File Units.hpp¶

Defines

ACTS_DEFINE_UNIT_LITERAL(name)¶

namespace Acts

Set the Geometry Context PLUGIN.

Set the Calibration Context PLUGIN.

Convenience functions to ease creation of and Acts::InterpolatedMaterialMap and to avoid code duplication.

Set the Mangetic Field Context PLUGIN.

Convenience functions to ease creation of and Acts::InterpolatedBFieldMap and to avoid code duplication.

Currently implemented for the two most common formats: rz and xyz.

namespace PhysicalConstants¶

Physical constants in native units.

Unit constants are intentionally not listed.

Variables

constexpr double hbar = 6.582119569509066e-25 * UnitConstants::GeV * UnitConstants::s ¶

Reduced Planck constant h/2*pi.

Computed from CODATA 2018 constants to double precision.

namespace UnitConstants¶

Unit definitions and conversions.

All physical quantities have both a numerical value and a unit. For the computations we always choose a particular unit so we only need to consider the numerical values as such. The chosen base unit for a particular physical quantity, e.g. length, time, or energy, within this code base is called the native unit.

Here, the following native units are used:

Length is expressed in mm.
Time is expressed in [speed-of-light * time] == mm. A consequence of this choice is that the speed-of-light expressed in native units is 1.
Angles are expressed in radian.
Energy, mass, and momentum are all expressed in GeV (consistent with a speed-of-light == 1).
Electric charge is expressed in e, i.e. units of the elementary charge.

The magnetic field is expressed in GeV/(e*mm). The magnetic field connects momentum to length, e.g. in SI units the radius of a charged particle trajectory in a constant magnetic field is given by

radius = - (momentum / charge) / magnetic-field

With the chosen magnetic field unit the expression above stays the
same and no additional conversion factors are necessary.

Amount of substance is expressed in mol.

To ensure consistent computations and results the following guidelines must be followed when handling physical quantities with units:

All unqualified numerical values, i.e. without a unit, are assumed to be expressed in the relevant native unit, e.g. mm for lengths or GeV for energy/momentum.

If a variable stores a physical quantity in a specific unit that is not the native unit, clearly mark this in the variable, i.e.

double momentum = 100.0; // momentum is stored as native unit GeV
double momentumInMeV = 10.0; // would be 0.01 in native units

All input values must be given as numerical_value * unit_constant or equivalently using the unit literals as value_unit. The resulting unqualified numerical value will be automatically converted to the native unit.
To output an unqualified numerical value in the native units as a numerical value in a specific unit divide by the unit constants as numerical_value / unit_constant or using the unit literals as value / 1_unit.

Examples:

#include "Acts/include/Definitions/Units.hpp"
using namespace Acts::UnitLiterals;

// define input values w/ units (via unit constants)
double width    = 12 * Acts::UnitConstants::mm;
double mmuon    = 105.7 * Acts::UnitConstants::MeV;
// define input values w/ units (via unit user literals)
double length   = 23_cm;
double time     = 1214.2_ns;
double angle    = 123_degree;
double momentum = 2.5_TeV;
double mass     = 511_keV;
double velocity = 345_m / 1_s;
double bfield   = 3.9_T;
double density  = 1_mol / 1_cm3;

// convert output values (via unit constants)
doube t_in_ns    = trackPars.time() / Acts::UnitConstants::ns;
// convert output values (via unit user literals)
double x_in_mm   = trackPars.position()[ePos0] / 1_mm;
double p_in_TeV = trackPars.absoluteMomentum() / 1_TeV;

Note

A helper script is available in Core/scripts/print_units_physical_constants.py to validate some of the numerical values.

Variables

constexpr double C = J / eV ¶

constexpr double cm = 10.0¶

constexpr double cm2 = cm * cm ¶

constexpr double cm3 = cm * cm * cm ¶

constexpr double degree = 0.017453292519943295¶

constexpr double e = 1.0¶

constexpr double eV = 1e-9¶

constexpr double fm = 1e-12¶

constexpr double fs = 1e-15 * s ¶

constexpr double g = 1.0 / 1.782662e-24¶

constexpr double Gauss = 1e-4 * T ¶

constexpr double GeV = 1.0¶

constexpr double h = 3600.0 * s ¶

constexpr double J = 6241509074.460763 * GeV ¶

constexpr double keV = 1e-6¶

constexpr double kg = 1.0 / 1.782662e-27¶

constexpr double kGauss = 1e-1 * T ¶

constexpr double km = 1e6¶

constexpr double m = 1e3¶

constexpr double m2 = m * m ¶

constexpr double m3 = m * m * m ¶

constexpr double MeV = 1e-3¶

constexpr double min = 60.0 * s ¶

constexpr double mm = 1.0¶

constexpr double mm2 = mm * mm ¶

constexpr double mm3 = mm * mm * mm ¶

constexpr double mol = 1.0¶

constexpr double mrad = 1e-3¶

constexpr double ms = 1e-3 * s ¶

constexpr double nm = 1e-6¶

constexpr double ns = 1e-9 * s ¶

constexpr double pm = 1e-9¶

constexpr double ps = 1e-12 * s ¶

constexpr double rad = 1.0¶

constexpr double s = 299792458000.0¶

constexpr double T = 0.000299792458¶

constexpr double TeV = 1e3¶

constexpr double u = 0.93149410242¶

constexpr double um = 1e-3¶

constexpr double us = 1e-6 * s ¶

namespace UnitLiterals¶