The Strong Coupling
Reference Value
The amount of QCD radiation in the shower is determined by
parm
Vincia:alphaSvalue
(default = 0.139
)
The effective αs value at
the scale mZ.
The default is chosen to obtain a reasonable agreement
with LEP event shapes for default shower settings,
and is appropriate for a
leading-order / leading-log shower (as compared, e.g., to leading-order
extractions of αs at LEP).
Order
mode
Vincia:alphaSorder
(default = 1
; minimum = 0
; maximum = 1
)
Order at which αs runs,
option
0 : zeroth order, i.e. αs is kept
fixed.
option
1 : first order. This option is recommended for LO
matrix elements and LL showers.
Argument of Running Coupling
When Vincia:alphaSorder
is non-zero,
the actual value is then regulated by running to the scale
k*μR, at which the shower evaluates
αs. The functional form of μR is given by
Vincia:alphaSmode
and the scale factor
kμ is given by Vincia:alphaSscaleFactor
.
mode
Vincia:alphaSmode
(default = 3
; minimum = 1
; maximum = 3
)
The functional form of μR is given by
option
1 : The evolution variable (specified by
VinciaShower: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 VinciaShower: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
)
If different from unity, αs is evaluated at the
scale defined by Vincia:alphaSmode
times this
scale factor, i.e., it gives the value of kμ in
the argument to
alphaS(kμ*μR). Thus, e.g., for transverse momentum, this scale
factor should be 0.5, since VINCIA uses 2pT as evolution variable.
Infrared Freezeout Scale
parm
Vincia:alphaSmuMin
(default = 0.5
; minimum = 0.0
)
Smallest kμ*μR scale at which alphaS will be
evaluated. I.e., the strong coupling is treated as frozen below this
scale.
Max Coupling
parm
Vincia:alphaSMax
(default = 1.0
; minimum = 0.0
)
Largest allowed numerical value for alphaS.