Sector Merging (VINCIA only)
- Matrix Elements
- Process Specification
- Merging Scale
- Expert Settings
The Vincia sector shower employs its own CKKW-L merging scheme, which
differs from the one implemented for the simple showers or Dire. The
biggest difference is that the Vincia sector shower only possesses a
single history (or branching tree), i.e., any given configuration
produced by it can be uniquely traced back to every intermediate state
the shower has produced on the way. Sector merging can therefore be
viewed as the midpoint between the CKKW-L and the CKKW scheme, as only
a single, deterministic history needs to be generated, which
nevertheless exactly reflects the shower history and
Sudakov factors are generated dynamically using trial showers. As
such, it is specifically designed for merging with high-multiplicity
matrix elements.
Sector merging may be enabled by using the VINCIA sector shower and
switching merging on:
PartonShowers:model = 2
Vincia:sectorShower = on
Merging:doMerging = on
In addition, the user should set
Vincia:kineMapFFsplit = 1
since the inverse kinematic map for other splitting maps are
not currently available. We also advise running with
Check:abortIfVeto = on
such that any errors which occur during merging will be flagged as an
aborted event (rather than a zero weight event for normally vetoed
events).
We note that, different to merging with the simple showers,
Merging:doMerging
always has to be set to on
to enable merging, no matter which merging scale definition is used.
The sector merging can generally replace CKKW-L merging with the
simple showers and is illustrated in the main program
main280.cc
. However, also other main280.cc
. However, also other tree-level merging main programs can be used
with the modifications to the command files as outlined below.
Matrix Elements
As for merging with the simple showers, the
user has to provide LHE files with pre-generated events with up to
N
additional jets. The maximal number of additional jets
relative to the Born that do not arise from resonance decays is
specified by setting
Merging:nJetMax = N
Process Specification
The hard process should be specified through
Merging:Process
, using the following rules:
- The whole string should be encased in curly brackets,
{
... }
.
- Specify particles one at a time, separated by whitespace, using
Pythia's naming conventions in ParticleData.
- The intial and final state should be separated by
>
.
- Exactly two initial state particles should be specified
- Resonance decays may be specifed also using
>
, but
if there is more than one particle in the intermediate final state it
should be encased in round brackets, ( ... )
. In
principle as many nested decays may be specified as required, with
additional brackets for each decay. However, we make no claims on how
physically sensible merging in such systems is.
- Resonances must decay to exactly 2 particles.
- In addition to Pythia's naming scheme, the following
"multiparticles" are defined:
p=p+=pbar=p-=n=nbar=j={1,2,3,4,5,-1,-2,-3,-4,-5,21}
q=Q=QUARK={1,2,3,4,5}
qbar=QBAR=ANTIQUARK={-1,-2,-3,-4,-5}
LEPTONS={11,-11,13,-13,15,-15}
l+={-11,-13,-15}
l-={11,13,15}
NEUTRINOS={12,-12,14,-14,16,-16}
nu={12,14,16}
nubar={-12,-14,-16}
gammaZ={22,23}
We note that the option Merging:Process = guess
is not
supported, as we believe that the user should be fully aware and in
control of what the hard process in the event is.
VBF and HEFT
Following the idea that the user needs to specify the event topology exactly,
the following two switches can be used to further refine the process:
flag
Vincia:MergeVBF
(default = off
)
Experimental switch to enable merging in VBF processes.
flag
Vincia:MergeHEFT
(default = off
)
Experimental switch to consider HEFT couplings in the history
construction.
Merging Scale
The sector merging presently supports three different merging scale
definitions. If only Merging:doMerging = on
is set, the
shower evolution variable (a generalised Ariadne
pT) is
used to define the merging scale. Additionally, the merging scale may be
defined in terms of a kT cut by setting
Merging:doKTMerging = on
and (optionally) specifying Merging:ktType
, see
CKKW-L Merging for details. In both cases,
the merging scale should be specified through Merging:TMS
while for merging with a kT cut, also the D
parameter should be specified by Merging:Dparameter
.
It is also possible to define the merging scale by a set of cuts
imposed on the events upon generation by setting
Merging:doCutBasedMerging = on
. The user then has to
provide the values of the three cuts: Merging:pTiMS
,
Merging:QijMS
, and Merging:dRijMS
representing the minimal transverse momentum pTi
of a single jet and the minimal invariant mass Qij
and minimal separation ΔRij of every pair of
jets, as used to generate the events.
Expert Settings
The Vincia sector shower supports dedicated merging in resonance
systems. At the current stage, it is, however, only an experimental
feature and should not yet be used for physics studies.
The expert user might want to experiment with the following settings.
flag
Vincia:MergeInResSystems
(default = off
)
Experimental switch to enable merging of additional jets in resonance
decay systems. Currenly only handles colour-singlet resonances only.
Intended for expert users only!
mode
Vincia:MergeNJetMaxRes
(default = 0
; minimum = 0
)
Analogue to Merging:nJetMax
, to communicate the maximum
number of additional jets that can be produced from a given resonance
decay system by the matrix-element generator. Only used if
Vincia:MergeInResSystems = on
.
mode
Vincia:MergeNResSys
(default = 0
; minimum = 0
)
The number of resonance systems allowed to produce jets if
Vincia:MergeInResSystems = on
and
Vincia:MergeNJetMaxRes > 0
.
Additionally, for simple topologies, resonances can be explicitly
inserted into the event record if they have not been written to the
event file by the matrix element generator. We do note that the sector
merging heavily relies on this information and that attaching
resonances to leptons is ambiguous for general processes.
flag
Vincia:InsertResInMerging
(default = off
)
If set to on
, Vincia tries to attach resonances to
final-state leptons and insert them into the event record before
constructing the shower history. If turned on, the hard process must
explicitly contain a resonance, e.g. Merging:Process = { p p >
Z0 }
.