We have measured the shift and width of the kaonic hydrogen 1s state due to the KN strong interaction. We have observed, for the first time, distinct K-series kaonic hydrogen x rays with good signal-to-noise ratio in the energy spectrum. The measured energy shift and width were determined to be DE͑1s͒ 2323 6 63͑stat͒ 6 11͑syst͒ eV (repulsive) and G͑1s͒ 407 6 208͑stat͒ 6 100͑syst͒ eV, respectively. [S0031-9007(97)02992-X] PACS numbers: 13.75. Jz, 25.80.Nv, 29.30.Kv, 36.10.Gv The determination of the strong-interaction energy level shift and width of the kaonic hydrogen x rays is one of the most important subjects for the understanding of the KN interaction. It is strongly affected by the presence of the L͑1405͒ subthreshold resonance. The study of the KN interaction is also relevant to the important question of K 2 condensation in dense matter [1,2].The observation of the shift and width of the kaonic hydrogen K a ͑2p ! 1s͒ x rays gives direct information about the KN s-wave interaction at the K 2 p threshold energy in a fairly model independent way [3]. The status of the study was quite puzzling due to the contradiction between the signs of the scattering lengths obtained by the previous x-ray measurements [4-6] and those extracted from the analyses of the low energy KN data, e.g., , as shown in Fig. 1. This contradiction is known to be almost impossible to reconcile within the conventional theoretical framework. Moreover, the x-ray signals of the previous experiments are very difficult to identify in their spectra. Therefore, a definitive experiment has been long awaited.We accumulated data for 760 hours at KEK-PS K3. A detailed description of our experimental setup is given in a separate paper [10]. Here we present a short summary.Optimization of the target density is quite important for this experiment. As a compromise between kaon stopping yield and kaon loss during the atomic cascade due to the Stark effect, we chose to operate the hydrogen FIG. 1. The energy shift and width of 1s state. One-standarddeviation region of shift and width of the previous experiments are plotted together with theoretical calculations. The present result is shown in bold.

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The violation of mirror symmetry in the weak force provides a powerful tool to study the internal structure of the proton. Experimental results have been obtained that address the role of strange quarks in generating nuclear magnetism. The measurement reported here provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. We also report evidence for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies in atomic systems such as cesium.

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 violation of baryon number, B , is an essential ingredient for the preferential creation of matter over antimatter needed to account for the observed baryon asymmetry in the Universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR program is a proposed two-stage experiment at the European Spallation Source to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units: free neutron–antineutron oscillation ( n → n ̄ ) via mixing, neutron–antineutron oscillation via regeneration from a sterile neutron state ( n → [ n ′ , n ̄ ′ ] → n ̄ ), and neutron disappearance (n → n′); the effective Δ B = 0 process of neutron regeneration ( n → [ n ′ , n ̄ ′ ] → n ) is also possible. The program can be used to discover and characterize mixing in the neutron, antineutron and sterile neutron sectors. The experiment addresses topical open questions such as the origins of baryogenesis and the nature of dark matter, and is sensitive to scales of new physics substantially in excess of those available at colliders. A goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches. The opportunity to make such a leap in sensitivity tests should not be squandered. The experiment pulls together a diverse international team of physicists from the particle (collider and low energy) and nuclear physics communities, while also including specialists in neutronics and magnetics.

Abstract. The electric dipole moment of the neutron, leptons, and atoms provide a unique window to Physics Beyond the Standard Model. We are currently developing a new neutron EDM experiment (the nEDM Experiment) [4]. This experiment, which will be run at the 8.9Å Neutron Line at the Fundamental Neutron Physics Beamline (FNPB) at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory, will search for the neutron EDM with a sensitivity two orders of magnitude better than the present limit. In this paper, the motivation for the experiment, the experimental method, and the present status of the experiment are discussed. IntroductionA nonzero permanent electric dipole moment (EDM) of a nondegenerate state of a system with spin J = 0 violates the invariance under time reversal as well as the invariance under parity operation. The violation of time reversal invariance implies a violation of invariance under CP operation (combined operations of parity and charge conjugation) through the CP T theorem.Within the standard model (SM), in the electroweak sector CP symmetry is broken by the complex phase (δ KM ) in the CKM quark mixing matrix (the KM mechanism). To date, in laboratory measurements CP violation has only been observed in K and B meson decays and the SM description of the CP violation agrees with all the laboratory measurements to date. However, the question remains whether or not there are additional sources of CP violation from new physics. Indeed, almost all extensions of the SM imply that there are such additional sources. Moreover, CP violation is one of the necessary conditions for the matter-antimatter asymmetry observed in the Universe, and the SM and its description of CP violation fail to accommodate the observed asymmetry. This discrepancy suggests that there are additional sources of CP violation beyond that in the SM.The current efforts to search for an electric dipole moment (EDM) of the neutron are motivated by the following two observations, which make the neutron EDM (as well as the EDM of other particles) an ideal place to search for new sources of CP violation:

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