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Background:The neutron β-decay asymmetry parameter A 0 defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A 0 permit an extraction of the ratio of the weak axial-vector to vector coupling constants, λ ≡ g A /g V , which under assumption of the conserved vector current hypothesis (g V = 1) determines g A . Precise values for g A are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron β-decay angular correlation performed with UCN. This article reports the most precise result for A 0 obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A 0 .

A new measurement of the neutron β-decay asymmetry A 0 has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to A 0 = −0.11954(55) stat (98) Precision measurements of neutron β decay are an essential ingredient in understanding the electro-weak interaction in the light quark sector. In particular the axial-vector weak coupling constant, g A , is an important input to understanding the spin and flavor structure of the nucleon [1,2] and is being actively studied in detailed lattice QCD calculations [3,4]. It also plays an important role in a variety of astrophysical processes, including solar fusion cross sections important for energy and neutrino production in the Sun [5].The angular distribution of emitted electrons from decays of a polarized neutron ensemble can be expressed as [6]where A(E) specifies the decay asymmetry for electron energy E, v ≡ βc is the electron velocity, P is the mean neutron polarization, and θ is the angle between the neutron spin and the electron momentum. can be expressed aswhere λ ≡ g A /g V is the ratio of the vector to axial-vector weak coupling constants. Combining g A with independent measurements of the Fermi coupling constant G F , the Cabibbo-Kobayashi-Maskawa matrix element V ud , and the neutron lifetime τ n allows a precision test of the consistency of measured neutron β-decay observables [7]. The ultracold neutron asymmetry (UCNA) experiment is the first experiment to use ultracold neutrons (UCNs) in a precision measurement of neutron decay correlations. Following the publication of our earlier results [7][8][9], the UCNA Collaboration implemented a number of experimental improvements that led to reductions in both statistical and systematic uncertainties. These improvements, described below, include enhanced UCN storage, improved electron energy reconstruction, and continuous monitoring of the magnetic field in the spectrometer. This refined treatment of the systematic corrections and uncertainties begins to address issues of consistency in the world data set for A 0 .The UCNA experiment ran in 2010 using the "thin window geometry D" as described in [7,9], and collected a total of 20.6 × 10 6 β-decay events after all cuts were applied. We used the UCN source [10] Copyright by the American Physical Society. Mendenhall, M. P. ; Pattie, R. W., Jr. ; Bagdasarova, Y. ; et al., Mar 25, 2013. "Precision measurement of the neutron beta-decay asymmetry," PHYSICAL REVIEW C 87(3): 032501.

The Pauli exclusion principle (PEP) has been tested for nucleons (n, p) in 12 C with the Borexino detector.The approach consists of a search for γ, n, p and β ± emitted in a non-Paulian transition of 1P 3/2shell nucleons to the filled 1S 1/2 shell in nuclei. Due to the extremely low background and the large mass (278 t) of the Borexino detector, the following most stringent up-to-date experimental bounds on PEP violating transitions of nucleons have been established: τ ( 12 C → 12 C + γ) ≥ 5.0 · 10 31 y, τ ( 12 C → 11 B + p) ≥ 8.9 · 10 29 y, τ ( 12 C → 11 C + n) ≥ 3.4 · 10 30 y, τ ( 12 C → 12 N + e − + νe) ≥ 3.1 · 10 30 y and τ ( 12 C → 12 B + e + + νe) ≥ 2.1 · 10 30 y, all at 90% C.L. The corresponding upper limits on the relative strengths for the searched non-Paulian electromagnetic, strong and weak transitions have been estimated: δ 2 γ ≤ 2.2 · 10 −57 , δ 2 N ≤ 4.1 · 10 −60 and δ 2 β ≤ 2.1 · 10 −35 .

Ultra-cold neutrons (UCN), neutrons with energies low enough to be confined by the Fermi potential in material bottles, are playing an increasing role in measurements of fundamental properties of the neutron. The ability to manipulate UCN with material guides and bottles, magnetic fields, and gravity can lead to experiments with lower systematic errors than have been obtained in experiments with cold neutron beams. The UCN densities provided by existing reactor sources limit these experiments. The promise of much higher densities from solid deuterium sources has led to proposed facilities coupled to both reactor and spallation neutron sources. In this paper we report on the performance of a prototype spallation neutron-driven solid deuterium source. This source produced bottled UCN densities of 145 +/-7 UCN/cm3, about three times greater than the largest bottled UCN densities previously reported. These results indicate that a production UCN source with substantially higher densities should be possible