<chapter name="VINCIA MEC Settings"> 
 
<h2>Matrix Element Corrections</h2> 
 
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. 
 
<p/> 
Born-level helicity selection can be enabled by setting the 
<code>modeMECs</code> and the respective <code>maxMECs</code> switches 
(see below) to <code>0</code>. Additionally, 
<code>Vincia:MEPlugin</code> must be set to a valid plugin library. By 
default, these plugins are not built and can be enabled with 
<code>./configure --enable-mg5mes</code>. Plugins exist as libraries 
of the form <code>libpythia8mg5PLUGIN.so</code> in the Pythia library 
directory where <code>PLUGIN</code> is the name of the plugin. A 
specific plugin is set via <code>Vincia:MEplugin = 
PLUGIN</code>. Further details on generating custom plugins can be 
found in <aloc href="MadGraph5Processes">Madgraph5 Processes</aloc>. 
Setting <code>modeMECs</code> to <code>-1</code> instead turns any use 
of matrix elements off completely.  Setting the respective 
<code>maxMECs</code> switches to <code>-1</code> only turns off the 
usage of matrix elements for that specific process type (e.g. for all 
2&rarr;1 processes when setting <code>maxMECs2to1 = -1</code>). 
 
<p/> 
The main MEC switch is given by: 
<modeopen name="Vincia:modeMECs" default="0" min="-1" max="1"> 
Selects the matrix element correction mode, with <code>0</code> 
switching any MECs off. 
<option value="-1">Off. Pure LL shower without any matrix element 
corrections.</option> 
<option value="0">No matrix element corrections, but allows to use 
matrix elements to select helicities at Born level.</option> 
<option value="1">Matrix-element corrections are switched on.</option> 
</modeopen> 
 
<p/> 
The matrix element library to be used has to be specified by: 
<word name = "Vincia:MEplugin" default = ""> 
Name of the plugin library to use for matrix element corrections and 
Born-level helicity selection. 
</word> 
<p/> 
 
Alternatively, for some processes, internal matrix elements are implemented. 
Their use can be enabled with 
<flag name = "Vincia:useInternalMEs" default = "off"> 
  Switch to use internal matrix elements. 
</flag> 
 
<h3>Matching Orders</h3> 
 
These modes select the order of tree-level matrix element corrections 
for different process types. 
 
<modepick name="Vincia:maxMECs2to1" default="0" min="-1" max="0"> 
Selects the order of tree-level matrix-element corrections for hard 
2&rarr;1 processes. The value <code>-1</code> is equivalent to 
switching matrix-element corrections off while <code>0</code> still 
allows to use matrix elements to select helicities at Born level. 
</modepick> 
<modepick name="Vincia:maxMECs2to2" default="0" min="-1" max="1"> 
Selects the order of tree-level matrix-element corrections for hard 
2&rarr;2 processes. The value <code>-1</code> is equivalent to 
switching matrix-element corrections off while <code>0</code> still 
allows to use matrix elements to select helicities at Born level. 
</modepick> 
<modepick name="Vincia:maxMECs2toN" default="0" min="-1" max="0"> 
Selects the order of tree-level matrix-element corrections for hard 
2&rarr;N processes. The value <code>-1</code> is equivalent to 
switching matrix-element corrections off while <code>0</code> still 
allows to use matrix elements to select helicities at Born level. 
</modepick> 
<modepick name="Vincia:maxMECsResDec" default="0" min="-1" max="0"> 
Selects the order of tree-level matrix-element corrections in 
resonance decays. The value <code>-1</code> is equivalent to 
switching matrix-element corrections off while <code>0</code> still 
allows to use matrix elements to select helicities at Born level. 
</modepick> 
<modepick name="Vincia:maxMECsMPI" default="-1" min="-1" max="0"> 
Selects the order of tree-level matrix-element corrections applied to 
the hardest MPI in the event. The value <code>-1</code> is equivalent 
to switching matrix-element corrections off while <code>0</code> still 
allows to use matrix elements to select helicities at Born level. 
</modepick> 
 
</chapter> 
 
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