SKKinematicsGenerator.cxx

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//____________________________________________________________________________
/*
 Copyright (c) 2003-2024, The GENIE Collaboration
 For the full text of the license visit http://copyright.genie-mc.org

 Chris Marshall <marshall \at pas.rochester.edu>
 University of Rochester

 Martti Nirkko
 University of Berne
*/
//____________________________________________________________________________

#include <cstdlib>

#include <TMath.h>

#include "Framework/Algorithm/AlgConfigPool.h"
#include "Framework/Conventions/GBuild.h"
#include "Framework/Conventions/Constants.h"
#include "Framework/Conventions/Controls.h"
#include "Framework/Conventions/Units.h"
#include "Framework/Conventions/KinePhaseSpace.h"
#include "Framework/EventGen/EVGThreadException.h"
#include "Framework/EventGen/EventGeneratorI.h"
#include "Framework/EventGen//RunningThreadInfo.h"
#include "Framework/GHEP/GHepRecord.h"
#include "Framework/GHEP/GHepFlags.h"
#include "Framework/Messenger/Messenger.h"
#include "Framework/Numerical/RandomGen.h"
#include "Framework/ParticleData/PDGCodes.h"
#include "Framework/ParticleData/PDGLibrary.h"
#include "Framework/Utils/KineUtils.h"
#include "Physics/Strange/EventGen/SKKinematicsGenerator.h"

using namespace genie;
using namespace genie::constants;
using namespace genie::controls;
using namespace genie::utils;

//___________________________________________________________________________
SKKinematicsGenerator::SKKinematicsGenerator() :
KineGeneratorWithCache("genie::SKKinematicsGenerator")
{
  //fEnvelope = 0;
}
//___________________________________________________________________________
SKKinematicsGenerator::SKKinematicsGenerator(string config) :
KineGeneratorWithCache("genie::SKKinematicsGenerator", config)
{
  //fEnvelope = 0;
}
//___________________________________________________________________________
SKKinematicsGenerator::~SKKinematicsGenerator()
{
  //if(fEnvelope) delete fEnvelope;
}
//___________________________________________________________________________
void SKKinematicsGenerator::ProcessEventRecord(GHepRecord * evrec) const
{
  if(fGenerateUniformly) {
    LOG("SKKinematics", pNOTICE)
          << "Generating kinematics uniformly over the allowed phase space";
  }

  //-- Access cross section algorithm for running thread
  RunningThreadInfo * rtinfo = RunningThreadInfo::Instance();
  const EventGeneratorI * evg = rtinfo->RunningThread();
  fXSecModel = evg->CrossSectionAlg();
  CalculateKin_AtharSingleKaon(evrec);
}
//___________________________________________________________________________
void SKKinematicsGenerator::CalculateKin_AtharSingleKaon(GHepRecord * evrec) const
{
  // Get the Primary Interacton object
  Interaction * interaction = evrec->Summary();
  interaction->SetBit(kISkipProcessChk);
  interaction->SetBit(kISkipKinematicChk);

  // Initialise a random number generator
  RandomGen * rnd = RandomGen::Instance();

  //-- For the subsequent kinematic selection with the rejection method:
  //   Calculate the max differential cross section or retrieve it from the
  //   cache. Throw an exception and quit the evg thread if a non-positive
  //   value is found.
  //   If the kinematics are generated uniformly over the allowed phase
  //   space the max xsec is irrelevant
  double xsec_max = (fGenerateUniformly) ? -1 : this->MaxXSec(evrec);

  // Determine lepton and kaon masses
  int leppdg = interaction->FSPrimLeptonPdg();
  const TLorentzVector pnuc4 = interaction->InitState().Tgt().HitNucP4(); // 4-momentum of struck nucleon in lab frame
  TVector3 beta = pnuc4.BoostVector();
  TLorentzVector P4_nu = *(interaction->InitStatePtr()->GetProbeP4(kRfHitNucRest)); // struck nucleon rest frame

  double enu = P4_nu.E(); // in nucleon rest frame
  int kaon_pdgc = interaction->ExclTag().StrangeHadronPdg();
  double mk = PDGLibrary::Instance()->Find(kaon_pdgc)->Mass();
  double ml = PDGLibrary::Instance()->Find(leppdg)->Mass();

  // Maximum possible kinetic energy
  const double Tkmax = enu - mk - ml;
  const double Tlmax = enu - mk - ml;

  // Tkmax <= 0 means we are below threshold for sure
  if( Tkmax <= 0.0 ) {
    LOG("SKKinematics", pWARN) << "No available phase space";
    evrec->EventFlags()->SetBitNumber(kKineGenErr, true);
    genie::exceptions::EVGThreadException exception;
    exception.SetReason("No available phase space");
    exception.SwitchOnFastForward();
    throw exception;
  }

  const double Tkmin = 0.0;
  const double Tlmin = 0.0;
  // for performance, use log( 1 - cos(theta_lepton) ) instead of cos(theta_lepton) because it is sharply peaked near 1.0
  const double xmin = fMinLog1MinusCosTheta; // set in LoadConfig
  const double xmax =  0.69314718056; // log(2) is physical boundary
  const double phikqmin = 0.0;
  const double phikqmax = 2.0 * kPi;
  const double dtk = Tkmax - Tkmin;
  const double dtl = Tlmax - Tlmin;
  const double dx = xmax - xmin;
  const double dphikq = phikqmax - phikqmin;

  //------ Try to select a valid tk, tl, costhetal, phikq quadruplet

  unsigned int iter = 0;
  bool accept = false;
  double xsec      = -1; // diff XS
  double tk        = -1; // kaon kinetic energy
  double tl        = -1; // lepton kinetic energy
  double costhetal = -1; // cosine of lepton angle
  double phikq     = -1; // azimuthal angle between kaon and q vector

  while(1) {
     iter++;
     if(iter > kRjMaxIterations) {
        LOG("SKKinematics", pWARN)
             << "*** Could not select a valid (tk, tl, costhetal) triplet after "
                                               << iter << " iterations";
        evrec->EventFlags()->SetBitNumber(kKineGenErr, true);
        genie::exceptions::EVGThreadException exception;
        exception.SetReason("Couldn't select kinematics");
        exception.SwitchOnFastForward();
        throw exception;
     }

     if(fGenerateUniformly) {
       tk = Tkmin + dtk * rnd->RndKine().Rndm();
       tl = Tlmin + dtl * rnd->RndKine().Rndm();
       double x = xmin + dx * rnd->RndKine().Rndm(); // log(1-costheta)
       costhetal = 1.0 - TMath::Exp(x);
       phikq = phikqmin + dphikq * rnd->RndKine().Rndm();
     } else {
       tk = Tkmin + dtk * rnd->RndKine().Rndm();
       tl = Tlmin + dtl * rnd->RndKine().Rndm();
       double x = xmin + dx * rnd->RndKine().Rndm(); // log(1-costheta)
       costhetal = 1.0 - TMath::Exp(x);
       phikq = phikqmin + dphikq * rnd->RndKine().Rndm();
     }

     LOG("SKKinematics", pDEBUG) << "Trying: Tk = " << tk << ", Tl = " << tl << ", cosThetal = " << costhetal << ", phikq = " << phikq;

     // nucleon rest frame! these need to be boosted to the lab frame before they become actual particles
     interaction->KinePtr()->SetKV(kKVTk, tk);
     interaction->KinePtr()->SetKV(kKVTl, tl);
     interaction->KinePtr()->SetKV(kKVctl, costhetal);
     interaction->KinePtr()->SetKV(kKVphikq, phikq);

     // lorentz invariant stuff, but do all the calculations in the nucleon rest frame
     double el = tl + ml;
     double pl = TMath::Sqrt(el*el - ml*ml);
     double M = interaction->InitState().Tgt().Mass();
     TVector3 lepton_3vector = TVector3(0,0,0);
     lepton_3vector.SetMagThetaPhi(pl,TMath::ACos(costhetal),0.0);
     TLorentzVector P4_lep( lepton_3vector, tl+ml );
     TLorentzVector q = P4_nu - P4_lep;
     double Q2 = -q.Mag2();
     double xbj = Q2/(2*M*q.E());
     double y = q.E()/P4_nu.E();
     double W2 = (pnuc4+q).Mag2();


     // computing cross section for the current kinematics
     xsec = fXSecModel->XSec(interaction, kPSTkTlctl);

     //-- decide whether to accept the current kinematics
     if(!fGenerateUniformly) {
        // Jacobian is 1-costheta for x = log(1-costheta)
        double max = xsec_max;
        double t   = max * rnd->RndKine().Rndm();
        double J   = TMath::Abs(1. - costhetal);

        this->AssertXSecLimits(interaction, xsec*J, max);

        if( xsec*J > xsec_max ) { // freak out if this happens
          LOG("SKKinematics", pWARN)
             << "!!!!!!XSEC ABOVE MAX!!!!! xsec= " << xsec << ", J= " << J << ", xsec*J = " << xsec*J << " max= " << xsec_max;
        }

        accept = (t< J*xsec);
     }
     else {
        accept = (xsec>0);
     }

     //-- If the generated kinematics are accepted, finish-up module's job
     if(accept) {

        // calculate the stuff

        // for uniform kinematics, compute an event weight as
        // wght = (phase space volume)*(differential xsec)/(event total xsec)
        if(fGenerateUniformly) {
          double wght = 1.0; // change this
          wght *= evrec->Weight();
          LOG("SKKinematics", pNOTICE) << "Current event wght = " << wght;
          evrec->SetWeight(wght);
        }
        LOG("SKKinematics", pWARN) << "\nLepton energy (rest frame) = " << el << " kaon = " << tl + mk;

        // reset bits
        interaction->ResetBit(kISkipProcessChk);
        interaction->ResetBit(kISkipKinematicChk);

        interaction->KinePtr()->SetKV(kKVSelTk, tk); // nucleon rest frame
        interaction->KinePtr()->SetKV(kKVSelTl, tl); // nucleon rest frame
        interaction->KinePtr()->SetKV(kKVSelctl, costhetal); // nucleon rest frame
        interaction->KinePtr()->SetKV(kKVSelphikq, phikq); // nucleon rest frame
        interaction->KinePtr()->SetQ2(Q2, true);
        interaction->KinePtr()->SetW(TMath::Sqrt(W2), true);
        interaction->KinePtr()->Setx( xbj, true );
        interaction->KinePtr()->Sety( y, true );
        interaction->KinePtr()->ClearRunningValues();

        // set the cross section for the selected kinematics
        evrec->SetDiffXSec(xsec*(1.0-costhetal),kPSTkTlctl); // phase space is really log(1-costheta)

        return;
     }
  }// iterations
}
//___________________________________________________________________________
double SKKinematicsGenerator::ComputeMaxXSec(const Interaction * in) const
{
// Computes the maximum differential cross section in the requested phase
// space. This method overloads KineGeneratorWithCache::ComputeMaxXSec
// method and the value is cached at a circular cache branch for retrieval
// during subsequent event generation.

#ifdef __GENIE_LOW_LEVEL_MESG_ENABLED__
  SLOG("SKKinematics", pDEBUG)
          << "Scanning the allowed phase space {K} for the max(dxsec/d{K})";
#endif

  double max_xsec = 0;
  double max_tk = -1;
  double max_tl = -1;
  double max_ctl = -1;

  const int Ntk = 100;
  const int Ntl = 100;
  const int Nctl = 100;
  // don't do phi_kq -- the maximum will always occur at phi_kq = pi

  int leppdg = in->FSPrimLeptonPdg();
  double enu = in->InitState().ProbeE(kRfHitNucRest); // Enu in nucleon rest frame
  int kaon_pdgc = in->ExclTag().StrangeHadronPdg();
  double mk = PDGLibrary::Instance()->Find(kaon_pdgc)->Mass();
  double ml = PDGLibrary::Instance()->Find(leppdg)->Mass();

  const double Tkmax = enu - mk - ml;
  const double Tlmax = enu - mk - ml;
  const double Tkmin = 0.0;
  const double Tlmin = 0.0;
  // cross section is sharply peaked at forward lepton
  // faster to scan over log(1 - cos(theta)) = x
  const double xmin = fMinLog1MinusCosTheta; // set in LoadConfig
  const double xmax =  0.69314718056; // physical limit -- this is log(2)
  const double dtk = (Tkmax - Tkmin)/Ntk;
  const double dtl = (Tlmax - Tlmin)/Ntl;
  const double dx = (xmax - xmin)/Nctl;

  // XS is 0 when the kinetic energies are == 0, so start at 1
  for(int i = 1; i < Ntk; i++) {
    double tk = Tkmin + dtk*i;
    for(int j = 1; j < Ntl; j++) {
      double tl = Tlmin + dtl*j;
      for(int k = 0; k < Nctl; k++) {
        double log_1_minus_cosine_theta_lepton = xmin + dx*k;
        // XS takes cos(theta_lepton), we are scanning over log(1-that) to save time, convert to cos(theta_lepton) here
        double ctl = 1.0 - TMath::Exp(log_1_minus_cosine_theta_lepton);

        // The cross section is 4D, but the maximum differential cross section always occurs at phi_kq = pi (or -pi)
        // this is because there is more phase space for the kaon when it is opposite the lepton
        // to save time in this loop, just set phi_kq to pi
        double phikq = kPi;

        in->KinePtr()->SetKV(kKVTk, tk);
        in->KinePtr()->SetKV(kKVTl, tl);
        in->KinePtr()->SetKV(kKVctl, ctl);
        in->KinePtr()->SetKV(kKVphikq, phikq);

        double xsec = fXSecModel->XSec(in, kPSTkTlctl);

        // xsec returned by model is d4sigma/(dtk dtl dcosthetal dphikq)
        // convert lepton theta to log(1-costheta) by multiplying by jacobian 1 - costheta
        xsec *= (1.0 - ctl);

        if( xsec > max_xsec ) {
          max_xsec = xsec;
          max_tk = tk;
          max_tl = tl;
          max_ctl = ctl;
        }
      }//ctl
    }//tl
  }//tk

  LOG("SKKinematics", pINFO) << "Max XSec is " << max_xsec << " for enu = " << enu << " tk = " << max_tk << " tl = " << max_tl << " cosine theta = " << max_ctl;

  // Apply safety factor, since value retrieved from the cache might
  // correspond to a slightly different energy.
  max_xsec *= fSafetyFactor;

#ifdef __GENIE_LOW_LEVEL_MESG_ENABLED__
  SLOG("SKKinematics", pDEBUG) << in->AsString();
  SLOG("SKKinematics", pDEBUG) << "Max xsec in phase space = " << max_xsec;
  SLOG("SKKinematics", pDEBUG) << "Computed using alg = " << fXSecModel->Id();
#endif



  return max_xsec;
}

//___________________________________________________________________________
double SKKinematicsGenerator::Energy(const Interaction * interaction) const
{
// Override the base class Energy() method to cache the max xsec for the
// neutrino energy in the LAB rather than in the hit nucleon rest frame.

  const InitialState & init_state = interaction->InitState();
  double E = init_state.ProbeE(kRfLab);
  return E;
}
//___________________________________________________________________________
void SKKinematicsGenerator::Configure(const Registry & config)
{
  Algorithm::Configure(config);
  this->LoadConfig();
}
//____________________________________________________________________________
void SKKinematicsGenerator::Configure(string config)
{
  Algorithm::Configure(config);
  this->LoadConfig();
}
//____________________________________________________________________________
void SKKinematicsGenerator::LoadConfig(void)
{
  // max xsec safety factor (for rejection method) and min cached energy
  this->GetParamDef("MaxXSec-SafetyFactor", fSafetyFactor, 1.5);
  this->GetParamDef("Cache-MinEnergy",      fEMin,         0.6);

  // Generate kinematics uniformly over allowed phase space and compute
  // an event weight?
  this->GetParamDef("UniformOverPhaseSpace", fGenerateUniformly, false);

  // Maximum allowed fractional cross section deviation from maxim cross
  // section used in rejection method
  this->GetParamDef("MaxXSec-DiffTolerance", fMaxXSecDiffTolerance, 999999.);
  assert(fMaxXSecDiffTolerance>=0);

  //
  this->GetParamDef("MaxXSec-MinLog1MinusCosTheta", fMinLog1MinusCosTheta, -20.0);
}
//____________________________________________________________________________