VINCIA implements its own instance of PYTHIA's AlphaStrong
class
for the strong coupling. You can find more documentation of the class in
the section on Standard-Model Parameters in the PYTHIA documentation.
Here, we list the specific parameters and switches governing its use in VINCIA.
The amount of QCD radiation in the shower is determined by
parm
Vincia:alphaSvalue
(default = 0.1215
; minimum = 0.06
; maximum = 2.0
)mode
Vincia:alphaSorder
(default = 2
; minimum = 0
; maximum = 2
)option
0 : zeroth order, i.e. αs is kept
fixed.
option
1 : first order, i.e., one-loop running.
option
2 : second order, i.e., two-loop running.
For both one- and two-loop running, the AlphaStrong
class
automatically switches from 3-, to 4-, and then to 5-flavour running as
one passes the s, c, and b thresholds, respectively,
with matching equations imposed at each flavour
treshold to ensure continuous values.
(See the manual page on Evolution
to see and change the threshold scales.)
By default, a change to 6-flavour running is also included above the t threshold, though this can be disabled using the following parameter:
mode
Vincia:alphaSnfmax
(default = 6
; minimum = 5
; maximum = 6
)option
5 : Use 5-flavour running for all scales above the b flavour threshold (old default).
option
6 : Use 6-flavour running above the t threshold (new default).
Resummation arguments [Cat91] indicate that a set of universal QCD corrections can be absorbed in coherent parton showers by applying the so-called CMW rescaling of the MSbar value of Lambda_QCD, defined by
By default, the CMW correction is applied in VINCIA. If desired, it can be switched off by using the following switch:
flag
Vincia:alphaScmw
(default = on
)option
on : CMW.
This option is recommended for the most accurate shower resummation.
option
off : MSbar.
Note: The CMW arguments were derived using two-loop running (the default in VINCIA).
Note 2: When this correction is switched on, the rescaling of the coupling away from its MSbar value is properly taken
into account by VINCIA's one-loop matching. When switched off, the
one-loop corrections generally become larger, to compensate for the missing
universal factor.
Note 3: If using VINCIA with an externally defined matching scheme, be
aware
that the CMW rescaling may need be taken into account in the context of
matrix-element matching. Note also that this option has only been made
available for timelike and spacelike showers, not for hard processes.
Note 4: tunes using this option need lower values of
alpha_strong(m_Z^2) than tunes that do not.
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
Vincia:alphaSmode
and the scale factor
kμ is given by Vincia:alphaSkMu
.
The functional form of μR is given by
mode
Vincia:alphaSmode
(default = 1
; minimum = 0
; maximum = 1
)option
0 : The invariant mass of the mother antenna, m(ijk).
option
1 : For gluon emissions: transverse momentum, defined as in ARIADNE, pT = m(ij)*m(jk)/m(ijk). For gluon splittings, g→qq, the qqbar invariant mass, m(qqbar).
For gluon emissions, the scale factor kμ is given by
parm
Vincia:alphaSkMu
(default = 1.0
; minimum = 0.1
; maximum = 10.0
)Vincia:alphaSmode
times this
scale factor, i.e., it gives the value of kμ in
the argument to
alphaS(kμ*μR).
parm
Vincia:alphaSkMuSplit
(default = 0.75
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:alphaSmuFreeze
(default = 0.5
; minimum = 0.0
; maximum = 10.0
)parm
Vincia:alphaSmax
(default = 1.5
; minimum = 0.1
; maximum = 10.0
)