Matrix Element Corrections
Tree-level Matrix element corrections (MECs) have not yet been
re-implemented in the VINCIA shower. This feature will become
available in the future. But since both the QCD helicity shower and
the EW shower require polarised particles, matrix elements can be used
to select helicities for the Born process.
Born-level helicity selection can be enabled by setting the
modeMECs and the respective maxMECs switches
(see below) to 0. Additionally,
Vincia:MEPlugin must be set to a valid plugin library. By
default, these plugins are not built and can be enabled with
./configure --enable-mg5mes. Plugins exist as libraries
of the form libpythia8mg5PLUGIN.so in the Pythia library
directory where PLUGIN is the name of the plugin. A
specific plugin is set via Vincia:MEplugin =
PLUGIN. Further details on generating custom plugins can be
found in Madgraph5 Processes.
Setting modeMECs to -1 instead turns any use
of matrix elements off completely. Setting the respective
maxMECs switches to -1 only turns off the
usage of matrix elements for that specific process type (e.g. for all
2→1 processes when setting maxMECs2to1 = -1).
The main MEC switch is given by:
mode Vincia:modeMECs
(default = 0; minimum = -1; maximum = 0)
Selects the matrix element correction mode, with 0
switching any MECs off.
option -1 : Off. Pure LL shower without any matrix element
corrections.
option 0 : No matrix element corrections, but allows to use
matrix elements to select helicities at Born level.
The matrix element library to be used has to be specified by:
word Vincia:MEplugin
Name of the plugin library to use for matrix element corrections and
Born-level helicity selection.
Matching Orders
These modes select the order of tree-level matrix element corrections
for different process types.
mode Vincia:maxMECs2to1
(default = 0; minimum = -1; maximum = 0)
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:maxMECs2to2
(default = 0; minimum = -1; maximum = 0)
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:maxMECs2toN
(default = 0; minimum = -1; maximum = 0)
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:maxMECsResDec
(default = 0; minimum = -1; maximum = 0)
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:maxMECsMPI
(default = -1; minimum = -1; maximum = 0)
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.