Standard-Model Parameters

The strong coupling

Currently the only class in this category is AlphaStrong, which is used to provide a first- or second-order running alpha_strong (or, trivially, a zeroth-order fixed one). Formulae are the standard ones found in [1]. The starting alpha_strong value is defined at the M_Z mass scale. The Lambda values are matched at the b and c flavour thresholds, such that alpha_strong is continuous, using an approximate iterative method for second-order matching. Since we allow alpha_strong to vary separately for hard processes, timelike showers, spacelike showers and multiple interactions, the relevant values are to be set in each of these classes.

The alpha_strong calculation is initialized by init( value, order), where value is the alpha_strong value at M_Z and order is the order of the running, 0, 1 or 2. Thereafter the value can be calculated by alphaS(scale), where scale is the Q^2 scale in GeV^2.

For applications inside shower programs, a second-order alpha_s value can be obtained as the product of the two functions alphaS1Ord(scale) and alphaS2OrdCorr(scale), where the first gives a simple first-order running (but with the second-order Lambda) and the second the correction factor, below unity, for the second-order terms. This allows a compact handling of evolution equations.

Electroweak couplings

In addition, this section contains a few further parameters that should normally not be touched.

parameter name="StandardModel:alphaEMfix" default="0.00729735" min="0.00729" max="0.00730"
Fixed alpha_em value used in QED vertices.

parameter name="StandardModel:sin2thetaW" default="0.232" min="0.225" max="0.240"
The weak mixing angle.

Eventually will appear a running alpha_em, and CKM matrix elements.

References

  1. Particle Data Group, S. Eidelman et al., Phys. Lett. B592 (2004) 1