EPACollinearFlux.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2023-2024  Laurent Forthomme
 *                2009-2012  Nicolas Schul, Jerome de Favereau de Jeneret
 *
 *  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 "CepGen/CollinearFluxes/CollinearFlux.h"
#include "CepGen/Core/Exception.h"
#include "CepGen/FormFactors/Parameterisation.h"
#include "CepGen/Modules/FormFactorsFactory.h"
#include "CepGen/Modules/PartonFluxFactory.h"
#include "CepGen/Physics/Constants.h"
#include "CepGen/Physics/PDG.h"
#include "CepGen/Physics/Utils.h"
#include "CepGen/Utils/Limits.h"
 
namespace cepgen {
  class EPACollinearFlux final : public CollinearFlux {
  public:
    explicit EPACollinearFlux(const ParametersList& params)
        : CollinearFlux(params), ff_(FormFactorsFactory::get().build(steer<ParametersList>("formFactors"))) {}
 
    static ParametersDescription description() {
      auto desc = CollinearFlux::description();
      desc.setDescription("EPA FF-dependent flux");
      desc.add<ParametersDescription>("formFactors", FormFactorsFactory::get().describeParameters("StandardDipole"));
      return desc;
    }
 
    bool fragmenting() const override { return ff_->fragmenting(); }
    pdgid_t partonPdgId() const override { return PDG::photon; }
 
    double fluxQ2(double x, double q2) const override {
      if (!x_range_.contains(x, true))
        return 0.;
      const auto q2min = utils::kt::q2(x, 0., mass2());
      if (q2min == 0. || q2 < q2min)
        return 0.;
      const auto form_factors = (*ff_)(q2);
      return prefactor_ * ((1. - x) * (1. - q2min / q2) * form_factors.FE + 0.5 * x * x * form_factors.FM) / x;
    }
 
  private:
    double mass2() const override { return mp2_; }
    const std::unique_ptr<formfac::Parameterisation> ff_;
  };
}  // namespace cepgen
REGISTER_COLLINEAR_FLUX("EPAFlux", EPACollinearFlux);