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Scattering.hpp
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1 // This file is part of the Acts project.
2 //
3 // Copyright (C) 2018-2020 CERN for the benefit of the Acts project
4 //
5 // This Source Code Form is subject to the terms of the Mozilla Public
6 // License, v. 2.0. If a copy of the MPL was not distributed with this
7 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
8 
9 #pragma once
10 
11 #include <array>
12 #include <random>
13 
18 
19 namespace ActsFatras {
20 namespace detail {
21 
25 template <typename scattering_model_t>
26 struct Scattering {
28  scattering_model_t angle;
29 
38  template <typename generator_t>
39  std::array<Particle, 0> operator()(generator_t &generator,
40  const Acts::MaterialProperties &slab,
41  Particle &particle) const {
42  // the scattered direction can be computed by rotating the initial
43  // direction around a vector orthogonal to the initial direction, i.e. the
44  // scattering deflector, by the scattering angle. there are an infinite
45  // number of vectors orthogonal to the initial direction. the deflector is
46  // rotated by some angle relative to some fixpoint.
47  //
48  // thus two random angles are required: the random deflector orientation
49  // angle drawn uniformly from the [-pi,pi) range and the scattering angle
50  // drawn from the specific scattering model distribution.
51 
52  // draw the random orientation angle
53  const auto psi =
54  std::uniform_real_distribution<double>(-M_PI, M_PI)(generator);
55  // draw the scattering angle
56  const auto theta = angle(generator, slab, particle);
57 
58  Acts::Vector3D direction = particle.unitDirection();
59  // construct the combined rotation to the scattered direction
60  Acts::RotationMatrix3D rotation(
61  // rotation of the scattering deflector axis relative to the reference
62  Acts::AngleAxis3D(psi, direction) *
63  // rotation by the scattering angle around the deflector axis
65  direction.applyOnTheLeft(rotation);
66  particle.setDirection(direction);
67 
68  // scattering is non-destructive and produces no secondaries
69  return {};
70  }
71 };
72 
73 } // namespace detail
74 } // namespace ActsFatras