mode
VinciaMatching:order
(default = 3
; minimum = 0
; maximum = 4
)2
would invoke tree-level matching
up to and including X+2
partons, to the extent the
relevant matrix elements are available in the code. The
value 0
is equivalent to switching matching off.
flag
VinciaMatching:fullColor
(default = true
)option
off : Leading Color. Only include matching to leading-color matrix
elements.
option
on : Full Color. Include the full color structure of the
matched matrix elements, absorbing the subleading-color pieces into
each leading-color one in proportion to the relative sizes of the
leading-color pieces. This procedure effectively diagonalizes the
full color 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
VinciaMatching:regOrder
(default = 3
; minimum = 0
; maximum = 4
)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. This is the
recommended option for the additive matching strategy. The
2nd-order matrix element correction generally contains subleading
logarithmic divergences which do not correspond exactly to those
generated by
the shower. Since these divergencies are not resummed, a regulator
is needed to keep the generated event weights from diverging,
starting from this order.
option
3 : On, starting from 3rd order in QCD. This is the
recommended option for the multiplicative and hybrid matching
strategies. Since the matrix-element corrections are exponentiated
in those approaches, the subleading divergencies in the 2nd-order
corrections are effectively resummed. In principle, this should
work to arbitrary order, but due to the LL nature of the shower in
other respects, 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.
mode
VinciaMatching:regShape
(default = 1
; minimum = 0
; maximum = 1
)VinciaMatching:RegOrder >= 1
,
choose the functional form of the regulator. (See below for how to
modify the choice of Q and Qmatch.)
option
0 : Step function at Q=Qmatch, i.e.,
option
1 : Suppress the shower-subtracted ME corrections by a
function that is unity above Q2 =
2*Q2match, zero below Q2 = Q2match/2, with a simple
interpolation (logarithmic in Q2) between those scales, i.e.,
mode
VinciaMatching:regType
(default = 1
; minimum = 1
; maximum = 2
)VinciaMatching:regOrder >= 1
, choose
argument of the regulator function (i.e., Q in the equations
listed under VinciaMatching:RegShape
).
option
1 : Impose matching scale in the type 1 evolution
variable, Q2=4*pT^2. The smallest pT scale of the current
branching and the color neighbor on
either side (if any) is used.
option
2 : Impose matching scale in the type 2 evolution
variable, Q2=4*pT^2. The smallest invariant mass of the current
branching and the color neighbor on
either side (if any) is used.
flag
VinciaMatching:regScaleIsAbsolute
(default = false
)option
false : Relative. The matching scale is determined automatically
in relation to the hard scale in the process (e.g., the Z mass) by
the factor VinciaMatching:regScaleRatio
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
true : Absolute. The matching scale is set by the
value VinciaMatching:regScale
(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
VinciaMatching:regScaleRatio
(default = 0.05
; minimum = 0.0
; maximum = 1.0
)VinciaMatching:regScaleIsAbsolute == 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
VinciaMatching:regScale
(default = 20.0
; minimum = 0.0
)VinciaMatching:regScaleIsAbsolute == 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.
A small library of matrix elements obtained with MADGRAPH (MG) are included with the standard VINCIA release. (In the future, matrix elements for additional processes will be available for download from the VINCIA homepage.) Through its MG interface, VINCIA is able to match to these matrix elements during the shower evolution.
flag
Vincia:MGInterface
(default = on
)Note: see the section on Useful References for additional references to include when using the matrix elements provided by this interface.
Note 2: parts of the MADGRAPH code (specifically the HELAS libraries) are needed at runtime to evaluate the MADGRAPH matrix elements. The Makefile distributed with VINCIA will automatically attempt to download and install the MADGRAPH standalone tarball for you, if it doesn't find it in the expected location, and will then compile the relevant libraries from it. If this fails, we recommend having a look in the Makefile before contacting the authors. Large parts of the Makefile have been made user-editable in hopefully straightforward ways, and it may be possible for you to solve the problem by trivial editing.
The procedure for including a new matrix element is not intended for general users at this point, but is documented here for reference:
The MADGRAPH matrix elements included with VINCIA are
located together with the actual interface code
in VINCIA's MGInterface/
subdirectory. They
are ordinary MADGRAPH matrix element routines, which have been
renamed and modified slightly for use by VINCIA. Most of this
conversion is done automatically, by a script. Implementing new
matrix elements should therefore be fairly trivial, as long as the
final state corresponds to something that VINCIA can actually
generate. (E.g., implementing matrix elements for DIS or
hadron-collider processes will not suddenly make VINCIA able to
handle such cases, since the shower is not yet developed for that.)
Procedure:
matrix.f
file for the new process, as follows:
Template/
directory,
e.g., tmp/.
Z>qq~
, then Z>qq~g, etc.
bin/newprocess
and wait for it to finish.SubProcesses
and
find the directory in there that corresponds to your chosen
process. If everything went well, that subdirectory will
contain a file matrix.f
, which is what we start
from in the steps below.
matrix.f
to replace all occurrences
of include "nexternal.inc"
by the actual contents of the
file "nexternal.inc"
, which is located in the same
subdirectory as matrix.f
. Trick: in emacs, use
Ctrl-x-i to insert the contents of a file directly into the current buffer.matrix.f
together with the diagram
file matrix.ps
to VINCIA's MGInterface/
subdirectory.MGInterface/
subdirectory and run
./convert.sh
. This replaces the
generic MadGraph names by unique ones that can be used
from inside VINCIA, and inserts a rotation in color space that VINCIA
relies on to include matching to subleading color.
The new names are automatically determined from the
contents of matrix.f
without intervention from you.convert.sh
should tell you what name
VINCIA has chosen for the modified version
of matrix.f
.z2duxudx.f
matrix element
included with VINCIA for an example and contact the authors if you need
more information.VinciaMGInterface.cc
and scroll down until you come
to a comment line that reads // Wrappers for MG decay matrix
elements
. Insert a new line here, defining the
function name contained in the modified MG matrix element. Be
careful to choose the right dimensionality of the input momentum array.MGInterface::ME2
to insert a call
to the new matrix element at an appropriate place. bool
Matching::isMatched()
in the
file src/VinciaMatching.cc
to specify the new
matching matrix element that is available.