Neutron lifetime puzzle

A free neutron eventually decays into a proton, an electron and an antineutrino. How long does it live on average? One method passes a neutron beam through a region and counts the protons produced there. Another stores very slow neutrons and compares how many survive after different waiting times. These methods give incompatible lifetimes. What causes the disagreement, and which lifetime do independent methods consistently support?[1] [2] [3]

A free neutron decays into a proton, an electron and an electron antineutrino. Proton-counting beam experiments and experiments measuring the survival of stored neutrons infer different mean lifetimes, the averages before decay. Which detection or loss effects, or other physics, explain this disagreement, and what value do independently controlled methods support?

A beam measurement infers the decay rate from detected products relative to the neutron population in a known volume. Missing protons, if not corrected, would undercount decays and bias the inferred lifetime upward. A bottle measurement compares survivors after different storage times. Losses other than decay, if not corrected, would bias the inferred lifetime downward. These are possible systematic effects of the apparatus or analysis, not established explanations. Magnetic storage avoids losses from striking material walls but still requires checks of residual gas, heating, population evolution and detector response.

The 2025 UCNτ analysis varied detection geometries and improved monitoring and gas-loss corrections. J-PARC instead counts decay electrons in a gas detector, normalizing them using neutron captures on helium-3. This has different background and efficiency uncertainties from counting protons. Its bottle-compatible result is an independent comparison, not a calibration that explains the proton-counting result.[1] [2] [3]

The proton-counting discrepancy is unexplained; a beam test counting electrons agrees with stored-neutron results.[1] [2] [3]

The 2025 bottle result does not reconcile proton-counting beam values. J-PARC’s 2024 manuscript reports 877.2 seconds with statistical and systematic uncertainties, compatible with bottles; it does not explain the proton-counting discrepancy.[2] [3]

Neutron decay helped set how many neutrons survived in the early universe to become bound in light nuclei. Its lifetime therefore affects predicted element abundances. It also sets a reference rate for testing the weak interaction that causes the decay.[1] [2]

Combining the lifetime with other neutron-decay observables tests the weak interaction without the nuclear-structure corrections needed for a bound nucleus. The lifetime also controls neutron survival in primordial light-element calculations. Resolving the measurement ambiguity lets those comparisons test particle physics and early-universe models with a consistent decay-rate input.[1] [2]