Combinatorial Analysis using Candidates

Complete:

Purpose

Perform combinatorial analysis using particle Candidates. The tools avoid double-counting and don't combine particles sharing the same component in one or more subdetectors.

Particles are composed according to a decay tree, to create CompositeCandidate objects. Daughter particles kinematics is copied into the composite object, and links to the orignlal "master" particles can also be stored using ShallowCloneCandidate.

CandCombiner Framework Modules

Different modules are provided, with possibility to read and save different types of collections. The supported combiner modules are specialization of a single module template, reco::modules::CandCombiner, defined in the package:

• CommonTools/CandAlgos

The type of decay is specified passing a string like in the following examples, where the character ' @ ' is used to specify the charge values after the collection name (zero charge is assumed if missing):

• decay = cms.string("muons@+ muons@-") for Z → μ⁺μ^-
• decay = cms.string("pions@+ pions@- pions") for η → π⁺π^-π⁰
• decay = cms.string("jPsi phi") for B_s → J/Ψ Φ

Charge conjugate decays are always implied.

Examples of specializations for this modules are:

CandViewShallowCloneCombiner

The module CandViewShallowCloneCombiner combines particle candidates to form composite objects. Daughter particles are shallow clones of the input particles. The cut is specified as a string.

• Input collections can be any type containing candidates (edm::View<reco::Candidate>)
• Output collection is of type std::vector<reco::CompositeCandidate>

Examples of configuration is:

  process.ZCandidates = cms.EDProducer("CandViewShallowCloneCombiner",
    decay = cms.string("muons@+ muons@-"),
    cut = cms.string("86.0 < mass < 96.0")
  )

More information on configurable string cuts can be found in:

• Physics Cut Parser.

CandViewCombiner

The module CandViewCombiner combines particle candidates to form composite objects. Daughter particles are clones of the input particles. The cut is specified as a string.

• Input collections can be any type containing candidates (edm::View<reco::Candidate>)
• Output collection is of type std::vector<reco::CompositeCandidate>

Examples of configuration are:

  process.ZCandidates = cms.EDProducer("CandViewCombiner",
    decay = cms.string("muons@+ muons@-"),
    cut = cms.string("86.0 < mass < 96.0")
  )

More information on configurable string cuts can be found in:

• Physics Cut Parser.

DeltaPhiMinCandCombiner

The module DeltaPhiMinCandCombiner produces combined candidate of pairs of particles (e.g.: jets) that have an acoplanarity angle Δφ larger than a given configurable value deltaPhiMin. This module is mainly used for min-bias and underlying event reconstruction.

DeltaRMinCandCombiner

The module DeltaRMinCandCombiner produces combined candidate of pairs of particles (e.g.: jets) that have a value of ΔR larger than a given configurable value deltaRMin. This module is mainly used for min-bias and underlying event reconstruction.

CandCombiner Utility

The template CandCombiner is defined in the following package:

• CommonTools/CandAlgos

Template Parameters

The utility has 5 template type arguments, some of which are optional: CandCombiner<Selector, OutputCollection, PairSelector, Cloner, Setup>. The argument types are:

• Selector: Selector type specifying which selection. The interface the one specified for the Generic Selector toolkit.
• OutputCollection: output collection type. Defaults are reco::CompositeCandidateCollection if InputCollection is edm::View<reco::Candidate> and reco::CandidateCollection if InputCollection is reco::CandidateCollection.
• Cloner: helper class to specify the cloning of daughter candidates. It is by default combiner::helpers::NormalClone, in which case the daughter candidates are stored in the composite candidates as clones of the original ones. It is also possible to specify Cloner as combiner::helper::ShallowClone, in which case daughters particles are shallow-clones of the original ones, which means that contain a new kinematics, with a reference to the original ("master") clone.
• Setup: specifies the way to set up the composite candidate kinematics. By default it is AddFourMomenta, which sums the momenta four-vectors. Other setup utilities could be specified, if needed (e.g.: a common vertex fitter).

CandCombiner Initialization via Constructor Parameters

Some optional information can be supplied to the CandCombiner constructor to specify how it should operate to perform the combinatorial analysis. The following are the possible information to pass:

• The charges of the daughter particles. This implies that the decay is accepted only if the daughters have the charges corresponding to the specified valies, or their charge conjugates. For instance, specifying charges +1, -1, only decays into oppositely charged particles are searched for; specifying charged +1, -1, 0 could be the proper way to search for a η→π⁺π^-π⁰. If not charges are specified, the combinatorial analysis will not check for particle charged. The number of charged particles to be reconstructed must match the number of charge values passed.
• The value of the selector of type S. If not passes, the default constructor of S is used.
• The value of the setup of type Setup. If not passed, the default constructor of Setup is used

Different constructors are provided with the possibility to pass or not the arguments of the types specified above. All the constructor are visible in the source file:

• CandCombiner.h.

Performing the Combinatorial Analysis

After creating a CandCombiner, it can perform combinatorial analysis on any candidate collection. The candidate collections should be passed via references of type reco::CandidateRefProd (a.k.a.: edm::RefProd<reco::CandidateCollection>). If the same reference is passed more than once, the module automatically avoid double-counting. The following methods are provided:

1. combine(const reco::CandidateBaseRefProd &) const: performs a search for a two-body decay of candidates all belonging to the same collection.
2. combine(const reco::CandidateBaseRefProd &, ...) const, with two to four references to collections arguments: performs a search for a two-body, three-body or four-body decay of candidates belonging to the specified collections.
3. combine(const std::vector<reco::CandidateRefProd> &) const: performs a search for a N -body decay, N being the size of the passed std::vector<reco::CandidateRefProd>.

In all cases, if a reference to the same colleciton is passed more than once, the particle decays are never double-counted. I.e.: if the decays X→A+B is reconstructed, the decay X→B+A is not added to the return collection.

The collection is returned as an auto-pointer of type:

• std::auto_ptr<reco::CompositeCandidateCollection>

Example of Usage of CandCombiner

To reconstruct decays Z→μ⁺μ^-, the following code can be used:

  edm::Handle<reco::CandidateCollection> muons;
  event.getByLabel( "allMuons", muons );
  reco::CandidateRefProd muonsRef( muons );
  MassRangeSelector<reco::Candidate> massRange( 70, 110 );
  CandCombiner<MassRangeSelector<reco::Candidate> > combiner( +1, -1 );
  std::auto_ptr<reco::CompositeCandidateCollection> zCands = combiner.combine( muonsRef );
  std::cout << "Reconstructed " << zCands->size() << " Z candidates" << endl;

Using CompositeCandidate Roles and Names

There is the capability of using CompositeCandidateCombiners with additional parameters which correspond to the name of the candidate (for instance, "ZtoMuMu") and the roles of the daughters (for instance, "muon1" and "muon2"). The syntax is

process.zToMuMu = cms.EDProducer("CandViewShallowCloneCombiner",
    decay = cms.string('muons@+ muons@-'),
    cut = cms.string('0.0 < mass < 20000.0'),
    name = cms.string('zToMuMu'),
    roles = cms.vstring('muon1', 'muon2')
)

Notice the additional two parameters name and roles are specified, and will be added to the candidates.

This allows the user to navigate the hierarchy of a CompositeCandidate event hypothesis like

Candidate * muon1 = zToMuMuCand->daughter("muon1");

As of 2.1.0, there is also support for "hierarchical" string access such as

Candidate * lepton = topCand->daughter("leptonicW")->daughter("lepton");

These additions work with any candidate combiners.

Review status

Reviewer/Editor and Date (copy from screen)	Comments
LucaLista - 4 Dec 2007	page author

Responsible: LucaLista
Last reviewed by: Reviewer