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. *
7 * Permission to use, copy, modify and distribute this software and its *
8 * documentation strictly for non-commercial purposes is hereby granted *
9 * without fee, provided that the above copyright notice appears in all *
10 * copies and that both the copyright notice and this permission notice *
11 * appear in the supporting documentation. The authors make no claims *
12 * about the suitability of this software for any purpose. It is *
13 * provided "as is" without express or implied warranty. *
14 **************************************************************************/
19 // Generator for slow nucleons in pA interactions.
20 // Source is modelled by a relativistic Maxwell distributions.
21 // This class cooparates with AliCollisionGeometry if used inside AliGenCocktail.
22 // In this case the number of slow nucleons is determined from the number of wounded nuclei
23 // using a realisation of AliSlowNucleonModel.
24 // Original code by Ferenc Sikler <sikler@rmki.kfki.hu>
27 #include <TDatabasePDG.h>
34 #include "AliCollisionGeometry.h"
35 #include "AliGenSlowNucleons.h"
36 #include "AliSlowNucleonModel.h"
38 ClassImp(AliGenSlowNucleons)
41 AliGenSlowNucleons::AliGenSlowNucleons()
67 // Default constructor
68 fCollisionGeometry = 0;
71 AliGenSlowNucleons::AliGenSlowNucleons(Int_t npart)
95 fSlowNucleonModel(new AliSlowNucleonModel())
98 fName = "SlowNucleons";
99 fTitle = "Generator for gray particles in pA collisions";
100 fCollisionGeometry = 0;
103 //____________________________________________________________
104 AliGenSlowNucleons::~AliGenSlowNucleons()
107 delete fSlowNucleonModel;
110 void AliGenSlowNucleons::SetProtonDirection(Float_t dir) {
111 // Set direction of the proton to change between pA (1) and Ap (-1)
112 fProtonDirection = dir / TMath::Abs(dir);
115 void AliGenSlowNucleons::Init()
120 Float_t kMass = TDatabasePDG::Instance()->GetParticle(kProton)->Mass();
121 fMomentum = fCMS/2. * fZTarget / fATarget;
122 fBeta = fMomentum / TMath::Sqrt(kMass * kMass + fMomentum * fMomentum);
124 fDebugHist1 = new TH2F("DebugHist1", "nu vs N_slow", 100, 0., 100., 20, 0., 20.);
125 fDebugHist2 = new TH2F("DebugHist2", "b vs N_slow", 100, 0., 100., 15, 0., 15.);
126 fCosThetaGrayHist = new TH1F("fCosThetaGrayHist", "Gray particles angle", 100, -1., 1.);
129 // non-uniform cos(theta) distribution
131 if(fThetaDistribution != 0) {
132 fCosTheta = new TF1("fCosTheta",
133 "(2./3.14159265358979312)/(exp(2./3.14159265358979312)-exp(-2./3.14159265358979312))*exp(2.*x/3.14159265358979312)",
138 void AliGenSlowNucleons::FinishRun()
141 // Show histogram for debugging if requested.
143 TCanvas *c = new TCanvas("c","Canvas 1",400,10,600,700);
150 fCosThetaGrayHist->Draw();
155 void AliGenSlowNucleons::Generate()
158 // Generate one event
161 // Communication with Gray Particle Model
163 if (fCollisionGeometry) {
164 Float_t b = fCollisionGeometry->ImpactParameter();
165 Int_t nn = fCollisionGeometry->NN();
166 Int_t nwn = fCollisionGeometry->NwN();
167 Int_t nnw = fCollisionGeometry->NNw();
168 Int_t nwnw = fCollisionGeometry->NwNw();
170 fSlowNucleonModel->GetNumberOfSlowNucleons(fCollisionGeometry, fNgp, fNgn, fNbp, fNbn);
172 printf("Nucleons %d %d %d %d \n", fNgp, fNgn, fNbp, fNbn);
173 fDebugHist1->Fill(Float_t(fNgp + fNgn + fNbp + fNbn), fCollisionGeometry->NwN(), 1.);
174 fDebugHist2->Fill(Float_t(fNgp + fNgn + fNbp + fNbn), b, 1.);
175 printf("AliGenSlowNucleons: Impact parameter from Collision Geometry %f %d %d %d %d\n",
176 b, nn, nwn, nnw, nwnw);
181 Float_t p[3], theta=0;
182 Float_t origin[3] = {0., 0., 0.};
183 Float_t polar [3] = {0., 0., 0.};
187 if(fVertexSmear == kPerEvent) {
189 for (j=0; j < 3; j++) origin[j] = fVertex[j];
196 for(i = 0; i < fNgp; i++) {
197 GenerateSlow(fCharge, fTemperatureG, fBetaSourceG, p, theta);
198 if (fDebug) fCosThetaGrayHist->Fill(TMath::Cos(theta));
199 PushTrack(fTrackIt, -1, kf, p, origin, polar,
200 0., kPNoProcess, nt, 1.);
208 for(i = 0; i < fNgn; i++) {
209 GenerateSlow(fCharge, fTemperatureG, fBetaSourceG, p, theta);
210 if (fDebug) fCosThetaGrayHist->Fill(TMath::Cos(theta));
211 PushTrack(fTrackIt, -1, kf, p, origin, polar,
212 0., kPNoProcess, nt, 1.);
220 for(i = 0; i < fNbp; i++) {
221 GenerateSlow(fCharge, fTemperatureB, fBetaSourceB, p, theta);
222 PushTrack(fTrackIt, -1, kf, p, origin, polar,
223 0., kPNoProcess, nt, 1.);
231 for(i = 0; i < fNbn; i++) {
232 GenerateSlow(fCharge, fTemperatureB, fBetaSourceB, p, theta);
233 PushTrack(fTrackIt, -1, kf, p, origin, polar,
234 0., kPNoProcess, nt, 1.);
242 void AliGenSlowNucleons::GenerateSlow(Int_t charge, Double_t T,
243 Double_t beta, Float_t* q, Float_t &theta)
247 Emit a slow nucleon with "temperature" T [GeV],
248 from a source moving with velocity beta
249 Three-momentum [GeV/c] is given back in q[3]
252 Double_t m, pmax, p, f, phi;
253 TDatabasePDG * pdg = TDatabasePDG::Instance();
254 const Double_t kMassProton = pdg->GetParticle(kProton) ->Mass();
255 const Double_t kMassNeutron = pdg->GetParticle(kNeutron)->Mass();
257 /* Select nucleon type */
258 if(charge == 0) m = kMassNeutron;
259 else m = kMassProton;
261 /* Momentum at maximum of Maxwell-distribution */
263 pmax = TMath::Sqrt(2*T*(T+sqrt(T*T+m*m)));
265 /* Try until proper momentum */
266 /* for lack of primitive function of the Maxwell-distribution */
267 /* a brute force trial-accept loop, normalized at pmax */
272 f = Maxwell(m, p, T) / Maxwell(m , pmax, T);
276 /* Spherical symmetric emission for black particles (beta=0)*/
277 if(beta==0 || fThetaDistribution==0) theta = TMath::ACos(2. * Rndm() - 1.);
278 /* cos theta distributed according to experimental results for gray particles (beta=0.05)*/
279 else if(fThetaDistribution!=0){
280 Double_t costheta = fCosTheta->GetRandom();
281 theta = TMath::ACos(costheta);
284 phi = 2. * TMath::Pi() * Rndm();
286 /* Determine momentum components in system of the moving source */
287 q[0] = p * TMath::Sin(theta) * TMath::Cos(phi);
288 q[1] = p * TMath::Sin(theta) * TMath::Sin(phi);
289 q[2] = p * TMath::Cos(theta);
291 /* Transform to system of the target nucleus */
292 /* beta is passed as negative, because the gray nucleons are slowed down */
293 Lorentz(m, -beta, q);
295 /* Transform to laboratory system */
296 Lorentz(m, fBeta, q);
297 q[2] *= fProtonDirection;
300 Double_t AliGenSlowNucleons::Maxwell(Double_t m, Double_t p, Double_t T)
302 /* Relativistic Maxwell-distribution */
304 ekin = TMath::Sqrt(p*p+m*m)-m;
305 return (p*p * exp(-ekin/T));
309 void AliGenSlowNucleons::Lorentz(Double_t m, Double_t beta, Float_t* q)
311 /* Lorentz transform in the direction of q[2] */
313 Double_t gamma = 1/sqrt(1-beta*beta);
314 Double_t energy = sqrt(m*m + q[0]*q[0] + q[1]*q[1] + q[2]*q[2]);
315 q[2] = gamma * (q[2] + beta*energy);