BetheHeitler.hpp

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// This file is part of the Acts project.
//
// Copyright (C) 2018-2021 CERN for the benefit of the Acts project
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#pragma once

#include "Acts/Material/MaterialSlab.hpp"
#include "Acts/Utilities/UnitVectors.hpp"
#include "ActsFatras/EventData/Particle.hpp"

#include <array>
#include <cmath>
#include <random>

namespace ActsFatras {

struct BetheHeitler {
  using Scalar = Particle::Scalar;
  using Vector3 = Particle::Vector3;

  double scaleFactor = 1.;

  // Simplified angle evaluation
  bool uniformHertzDipoleAngle = false;

  Particle bremPhoton(const Particle &particle, Scalar gammaE, Scalar rndPsi,
                      Scalar rndTheta1, Scalar rndTheta2,
                      Scalar rndTheta3) const;

  template <typename generator_t>
  std::array<Particle, 1> operator()(generator_t &generator,
                                     const Acts::MaterialSlab &slab,
                                     Particle &particle) const {
    // Take a random gamma-distributed value - depending on t/X0
    std::gamma_distribution<double> gDist(slab.thicknessInX0() / std::log(2.0),
                                          1.0);

    const auto u = gDist(generator);
    const auto z = std::exp(-u);  // MARK: fpeMask(FLTUND, 1, #2346)
    const auto sampledEnergyLoss =
        std::abs(scaleFactor * particle.energy() * (z - 1.));

    std::uniform_real_distribution<Scalar> uDist(0., 1.);
    // Build the produced photon
    Particle photon =
        bremPhoton(particle, sampledEnergyLoss, uDist(generator),
                   uDist(generator), uDist(generator), uDist(generator));
    // Recoil input momentum
    particle.setDirection(particle.direction() * particle.absoluteMomentum() -
                          photon.energy() * photon.direction());

    // apply the energy loss
    particle.correctEnergy(-sampledEnergyLoss);

    return {photon};
  }
};

}  // namespace ActsFatras