively. However there is the option to switch to the above transverse
momentum ordering by switching Vincia:evolveAllInPT
to on.
Note that with transverse momentum ordering
the ordering variable is no longer the inverse of the singularity associated
with the branching process. Also the mass corrections
are not applied correctly since they rely on ordering in invariant mass.
flag
Vincia:evolveAllInPT
(default = off
)The contours below illustrate the progression of the evolution variable over the dipole-antenna phase space for four fixed values, with sAB=mH^2 for the initial-initial case and xA=0.6 and sAK=25.2 GeV^2 for the initial-final case.
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Important Note: All flavours will be treated with massless kinematics. However we apply certain corrections.
mode
Vincia:nFlavZeroMassI
(default = 3
; minimum = 3
; maximum = 5
)
mode
Vincia:massCorrectionLevelI
(default = 1
)Vincia:nFlavZeroMassI
flavours.
option
0 : No mass effects. Equivalent to
using Vincia:nFlavZeroMassI=5
.
option
1 : All heavy flavours
in the initial state will undergo a conversion into a gluon when the
evolution variable goes towards their mass threshold, using the
vanishing PDFs. A quark mass is determined by using the maximum of
the input value given below and the mass extracted from the PDFs
(if possible).
The following mass thresholds are imposed on the evolution for quarks in the initial state:
parm
Vincia:ThresholdMB
(default = 4.8
)parm
Vincia:ThresholdMC
(default = 1.5
)The post-branching momenta are fixed by the following requirements:
1) The direction of the initial state partons is aligned with the beam axis
(z-axis).
2) The invariant mass and the rapidity of the final state recoiler are not
changed by the branching. This allows a direct construction of the
post-branching momenta in the lab frame.
The post-branching momenta are fixed by the following requirements:
1) The direction of the initial state parton is aligned with the beam axis
(z-axis).
2) There are no recoils outside of the antenna. This allows a construction
of the post-branching momenta in the centre-of-mass frame of the
initial-final antenna.