AlphaSAnalytic.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 <LHAPDF/LHAPDF.h>
 
#include "CepGen/Modules/CouplingFactory.h"
#include "CepGen/Physics/Coupling.h"
#include "CepGen/Physics/PDG.h"
 
#if defined LHAPDF_MAJOR_VERSION && LHAPDF_MAJOR_VERSION == 6
 
namespace cepgen {
  namespace lhapdf {
    class AlphaSAnalytic final : public Coupling {
    public:
      explicit AlphaSAnalytic(const ParametersList& params) : Coupling(params), ana_(new LHAPDF::AlphaS_Analytic) {
        ana_->setOrderQCD(steer<int>("order"));
        for (int i = 1; i <= 6; ++i)  // set all quarks masses for evolution
          ana_->setQuarkMass(i, PDG::get().mass(i));
        // set gradients for evolution
        size_t i = 3;
        for (const auto& lambda : steer<std::vector<double> >("lambdas"))
          ana_->setLambda(i++, lambda);
      }
 
      static ParametersDescription description() {
        auto desc = Coupling::description();
        desc.setDescription("Analytic LHAPDF perturb.algo.");
        desc.add<std::string>("pdfSet", "cteq66");
        desc.add<int>("order", 4).setDescription("QCD order");
        desc.add<std::vector<double> >("lambdas", {0.339, 0.296, 0.213});
        return desc;
      }
 
      double operator()(double q) const override { return ana_->alphasQ(q); }
 
    private:
      std::unique_ptr<LHAPDF::AlphaS_Analytic> ana_;
    };
  }  // namespace lhapdf
}  // namespace cepgen
using AlphaSLHAPDFAnalytic = cepgen::lhapdf::AlphaSAnalytic;
REGISTER_ALPHAS_MODULE("lhapdfAnalytic", AlphaSLHAPDFAnalytic);
 
#endif