SuriYennie.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2013-2024  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/Modules/StructureFunctionsFactory.h"
#include "CepGen/Physics/Utils.h"
#include "CepGen/StructureFunctions/Parameterisation.h"
 
namespace cepgen {
  namespace strfun {
    class SuriYennie : public Parameterisation {
    public:
      explicit SuriYennie(const ParametersList& params)
          : Parameterisation(params),
            c1_(steer<double>("C1")),
            c2_(steer<double>("C2")),
            d1_(steer<double>("D1")),
            rho2_(steer<double>("rho2")),
            cp_(steer<double>("Cp")),
            bp_(steer<double>("Bp")) {}
 
      static ParametersDescription description() {
        auto desc = Parameterisation::description();
        desc.setDescription("Suri-Yennie");
        desc.add<double>("C1", 0.86926);
        desc.add<double>("C2", 2.23422);
        desc.add<double>("D1", 0.12549);
        desc.add<double>("rho2", 0.585);
        desc.add<double>("Cp", 0.96);
        desc.add<double>("Bp", 0.63);
        return desc;
      }
 
      void eval() override {
        const double mx2 = utils::mX2(args_.xbj, args_.q2, mp2_), dm2 = mx2 - mp2_;  // [GeV^2]
        const double en = args_.q2 + dm2;                                            // [GeV^2]
        const double nu = 0.5 * en / mp_, x_pr = args_.q2 / (args_.q2 + mx2), tau = 0.25 * args_.q2 / mp2_;
        const double mq = rho2_ + args_.q2;
 
        const double inv_q2 = 1. / args_.q2;
 
        const double fm = inv_q2 * (c1_ * dm2 * pow(rho2_ / mq, 2) +
                                    c2_ * mp2_ * pow(1. - x_pr, 4) / (1. + x_pr * (x_pr * cp_ - 2. * bp_)));
        const double fe =
            (tau * fm + d1_ * dm2 * args_.q2 * rho2_ / mp2_ * pow(dm2 / mq / en, 2)) / (1. + nu * nu * inv_q2);
 
        setFE(fe);
        setFM(fm);
        setW1(0.5 * fm * args_.q2 / mp_);
        setW2(2. * mp_ * fe);
        setF2(2. * nu * fe);
      }
 
    private:
      double c1_{0.}, c2_{0.};
      double d1_{0.};
      double rho2_{0.};
      double cp_{0.}, bp_{0.};
    };
 
    struct SuriYennieAlt final : public SuriYennie {
      using SuriYennie::SuriYennie;
      static ParametersDescription description() {
        auto desc = SuriYennie::description();
        desc.setDescription("Suri-Yennie (alternative)");
        desc.add<double>("C1", 0.6303);
        desc.add<double>("C2", 2.3049);
        desc.add<double>("D1", 0.04681);
        desc.add<double>("rho2", 1.05);
        desc.add<double>("Cp", 1.23);
        desc.add<double>("Bp", 0.61);
        return desc;
      }
    };
  }  // namespace strfun
}  // namespace cepgen
using cepgen::strfun::SuriYennie;
using cepgen::strfun::SuriYennieAlt;
REGISTER_STRFUN("SuriYennie", 11, SuriYennie);
REGISTER_STRFUN("SuriYennieAlt", 14, SuriYennieAlt);