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 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.
When Vincia:alphaSorder
is non-zero,
the actual value of alphaS used for shower branchings is governed by
the choice of scheme (MSbar or CMW, see the section on SM Couplings
and then 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 = 0.72
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:alphaSkMuSplitI
(default = 0.72
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:alphaSkMuConv
(default = 0.72
; 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.)
For theory tests, individual antenna functions can be switched off by setting the corresponding colour-charge factor to zero.
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
)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.