Decay Of Polarized Neutrons Research Articles

Ultracold neutron depolarization in magnetic bottles

We analyze the depolarization of ultracold neutrons confined in a magnetic field configuration similar to those used in existing or proposed magneto-gravitational storage experiments aiming at a precise measurement of the neutron lifetime. We use an extension of the semi-classical Majorana approach as well as an approximate quantum mechanical analysis, both pioneered by Walstrom et al. [Nucl. Instr. Meth. Phys. Res. A 599, 82 (2009)]. In contrast with this previous work we do not restrict the analysis to purely vertical modes of neutron motion. The lateral motion is shown to cause the predominant depolarization loss in a magnetic storage trap. The system studied also allowed us to estimate the depolarization loss suffered by ultracold neutrons totally reflected on a non-magnetic mirror immersed in a magnetic field. This problem is of preeminent importance in polarized neutron decay studies such as the measurement of the asymmetry parameter A using ultracold neutrons, and it may limit the efficiency of ultracold neutron polarizers based on passage through a high magnetic field.

Search for aT-odd,P-even triple correlation in neutron decay

Background: Time-reversal-invariance violation, or equivalently $CP$ violation, may explain the observed cosmological baryon asymmetry as well as indicate physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation $D\ensuremath{\langle}{\stackrel{P\vec}{J}}_{n}\ensuremath{\rangle}/{J}_{n}\ifmmode\cdot\else\textperiodcentered\fi{}({\stackrel{P\vec}{\ensuremath{\beta}}}_{e}\ifmmode\times\else\texttimes\fi{}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{p}}_{\ensuremath{\nu}})$ is a parity-even, time-reversal-odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects contribute to $D$ at the level of 10${}^{\ensuremath{-}5}$ and can be calculated with a precision of 1$%$ or better.Purpose: We have improved the sensitivity to $T$-odd, $P$-even interactions in nuclear $\ensuremath{\beta}$ decay.Methods: We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract $D$ and study systematic effects in a blind analysis.Results: We find $D=[\ensuremath{-}0.94\ifmmode\pm\else\textpm\fi{}1.89(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.97(\mathrm{sys})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This differs from the result of our recent paper [Phys. Rev. Lett. 107, 102301 (2011)] due to refinement of corrections for background and backscattering.Conclusions: This is the most sensitive measurement of $D$ in nuclear $\ensuremath{\beta}$ decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants (${C}_{A}/{C}_{V}=|\ensuremath{\lambda}|{e}^{i{\ensuremath{\varphi}}_{AV}}$) with ${\ensuremath{\varphi}}_{AV}=180.{012}^{\ensuremath{\circ}}\ifmmode\pm\else\textpm\fi{}0.{028}^{\ensuremath{\circ}}$ (68$%$ confidence level). This result can also be used to constrain time-reversal-violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks.

T-odd correlation in polarized neutron decay

T-odd correlation [pe, pν]η appears in the lowest order radiative correction to the polarized-neutron decay width, where δ is the polarization vector of a neutron. It appears as a consequence of the final-state interaction and should not be confused with real CP violation of the Standard Model. The asymmetry has the order of 10−5. Final-state interaction in the decay of an unpolarized neutron causes the appearance of transverse polarization of a recoil proton and electron. Explicit expressions are given.

New limit on theDcoefficient in polarized neutron decay

We describe an experiment that has set new limits on the time reversal invariance violating D coefficient in neutron $\ensuremath{\beta}$ decay. The emiT experiment measured the angular correlation $〈\mathbf{J}〉\ensuremath{\cdot}({\mathbf{p}}_{e}\ifmmode\times\else\texttimes\fi{}{\mathbf{p}}_{p})$ using an octagonal symmetry that optimizes electron-proton coincidence rates. The result is $D=[\ensuremath{-}0.6\ifmmode\pm\else\textpm\fi{}1.2(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.5(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$ This improves constraints on the phase of ${g}_{A}{/g}_{V}$ and limits contributions to T violation due to leptoquarks. This paper presents details of the experiment, data analysis, and the investigation of systematic effects.

Investigation of the antineutrino angular distribution in experiments on the β decay of polarized neutrons

Since the emission of γ grays unavoidably accompanies β decay, the final state after the β decay of a neutron includes a photon along with a proton, an electron, and an antineutrino, i.e., four particles, rather than three. Therefore, when only the electron and proton momenta are detected and the γ-ray momentum is not detected in an experiment, the antineutrino momentum cannot be uniquely reconstructed, and only its mean value over a γ-ray momentum distribution determined from corresponding calculations can be considered. The γ grays are significant for finding the asymmetry parameter B of the antineutrino angular distribution from experiments on the β decay of polarized neutrons, where the electron momentum p directed along the x axis and the projection of the proton momentum Px onto the x axis are detected, and the neutron polarization vector ξ is parallel or antiparallel to x. Since the γ rays are not detected in such experiments, the antineutrino kinematics are not uniquely specified by the observables p and Px and can be reconstructed only on the average, so that the antineutrino momentum distribution averaged over a γ-ray momentum distribution is considered. Thus, the exact value of B cannot be obtained from these experiments, but the true value of B can be estimated on the average by considering the mean (most likely) value 〈B〉 and the dispersion (rms deviation) ΔB. The unavoidable uncertainty in the estimate of B amounts to several percent and is thus significant for present-day experiments, which are intended to obtain the value of B to a very high accuracy of ∼ (0.1–1)%. If electromagnetic interactions are taken into account, measurements of the electron and proton momentum distributions can also be used to obtain gA, i.e., the axial β-decay amplitude, to high accuracy.

Electron spectra and electron-proton asymmetries in polarized neutron decay

Numerical results of recoil-order and model independent radiative correction calculations are presented for 4 electron spectra and 2 electron-proton asymmetry functions in polarized neutron decay. The inapplicability of previous analytical results for the experimental analyses is explained. A simple and precise formula for the Coulomb correction is also given.

Electron-neutrino angular correlation coefficientameasured from free-neutron decay

Measurements of the electron-neutrino angular correlation coefficient $a$ in the free-neutron decay are reported. The method is based on the measurement of the energy spectrum of recoil protons obtained from neutrons inside a highly evacuated tangential reactor beam tube. Spectroscopy was done outside the reactor shielding by means of an electrostatic condenser and an ion-electron converter detector of the coincidence type. The result, based on 30 measured proton-energy spectra, is $a=\ensuremath{-}0.099\ifmmode\pm\else\textpm\fi{}0.011$, giving $|\frac{{G}_{A}}{{G}_{V}}|=1.250\ifmmode\pm\else\textpm\fi{}0.036$. Thus a considerable improvement in accuracy concerning the measurement of $a$ of the free unpolarized neutron has been achieved. The result is in good agreement with the data obtained from polarized neutron decay.

New Experimental Limit onTInvariance in Polarized-NeutronβDecay

We report an improved experimental upper limit for D, the triple-correlation coefficient in polarized-neutron decay. A nonzero value for this coefficient implies a breakdown of T invariance. We find that D=−(1.1±1.7)×10−3, consistent with T invariance.

Measurement of Angular Correlations in the Decay of Polarized Neutrons

Measurement of Angular Correlations in the Decay of Polarized Neutrons

Electric Dipole Moment of a Nucleon

A violation of time-reversal invariance of any of the elementary particle interactions would imply the existence of a static electric-dipole moment for all these particles. We make here a theoretical estimate of the electric-dipole moment (${\ensuremath{\mu}}_{e}$) of a bare nucleon, assuming the existence of an intermediate vector boson ($W$) and a small violation of $T$ invariance for the weak interactions. With the existing experimental upper limit for $T$ violation from polarized neutron decay, we find that ${\ensuremath{\mu}}_{e}\ensuremath{\lesssim}\frac{e}{{m}_{N}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. Since a value for ${\ensuremath{\mu}}_{e}$ somewhat smaller than this upper limit seems to be experimentally observable, such an experiment would provide a good test for $T$ invariance as well as for the existence of $W$ mesons.

Measurements of Spatial Asymmetries in the Decay of Polarized Neutrons

A series of experiments has been carried out to examine the spatial symmetry properties of the beta decay of the free neutron. Measurements of angular distributions of the electrons and protons coming from the decay of polarized neutrons have shown that the correlation coefficient between the directions of electron momentum and neutron spin is -0.11\ifmmode\pm\else\textpm\fi{}0.02 and that between the antineutrino momentum and the neutron spin is 0.88\ifmmode\pm\else\textpm\fi{}0.15. The coefficient of the term proportional to the scalar triple product of antineutrino momentum, electron momentum, and neutron spin (which is not invariant under time reversal) is 0.04\ifmmode\pm\else\textpm\fi{}0.05. All these results are consistent with the $A\ensuremath{-}V$ theory of beta decay. Details of measurement techniques and the method of producing a beam of polarized neutrons are given.

Measurements of Asymmetries in the Decay of Polarized Neutrons

Previous measurements on the decay of polarized neutrons showed it to be non-isotropic with respect to the neutron spin. The coefficient for the correlation between the direction of emission of the beta particles and the neutron spin was shown to be too large in absolute value because of an interference from the correlation of neutrino direction with neutron spin. This correlation, of fundamental interest in beta-decay theory, was measured. The experimental apparatus is shown, and the results are tabulated. (D.E.B.)

Measurement of Beta Asymmetry in the Decay of Polarized Neutrons

The relative probability of beta emission by the neutron was measured in the directions respectively parallel and antiparallel to their spin. The angular distribution relative to the neutron spin direction is W( theta ) = 1- (0.37 plus or minus 0.11)(v/c) cos theta . (M.H.R.)