Gabriele Cosmo
234 lines
7.9 KiB
C++
234 lines
7.9 KiB
C++
//
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// ********************************************************************
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// * License and Disclaimer *
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// * *
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// * The Geant4 software is copyright of the Copyright Holders of *
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// * the Geant4 Collaboration. It is provided under the terms and *
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// * conditions of the Geant4 Software License, included in the file *
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// * LICENSE and available at http://cern.ch/geant4/license . These *
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// * include a list of copyright holders. *
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// * *
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// * Neither the authors of this software system, nor their employing *
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// * institutes,nor the agencies providing financial support for this *
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// * work make any representation or warranty, express or implied, *
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// * regarding this software system or assume any liability for its *
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// * use. Please see the license in the file LICENSE and URL above *
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// * for the full disclaimer and the limitation of liability. *
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// * *
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// * This code implementation is the result of the scientific and *
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// * technical work of the GEANT4 collaboration. *
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// * By using, copying, modifying or distributing the software (or *
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// * any work based on the software) you agree to acknowledge its *
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// * use in resulting scientific publications, and indicate your *
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// * acceptance of all terms of the Geant4 Software license. *
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// ********************************************************************
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//
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//
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// Multibody "phase space" generator using GENBOD (CERNLIB W515) method.
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//
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// Author: Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>
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#include "G4HadPhaseSpaceGenbod.hh"
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#include "G4LorentzVector.hh"
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#include "G4PhysicalConstants.hh"
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#include "G4ThreeVector.hh"
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#include "Randomize.hh"
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#include <algorithm>
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#include <functional>
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#include <iterator>
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#include <numeric>
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#include <vector>
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namespace {
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// Wrap #define in a true function, for passing to std::fill
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G4double uniformRand() { return G4UniformRand(); }
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}
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// Constructor initializes everything to zero
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G4HadPhaseSpaceGenbod::G4HadPhaseSpaceGenbod(G4int verbose)
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: G4VHadPhaseSpaceAlgorithm("G4HadPhaseSpaceGenbod",verbose),
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nFinal(0), totalMass(0.), massExcess(0.), weightMax(0.), nTrials(0) {;}
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// C++ re-implementation of GENBOD.F (Raubold-Lynch method)
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void G4HadPhaseSpaceGenbod::
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GenerateMultiBody(G4double initialMass,
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const std::vector<G4double>& masses,
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std::vector<G4LorentzVector>& finalState) {
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if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;
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finalState.clear();
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Initialize(initialMass, masses);
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const G4int maxNumberOfLoops = 10000;
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nTrials = 0;
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do { // Apply accept/reject to get distribution
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++nTrials;
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FillRandomBuffer();
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FillEnergySteps(initialMass, masses);
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} while ( (!AcceptEvent()) && nTrials < maxNumberOfLoops ); /* Loop checking, 02.11.2015, A.Ribon */
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if ( nTrials >= maxNumberOfLoops ) {
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G4ExceptionDescription ed;
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ed << " Failed sampling after maxNumberOfLoops attempts : forced exit" << G4endl;
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G4Exception( " G4HadPhaseSpaceGenbod::GenerateMultiBody ", "HAD_GENBOD_001", FatalException, ed );
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}
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GenerateMomenta(masses, finalState);
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}
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void G4HadPhaseSpaceGenbod::
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Initialize(G4double initialMass, const std::vector<G4double>& masses) {
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if (GetVerboseLevel()>1) G4cout << GetName() << "::Initialize" << G4endl;
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nFinal = masses.size();
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msum.resize(nFinal, 0.); // Initialize buffers for filling
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msq.resize(nFinal, 0.);
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std::partial_sum(masses.begin(), masses.end(), msum.begin());
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std::transform(masses.begin(), masses.end(), masses.begin(), msq.begin(),
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std::multiplies<G4double>());
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totalMass = msum.back();
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massExcess = initialMass - totalMass;
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if (GetVerboseLevel()>2) {
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PrintVector(msum, "msum", G4cout);
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PrintVector(msq, "msq", G4cout);
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G4cout << " totalMass " << totalMass << " massExcess " << massExcess
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<< G4endl;
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}
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ComputeWeightScale(masses);
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}
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// Generate ordered list of random numbers
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void G4HadPhaseSpaceGenbod::FillRandomBuffer() {
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if (GetVerboseLevel()>1) G4cout << GetName() << "::FillRandomBuffer" << G4endl;
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rndm.resize(nFinal-2,0.); // Final states generated in sorted order
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std::generate(rndm.begin(), rndm.end(), uniformRand);
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std::sort(rndm.begin(), rndm.end());
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if (GetVerboseLevel()>2) PrintVector(rndm, "rndm", G4cout);
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}
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// Final state effective masses, min to max
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void
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G4HadPhaseSpaceGenbod::FillEnergySteps(G4double initialMass,
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const std::vector<G4double>& masses) {
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if (GetVerboseLevel()>1) G4cout << GetName() << "::FillEnergySteps" << G4endl;
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meff.clear();
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pd.clear();
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meff.push_back(masses[0]);
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for (size_t i=1; i<nFinal-1; i++) {
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meff.push_back(rndm[i-1]*massExcess + msum[i]);
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pd.push_back(TwoBodyMomentum(meff[i], meff[i-1], masses[i]));
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}
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meff.push_back(initialMass);
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pd.push_back(TwoBodyMomentum(meff[nFinal-1], meff[nFinal-2], masses[nFinal-1]));
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if (GetVerboseLevel()>2) {
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PrintVector(meff,"meff",G4cout);
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PrintVector(pd,"pd",G4cout);
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}
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}
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// Maximum possible weight for final state (used with accept/reject)
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void
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G4HadPhaseSpaceGenbod::ComputeWeightScale(const std::vector<G4double>& masses) {
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if (GetVerboseLevel()>1)
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G4cout << GetName() << "::ComputeWeightScale" << G4endl;
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weightMax = 1.;
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for (size_t i=1; i<nFinal; i++) {
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weightMax *= TwoBodyMomentum(massExcess+msum[i], msum[i-1], masses[i]);
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}
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if (GetVerboseLevel()>2) G4cout << " weightMax = " << weightMax << G4endl;
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}
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// Event weight computed as either constant or Fermi-dependent cross-section
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G4double G4HadPhaseSpaceGenbod::ComputeWeight() const {
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if (GetVerboseLevel()>1) G4cout << GetName() << "::ComputeWeight" << G4endl;
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return (std::accumulate(pd.begin(), pd.end(), 1./weightMax,
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std::multiplies<G4double>()));
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}
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G4bool G4HadPhaseSpaceGenbod::AcceptEvent() const {
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if (GetVerboseLevel()>1)
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G4cout << GetName() << "::AcceptEvent? " << nTrials << G4endl;
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return (G4UniformRand() <= ComputeWeight());
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}
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// Final state momentum vectors in CMS system, using Raubold-Lynch method
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void G4HadPhaseSpaceGenbod::
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GenerateMomenta(const std::vector<G4double>& masses,
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std::vector<G4LorentzVector>& finalState) {
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if (GetVerboseLevel()>1) G4cout << GetName() << "::GenerateMomenta" << G4endl;
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finalState.resize(nFinal); // Preallocate vectors for convenience below
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for (size_t i=0; i<nFinal; i++) {
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AccumulateFinalState(i, masses, finalState);
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if (GetVerboseLevel()>2)
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G4cout << " finalState[" << i << "] " << finalState[i] << G4endl;
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}
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}
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// Process final state daughters up to current index
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void G4HadPhaseSpaceGenbod::
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AccumulateFinalState(size_t i,
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const std::vector<G4double>& masses,
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std::vector<G4LorentzVector>& finalState) {
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if (GetVerboseLevel()>2)
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G4cout << GetName() << "::AccumulateFinalState " << i << G4endl;
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if (i==0) { // First final state particle left alone
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finalState[i].setVectM(G4ThreeVector(0.,pd[i],0.),masses[i]);
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return;
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}
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finalState[i].setVectM(G4ThreeVector(0.,-pd[i-1],0.),masses[i]);
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G4double phi = G4UniformRand() * twopi;
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G4double theta = std::acos(2.*G4UniformRand() - 1.);
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if (GetVerboseLevel() > 2) {
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G4cout << " initialized Py " << -pd[i-1] << " phi " << phi
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<< " theta " << theta << G4endl;
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}
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G4double esys=0.,beta=0.,gamma=1.;
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if (i < nFinal-1) { // Do not boost final particle
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esys = std::sqrt(pd[i]*pd[i]+meff[i]*meff[i]);
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beta = pd[i] / esys;
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gamma = esys / meff[i];
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if (GetVerboseLevel()>2)
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G4cout << " esys " << esys << " beta " << beta << " gamma " << gamma
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<< G4endl;
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}
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for (size_t j=0; j<=i; j++) { // Accumulate rotations
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finalState[j].rotateZ(theta).rotateY(phi);
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finalState[j].setY(gamma*(finalState[j].y() + beta*finalState[j].e()));
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if (GetVerboseLevel()>2) G4cout << " j " << j << " " << finalState[j] << G4endl;
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}
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}
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