constants.h

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/*
 *    Copyright (c) 2013-2021,2023,2025-2026
 *      SMASH Team
 *
 *    GNU General Public License (GPLv3 or later)
 */
#ifndef SRC_INCLUDE_SMASH_CONSTANTS_H_
#define SRC_INCLUDE_SMASH_CONSTANTS_H_
 
#include <cmath>
#include <cstdint>
#include <limits>
 
/**
 * \file
 *
 * Collection of useful constants that are known at compile time.
 */
 
namespace smash {
 
/// Centralized alias for a quiet NaN.
template <typename T>
constexpr T smash_NaN = std::numeric_limits<T>::quiet_NaN();
 
/**
 * GeV <-> fm conversion factor.
 */
constexpr double hbarc = 0.197327053;
 
/// mb <-> fm^2 conversion factor.
constexpr double fm2_mb = 0.1;
 
/// GeV^-2 <-> mb conversion factor.
constexpr double gev2_mb = hbarc * hbarc / fm2_mb;
 
/// MeV to GeV conversion factor.
constexpr double mev_to_gev = 1.e-3;
 
/// Numerical error tolerance.
constexpr double really_small = 1.0e-6;
 
/// A very small double, used to avoid division by zero
constexpr double very_small_double = 1.0e-15;
 
/**
 * \f$ 2\pi \f$.
 */
constexpr double twopi = 2. * M_PI;
 
/// Ground state density of symmetric nuclear matter [fm^-3].
constexpr double nuclear_density = 0.168;
 
/// Physical error tolerance.
constexpr double small_number = 1.0e-4;
 
/**
 * Electron mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double electron_mass = 0.000511;
 
/**
 * Nucleon mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double nucleon_mass = 0.938;
 
/**
 * Pion mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double pion_mass = 0.138;
 
/**
 * Kaon mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double kaon_mass = 0.494;
 
/**
 * omega mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double omega_mass = 0.783;
 
/**
 * Delta mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double delta_mass = 1.232;
/**
 * Deuteron mass in GeV.
 *
 * Note that this should be the same as in particles.txt.
 */
constexpr double deuteron_mass = 1.8756;
 
/// Fine-struture constant, approximately 1/137.
constexpr double fine_structure = 7.2973525698e-3;
 
/// Elementary electric charge in natural units, approximately 0.3
const double elementary_charge = std::sqrt(fine_structure * 4 * M_PI);
 
/**
 * The maximum value of the random seed used in PYTHIA.
 */
constexpr int maximum_rndm_seed_in_pythia = 900000000;
 
/**
 * Energy in GeV, below which hard reactions via pythia are impossible.
 * This constraint is technical and comes from the pythia model itself.
 * At the same time, physics-wise, hard cross-sections at the low
 * energies are so small, that this constrant is well justified.
 */
constexpr double minimum_sqrts_pythia_can_handle = 10.0;  // GeV
 
/**
 * Process ID for any photon process.
 *
 * It is chosen such that it will not conflict with any other process.
 */
constexpr std::uint32_t ID_PROCESS_PHOTON =
    std::numeric_limits<std::uint32_t>::max();
 
}  // namespace smash
 
#endif  // SRC_INCLUDE_SMASH_CONSTANTS_H_