Couplings and Masses

Quark and Lepton Masses

The following switches control which quarks and leptons are treated as massive during the perturbative evolution. Note: the values of the masses are in all cases taken from PYTHIA's particle database, minimizing the risk of conflict. The parameters here only control whether the corresponding particles are treated as massive or massless by VINCIA.

flag  Vincia:isMassiveS   (default = false)

flag  Vincia:isMassiveC   (default = true)

flag  Vincia:isMassiveB   (default = true)

flag  Vincia:isMassiveTau   (default = true)
Note: if the tau is treated as massless, its decay width will be put to zero in the MadGraph interface.

Note: New-physics particles will generally be treated as massive if their masses are larger than the lightest quark for which mass corrections are switched on.

Note 2: For particles that are treated as massive, the mass corrections to the antenna functions are discussed separately, in the section on Antenna Functions.

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 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 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(kμR). 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 kμR 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.