Currently, the only SM parameter that can be configured in VINCIA is the definition of the strong coupling constant, specified by providing its reference value (interpreted as given at the Z pole in the MSbar scheme) and running properties (loop order, behaviour at top threshold, and any low-scale regularisation/dampening). All other parameters are taken from the PYTHIA Couplings database.
Note that VINCIA only uses one global value for the definition of the strong coupling constant. The effective couplings used in shower branchings (renormalisation scheme and scale) are governed by separate parameters which are specified under initial- and final-state showers respectively.
VINCIA implements its own instance of PYTHIA's
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
default = 0.118;
minimum = 0.06;
maximum = 2.0)
default = 2;
minimum = 0;
maximum = 2)
option0 : zeroth order, i.e. αs is kept fixed.
option1 : first order, i.e., one-loop running.
option2 : second order, i.e., two-loop running.
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
default = true)
Note 1: If using VINCIA with an externally defined matching scheme, be
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 2: Tunes using this option need roughly 10% lower values of alphas(mZ) than tunes that do not.
For both one- and two-loop running, the
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:
default = 6;
minimum = 5;
maximum = 6)
option5 : Use 5-flavour running for all scales above the b flavour threshold (old default).
option6 : Use 6-flavour running above the t threshold (new default).
default = 0.5;
minimum = 0.0;
maximum = 10.0)
default = 1.5;
minimum = 0.1;
maximum = 10.0)