Introductory comments

ZvTopVertexer has so far been run on fast Monte Carlo tracks from Monte Carlo truth level information in stdhep format.  The events have been generated with Pythia, Pandora/Pythia or the Diagnostic Generator (from the Java hep package).

Feel free to contact me in case of problems or for feedback.

Important remarks

• ZvTopVertexer needs the parametrization of the tracks with five helical track parameters as defined by the Track interface in the lcd environment.
• In addition it needs the coveriance matrices for those tracks parametrized accordingly as defined by the Track interface in the lcd environment.
• ZvTopVertexer operates on a list of tracks, which are assumed to form a jet.  The default is to only look for vertices in the forward direction of the jet as seen from the interaction point.  The user has to make sure, that the sets of tracks form (approximately) a jet.  The default forward direction is given by the sum of the momentum vectors of the tracks in the list passed to ZvTopVertexer.  Optionally, a vector can be given for the forward direction.
• ZvTopVertexer needs two helpers: a swimmer as specified by the Swimmer interface and a track vertex fitter as specified by the VtxFitter interface.  One concrete implementation is supplied for each of them, called ZvSwimmer and ZvFitter, respectively.  Both contain tunable parameters.
• There needs to be an object containing general parameters for the main ZvTopVertexer, instanciated from the ZvTopParam class.
• The resulting vertices are returned in a list of ZvVertex objects.  There is a ZvTopStatus status object, which can be inspected after the fit.

Package import statement

import hep.lcd.vertexing.zvtop.*;

Initialization

These three objects and the (constant) beamspot definition are best declared in the global scope of your Processor class:

 // general parameter object for ZvTopVertexer

 private ZvTopParam  vtxParam = new ZvTopParam();

 // swimmer
 private ZvSwimmer   swimmer  = new ZvSwimmer();

 // track vertex fitter
 private ZvFitter    fitter   = new ZvFitter();

 // beamspot definition
 private ZvBeamSpot  beamSpot = new ZvBeamSpot(new double[]{0.,0.,0.});
 

• For the ZvTopVertexer parameters (ZvTopParam object):

   //
  // general parameters for ZvTopVertexer
  //
  // of general interest:
  vtxParam.setJetMomentum(new double[]{0.,0.,0.}); // use p_track vector sum instead
  vtxParam.setIPWeight(1.0);             // weight for IP in vertex significance: 1.0 full, 0.0 no weight
  vtxParam.setBackwardCutoff(-0.01);     // default: -0.01 cm // don't allow vertices behind
  vtxParam.setForwardCutoff(10.);        // default: 10.0 cm  // don't allow vertices beyond
  vtxParam.setTubeScaleFac(1.0);         // default: 0.0 = const; 1.0 = parabolic approximation

  // parameters for general tuning:
  vtxParam.setCang(0.);                  // no weighting with angle(momentum_track, momentum_jet)
  vtxParam.setVSigMinFrac(0.1);          // min vertex significance at spatial point creation time
  vtxParam.setResolMin(0.001);           // default: 10 um; minimum distance required for resolved vertices
  vtxParam.setResolNSteps(16);           // default: 16; number of steps used to check valley between vertices
  vtxParam.setChi2TrackCut(10.);         // maximum chi2 a single track may contribute to a vertex

  // specialties:
  vtxParam.setUseJetAxis(false);         // don't use jet axis for trdi and lodi calculation
  vtxParam.setMode(0);                   // normal mode, no high impact parameter/lepton track plane
  vtxParam.setPlaneWidth(1.0);           // not used, if not in high impact/lepton track mode
 
 

• For the swimmer:

   //
  // setup ZvSwimmer
  //
  // general
  double[] beamDir = {0.,0.,1.};      // beam direction (positive z-axis)
  swimmer.setBeamDirection(beamDir);

  // fiducial volume
  swimmer.setMaxSinL(1.0);            // max sin(dip_angle)
  swimmer.setVolume(10.0,10.0);       // fiducial volume (r,z for cylinder in cm)

  // tuning specialties (speed vs accuracy)
  swimmer.setStepMin(0.000001);       // minimum step size for convergence
  swimmer.setStepSize(0.01);          // initial step size
  swimmer.setMaxTry(20);              // max tries for convergence
 

• For the vertex fitter:

  //
  // setup ZvFitter
  //
  // general
  fitter.setSwimmer(swimmer);         // vertex fitter needs swimmer
  fitter.takeNewTrackList(true);      // output re-swum track parameters with vertex tracklist

  // tuning
  fitter.setDistMin(0.1);             // min distance for re-swimming tracks (in cm)
  fitter.setVtxResMax(0.001);         // max vertex resolution for convergence

  // special tuning
  fitter.setMaxTry(5);                // max number of fit tries
  fitter.setMaxTryInner(2);           // max number of tries for inner fit loop

Event processing

• Definition of a ZvTrackList with the tracks to be considered:

//
ZvTrackList forwardList  = new ZvTrackList(new Filter(event.getTrackList().getTracks(),
                                           new ZvtThrustPredicate(es.thrustAxis(),+1)));
 
• Creation of a  ZvTopVertexer object:

//
// setup vertexer
ZvTopVertexer vertexer = new ZvTopVertexer(vtxParam);
vertexer.setSwimmer(swimmer);
vertexer.setVtxFitter(fitter);
vertexer.setBeamSpot(beamSpot);
 
• Run of ZvTopVertexer on this tracklist:

//
vertexer.setTrackList(forwardList);
ZvTopStatus  fwdVtxStatus   = vertexer.findVertices();
 
• Check of  the vertex finding status with the ZvTopStatus object:

//
out.println("Forward: "+fwdVtxStatus.toString());
 
• Retrieving the list of ZvVertex objects:

//
ZvVertexList forwardVtxList = vertexer.finalVertices();    // could be a null pointer or empty
if ( fwdVtxStatus.success() )
    out.println("  #vertices = "+forwardVtxList.getNVertices());

Complete example

• ZvtExample1-- runs ZvTopVertexer on e+e--> qqbar events and produces flavor dependent vertex multiplicity histograms.

Use the file udscb-atZ-2.stdhep from  the /YOURCDROMPATH/Data/Generator Level/pyqqatZ/ directory on the disk for some sample qqbar events at the Z pole. (Usefull for technical checks.)

Contact