4.3 Particle Candidates Utilities and Framework Modules

More details on available candidate selectors can be found in the following document:

• Combinatorial Analysis tools using particle candidates.

It is also possible to specify an optional name and daughter roles for these tools, like:

process.zToMuMu = cms.EDProducer("CandViewShallowCloneCombiner",
    decay = cms.string('selectedLayer1Muons@+ selectedLayer1Muons@-'),
    cut = cms.string('50.0 < mass < 120.0'),
    name = cms.string('zToMuMu'),
    roles = cms.vstring('muon1', 'muon2')
)

This will automatically assign names and roles as described here. The rest of the functionality is the same as the previous example.

Other Modules

A more complete list of the available modules to manipulate collections of candidates can be found in:

• Candidate Common Modules SW Guide

Candidate Utilities

Overlap Checking

Overlap between two candidates occurs if the two candidates, or any of their daughters, share one of the components (a track, a super-cluster, etc.). The utility OverlapChecker checks for overlap occurrences:

  #include "DataFormats/Candidate/interface/OverlapChecker.h"

  OverlapChecker overlap;
  const Candidate & c1 = ..., & c2 = ...;
  if (overlap( c1, c2 ) )  { ... }

Note: this overlap-checking mechanism only looks for identical components in the candidate decay chain, but has no way to check, for instance, if two candidates are made from tracks or clusters that share a common set of hits. More advanced utilities should be used for such, more refined overlap checking.

Candidate Setup Utilities

• AddFourMomenta: sets a candidate's 4-momentum adding 4-momenta of its daughters. In the following example, a Composite Candidate is created, its two daughters are added to it, and its momentum is set as the sum of the daughters' four-momenta:

      AddFourMomenta addFourMomenta;
      reco::CompositeCandidate recHadt;
      reco::CompositeCandidate recHadW;
      recHadt.addDaughter(recHadW, "hadW");
      addFourMomenta.set(recHadW);
      addFourMomenta.set(recHadt);

Boosting Candidates

If you want to boost a candidate, you should get sure you can modify it. Candidates taken from event collections are immutable, so you need to clone them before boosting.

If you want to boost a candidate to another candidate centre of mass, you can do the following:

   Candidate * c1clone = c1->clone();
   Booster boost(c2->boostToCM());
   booster.set(*c1clone);

Particle::Vector boost = mother.p4().BoostToCM();

If you want to boost a ComposteCandidate and its daughters to its centre of mass, you can use the following example:

   // create booster object
   CenterOfMassBooster boost(h);
   // clone object and update its kinematics
   Candidate * higgs = h.clone();
   boost.set(*higgs);
   // get boosted Z as Higgs daughters
   const Candidate * Z1 = higgs->daughter(0);
   const Candidate * Z2 = higgs->daughter(1);

In the above example, the Z daughters (leptons) will also be boosted.

Booster utilities are defined in CMS.PhysicsTools/CandUtils.

Common Vertex Fitter

Common vertex fitter is a "setup" operator using the Vertex Fitter tools for track collections. It requires the magnetic field map to be passed to the algorithm, which can be obtained from the EventSetup:

    ESHandle<CMS.MagneticField> B;
    es.get<IdealMagneticFieldRecord>().get(B);

    CandCommonVertexFitter<KalmanVertexFitter> fitter;
    fitter.set(B.product());

    const Candidate & zCand = ...; // get a Z candidate
    VertexCompositeCandidate fittedZ(zCand);
    fitter.set(fittedZ);

More details on:

• Constrained vertex fit with candidates

Candidates and Monte Carlo Truth

The candidate used to represent Monte Carlo truth from generator output can be matched to RECO objects using a set of common tools described in the document below:

• MC Truth matching tools

Review status

Responsible: LucaLista
Last reviewed by: PetarMaksimovic - 28 Feb 2008