VINCIA QED and EW Antenna Shower Settings

  1. Main Switches
  2. QED Shower Settings
  3. EW Shower Settings

Here, parameters specific to VINCIA's QED and EW antenna showers are collected, including VINCIA's interleaved treatment of resonance decays. See the main VINCIA antenna shower page for more general parameters that are common to both the QCD and QED/EW showers.

Main Switches

VINCIA contains two alternative implementations of QED/EW shower effects. One is restricted to pure QED but allows for a fully coherent (multipole) treatment of that sector. The other is limited to dipole-style coherence in the photon sector but includes a full set of electroweak (EW) branching kernels with the appropriate (quasi-)collinear limits.

mode  Vincia:EWmode   (default = 2; minimum = 0; maximum = 3)
Main switch for QED and Weak showers in VINCIA, in particular for the hard process. (See Vincia:QEDmodeMPI below for the handling of QED corrections in MPI systems.)
option 0 : No QED or weak showers at all.
option 1 : Dipole QED showers. Partially coherent, based on maximally screening dipole pairs. Spin-dependent antenna terms can be switched on/off using Vincia:QEDfullKernels, see below.
option 2 : Multipole QED showers. Fully coherent but computationally slower than the dipole approximation. Spin-dependent antenna terms can be switched on/off using Vincia:QEDfullKernels, see below.
option 3 : Dipole QED + Weak showers. All electroweak branchings are enabled, comprising emissions, splittings, and self-interactions of W, Z, and H bosons, in addition to QED. Note that the weak shower operates on helicity eigenstates; this is particularly relevant for W and Z bosons for which the longitudinal polarisations have different splitting amplitudes than the transverse ones. Typically, the hard process does not provide helicity information for the in- and outgoing legs in Pythia. In this case, VINCIA will attempt to use its MG5 matrix-element interface and its internal EW splitting amplitudes to assign such helicities. Note that this requires Pythia to be configured using the --with-mg5mes option (see also the example program main404.cc). If the given hard process is not available in the linked MG5 library, or if the helicity assignment fails for any other reason, then the QED dipole shower is used as a fallback.

mode  Vincia:QEDmodeMPI   (default = 1; minimum = 0; maximum = 2)
Treatment of QED showers for MPI systems. Note that QEDmodeMPI is forced to be less than or equal to the main EWmode switch above so that the treatment of QED corrections for MPI cannot be more sophisticated than that of the hard interaction. Also note that there is currently no option to include weak showers for MPI.
option 0 : No QED showers in MPI systems.
option 1 : Dipole QED showers in MPI systems.
option 2 : Multipole QED showers in MPI systems. This is the most advanced option, with full coherence, but is somewhat computationally slower and would normally be overkill for MPI.

mode  Vincia:QEDmodeHadDec   (default = 2; minimum = 1; maximum = 2)
When HadronLevel:QED = on, this switch determines whether QED multipole interference effects (similar to YFS) are taken into account or not, for QED showers in hadron and tau decays (with spin dependence according to Vincia:useSpinsQEDHadDec).
option 1 : Dipole QED showers in hadron and tau decays.
option 2 : Multipole QED showers in hadron and tau decays.

QED Shower Settings

mode  Vincia:nGammaToQuark   (default = 5; minimum = 0; maximum = 6)
Number of allowed quark flavours in final-state photon splitting.

mode  Vincia:nGammaToLepton   (default = 3; minimum = 0; maximum = 3)
Number of allowed lepton flavours in final-state photon splitting.

flag  Vincia:convertGammaToQuark   (default = on)
Allow incoming photons to backwards-evolve into (anti)quarks during the initial-state shower evolution.

flag  Vincia:convertQuarkToGamma   (default = on)
Allow incoming (anti)quarks to backwards-evolve into photons during the initial-state shower evolution.

The QED coupling in the VINCIA Shower

mode  Vincia:alphaEMorder   (default = 1; minimum = 0; maximum = 1)

option 0 : zeroth order, i.e. αem is kept fixed.
option 1 : first order, i.e., one-loop running.

parm  Vincia:alphaEM0   (default = 0.00729735; minimum = 0.0072973; maximum = 0.0072974)
The alpha_em value at vanishing momentum transfer (and also below m_e).

parm  Vincia:alphaEMmZ   (default = 0.00781751; minimum = 0.00780; maximum = 0.00783)
The alpha_em value at the M_Z mass scale.

Lower Cutoffs for the QED evolution

parm  Vincia:QminChgQ   (default = 0.5; minimum = 0.1; maximum = 2.0)
Parton shower cut-off scale for photon coupling to coloured particles.

parm  Vincia:QminChgL   (default = 1e-6; minimum = 1e-10; maximum = 2.0)
Parton shower cut-off scale for pure QED branchings. Assumed smaller than (or equal to) QminChgQ.

fvec  Vincia:useSpinsQED   (default = {on, on, on})
Switch to incorporate the full QED antenna functions (on) or just their (spin-independent, YFS-style) scalar eikonal terms (off), for QED in hard processes, resonance decays, and MPI. The first entry switches the spin-dependent collinear terms on/off for spin-1/2 radiators, the second for spin-1 radiators, and the third for spin-3/2 ones. Note that the spin-3/2 case is not fully implemented and currently defaults to use the same terms as the spin-1/2 case.

fvec  Vincia:useSpinsQEDHadDec   (default = {on, on, on})
Same as Vincia:useSpinsQED for QED in hadron and tau decays.

parm  Vincia:mMaxGamma   (default = 10.; minimum = 0.001; maximum = 5000.0)
Maximum invariant mass allowed for the created fermion pair by photon splitting in the shower.

EW Shower Settings

mode  Vincia:kineMapEWFinal   (default = 3; minimum = 1; maximum = 3)
Kinematics Map for EW emissions (See section on kinematics.)
option 1 : The Ariadne angle.
option 2 : Longitudinal (dipole) map.
option 3 : The Kosower map.

flag  Vincia:doBosonicInterference   (default = on)
If switched on, apply an event weight to correct for interference between $\gamma/Z_T$ and $h/Z_L$.

mode  Vincia:bwMatchingMode   (default = 2; minimum = 1; maximum = 3)
Whenever a resonance is produced as part of the hard process or as part of the electroweak shower, its kinematic mass is distributed according to a Breit-Wigner distribution. When a shower branching of a resonance type occurs, the Breit-Wigner-generated mass is replaced with an off-shellness acquired through the shower as normal. Otherwise, it is decayed in a regular $1 \rightarrow 2$ decay process.
option 1 : Resonance-type branchings in the shower are disabled. Resonances are instead decayed according to a Breit-Wigner distribution exclusively.
option 2 : A suppression factor $\frac{Q^4}{(Q^2 + Q^2_{\mathrm{EW}})^2} is applied to the resonance decay-type branchings in the shower. Any resonance that does not disappear due to a shower branching before its Breit-Wigner-sampled off-shellness is instead decayed according to the Breit-Wigner distribution.
option 3 : No off-shellness is sampled from a Breit-Wigner. Only the shower is ran without suppression factor. Does not guarantee that all resonances decay.

parm  Vincia:EWScale   (default = 100.; minimum = 80.; maximum = 175.)
Determines the value of $Q_\mathrm{EW}$ used with Vincia:bwMatchingMode = 2.

flag  Vincia:EWoverlapVeto   (default = off)
Settings to prevent double counting that may appear when running the QCD and EW showers. For example, configurations with 2 jets and 1 vector boson may be reached by either an EW emission from pure QCD processes or from QCD emissions from V + j processes. If set to true, hard EW emissions off QCD processes and hard QCD emissions off EW processes are vetoed, as determined by the kT algorithm to prevent overlap in all regions of phase space. Note that the current implementation of the overlap veto does not take interleaved resonance decays into account. This switch should therefore only be set to on in conjunction with Vincia:interleaveResDec = off.

parm  Vincia:EWoverlapVetoDeltaR   (default = 0.6; minimum = 0.1)
The deltaR parameter used in the kT clustering for the overlap veto algorithm used to avoid double counting.

flag  Vincia:BWstrongOrdering   (default = off)
Controls the treatment of resonance production with Breit-Wigner generated off-shellness larger than the current shower scale. If set to "off", resonances with such offshellness are immediately decayed. If set to 'on', the branching that produces the resonance is vetoed.

parm  Vincia:EWheadroomF   (default = 1.1; minimum = 1.; maximum = 2.)
Multiplicative factor to increase overestimate function for final state electroweak branchings.

parm  Vincia:EWheadroomI   (default = 3; minimum = 1.; maximum = 5.)
Multiplicative factor to increase overestimate function for initial state electroweak branchings.

Overestimate Determination for Electroweak Branchings

As there are such a very large number of electroweak branchings, the technical problem of determining the overestimate function (used for generating trial branchings) has been automated. The overestimate function is first parameterised in terms of four functions; the coefficients of these are then extracted by numerically minimising the difference between the corresponding functional and the branching kernels themselves. The coefficients determined by this procedure are collected in the XML file and are read in during initialisation.