openloops – Hepforge

Parameters

Here you find a list of parameters available in OpenLoops 1. Within the general purpose Monte Carlo frameworks (e.g. Sherpa or Herwig++) these parameters are synchronised automatically.

Model parameters

Parameter	Decription	Options
mu	renormalization scale	double
alphas	strong coupling constant	double
alpha	electromagnetic 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 α_s (only affects α_s counterterm)	int, default=0

Unstable particles with a finite width are by default treated in the complex mass scheme. The cosine of the weak mixing angle c_W is always defined as c_W=M_W/M_Z and thus in general a complex valued parameter.

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 α_s 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 α_s 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 μ_R/Mq that are not present in the evolution of α_s 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).

A more detailed documentation will soon be available.

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
redlib1	select primary reduction library	1: Coli 5: CutTools (default) 6: Samurai 7: DD
redlib2	select secondary reduction library	1: Coli 5: CutTools (default) 6: Samurai 7: DD
redlib_qp	select quad-precision reduction library	5: CutTools (default) 6: Samurai 7: DD
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.