CollinearFlux.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 <APFEL/APFEL.h>
 
#include <cmath>
 
#include "CepGen/CollinearFluxes/CollinearFlux.h"
#include "CepGen/Modules/PartonFluxFactory.h"
#include "CepGen/Physics/PDG.h"
#include "CepGen/Utils/Message.h"
 
namespace cepgen {
  namespace apfel {
    class CollinearFlux final : public cepgen::CollinearFlux {
    public:
      explicit CollinearFlux(const ParametersList& params)
          : cepgen::CollinearFlux(params), pdgid_(steer<pdgid_t>("partonPdgId")), q_range_(steer<Limits>("qrange")) {
        APFEL::SetPerturbativeOrder(steer<int>("perturbativeOrder"));
        const auto pdfset = steer<std::string>("set");
        if (!pdfset.empty())
          APFEL::SetPDFSet(pdfset);
        //APFEL::SetMaxFlavourPDFs(7);
        APFEL::SetFastEvolution(steer<bool>("fastEvolution"));
        APFEL::InitializeAPFEL();
        APFEL::EvolveAPFEL(q_range_.min(), q_range_.max());
        APFEL::CachePDFsAPFEL(q_range_.min());
        CG_INFO("apfel:CollinearFlux") << "Partonic collinear parton flux evaluator successfully built.\n"
                                       << " * APFEL version: " << APFEL::GetVersion() << "\n"
                                       << " * Parton PDG identifier: " << pdgid_
                                       << ", max flavours: " << APFEL::GetMaxFlavourPDFs() << "\n"
                                       << " * Q range: " << q_range_ << " ("
                                       << Limits{APFEL::GetMuF0(), APFEL::GetMuF()} << ") GeV\n"
                                       << " * perturbative order: " << APFEL::GetPerturbativeOrder() << ".";
      }
 
      cepgen::pdgid_t partonPdgId() const override { return pdgid_; }
      bool fragmenting() const override { return true; }
      double mass2() const override { return mp2_; }
 
      double fluxQ2(double x, double q2) const override {
        if (!x_range_.contains(x, true))
          return 0.;
        const auto q = std::sqrt(q2);
        if (!q_range_.contains(q))
          return 0.;
        return prefactor_ * APFEL::xPDFxQ(pdgid_, x, q) / x;
      }
 
      static ParametersDescription description() {
        auto desc = cepgen::CollinearFlux::description();
        desc.setDescription("APFEL coll.flux");
        desc.add<std::string>("set", "").setDescription("LHAPDF set to use at the initial scale");
        desc.add<pdgid_t>("partonPdgId", PDG::photon).setDescription("parton PDG identifier");
        desc.add<Limits>("qrange", {1., 100.});
        desc.add<int>("perturbativeOrder", 2);
        desc.add<bool>("fastEvolution", false);
        return desc;
      }
 
    private:
      const pdgid_t pdgid_;
      const Limits q_range_;
    };
  }  // namespace apfel
}  // namespace cepgen
using CollinearFluxAPFEL = cepgen::apfel::CollinearFlux;
REGISTER_COLLINEAR_FLUX("apfel", CollinearFluxAPFEL);