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 framework(1-3) and heavy baryon chiral perturbation theory(4) (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) x 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62639/1/nature05390.pd

How child‐centred education favours some learners more than others

Debates on how best to educate young children have been raging over the last 100 years—more often fuelled by ideological preferences rather than empirical evidence. To some extent this is hardly surprising given the difficulty of examining pupil progress in a systematic and comparative way. However, the introduction of a new child‐centred curriculum in Wales provides the opportunity to undertake just such an examination. The Foundation Phase curriculum, introduced in 2008, is designed to provide all 3‐ to 7‐year‐olds with a developmental, experiential, play‐based approach to learning. Evidence from a major 3‐year evaluation of this intervention finds that, overall, pupil progress and well‐being is fostered in those settings where the principles of the Foundation Phase have been most closely followed. However, the evidence also suggests that even within these contexts, progress is uneven and that some kinds of children seem to gain more from this approach than others. The ‘losers’ appear to be boys and those living in poverty. Drawing on the theories of Basil Bernstein, the paper explores why this may be the case and examines the relative significance of teacher dispositions, teacher–learner dynamics and the availability of resources. The paper concludes by arguing that these issues will need to be addressed if the benefits of child‐centred approaches are to benefit all

Searching for time reversal invariance violation in polarized neutron decay

Time reversal invariance violation is tightly constrained in the Standard Model, and the existence of a T-violating effect above the predicted level would be an indication of new physics. A sensitive probe of this symmetry in the weak interaction is the measurement of the D-coefficient in neutron decay. This parameter characterizes the triple-correlation of neutron spin, electron momentum, and neutrino (or proton) momentum, which changes sign under time reversal. The emiT experiment, now on line, attempts to improve the measurement of D,D, whose current average is 0.3±1.5×10−3.0.3±1.5×10−3. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87907/2/399_1.pd