Sector Merging (VINCIA only)

  1. Matrix Elements
  2. Process Specification
  3. Merging Scale
  4. 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: 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 }.