Note that this version of VINCIA only has very limited matrix-element capabilities, since it is in the process of migrating from a matrix-element interface based on MadGraph 4 to one based on MadGraph 5. The options and switches on this page are therefore mainly for development purposes and should be ignored by ordinary users.

`mode `

** Vincia:MECs2to1 **
(`default = `

; **-1**`minimum = -1`

; `maximum = 2`

)

Selects the order of tree-level matrix-element corrections for hard
2→1 processes. The value `-1`

is equivalent to
switching matrix-element corrections off while `0`

still
allows to use matrix elements to select helicities at Born level.

`mode `

** Vincia:MECs2to2 **
(`default = `

; **-1**`minimum = -1`

; `maximum = 2`

)

Selects the order of tree-level matrix-element corrections for hard
2→2 processes. The value `-1`

is equivalent to
switching matrix-element corrections off while `0`

still
allows to use matrix elements to select helicities at Born level.

`mode `

** Vincia:MECs2toN **
(`default = `

; **-1**`minimum = -1`

; `maximum = 2`

)

Selects the order of tree-level matrix-element corrections for hard
2→N processes. The value `-1`

is equivalent to
switching matrix-element corrections off while `0`

still
allows to use matrix elements to select helicities at Born level.

`mode `

** Vincia:MECsResDec **
(`default = `

; **-1**`minimum = -1`

; `maximum = 2`

)

Selects the order of tree-level matrix-element corrections in
resonance decays. The value `-1`

is equivalent to
switching matrix-element corrections off while `0`

still
allows to use matrix elements to select helicities at Born level.

`mode `

** Vincia:MECsMPI **
(`default = `

; **-1**`minimum = -1`

; `maximum = 1`

)

Selects the order of tree-level matrix-element corrections applied to
the hardest MPI in the event. The value `-1`

is equivalent to
switching matrix-element corrections off while `0`

still
allows to use matrix elements to select helicities at Born level.

`flag `

** Vincia:matchingFullColour **
(`default = `

)**true**

`option `

** off** : Leading Colour.
`option `

** on** : Full Colour. Include the full colour structure of the
matched matrix elements, absorbing the subleading-colour pieces into
each leading-colour one in proportion to the relative sizes of the
leading-colour pieces. This procedure effectively diagonalises the
full colour matrix and guarantees positive-weight corrections.

We use the term matching regulator to refer to a generic sharp or smooth dampening of the ME corrections as one crosses into a specified region of phase space. The purpose of this is to restrict the matching to regions of phase space that are free from subleading logarithmic divergences in the matrix elements. This is familiar from the CKKW and MLM approaches, where the matching scale is imposed as a step function in pT, with full ME corrections above that scale and no ME corrections below it. We explore a few alternatives to this approach.

`mode `

** Vincia:matchingRegOrder **
(`default = `

; **3**`minimum = 0`

; `maximum = 5`

)

Choose starting order from which matrix element corrections are regulated.
`option `

** 0** : Off. Matrix element corrections are not regulated at
all. Not advised for production runs, but can be useful for theory
studies.
`option `

** 1** : On, starting from 1st order in QCD. This would
normally be overkill since the LL shower exactly reproduces the
1st order matrix-element singularities - the first-order correction
should therefore normally be free of divergencies and should not
need to be regulated.
`option `

** 2** : On, starting from 2nd order in QCD. The
2nd-order matrix element correction generally contains subleading
logarithmic divergences which do not correspond exactly to those
generated by the pure shower. Nonetheless, due to the unitary properties of VINCIA's matching formalism and the close approximation of its shower expansions to 2nd order matrix elements, however, 2nd order corrections can typically be applied over all of phase space, without ill effects.
`option `

** 3** : On, starting from 3rd order in QCD. This is the
recommended option for the multiplicative matching
strategy. Since the matrix-element corrections are exponentiated,
the subleading divergencies in the higher-order
corrections are effectively resummed. However,
due to the LL nature of the underlying shower,
it appears from empirical studies that a matching
scale is still needed starting from 3rd order even in the
multiplicative case.
`option `

** 4** : On, starting from 4th order in QCD. Not recommended
for production runs, but can be useful for theory studies.
`option `

** 5** : On, starting from 5th order in QCD. Not recommended
for production runs, but can be useful for theory studies.

`mode `

** Vincia:matchingRegShape **
(`default = `

; **1**`minimum = 0`

; `maximum = 1`

)

When `Vincia:matchingRegOrder >= 1`

,
choose the functional form of the regulator. (See below for how to
modify the choice of *Q* and *Q _{match}*.)

`option `

*Damp = 0*for*Q < Q*_{match}*Damp = 1*for*Q >= Q*_{match}

`option `

*Damp = 0*for*Q2 < Q2match/2**Damp = 0.5*(log2(Q2/Q2match)+1)*for*Q2match/2 < Q2 < 2*Q2match**Damp = 1*for*Q2 > 2*Q2match*

`flag `

** Vincia:matchingRegScaleIsAbsolute **
(`default = `

)**false**

Selects whether the user wants to input the value of *Q _{match}*
either by giving an an absolute number in GeV or by giving a ratio
with respect to the hard scale.

`option `

`Vincia:matchingRegScaleRatio`

below. This is the
default option and the one recommended for non-experts. It should
allow a wide range of processes to be considered without having to
manually adjust the matching scale.
`option `

`Vincia:matchingRegScale`

(in GeV). Care must then be
taken to select a matching scale appropriate to the specific process
and hard scales under consideration. For non-experts, the relative
method above is recommended instead.
`parm `

** Vincia:matchingRegScaleRatio **
(`default = `

; **0.05**`minimum = 0.0`

; `maximum = 1.0`

)

When `Vincia:matchingRegScaleIsAbsolute == false`

(default),
this sets the ratio of the matching scale to the process-dependent
hard scale; inactive otherwise. Since the
unresummed logarithms depend on ratios of scales, it is more natural
to express the matching scale in this way than as an absolute number
in GeV. Note that this parameter
should normally not be varied by more than a factor of 2 in either
direction. The default value has been chosen so as to allow one order
of magnitude between the hard scale and the matching scale. Setting it
too close to unity will effectively switch off the matching, even at
high scales. Settings around 0.01 and below risk re-introducing large
unresummed logarithms in the matching coefficients.

`parm `

** Vincia:matchingRegScale **
(`default = `

; **20.0**`minimum = 0.0`

)

When `Vincia:matchingRegScaleIsAbsolute == true`

, this sets the
absolute value of the matching scale, in GeV; inactive otherwise.
Care must be taken to select a matching scale appropriate to the
specific process and hard scales under consideration.

Due to the freezing of *alphaS* in the infrared, it is possible
to run VINCIA with very low hadronisation cutoffs. Though this
formally continues the perturbative treatment into the infrared,
allowing the emission of gluons with very soft momenta, it is doubtful
whether ME corrections would be of any value in that region.

Our intuition is
that, at best, continuing such corrections into the region below ~ 1
GeV would merely slow down the code. At worst they could generate
unphysically large corrections (e.g., the scale-dependent terms in the
NLO corrections are unphysical at scales near
Λ_{QCD}).

The parameter below sets an absolute lower scale for the evolution variable, in GeV, below which matrix-element corrections are not applied. Note that the normalisation of the evolution variable will affect how this translates to invariants.

`parm `

** Vincia:matchingIRcutoff **
(`default = `

; **2.0**`minimum = 0.0`

; `maximum = 100.0`

)