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 value of αs at the scale mZ, in the MSbar scheme. The default is chosen to obtain a reasonable agreement with LEP event shapes for default shower settings.

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.

Effective Renormalization Scheme

All-orders resummation arguments [CMW91] show that a universal next-to-leading correction factor associated with soft gluon emission can be absorbed in coherent parton showers by applying the so-called CMW rescaling to the MSbar value of alphaS. This translation amounts to an nf-dependent rescaling of the value of alphaS, relative to its MSbar value, as follows:

αs(CMW) = αs(MSbar) * (1 + K * αs(MSbar) / 2π)
with K = CA * (67/18 - π2/6) - 5/9nf. By default, this 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.
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.

Argument of Running Coupling

When Vincia:alphaSorder is non-zero, the actual value is regulated by running to the scale kμR, 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).

The scale factor kμ is given by

parm  Vincia:alphaSkMu   (default = 1.0; minimum = 0.1; maximum = 10.0)
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).

Infrared Freezeout Scale

parm  Vincia:alphaSmuMin   (default = 0.5; minimum = 0.1; maximum = 10.0)
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 = 0.1; maximum = 10.0)
Largest allowed numerical value for alphaS.