Global VINCIA Parameters

  1. The Strong Coupling

The Strong Coupling

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.

Reference Value

The free parameter of the strong coupling constant is specified by

parm  Vincia:alphaSvalue   (default = 0.118; minimum = 0.06; maximum = 2.0)
The value of αs at the scale mZ, in the MSbar scheme. Chosen to be in agreement with the current world average. The effective value used for showers is further affected by the parameters given for FSR and ISR showers separately.

Running Order

mode  Vincia:alphaSorder   (default = 2; minimum = 0; maximum = 2)
Order at which αs runs,
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.

Flavour Thresholds

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).

Infrared Freezeout Scale

parm  Vincia:alphaSmuFreeze   (default = 0.4; minimum = 0.0; maximum = 10.0)
In the far infrared, the behaviour of the running coupling is regulated by a shift in the effective renormalisation scale, to μeff 2 = μfreeze2 + (kμR)2.

Max Coupling

parm  Vincia:alphaSmax   (default = 1.2; minimum = 0.1; maximum = 10.0)
Largest allowed numerical value for alphaS.