DØ "How To" Guide to
How To Analyze a DST

This document contains a semi-random collection of examples of how to analyze the DØ DST.

The following code snipets show examples of how to get at information. It is left as an exercise to the reader to figure out what include files are needed (hint - you need at least the includes of the objects described below, which you can find by following the links).

• Charged Particle Information
  • The ChargedParticle information is contained in the ChargedParticleChunk. To access this chunk:

    TKey<ChargedParticleChunk> chKey;
THandle<ChargedParticleChunk> chChunk = chKey.find(event);
if ( chChunk.is_valid() ) { ... }

  • To iterate (loop) over all charged particles, try

    vector<ChargedParticle>::const_iterator chpartPtr;
const vector<ChargedParticle>* chparts = chpartChunk->getParticles();
for ( chpartPtr = chparts->begin(); chpartPtr != chparts->end(); chpartPtr++ ) { ... }

  • Each ChargedParticle has an associated GTrack (global track). To access a ChargedParticle GTrack:

    ChargedParticle::GtrIndex gtrackIndex = chpartPtr->gtrackindex();
LinkPtr<GTrackChunk, GTrack> gtrackPtr(gtrackIndex);
if ( gtrackPtr->is_valid() ) { ... }

  • The track parameters of a ChargedParticle are calculated at the DCA of the best primary vertex. To access these track parameters:

    const ETrack etrack = chpartPtr->get_etrack();
float r_dca = etrack.get_vector(SurfDCA::IRSIGNED);
float z_dca = etrack.get_vector(SurfDCA::IZ);
float phi_dca = etrack.get_vector(SurfDCA::IPHID);
float tanlam_dca = etrack.get_vector(SurfDCA::ITLM);
float qpt_dca = etrack.get_vector(SurfDCA::IQPT);

• Track Information
  • To access the hits in a GTrack, you may use a D0Track. For example,

    const D0Track d0track(*gtrackPtr);
int nHits = d0track.num_hits();

  • The number of hits on a track is different than the number of measurements. Hits may be 3D, being composed of two measurements. The number of degrees of freedom for a track is the number of measurements - 5 (number of track parameters). To access the number of measurements:

    int nMeas = gtrackPtr->get_num_measurements();

• Vertex information
  • The vertex information is contained in the VertexCollChunk. To access this chunk:

    TKey<VertexCollChunk> vtxKey;
THandle<VertexCollChunk> vtxChunk = vtxKey.find(event);
if ( vtxChunk.is_valid() ) { ... }

  • Methods contained in "vertex_evt/VertexInfo.hpp" give access to various types of vertices.
    • For example, get_PrimaryVertex(event, vtx) returns the primary vertex.
  • The Vertex object is defined in "vertexutil/Vertex.hpp"
  • Vertex position is stored as a ColumnVector:

    xyz[0] = vtx->get_vertexpos()(0); // x
xyz[1] = vtx->get_vertexpos()(1); // y
xyz[2] = vtx->get_vertexpos()(2); // z

• Calorimeter information
  • The calorimeter information is contained in the CalDataChunk. To access this chunk:

    TKey<CalDataChunk> calKey;
THandle<CalDataChunk> calChunk = calKey.find(event);
if ( calChunk.is_valid() ) { ... }

  • To get information about the calorimeter cells (CalCell), you should use the getCalData method of the CalDataChunk. This method requires you to supply the position of the vertex, so it can calculate pt, etc for the cells. You can use the primary vertex position (see above), (0,0,0) or some other position:

    const CalData* calDataPtr = calChunk->getCalData(xyz);
if ( calDataPtr != NULL ) {

list <const CalCell*> cellList;
calDataPtr->getCells(cellList);


  • You can get kinematic and geometric information from a calorimeter cell:

    list <const CalCell*>::iterator cellItr;
for (cellItr = cellList.begin(); cellItr != cellList.end(); cellItr++) {
const CalCell* cellPtr = *cellItr;
if ( cellPtr != NULL ) {
float e = cellPtr->E();
float pt = cellPtr->pT();
float phi = cellPtr->phi();
float eta = cellPtr->eta();
CellAddress address = cellPtr->address();
int ieta = address.ieta();
int iphi = address.iphi();
int ilyr = address.layer();
...

• MET information
  • The missing ET information is contained in the MissingETChunk. To access this chunk:

    TKey<MissingETChunk> metKey;
THandle<MissingETChunk> metChunk = metKey.find(event);
if ( metChunk.is_valid() ) { ... }

  • To get at the MissingET object, use

    MissingET* metPtr = metChunk.ptr()->getMissingET();

  • Example of using this object:

    if ( metPtr != NULL ) {
float mex;
float mey;
metPtr->getMExy(mex, mey);
float met = metPtr->getMET();
...

This page maintained by Harry Melanson