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Parameters

Here you find a list of parameters available in OpenLoops 2. Within the general purpose Monte Carlo frameworks (e.g. Sherpa or Herwig++) these parameters are synchronised automatically.

A more detailed documentation will soon be available.

Model parameters

Parameter Decription Options
mu renormalization scale double
alphas strong coupling constant double
mass(PDG) mass of particle with given PDG code double
width(PDG) width of particle with given PDG code double
n_quarks Number of active quarks (only affects UV and R2 counterterms) int, default=6
minnf_alphasrun Minimum number of quark flavours that contribute to the runnning of αs (only affects αs counterterm) int, default=0

OpenLoops allows the choice between three EW input schmemes, which can be selected via the parameter ew_scheme according to the following table.

Name ew_scheme Input Description
α(0) 0 alpha_qed_0 α=alpha_qed_0
Gμ 1 (default) Gmu α=sqrt2/pi*Gmu*|MW2*sw2|
α(MZ) 2 alpha_qed_mz α=alpha_qed_mz

Unstable particles with a finite width are by default treated in the complex mass scheme, i.e. M2 = rM2-i rM*Γ. Here rM and Γ are the real valued on-shell mass and width of the unstable particle. The cosine of the weak mixing angle cW is always defined as cW=MW/MZ and thus in general a complex valued parameter. The chosen EW input scheme also fixes the EW renormalisation scheme.

By default all quark flavours are included at one-loop. More precisely, the number of quark flavours that contribute to the renormalisation (and evolution) of αs is chosen as max(number of massless quarks, minnf_alphasrun). Thus, by default only massless quarks are treated as active flavours, while the contribution of massive quarks is effectively decoupled from the running of αs by renormalising it via zero-momentum subtraction. In this approach, above threshold the heavy-quark (with mass Mq) contributions to any renormalised one-loop amplitudes generate logarithms of μR/Mq that are not present in the evolution of αs above threshold. This behaviour can be altered specifying the parameter minnf_alphasrun. If set to a value larger then the number of massless quark flavours, also the corresponding heavy quarks are assumed to contribute to the running of the strong coupling (above the respective thresholds).

Process parameters

Parameter Decription Options
order_ew select fixed EW coupling order of squared amplitude (for NLO QCD) int
order_qcd select fixed QCD coupling order of squared amplitude (for NLO EW) int

These order_ew, and order_qcd selectors are used for the bookkeping of different perturbative orders as explained here.

Technical parameters

Parameter Decription Options
install_path set the (absolute) path of the directory containing the proclib directory (including all process libraries) string
ct_on switch on/off UV counterterms 1 (default) / 0
r2_on switch on/off R2 counterterms 1 (default) / 0
verbose set verbosity level for the process loader 0 (default) / 1 / 2
psp_tolerance numerical tolerance of on-shell condition for external particles double, default=E-9

 

Further technical parameters affecting the stability system are listed here.