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 <Pythia8/PartonDistributions.h>
 
#include <numeric>
 
#include "CepGen/CollinearFluxes/CollinearFlux.h"
#include "CepGen/Core/Exception.h"
#include "CepGen/Modules/PartonFluxFactory.h"
#include "CepGen/Physics/PDG.h"
 
namespace cepgen {
  namespace pythia8 {
    class CollinearFlux final : public cepgen::CollinearFlux {
    public:
      explicit CollinearFlux(const ParametersList& params)
          : cepgen::CollinearFlux(params), type_(steer<std::string>("type")), pdgid_(steer<pdgid_t>("partonPdgId")) {
        if (type_ == "Lepton") {
          auto lepton_params = steer<ParametersList>("leptonParams");
          info_.reset(new Pythia8::Info);
          if (const auto dil_sqrt_s = lepton_params.get<double>("sqrtS"); dil_sqrt_s > 0.)
            info_->setECM(dil_sqrt_s);
          else
            CG_WARNING("pythia8:CollinearFlux") << "Beam-beam centre-of-mass energy is required (through the 'sqrtS' "
                                                   "parameter) for the 'Lepton' collinear flux mode.";
          pdf_.reset(new Pythia8::Lepton(
              lepton_params.get<pdgid_t>("beamPdgId"), lepton_params.get<double>("Q2max"), info_.get()));
        } else if (type_ == "LHAGrid1")
          pdf_.reset(new Pythia8::LHAGrid1);
        else if (type_ == "MSTWpdf")
          pdf_.reset(new Pythia8::MSTWpdf);
        else if (type_ == "Proton2gammaDZ")
          pdf_.reset(new Pythia8::Proton2gammaDZ);
        else if (type_ == "ProtonPoint")
          pdf_.reset(new Pythia8::ProtonPoint);
        else
          throw CG_FATAL("pythia8:CollinearFlux") << "Failed to initialise the Pythia 8 evaluator!\n"
                                                  << "Parameters: " << params_;
 
        CG_INFO("pythia8:CollinearFlux") << "Pythia 8 '" << type_ << "' evaluator for collinear parton "
                                         << "(" << (PDG::Id)pdgid_ << ") flux initialised.";
      }
 
      static ParametersDescription description() {
        auto desc = cepgen::CollinearFlux::description();
        desc.setDescription("Pythia 8 coll.flux");
        desc.add<std::string>("type", "Proton2gammaDZ").setDescription("type of PDF evaluator to use");
        desc.add<pdgid_t>("partonPdgId", PDG::photon).setDescription("parton PDG identifier");
        auto lepton_desc = ParametersDescription();
        lepton_desc.add<pdgid_t>("beamPdgId", PDG::electron).setDescription("beam particle PDG identifier");
        lepton_desc.add<double>("sqrtS", -1.);
        lepton_desc.add<double>("Q2max", 50.);
        desc.add<ParametersDescription>("leptonParams", lepton_desc);
        return desc;
      }
 
      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 == 0. || x < pdf_->getXmin())
          return 0.;
        return pdf_->xf((int)pdgid_, x, q2);
      }
 
    private:
      std::unique_ptr<Pythia8::PDF> pdf_;
      std::unique_ptr<Pythia8::Info> info_;
      const std::string type_;
      const pdgid_t pdgid_;
    };
  }  // namespace pythia8
}  // namespace cepgen
using PythiaCollinearFlux = cepgen::pythia8::CollinearFlux;
REGISTER_COLLINEAR_FLUX("pythia8", PythiaCollinearFlux);