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 free parameter of the strong coupling constant is specified by
parm
Vincia:alphaSvalue
(default = 0.118
; 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.
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).
parm
Vincia:alphaSmuFreeze
(default = 0.5
; minimum = 0.0
; maximum = 10.0
)parm
Vincia:alphaSmax
(default = 1.5
; minimum = 0.1
; maximum = 10.0
)