AlphaSWebber.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 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/CouplingFactory.h"
#include "CepGen/Physics/Coupling.h"
#include "CepGen/Utils/Message.h"
 
namespace cepgen {
  class AlphaSWebber : public Coupling {
  public:
    explicit AlphaSWebber(const ParametersList& params)
        : Coupling(params),
          nc_(steer<int>("Nc")),
          nf_(steer<int>("nf")),
          lambda_(steer<double>("Lambda")),
          beta0_((11. * nc_ - 2. * nf_) / 3.),
          prefac_(4. * M_PI / beta0_) {
      CG_INFO("AlphaSWebber:init") << "Webber et al. alpha(S) evolution algorithm initialised with parameters:\n\t"
                                   << "Nc: " << nc_ << ", nf: " << nf_ << " -> beta0: " << beta0_
                                   << ", Lambda: " << lambda_ << ".";
    }
 
    static ParametersDescription description() {
      auto desc = Coupling::description();
      desc.setDescription("Webber alpha(S) evolution algorithm");
      desc.add<int>("Nc", 3).setDescription("number of colours considered");
      desc.add<int>("nf", 3).setDescription("number of fermion flavours considered");
      desc.add<double>("Lambda", 0.25).setDescription("evolution scale (in GeV)");
      return desc;
    }
 
    double operator()(double q) const override {
      const auto mun = q * q / lambda_ / lambda_;
      return prefac_ * (1. / std::log(mun) + 125. * (1. + 4. * mun) / (1. - mun) / std::pow(4. + mun, 4));
    }
 
  private:
    const int nc_, nf_;
    const double lambda_, beta0_;
    const double prefac_;
  };
}  // namespace cepgen
 
REGISTER_ALPHAS_MODULE("webber", AlphaSWebber);