# HEP System **Namespace:** `mp_units::hep` **Module:** `mp_units.systems` **Header:** `` ## Units | Unit Name | Symbol | unit_symbol | Definition | |-----------|:------:|:-----------:|------------| | atomic_mass_unit | u | `u` | mag_ratio<1'660'539'066'605, 1'000'000'000'000> * mag_power<10, -27> * [si::kilogram](si.md#kilogram) | | barn | b | `b` | mag_power<10, -28> * square([si::metre](si.md#metre)) | | electron_mass | m_e | `m_e` | mag_ratio<9'109'383'701'528, 1'000'000'000'000> * mag_power<10, -31> * [si::kilogram](si.md#kilogram) | | elementary_charge | — | — | alias to [si::elementary_charge](#elementary_charge) | | fermi | — | `fm` | [si::femto](si.md#femto)<[si::metre](si.md#metre)> | | neutron_mass | m_n | `m_n` | mag_ratio<1'674'927'498'049, 1'000'000'000'000> * mag_power<10, -27> * [si::kilogram](si.md#kilogram) | | planck_constant | — | — | alias to [si::planck_constant](#planck_constant) | | proton_mass | m_p | `m_p` | mag_ratio<1'672'621'923'695, 1'000'000'000'000> * mag_power<10, -27> * [si::kilogram](si.md#kilogram) | | speed_of_light | — | — | alias to [si::speed_of_light_in_vacuum](si.md#si2019-speed_of_light_in_vacuum) |