The e- and e+ identification by using HPidAlgStandCuts algorithm

There are several PID algorithms implemented in the HYDRA package which are used to identify particle species. One of them is HPidAlgStandCuts algorithm which can be used to identify e+/e- from elementary and heavy ion collisions. At the moment this algorithm uses information (observables) provided by RICH (ring properties), MDC (momentum, track length), TOF/TOFino (tof) and PreShower (elctromagnetic shower properties) detectors. The RICH detector plays a crutial role in e+/e- identification in the HADES spectrometer because it is almost blind to hadrons at beam energies of 1-2 AGeV. Thus, each electron candidate must have correlated signal in the RICH detector. It is realised by requiring the angular correlation between RICH and inner MDC segment.


e+/e- signal eveluation in RICH

The RICH ring properties used in HPidAlgStandCuts for e+/e- identification.


e+/e- identification in PreShower

The PreShower observables used in HPidAlgStandCuts for e+/e- identification.

HPidAlgStandCutsPar parameter container

The HPidAlgStandCutsPar is a parameter container which is used by HPidAlgStandCuts reconstructor for e+/e- identification. It derives from HPidHistogramsCont and currently contains 7 parameter sets (histograms) for 7 different e+/e- identification algorithms. These histograms are stored in HPidAlgStandCutsPar in the following way:

HPidAlgStandCutsPar *par = rtdb->getContainer("PidAlgStandCutsPar");
par->addHistogram(particleID, algorithmOffset, sectorNb, pParamHist);

// set parameter context

where particleID is particle ID (Geant convension: 2 - e+, 3 - e- ...) , algorithmOffset is an algorithm offset, sectorNb is a sector number and pParamHist is a pointer to histogram with algorithm parameters. The parameter context defines the track reconstruction algortihm. The algorithmOffset and pParamHist for HPidAlgStandCuts are following:

algorithmOffset pParamHist
0 Rich ring parameters
1 PreShower F_thr vs. momentum parametrization
2 PreShower Sum0_up vs momentum parametrization
3 system0 (momentum vs. beta - lower limits)
4 system0 (momentum vs. beta - upper limits)
5 system1 (momentum vs. beta - lower limits)
6 system1 (momentum vs. beta - upper limits)
7 Pre-Shower SumDiff_thr vs. momentum parametrization

The following steps should be made to generate HPidAlgStandCutsPar parametrs:

  1. Create and fill histograms (sectorwise, particleID) with
    • Rich ring properties (PM, RC, NP, AC)
    • momentum vs beta for the system0 (PreShower polar angles) and for system1 (TOF polar angles)
    • PreShower (F vs momentum, Sum0 vs momentum and  SumDiff vs momentum)
  2. Define and calculate parameter thresholds, upper or lower limits (e.g. Rich minimum bias cuts, momentum vs. beta (3 sigma), F_thr (CL =80%)...)
  3. Parametrize F_thr(momentum), Sum0_up(momentum) and SumDiff_thr(momentum) dependencies by fitting the functions
  4. Store histograms with algorithm parameters in the HPidAlgStandCutsPar as described above

Example of the macros to make these steps are in official HYDRA package in pidutil/tools/HardCutParams

How to run HPidAlgStandCuts algorithm in the HYDRA framework

In order to make PID by using HPidAlgStandCuts algorithm two HYDRA reconstructors HPidReconstructor and HPidParticleFiller must be used. The HPidReconstructor is a master reconstructor which calls algorithms (e.g. HPidAlgStandCuts algorithm) making particle identification (PID) while the HPidParticleFiller selects identified particles and stores them in the output HPidParticle category.

There are following flags implemented in the HPidAlgStandCuts algorithm:

RICHRINGCUTS  - Rich ring properties
TOFPVSBETA    - momentum vs. beta window (TOF)
TOFINOPVSBETA - momentum vs beta lower limit (TOFINO)
SHOWERMAXFVSP - threshold on charge multiplication factor max(Sum1/Sum0,Sum2/Sum0) in PreShower 
SHOWERSUM0VSP - upper limit on charge Sum0 in the first chamber of PreShower
SHOWERSUMDIFFVSP - threshold on charge difference Sum1+Sum2-Sum0 in PreShower

User can select conditions which should be used by HPidAlgStandCuts for PID. By default the "RICHRINGCUTS,TOFPVSBETA,TOFINOPVSBETA,SHOWERMAXFVSP,SHOWERSUM0VSP" flags are used in HPidAlgStandCuts algorithm.

The HPidAlgStandCuts was designed in such a way that it returns 0 (is not lepton) or 1 (is lepton) for the selected track/momentum reconstruction algorithm which has to be specified by user in the HPidReconstructor constructor. The identification of other paricles pions/protons/deuterons is not done by this algorithm and decision is not taken.

To run PID by using HPidAlgStandCuts algorithm you need the HPidTrackCand or HPidTrackCandSim category in the input DST file and the set of parameters HPidAlgStandCutsPar which are initialised from ORACLE. They can be different for different track/momentum reconstruction algorithms (KICK,SPLINE, RUNGEKUTTA) and must be stored in ORACLE with suitable context.

The HPidAlgStandCuts algorithm creates the output control ntuple (optional) which contains useful information about standard cuts performance.

The HPidParticleFiller task rejects particles for which HPidAlgStandCuts returns 0. It is done for selected track/momentum reconstruction algorithm which must be set by user. Only tracks reconstructed by this algorithm are stored in the output HPidParticle data category.

User must select the same track/momentum reconstruction algorithm for both HPidReconstructor and HPidParticleFiller reconstructors.

In orer to use the HPidAlgStandCuts algorithm (experiment) in the HYDRA framework the following lines of code have to be added to the analysis program:

// create HPidReconstructor reconstructor (must be added later to task set) 
  HPidReconstructor *pPidRec=new HPidReconstructor("pdf,ALG_RUNGEKUTTA")
  Short_t nParticles[2]={2,3}; // only lepton ID

// create HPidAlgStandCuts algorithm and add it to HPidReconstructor (default cuts)
  HPidAlgStandCuts *pPidAlgStandCuts=new HPidAlgStandCuts("nameOutputNtuple.root");

// create HPidParticleFiller (must be added later to the task list)
  HPidParticleFiller *PartFiller=new HPidParticleFiller("RUNGEKUTTA");
  pPartFiller->setAlgorithm(7); // 7-check values from HPidAlgStandCuts algorithm

// add user reconstructors/tasks to the master task set
  HTaskSet *pTaskSet=gHades->getTaskSet("real");

Example of programs

executable program to run PID by using HPidAlgStanCuts algorithm

Exercises (HPidAlgStandCutsPar paramter container)

  1. Initialise HPidAlgStandCutsPar parameter container from ORACLE and store it in ROOT file (runID=7006)
    • Set your HYDRA environement by using
    • Copy /d/hades/jacek/hydra-8.13/tutorials/initParameters/initPidFromOra.C macro to your working directory
    • Open the macro and identify: paramater container, runId and output file
    • Run the macro (root -l initPidFromOra.C)
  2. Draw parameter histograms stored in HPidAlgStandCutsPar
    • Open the output file from produced by initPidFromOra.C
    • Draw parameter histograms by using PidAlgStandCutsPar->getHistogram(particleID,algorithmOffset,sectorNb)->Draw() function

Exercises (control PID ntuple with C+C@2GeV events)

  1. Open wiki page with ntuple variable description (
  2. Make q*p vs. cut number plots
    • Open /d/hades06/data/sudol/nov02/gen4/leptons/new/leptons_gen4.root root ntuple
    • Create and draw q*p histograms vs. cuts numbers:
      • pLep->Draw("charge*mom>>h0(201,-1000,1000)")
      • pLep->Draw("charge*mom>>h1(201,-1000,1000)","isrich")
      • pLep->Draw("charge*mom>>h2(201,-1000,1000)","isrich&&(istof||istofino)")
      • pLep->Draw("charge*mom>>h3(201,-1000,1000)","isrich&&(istof||istofino&&isshower)")
    • Draw all histograms in one ROOT canvas (e.g. h0->Draw(); h1->Draw("same");)
  3. Determine purity and efficiency of e+/e- standard cuts reconstruction algorithm
    • Open /d/hades06/data/sudol/nov02/gen4/urqmd/leptons_urqmd_gen4.root simulation ntuple
    • Draw gcomtrk ntuple variable pLep->Draw("gcomtrk")
                //piddef.h file
                //Enumerated variables indicating which detector has seen a geant particle
                //and combinations thereof (to be used as shorthand-notation). These are gcomtrk values
                enum enumDetBits
                  //individual detectors
                  kTrackNotSet         = 0x00,  // This object does not contain valid data
                  kSeenInRICHIPU       = 0x01,  // RICH IPU has seen this trackid
                  kSeenInOuterMDC      = 0x02,  // Outer Mdc has seen this trackid
                  kSeenInRICH          = 0x04,  // RICH has seen this trackid
                  kSeenInInnerMDC      = 0x08,  // Inner Mdc has seen this trackid
                  kSeenInTOF           = 0x10,  // TOF has seen this trackid
                  kSeenInShower        = 0x20,  // Shower has seen this trackid
                  kSeenInMETA          = 0x40,  // One of the Meta dets has seen this trackid
    • Create and draw q*p histograms before cuts (only pure tracks)
      • pLep->Draw("charge*mom>>h0(201,-1000,1000)","gcomtrk>74")
    • Create and draw q*p histograms after all cuts (only pure tracks)
      • pLep->Draw("charge*mom>>h1(201,-1000,1000)","isrich&&(istof||istofino&&isshower)&&gcomtrk>74")
    • Create and draw q*p histograms after all cuts
      • pLep->Draw("charge*mom>>h2(201,-1000,1000)","isrich&&(istof||istofino&&isshower)")
    • Efficiency is difined as (Eff = h1/h0) while Purity is defined as (Pur = h1/h2)
      • Use ROOT Clone() function to create clone of h0, h1 and h2 histograms (e.g. TH1F* h0_clone=h0->Clone())
      • Use ROOT Sumw2() function for h1 and h1_clone histograms to propagate properly errors (e.g. h1->Sumw2())
      • Calculate efficiency (h1->Divide(h0_clone)) and purity (h1_clone->Divide(h2_clone))
      • Draw efficiency (h1->Draw()) and purity (h1_clone->Draw())

-- JacekOtwinowski - 25 Jun 2007
Topic revision: r14 - 2007-07-03, JacekOtwinowski
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