Nature 444, 1059-1062 (21 December 2006)
Observation of the radiative decay mode of the free neutron
The theory of quantum
electrodynamics (QED) predicts that beta decay of the neutron into a proton,
electron and antineutrino should be accompanied by a continuous spectrum of soft
photons. While this inner bremsstrahlung branch has been previously measured in
nuclear beta and electron capture decay, it has never been observed in free
neutron decay. Recently, the photon energy spectrum and branching ratio for
neutron radiative decay have been calculated using two approaches: a standard
QED framework1, 2, 3 and heavy baryon chiral perturbation theory4 (an effective theory of hadrons based on the
symmetries of quantum chromodynamics). The QED calculation treats the nucleons
as point-like, whereas the latter approach includes the effect of nucleon
structure in a systematic way. Here we observe the radiative decay mode of free
neutrons, measuring photons in coincidence with both the emitted electron and
proton. We determined a branching ratio of (3.13 0.34)
10-3 (68 per cent level
of confidence) in the energy region between 15 and 340 keV, where the
uncertainty is dominated by systematic effects. The value is consistent with the
predictions of both theoretical approaches; the characteristic energy spectrum
of the radiated photons, which differs from the uncorrelated background
spectrum, is also consistent with the calculated spectrum. This result may
provide opportunities for more detailed investigations of the weak interaction
processes involved in neutron beta decay.