Tau lepton mass
| Value | 3.16754e-27 kg |
| Status | Measured: ± 2.10e-32 kg (0.0000066 relative) |
| Source | CODATA 2022 |
| Categories | Universal & Atomic |
| unified atomic mass unit (dalton) | 1.9075372 u |
| picogram | 3.1675400e-12 pg |
| nanogram | 3.1675400e-15 ng |
| microgram | 3.1675400e-18 μg |
| milligram | 3.1675400e-21 mg |
| centigram | 3.1675400e-22 cg |
| grain | 4.8882612e-23 gr |
| metric carat | 1.5837700e-23 ct |
| gram | 3.1675400e-24 g |
| pennyweight | 2.0367755e-24 dwt |
| dram | 1.7877070e-24 dr |
| ounce | 1.1173169e-25 oz |
| troy ounce | 1.0183878e-25 oz t |
| troy pound | 8.4865646e-27 lb t |
| pound | 6.9832303e-27 lb |
| kilogram | 3.1675400e-27 kg |
| stone | 4.9880217e-28 st |
| slug | 2.1704543e-28 slug |
| US hundredweight | 6.9832303e-29 cwt |
| UK long hundredweight | 6.2350271e-29 cwt (UK) |
| quintal | 3.1675400e-29 q |
| US short ton | 3.4916152e-30 ton |
| metric tonne | 3.1675400e-30 t |
| UK long ton | 3.1175135e-30 long ton |
| kilotonne | 3.1675400e-33 kt |
| megatonne | 3.1675400e-36 Mt |
| gigatonne | 3.1675400e-39 Gt |
| Earth mass | 5.3038076e-52 Mearth |
| solar mass | 1.5930014e-57 Msun |
Learning zone
Martin Perl's group found the tau at SLAC between 1974 and 1977 in events producing an electron and a muon with missing energy, establishing a third generation of matter before anyone had seen the bottom or top quark. At 1776.86 MeV/c² it outweighs the proton, which is why it is the only lepton heavy enough to decay into hadrons — about 65 % of taus end as pions rather than as an electron or muon.
Its mean life is 2.9 × 10⁻¹³ s, so even at relativistic speeds a tau travels well under a millimetre before decaying, and detectors identify it by a displaced vertex rather than a track. Note that the tau's mass is known to only about 7 parts per 100 000 — feeble by CODATA standards but limited by threshold-scan statistics at electron-positron colliders, not by any technique that could be pushed much further.