Geometry & Materials

Simulation

1. select a file name for the root output:

   	//=======================================================================
   	//  Create the output file
   	sprintf(filename,"myfilename.root");
   	

2. Define your event generator and simulations parameters:

            AliGenCocktail *gener = new AliGenCocktail(); // Cocktail class
            gener->SetPtRange(.02,10.00);                 // Transverse momentum range   
            gener->SetPhiRange(180.,360.);                // Azimuthal angle range  
            gener->SetYRange(-0.25,0.25);                 // Pseudorapidity range
            gener->SetOrigin(0,0,0);                      // Vertex position
            gener->SetSigma(0,0,5.6);                     // Sigma in (X,Y,Z) (cm) on IP position
            gener->Init();                                // Initialize the generator
   	

3. Select the detector seen by the tracking:

            Int_t iPHOS=1;    // PHOS is in
            Int_t iPMD=0;     // PMD  is out
   	

4. Create a PHOS object:

             Int_t iPHOS=1;
             if(iPHOS) {
               AliPHOS * phos = new AliPHOSv1("PHOS", char * title) ;
             }
           

   Its second parameter title defines the configuration of the charged-particle detector in front of the electromagnetic calorimeter. The title "GPS2" stands for the PHOS PreShoweris Detector (PPDS) by the Subatech IN2P3, Nantes, while the title "IHEP" defines the Charged Particle Veto (CPV) detector by IHEP, Protvino:

    
             if(iPHOS) {
               AliPHOS *phos = new AliPHOSv1("PHOS","GPS2");
             }
   	

   or

    
             if(iPHOS) {
               AliPHOS *phos = new AliPHOSv1("PHOS","IHEP");
             }
   	

You van now run the simulation by entering the following command at the aliroot prompt:

       aliroot> gAlice->SetConfigFunction("Config(11,0);"); // 11 = particle PDG number, 0 = absorber thickness    
       aliroot> gAlice->Run(1,"PHOSConfig_RealParticle.C"); 
    

Geometry

Definition

Four versions of the PHOS detectors have been defined:

1. AliPHOSv0 ( inheritance tree ) describes the standart geometry. Hits are NOT saved on the hit tree (TREEH) and digits are NOT saved on the digit tree (TREED). To be used only if PHOS is to be considered as a silent detector (tracking is done but no output);
2. AliPHOSv1 same geometry as AliPHOSv0 but digits are saved on the digit tree (TREED). In addition, for the option IHEP hits are saved for the CPV detector ;
3. AliPHOSv2 same geometry as AliPHOSv0 but hits are saved on the hit tree (TREEH) and digits on the digit tree (TREED);
4. AliPHOSv3 same geometry as AliPHOSv0 but the response of the PIN diode has been added to the digit; cumulative hits and digits are saved and;
5. AliPHOSv4 only outlines the geometry of AliPHOSv0 and does no tracking inside PHOS, a parametrized response function is used instead.

The parameters of the geometry are given in class AliPHOSGeometry ( inheritance tree )

Figure 1.a: GEANT Tree which describes the EMC-PHOS geometry

Figure 1.b: GEANT Tree which describes the PPSD-PHOS geometry

Implementation

Figure 2.: Class diagram for the geometry/simulation package

AliPHOS: This is the base class. It derives from AliDetector. It's only purpose is to describe the materials (à la AliMC) needed for the EMCA and PPSD construction.

AliPHOSv0: It derives from AliPHOS.

• It sets up the geometry for the Root display (AliPHOSv0::BuildGeometry()) and for the GEANT tracking (AliPHOSv0::CreateGeometry()).
• It produces no output although particles are transported through PHOS

AliPHOSv1: It derives from AliPHOSv0.

• It watches the tracks passing through the active media of EMCA and PPSD (AliPHOSv1::StepManager()).
• It stores the cumulative hits (AliPHOSv1::AddHit()), using the AliPHOSHit class.
• Besides, in the IHEP option, CPV hits are stored into the tree TREEH in branches CPV1 - CPV5, one branch per CPV module.

AliPHOSv2: It derives from AliPHOSv0.

• It watches the tracks passing through the active media of EMCA and PPSD (AliPHOSv1::StepManager()).
• It stores the hits (AliPHOSv2::AddHit()), using the AliPHOSHit class.

AliPHOSv3 : derives from AliPHOSv0 and includes the light transport in the crystal and the response of the PIN photodiode à la O. H. Oddland.

AliPHOSv4 : used for fast simulation.

AliPHOSGeometry : It derives from TObject to make it persistent. It is a singleton, i.e., a pointer to the unique instance of this class is obtained by:

        AliPHOSGeometry * Geom = AliPHOSGeometry::GetInstance()
      

• It sets the various parameters for the geometry description and provides the method to access all the parameters. The geometry parameters are split into 3 objects: EMCA geometry (class AliPHOSEMCAGeometry), PPSD or CPV geometry (classes AliPHOSPPSDGeometry or AliPHOSCPVGeometry derived from AliPHOSCPVBaseGeometry) and PHOS support geometry (class AliPHOSSupportGeometry). To avoid cumbersome macros at run time these parameters can only be changed manually in the source code.
• It provides the method to convert the absolute detector Id (crystal in EMCA or pad in PPSD/CPV) into a relative Id : PHOS module number, PPSD/CPV module number, row, column (AliPHOSGeometry::AbsToRelNumbering()) and the reverse operation (AliPHOSGeometry::RelToAbsNumbering()).
• It provides the method to convert an absolute Id into a three-vector giving the position of the detector in ALICE (AliPHOSGeometry::RelPosInAlice()).

AliPHOSHit: It derives from AliHit. It stores the hits as the pair (absolute Id, deposited energy). The hits presently are stored in the hit TTree as one hit per elementary cell (EMC crystal or PPSD gas cell) (See the the AddHit() method in AliPHOSv0).

Hits

During the simulation phase, the hits are stored into the root file. Now there 2 kind of hits in PHOS.

• Cumulative hits. These hits are of the class AliPHOSHit stored into the array AliPHOSv1::fHits which is written to the branch PHOSCH of the tree TreeH. These hits are accumulated for each primary track in each cell of PHOS (crystal of EMCA, gas cell of PPSD or pad of CPV). Here "accumulated" means that these hits contain the sum of amplitudes over all steps originated by the same primary track in the same cell.
• "Exact" hits. These hits are of the class AliPHOSCPVHit stored into the arrays per PHOS module which are written to the separate branches EMC1-EMC5 and CPV1-CPV5 of the tree TreeH. These hits contain exact information about the position, 4-momentum and particle type of the track which enters EMCA of CPV from the side of the interaction point. These hits can be used to study the the true occupancy of the detectors. Comparison of the resonstructed points with these "exact" hits gives the coordinate and energy resolutions.

Collection of pictures

Figure 3a.: ROOT Display of ALICE: All Views

Figure 3b.: ROOT Display of ALICE: Front Views

Figure 3c.: ROOT Display of ALICE: All View3

Figure 4.a: ROOT Display of ALICE: zoom on PPSD, Front Views

Figure 4.b: ROOT Display of ALICE: zoom on PPSD, Perspective View

Result