KleinElasticHeavyIonKTFlux.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2023-2024  Laurent Forthomme
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#include <cmath>
 
#include "CepGen/FormFactors/Parameterisation.h"
#include "CepGen/KTFluxes/KTFlux.h"
#include "CepGen/Modules/FormFactorsFactory.h"
#include "CepGen/Modules/PartonFluxFactory.h"
#include "CepGen/Physics/HeavyIon.h"
#include "CepGen/Physics/PDG.h"
#include "CepGen/Physics/Utils.h"
 
namespace cepgen {
  class KleinElasticHeavyIonKTFlux final : public KTFlux {
  public:
    explicit KleinElasticHeavyIonKTFlux(const ParametersList& params)
        : KTFlux(params),
          hi_(HeavyIon::fromPdgId(steer<pdgid_t>("heavyIon"))),
          ff_(FormFactorsFactory::get().build(params.get<ParametersList>("formFactors"))) {}
 
    static ParametersDescription description() {
      auto desc = KTFlux::description();
      desc.setDescription("Elastic photon emission from heavy ion (from Starlight)");
      desc.addAs<pdgid_t, HeavyIon>("heavyIon", HeavyIon::Pb());
      desc.add<ParametersDescription>("formFactors", FormFactorsFactory::get().describeParameters("HeavyIonDipole"));
      return desc;
    }
 
    bool fragmenting() const override { return false; }
    double mass2() const override { return hi_.A * hi_.A * mp2_; }
    pdgid_t partonPdgId() const override { return PDG::photon; }
 
    double fluxQ2(double x, double kt2, double q2) const override {
      if (!x_range_.contains(x))
        return 0.;
 
      const auto ff = (*ff_)(q2);
      const double ela1 = pow(kt2 / q2 / (1. - x), 2);
      const double ela2 = pow(ff.GE, 2);
      //const double ela3 = kt2 / q2;
      const auto z = (unsigned short)hi_.Z;
      return prefactor_ * z * z * ela1 * ela2 / q2;
    }
 
    double fluxMX2(double x, double kt2, double) const override {
      return fluxQ2(x, kt2, utils::kt::q2(x, kt2, mass2()));
    }
 
  private:
    const HeavyIon hi_;
    std::unique_ptr<formfac::Parameterisation> ff_;
  };
}  // namespace cepgen
 
REGISTER_KT_FLUX("KleinElasticHI", 101, KleinElasticHeavyIonKTFlux);