Setting parameters

Couplings and masses

Setting parameters can be done by invoking set_parameter_rcl(). For SM-like theories Recola2 comes with dedicated functions to set certain parameters which are all listed below.

set_parameter_rcl(pname,pvalue)	Sets value for the parameter with name pname to pvalue.
set_pole_mass_w_rcl(m,g)	Sets the pole mass and width of the W boson.
set_pole_mass_z_rcl(m,g)	Sets the pole mass and width of the Z boson.
set_pole_mass_h_rcl(m,g)	Sets the pole mass and width of the Higgs boson.
set_pole_mass_top_rcl(m,g)	Sets the pole mass and width of the top-quark.
set_pole_mass_bottom_rcl(m,g)	Sets the pole mass and width of the bottom-quark.
set_pole_mass_charm_rcl(m,g)	Sets the pole mass and width of the charm-quark.
set_pole_mass_strange_rcl(m)	Sets the pole mass of the strange-quark.
set_pole_mass_up_rcl(m)	Sets the pole mass of the up-quark.
set_pole_mass_down_rcl(m)	Sets the pole mass of the down-quark.
set_pole_mass_tau_rcl(m,g)	Sets the pole mass and width of the tau.
set_pole_mass_muon_rcl(m,g)	Sets the pole mass and width of the muon.
set_pole_mass_electron_rcl(m)	Sets the pole mass of the electron.

Complex-Mass scheme

By default, Recola uses the Complex-Mass scheme for any process. When unstable final state particles are present one should switch to the on-shell scheme by invoking:

set_complex_mass_scheme_rcl()	Selects the Complex-Mass scheme for the renormalization of particles.
set_on_shell_scheme_rcl()	Selects the on-shell scheme for the renormalization of particles.

Renormalization of alphas

A running \(\alpha_\mathrm{s}\) is implemented for various different scenarios.

1. The running is given externally (obtained from pdfs or other sources). In this case it is enough to update the value \(\alpha_\mathrm{s}(Q)\) at the scale \(Q\) before computing the next phase-space-point using set_alphas_rcl().

2. The running can be computed by Recola to 1 and 2-loop order via compute_running_alphas_rcl().

Computations are typically performed for different scale choices at the same time in order to give an estimate of missing higher orders. The additional scale variation can be obtained at zero cost by using the following sequence of calls:

First the a default scale is computed via the squence:

• set_alphas_rcl()

• compute_process_rcl()

and then other scales are obtained by rescaling:

• set_alphas_rcl()

• rescale_process_rcl()

set_alphas_rcl(as,Q,Nf)	Sets the values of \(\alpha_\mathrm{s}\) to as, the renormalization scale \(\mu_\mathrm{MS}\) to Q and the number of active quark flavours to Nf.
get_alphas_rcl()	return Returns the value of \(\alpha_\mathrm{s}\)
compute_running_alphas_rcl(Q,Nf,lp)	Computes the value for \(\alpha_\mathrm{s}\) at the scale Q employing the renormalization-group evolution at lp loops (lp can take the value lp=1 or lp=2).
set_alphas_masses_rcl(mc,mb,mt,gc=0.,gb=0.,gt=0.)	Overwrites the values of the quark masses for the running of \(\alpha_\mathrm{s}\) used in the rest of the computations.

Renormalization of alpha

use_gfermi_scheme_rcl([g])	Sets the EW renormalization scheme to the gfermi scheme.
use_alpha0_scheme_rcl([a])	Sets the EW renormalization scheme to the \(\alpha_0\) scheme.
use_alphaZ_scheme_rcl([a])	Sets the EW renormalization scheme to the \(\alpha_\mathrm{Z}\) scheme.
get_alpha_rcl()	return Returns the value of \(\alpha\)

Scales

set_delta_uv_rcl(d)	This subroutine sets the finite part of the UV subtracted term: \(\Delta_\mathrm{UV} = \frac{1}{\epsilon} - \gamma + \log (4 \pi)\) to d.
get_delta_uv_rcl()	Gets the finite part of the UV subtracted term \(\Delta_\mathrm{UV}\).
set_delta_ir_rcl(d1, d2)	This subroutine sets the finite part of the IR subtracted term \(\Delta_\mathrm{IR} = \frac{1}{\epsilon} - \gamma + \log (4 \pi)\) to d1 \(\Delta_\mathrm{IR2} = \frac{(4 \pi)^\epsilon \Gamma(1+\epsilon)}{\epsilon^2}\) to d2.
get_delta_ir_rcl()	Gets the finite parts of the IR subtracted terms \(\Delta_\mathrm{IR}\), \(\Delta_\mathrm{IR2}\).
set_mu_uv_rcl(m)	Sets the UV scale \(\mu_\mathrm{UV}\) to m.
get_mu_uv_rcl()	Returns the UV scale \(\mu_\mathrm{UV}\).
set_mu_ms_rcl(m)	Sets the \(\overline{\mathrm{MS}}\) scale \(\mu_{\overline{\mathrm{MS}}}\) to m.
get_mu_ms_rcl()	Returns the \(\overline{\mathrm{MS}}\) scale \(\mu_{\overline{\mathrm{MS}}}\).
set_dynamic_settings_rcl(i)	Sets the dynamic_settings flag to i.
set_compute_ir_poles_rcl(mode)	Sets whether IR poles are computed when calling compute_process_rcl().

Dimensionional and mass regularisation

Recola + Collier supports amplitudes in dimensional and mass regularisation for collinear singularties. In order to perform the computation in mass regularisation one has to assign mass values to the corresponding charged particles and declare them as light particles. E.g. for the muon call set_pole_mass_muon_rcl() (with zero width) and then set_light_muon_rcl(). In the output the muon is then declared as light next to the mass (regulator) value. Alternatively, all particles with a mass lower than some cut (set_light_fermions_rcl()) can be tagged as light. Note that the regulator mass does not need to be light in the strict sense, and can take any positive value. Finally, it is important that the phasespace points passed to Recola are onshell with light particles being treated massless.

set_light_fermions_rcl(mcut)	Sets a masscut mcut below which massive fermions are treated as light particles, i.e.
set_light_electron_rcl	Sets the electron as light, regularizing IR singularities with its mass.
unset_light_electron_rcl	unsets the electron as light, keeping the full mass dependence.
set_light_muon_rcl	Sets the muon as light, regularizing IR singularities with its mass.
unset_light_muon_rcl	unsets the muon as light, keeping the full mass dependence.
set_light_tau_rcl	Sets the tau as light, regularizing IR singularities with its mass.
unset_light_tau_rcl	unsets the tau as light, keeping the full mass dependence.
set_light_down_rcl	Sets the down-quark as light, regularizing IR singularities with its mass.
unset_light_down_rcl	unsets the down-quark as light, keeping the full mass dependence.
set_light_up_rcl	Sets the up-quark as light, regularizing IR singularities with its mass.
unset_light_up_rcl	unsets the up-quark as light, keeping the full mass dependence.
set_light_strange_rcl	Sets the strange-quark as light, regularizing IR singularities with its mass.
unset_light_strange_rcl	unsets the strange-quark as light, keeping the full mass dependence.
set_light_charm_rcl	Sets the charm-quark as light, regularizing IR singularities with its mass.
unset_light_charm_rcl	unsets the charm-quark as light, keeping the full mass dependence.
set_light_bottom_rcl	Sets the bottom-quark as light, regularizing IR singularities with its mass.
unset_light_bottom_rcl	unsets the bottom-quark as light, keeping the full mass dependence.
set_light_top_rcl	Sets the top-quark as light, regularizing IR singularities with its mass.
unset_light_top_rcl	unsets the top-quark as light, keeping the full mass dependence.