UserPhotonFlux.h

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
#ifndef PYTHIA8_I_USERPHOTONFLUX_H
#define PYTHIA8_I_USERPHOTONFLUX_H
 
#include "Pythia8_i/IPythia8Custom.h"
#include "AthenaBaseComps/AthAlgTool.h"
 
#include "Pythia8/Pythia.h"
 
 
// Photon flux from lead-ions. Integrated over impact parameters > 2*r_Pb.
// Suitable for photo-nuclear processes but not for photon-photon.
// This should be considered as an experimental setup and used with caution.
 
 
// Photon flux from leptons, corresponds to internal Lepton2gamma.
 
class Lepton2gamma2 : public Pythia8::PDF {
 
public:
 
  // Constructor.
  Lepton2gamma2(int idBeamIn) : Pythia8::PDF(idBeamIn) {}
 
  // Update the photon flux.
  void xfUpdate(int , double x, double Q2) {
    xgamma = 0.5 * 0.007297353080 / M_PI * (1. + Pythia8::pow2(1. - x)) / Q2;
  }
};
 
class Nucleus2gamma2 : public Pythia8::PDF {
 
public:
 
  // Constructor.
  //defaults for m_Z m_min_b=2*nuclear radius are for Pb208 (Z=82, R=6.62fm)
  Nucleus2gamma2(int idBeamIn) : Pythia8::PDF(idBeamIn),
                m_Z(82.),
                m_min_b(13.24),
                m_min_x(-1)
  {}
 
  // Update the photon flux.
  void xfUpdate(int , double x, double ) {
 
 
    if(x < m_min_x)
    {
      xgamma=0;
      return;
    }
    using Pythia8::pow2;
    double xi = x * M_NUCLEON_EFF * m_min_b / HBAR_TIMES_C;
    double bK0 = Pythia8::besselK0(xi);
    double bK1 = Pythia8::besselK1(xi);
    double intB = xi * bK1 * bK0 - 0.5 * pow2(xi) * ( pow2(bK1) - pow2(bK0) );
    xgamma = 2. * ALPHA_EM * pow2(m_Z) / M_PI * intB;
  }
  void setZ(double in_Z) {m_Z=in_Z;};
  void setMinB(double in_min_b) {m_min_b=in_min_b;};
  void setMinX(double in_min_x) {m_min_x=in_min_x;};
private:
  double m_Z;
  double m_min_b;
  double m_min_x;
 
 
  //GeV equivalent of 1 amu, e.g. mass of nucleon bound in nucleus (not free) = 0.9314 GeV 
  static constexpr double M_NUCLEON_EFF=0.9315;
  static constexpr double ALPHA_EM=7.2973525664e-3;
  static constexpr double HBAR_TIMES_C=0.1973269788; // in GeV x fm
 
};

class UserPhotonFlux: public extends<AthAlgTool, IPythia8Custom> {
  
  public:
  
  using base_class::base_class;

  virtual ~UserPhotonFlux(){};
  
  StatusCode initialize()  override;
  StatusCode finalize() override;
  
  StatusCode ModifyPythiaEvent(Pythia8::Pythia& pythia) const  override;

  virtual double CrossSectionScaleFactor() const  override;
 
  StatusCode InitializePythiaInfo(Pythia8::Pythia& ) const override;
 
 private:
 
  Gaudi::Property<int> m_process{this, "Process", 1};
  Gaudi::Property<double> m_flux_Z{this, "NuclearCharge", 82.};
  Gaudi::Property<double> m_flux_min_b{this, "MinimumB", 13.24};
  Gaudi::Property<double> m_flux_min_x{this, "MinimumX", -1};//default applies no cut since sampled x's always >0.
};
 
#ifdef PYTHIA_VERSION_INTEGER
  #if PYTHIA_VERSION_INTEGER > 8300
    typedef std::shared_ptr<Pythia8::PDF> PDFPtr;
    typedef std::shared_ptr<Nucleus2gamma2> Nucleus2gammaPtr;
    #define PNEW(X, Y) std::make_shared<X>(Y);
  #else
    typedef Nucleus2gamma2* Nucleus2gammaPtr;
    typedef Pythia8::PDF* PDFPtr;
    #define PNEW(X, Y) new X(Y);
  #endif
#else
  typedef Nucleus2gamma2* Nucleus2gammaPtr;
  typedef Pythia8::PDF* PDFPtr;
  #define PNEW(X, Y) new X(Y);
#endif
 
 
#endif