Vertex Fitting with Candidates

Complete: 5

Goals of this page

Describe common tool to perform common vertex constrained fits with particle candidates.

Introduction

The utilities documented here are released in 1.6.11. Earlier releases may have different interface and reduced functionalities.

Vertex fitting tools can be easily used with composite particle candidates representing a decay tree. The tools described below allow to apply common vertex fit to all charged particle candidates present in the decay tree, such as Z→μ+μ-, Z→e+e-, H→ZZ(*)→4l, etc. Neutral particles in the final state of the decay tree are left unchanged by the fitter.

In the EDM, objects are immutable once they are stored into the event, so composite particles must be cloned before applying a constrained fit.

Constrained Fitter Framework Modules

Modules are provided to perform vertex constrained fit on all particles in input collection and produce a collection of new fitted particles.

KalmanVertexFitCompositeCandProducer

Performs a constrained common vertex fit to a collection of composite candidates usint the kalman fitter. the outout collections is a reco::VertexCompositeCandidateCollection. Below an example of cfg fragment:

  module vtxZToMuMu =  KalmanVertexFitCompositeCandProducer {
    InutTag src = zToMuMu
  }

GsfVertexFitCompositeCandProducer

Performs a constrained common vertex fit on a collection of composite candidates using the gaussian-sum fitter. The parameters of the fitter should also be added. The input collection is any matching edm::View<reco::Candidate>, the output collections is a reco::VertexCompositeCandidateCollection. Below an example of cfg fragment:

  module vtxZToEE =  GsfVertexFitCompositeCandProducer {
    InutTag src = zToEE
    double maxDistance = 0.01
    int32 maxNbrOfIterations = 10
    bool limitComponents = true
    bool smoothTracks = false
    PSet GsfMergerParameters = {
      int32 maxNbrComponents = 4
      string merger = "CloseComponentsMerger"
      string distance = "KullbackLeiblerDistance"
    }
  }

KinematicVertexFitCompositeCandProducer NEW

Performs a constrained common vertex fit on a collection of composite candidates using the kinematic fitter, that can handle long-lived particles. The input collection is any matching edm::View<reco::Candidate>, the output collections is a reco::VertexCompositeCandidateCollection. Particles can be marked as long-lived adding to the combiner or fitter module the following flag: bool setLongLived = true. Below an example of cfg fragment:

  module kShortsBare = CandViewShallowCloneCombiner {
    string decay = "goodPions@+ goodPions@-"
    string cut = "0.3 < mass < 0.7"
    bool setLongLived = true
    int32 setPdgId = 310
  }

  module kShorts = KinematicVertexFitCompositeCandProducer {
    InputTag src = kShortsBare
    bool setLongLived = true
    int32 setPdgId = 310
  }

  module BdBare = CandViewShallowCloneCombiner {
    string decay = "kShorts goodMuons@+ goodMuons@-"
    string cut = "3.0 < mass < 7.0"
    int32 setPdgId = 511
  }

  module Bd = KinematicVertexFitCompositeCandProducer {
    InputTag src = BdBare
    int32 setPdgId = 511
  }

Constrained Fitter Utility

A generic utilities to perform vertex fix on reco::CompositeCandidate objects in analyzer code is defined in:

  • RecoCandUtils/interface/CandCommonVertexFitter.h

It is a class template:

  • template<typename Fitter> class CandCommonVertexFitter;

where Fitter can be any vertex fitter object provided with the vertexing suite.

Common vertex fitter interface is similar to other candidate utilities. In addition, it requires the magnetic field from the EventSetup. One example of usage is the following:

    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);

Z kinematics as well as daughters' kinematics is updated according to the fit. The output type, VertexCompositeCandidate, will contain error matrix on the fitted vertex and fit information (χ2/n.d.o.f).

Edit | Attach | Watch | Print version | History: r9 < r8 < r7 < r6 < r5 | Backlinks | Raw View | WYSIWYG | More topic actions
Topic revision: r9 - 2008-10-27 - PhilipAllfrey



 
    • Cern Search Icon Cern Search
    • TWiki Search Icon TWiki Search
    • Google Search Icon Google Search

    CMSPublic All webs login

This site is powered by the TWiki collaboration platform Powered by PerlCopyright & 2008-2019 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback