crosssectionsphoton.h

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/*
 *
 *    Copyright (c) 2017-2023
 *      SMASH Team
 *
 *    GNU General Public License (GPLv3 or later)
 *
 */
 
#ifndef SRC_INCLUDE_SMASH_CROSSSECTIONSPHOTON_H_
#define SRC_INCLUDE_SMASH_CROSSSECTIONSPHOTON_H_
 
#include "kinematics.h"
 
namespace smash {
/** Cross section after cut off.
 *
 * Photon cross sections diverge tremendously at the threshold which
 * becomes particularly problematic when running with broad rho
 * mesons. Then the actual photon cross section is used for the
 * weight: W = Sigma_photon/Sigma_hadron.  If the photon cross
 * section diverges, the weight becomes huge and we significantly
 * overestimate photon production. This cutoff fixes the problem.
 *
 * Either the cross section is returned or, if the cross section i
 * larger than the cut off, the cut off value is returned.
 *
 * \param[in] sigma_mb cross section before cut off [mb]
 * \return Cross section after cut off [mb]
 */
double cut_off(const double sigma_mb);
 
/**
 * Calculation method for the cross sections. It has only one member at the
 * moment. In the future there will be more options.
 */
enum class ComputationMethod { Analytic };
 
template <ComputationMethod method>
class CrosssectionsPhoton {};
 
/**
 * Class to calculate the cross-section of a meson-meson to meson-photon
 * process. This template specialization uses the analytic formulas for the
 * cross-sections.
 */
template <>
class CrosssectionsPhoton<ComputationMethod::Analytic> {
 public:
  /** @name Total cross-section
   * The functions in this group calculate the analytical value of the total
   * cross-section for a photon process.
   */
  ///@{
  /**
   * Total cross sections for given photon process:
   *
   * \param[in] s Mandelstam-s [GeV^2]
   * \param[in] m_rho Mass of participating rho-meson [GeV]
   * \returns photon cross-section [mb]
   */
  static double xs_pi_pi_rho0(const double s, const double m_rho);
  static double xs_pi_pi0_rho(const double s, const double m_rho);
  static double xs_pi0_rho0_pi0(const double s, const double m_rho);
  static double xs_pi_rho0_pi(const double s, const double m_rho);
 
  static double xs_pi_rho_pi0(const double s, const double m_rho);
  static double xs_pi_rho_pi0_rho_mediated(const double s, const double m_rho);
  static double xs_pi_rho_pi0_omega_mediated(const double s,
                                             const double m_rho);
 
  static double xs_pi0_rho_pi(const double s, const double m_rho);
  static double xs_pi0_rho_pi_rho_mediated(const double s, const double m_rho);
  static double xs_pi0_rho_pi_omega_mediated(const double s,
                                             const double m_rho);
  ///@}
 
  /** @name Differential cross-section
   * The functions in this group calculate the analytical value of the
   * differential cross-section for a photon process.
   */
  ///@{
  /**
   * Differential cross section for given photon process.
   *
   * \param[in] s Mandelstam-s [GeV^2]
   * \param[in] t Mandelstam-t [GeV^2]
   * \param[in] m_rho Mass of participating rho-meson [GeV]
   * \returns photon cross-section [mb]
   */
  static double xs_diff_pi_pi_rho0(const double s, const double t,
                                   const double m_rho);
  static double xs_diff_pi_pi0_rho(const double s, const double t,
                                   const double m_rho);
  static double xs_diff_pi0_rho0_pi0(const double s, const double t,
                                     const double m_rho);
  static double xs_diff_pi_rho0_pi(const double s, const double t,
                                   const double m_rho);
 
  static double xs_diff_pi_rho_pi0_rho_mediated(const double s, const double t,
                                                const double m_rho);
  static double xs_diff_pi_rho_pi0_omega_mediated(const double s,
                                                  const double t,
                                                  const double m_rho);
 
  static double xs_diff_pi0_rho_pi_rho_mediated(const double s, const double t,
                                                const double m_rho);
  static double xs_diff_pi0_rho_pi_omega_mediated(const double s,
                                                  const double t,
                                                  const double m_rho);
  ///@}
 
  // Todo{JR}: Needed for tabulation.
  /// @cond
  static double s_min, s_max, t_min, t_max;
  /// @endcond
 
 private:
  /** @name Constants
   * The choice of these parameters, necessary to determine the photon cross
   * sections, follows from (\iref{Turbide:2006zz}). Here, different
   * combinations of parameters were proposed and investigated. We decided to
   * use the parameters of set II in their categorization. They are named
   * identically, except for "Const", which corresponds to "C" in
   * \iref{Turbide:2006zz}.
   */
  ///@{
  /**
   * Constant in the computation of photon cross sections.
   */
  constexpr static double Const = 0.059;
  constexpr static double g_POR = 22.6;
  constexpr static double ghat = 6.4483;
  constexpr static double eta1 = 2.3920;
  constexpr static double eta2 = 1.9430;
  constexpr static double delta = -0.6426;
  constexpr static double C4 = -0.14095;
  constexpr static double Gammaa1 = 0.4;
  ///@}
  /// Value of \f$ \pi \f$
  constexpr static double Pi = M_PI;
  /**
   * Value of a1 mass in GeV.
   *
   * \note The cross section calculated here relies on the following value of
   * the a1 particle, which is slightly different from the value used in the
   * particles.txt file. Other mass values (like the pion mass) are globally
   * defined and imposed to be identical to those provided in the particles.txt
   * file. For a1 we want to allow in general different values from 1.26 GeV.
   * This is why we declare the value here.
   */
  constexpr static double a1_mass = 1.26;
};
 
}  // namespace smash
 
#endif  // SRC_INCLUDE_SMASH_CROSSSECTIONSPHOTON_H_