Generator

Event generation

One of the main ingredients to simulate the physic performance of PHOS is the generation of a realistic background in central Pb+Pb collisions at LHC energies.

However, most of ALICE subdetectors are sensitive to charged particles and, in consequence, the charged hadronic background is the most important one (see for instance AliGenHIJINGPara class).
However, PHOS detector is sensitive to most of particles: electrons, muon, photons, charged and neutral hadrons. In particular the main background is generated by the electromagnetic decay of the neutral pion, which was omitted in the background generators classes defined in aliroot package 3.01.

Within this respect, we have defined a new class: AliGenPHOSlib class, and adapted the AliGenParam class class in order to be able to simulate the full neutral and charged background which is present in heavy-ion collisions. All these stuff has been included in the aliroot version 3.02.

Enclosed, an exemple of the aliroot config file is reported. This neutral and charged background contains pi+, pi-,pi0,K+,K-,kshort,Klong,eta,omega,p. p bar, n and n bar with relative yields given by hijing in Pb+Pb central collisions (results from venus or shaker are close to the hijing ones). Pion transverse momentum (Pt) distribution is taken from AliGenMUONlib class, version 3.01 of aliroot: Pt Parameterization CDF (PRL 61(88) 1819): Power law for Pt > 500 MeV and Mt scaling below (slope T=160 MeV). Meson Pt distributions are calculated by mt-scaling with pions (see AliGenMUONlib or AliGenPHOSlib class).

You will notice that, in the case of baryons, the mt-scaling is not the best choice. For instance, the slope of protons and antiprotons is around 300 MeV at SPS energies (Pb+Pb @ 158A GeV) and the mt slope of negative hadrons (mainly negative pions) is 190 MeV (NA49 coll. Nucl.Phys. A610 (1996) 188c ). In consequence, that relative ratio baryon/pion noticeable increases a funciton of the transverse mass. This effect is not included in the present event generator...

           // The following Cocktail generator is defined to simulate the neutral and
           // charged background in the ALICE detector. This background is important 
           // in the case of photon detector as PHOS. We simulated a cocktail of 
           // pions (pi+, pi- and pi0) , kaons (K+, K-, Kshort and Klong), eta mesons, 
           // omega mesons and main baryons (protons, antiprotons, neutrons and
           // antineutrons) 
           //
           // 1-Nov-1999 Gines MARTINEZ, GPS @ SUBATECH, Nantes, France  
           //
           Int_t ParticleDensity =  1100;    // Number of particles created in the selected rapidity range
           Int_t NumberOfPions   =  0.77 * ParticleDensity ;  // ~77% of pions: pi^+, pi^- and pi^0 with equal probability 
           Int_t NumberOfKaons   =  0.11 * ParticleDensity ;  // ~11% of kaons: K^+, K^-, K_short and K_long with equal probability
           Int_t NumberOfEtas    =  0.04 * ParticleDensity ;  // ~4.0% of eta mesons
           Int_t NumberOfOmegas  =  0.01 * ParticleDensity ;  // ~1.0% of omega mesons
           Int_t NumberOfBaryons =  0.07 * ParticleDensity ;  // ~7.0% of baryons: p, pbar, n and nbar with equal probability

           AliGenCocktail *gener = new AliGenCocktail();                                // Cocktail class
             gener->SetPtRange(.1,5.);       // Transverse momentum range   
             gener->SetPhiRange(220.,320.);    // 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

           // P  I  O  N  S
               AliGenParam *generpion = new AliGenParam(NumberOfPions,Pion);
//                            AliGenPHOSlib::GetPt(Pion), AliGenPHOSlib::GetY(Pion), AliGenPHOSlib::GetIp(Pion)  );
               generpion->SetWeighting(non_analog);   // Selecting the Pt distribution provided by AliGenPHOSlib::GetPt
               generpion->SetForceDecay(nodecay);     // Requiring that the generated particle is directly tracked by GEANT
           //
           //  K  A  O  N  S
              AliGenParam *generkaon = new AliGenParam(NumberOfKaons,Kaon); 
//                           AliGenPHOSlib::GetPt(Kaon), AliGenPHOSlib::GetY(Kaon), AliGenPHOSlib::GetIp(Kaon)   );
               generkaon->SetWeighting(non_analog);   // Selecting the Pt distribution provided by AliGenPHOSlib::GetPt
               generkaon->SetForceDecay(nodecay);     // Requiring that the generated particle is directly tracked by GEANT
           //
           // E  T  A  S
               AliGenParam *genereta = new AliGenParam(NumberOfEtas,Eta);            
//                            AliGenPHOSlib::GetPt(Eta), AliGenPHOSlib::GetY(Eta), AliGenPHOSlib::GetIp(Eta) );
               genereta->SetWeighting(non_analog);   // Selecting the Pt distribution provided by AliGenPHOSlib::GetPt
               genereta->SetForceDecay(nodecay);     // Requiring that the generated particle is directly tracked by GEANT
           //
           // O M E G A S
               AliGenParam *generomega = new AliGenParam(NumberOfOmegas,Omega);            
//                            AliGenPHOSlib::GetPt(Omega), AliGenPHOSlib::GetY(Omega), AliGenPHOSlib::GetIp(Omega) );
               generomega->SetWeighting(non_analog);   // Selecting the Pt distribution provided by AliGenPHOSlib::GetPt
               generomega->SetForceDecay(nodecay);    // Requiring that the generated particle is directly tracked by GEANT
           //
           // B A R Y O N S

               AliGenParam *generbaryon = new AliGenParam(NumberOfBaryons,Baryon);            
//                            AliGenPHOSlib::GetPt(Baryon), AliGenPHOSlib::GetY(Baryon), AliGenPHOSlib::GetIp(Baryon) );
               generbaryon->SetWeighting(non_analog);   // Selecting the Pt distribution provided by AliGenPHOSlib::GetPt
               generbaryon->SetForceDecay(nodecay);    // Requiring that the generated particle is directly tracked by GEANT
               
            
           //               
             gener->AddGenerator(generpion,"pion",1.);
             gener->AddGenerator(generkaon,"kaon",1.);
             gener->AddGenerator(genereta,"eta",1.);
             gener->AddGenerator(generomega,"omega",1.);
             gener->AddGenerator(generbaryon,"baryon",1.);
             gener->Init();