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.