1 /**************************************************************************
2 * Copyright(c) 1998-1999, ALICE Experiment at CERN, All rights reserved. *
4 * Author: The ALICE Off-line Project. *
5 * Contributors are mentioned in the code where appropriate. *
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8 * documentation strictly for non-commercial purposes is hereby granted *
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11 * appear in the supporting documentation. The authors make no claims *
12 * about the suitability of this software for any purpose. It is *
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14 **************************************************************************/
21 Generator for gray nucluons in pA interactions.
22 Source is modelled by a relativistic Maxwell distributions.
23 Original code by Ferenc Sikler <sikler@rmki.kfki.hu>
25 #include "AliGenGrayParticles.h"
27 #include <TDatabasePDG.h>
29 ClassImp(AliGenGrayParticles)
31 AliGenGrayParticles::AliGenGrayParticles():AliGenerator(-1)
33 // Default constructor
36 AliGenGrayParticles::AliGenGrayParticles(Int_t npart)
40 fName = "GrayParticles";
41 fTitle = "Generator for gray particles in pA collisions";
44 SetNominalCmsEnergy();
48 //____________________________________________________________
49 AliGenGrayParticles::~AliGenGrayParticles()
55 void AliGenGrayParticles::Init()
60 Float_t kMass = TDatabasePDG::Instance()->GetParticle(kProton)->Mass();
61 fMomentum = fCMS/2. * fZTarget / fATarget;
62 fBeta = fMomentum / TMath::Sqrt(kMass * kMass + fMomentum * fMomentum);
66 void AliGenGrayParticles::Generate()
72 Float_t origin[3] = {0., 0., 0.};
73 Float_t polar [3] = {0., 0., 0.};
75 for(i = 0;i < fNpart; i++) {
77 GenerateSlow(1, fTemperature, 0., p);
79 SetTrack(fTrackIt, -1, kf, p, origin, polar,
80 0., kPNoProcess, nt, 1.);
89 void AliGenGrayParticles::GenerateSlow(Int_t charge, Double_t T, Double_t beta, Float_t* q)
91 Emit a slow nucleon with "temperature" T [GeV],
92 from a source moving with velocity beta
93 Three-momentum [GeV/c] is given back in q[3]
97 Double_t m, pmax, p, f, theta, phi;
99 TDatabasePDG * pdg = TDatabasePDG::Instance();
100 const Double_t kMassProton = pdg->GetParticle(kProton) ->Mass();
101 const Double_t kMassNeutron = pdg->GetParticle(kNeutron)->Mass();
103 /* Select nucleon type */
104 if(charge == 0) m = kMassNeutron;
105 else m = kMassProton;
107 /* Momentum at maximum of Maxwell-distribution */
109 pmax = TMath::Sqrt(2*T*(T+sqrt(T*T+m*m)));
111 /* Try until proper momentum */
112 /* for lack of primitive function of the Maxwell-distribution */
113 /* a brute force trial-accept loop, normalized at pmax */
118 f = Maxwell(m, p, T) / Maxwell(m , pmax, T);
122 /* Spherical symmetric emission */
123 theta = TMath::ACos(2. * Rndm() - 1.);
124 phi = 2. * TMath::Pi() * Rndm();
126 /* Determine momentum components in system of the moving source */
127 q[0] = p * TMath::Sin(theta) * TMath::Cos(phi);
128 q[1] = p * TMath::Sin(theta) * TMath::Sin(phi);
129 q[2] = p * TMath::Cos(theta);
131 /* Transform to system of the target nucleus */
132 /* beta is passed as negative, because the gray nucleons are slowed down */
133 Lorentz(m, -beta, q);
135 /* Transform to laboratory system */
136 Lorentz(m, fBeta, q);
139 Double_t AliGenGrayParticles::Maxwell(Double_t m, Double_t p, Double_t T)
141 /* Relativistic Maxwell-distribution */
143 ekin = TMath::Sqrt(p*p+m*m)-m;
144 return (p*p * exp(-ekin/T));
148 void AliGenGrayParticles::Lorentz(Double_t m, Double_t beta, Float_t* q)
150 /* Lorentz transform in the direction of q[2] */
152 Double_t gamma = 1/sqrt(1-beta*beta);
153 Double_t energy = sqrt(m*m + q[0]*q[0] + q[1]*q[1] + q[2]*q[2]);
154 q[2] = gamma * (q[2] + beta*energy);