Kelly.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2023  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/Modules/FormFactorsFactory.h"
 
namespace cepgen {
  namespace formfac {
    class Kelly final : public Parameterisation {
    public:
      explicit Kelly(const ParametersList& params)
          : Parameterisation(params),
            ae_(steer<std::vector<double> >("aE")),
            be_(steer<std::vector<double> >("bE")),
            am_(steer<std::vector<double> >("aM")),
            bm_(steer<std::vector<double> >("bM")) {}
 
      static ParametersDescription description() {
        auto desc = Parameterisation::description();
        desc.setDescription("Kelly");
        desc.add<std::vector<double> >("aE", {1., -0.24});
        desc.add<std::vector<double> >("bE", {10.98, 12.82, 21.97});
        desc.add<std::vector<double> >("aM", {1., 0.12});
        desc.add<std::vector<double> >("bM", {10.97, 18.86, 6.55});
        return desc;
      }
 
    private:
      double computeFF(double tau, const std::vector<double>& as, const std::vector<double>& bs) const {
        double num{0.}, den{1.};
        for (size_t i = 0; i < as.size(); ++i)
          num += as.at(i) * std::pow(tau, i);
        for (size_t i = 0; i < bs.size(); ++i)
          den += bs.at(i) * std::pow(tau, i + 1);
        return num / den;
      }
      void eval() override {
        const auto ta = tau(q2_);
        setGEGM(computeFF(ta, ae_, be_), MU * computeFF(ta, am_, bm_));
      }
 
      const std::vector<double> ae_, be_, am_, bm_;
    };
  }  // namespace formfac
}  // namespace cepgen
using cepgen::formfac::Kelly;
REGISTER_FORMFACTORS("Kelly", Kelly);