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The Nab experiment will measure the correlation a between the momenta of the beta particle and antineutrino in neutron decay as well as the Fierz term b which distorts the beta spectrum.

We are developing an experiment to measure the correlations a, A, and B, and the Fierz interference term b in neutron decay, with a precision of approximately 10(-4). The experiment uses an electromagnetic spectrometer in combination with two large-area segmented silicon detectors to detect the proton and electron from the decay in coincidence, with 4π acceptance for both particles. For the neutron-polarization-dependent observables A and B, precision neutron polarimetry is achieved through the combination of a pulsed neutron beam, under construction at the SNS, and a polarized (3)He neutron polarizer. Measuring a and A in the same apparatus provides a redundant determination of λ = gA/gV . Uncertainty in λ dominates the uncertainty of CKM unitarity tests.

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, \\(\\lambda = g_A / g_V\\), through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter \\(a\\) with a precision of \\(\\delta a / a = 10^{-3}\\) and the Fierz interference term \\(b\\) to \\(\\delta b = 3\\times10^{-3}\\) in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio \\(\\lambda\\) with a precision of \\(\\delta \\lambda / \\lambda = 0.03\\%\\) that will allow an evaluation of \\(V_{ud}\\) and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.

The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, \\(R_{2\\gamma}\\), a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of \\(\\approx 20\\degree\\) to \\(80\\degree\\). The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at \\(12\\degree\\), as well as symmetric Møller/Bhabha calorimeters at \\(1.29\\degree\\). A total integrated luminosity of 4.5~fb\\(^{-1}\\) was collected. In the extraction of \\(R_{2\\gamma}\\), radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of \\(R_{2\\gamma}\\), presented here for a wide range of virtual photon polarization \\(0.456<\\epsilon<0.978\\), are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.

Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.

The roles played by mesons in the electromagnetic form factors of the nucleon are explored using as a basis a model containing vector mesons with coupling to the continuum together with the asymptotic \\(Q^2\\) behavior of perturbative QCD. Specifically, the vector dominance model (GKex) developed by Lomon is employed, as it is known to be very successful in representing the existing high-quality data published to date. An analysis is made of the experimental uncertainties present when the differences between the GKex model and the data are expanded in orthonormal basis functions. A main motivation for the present study is to provide insight into how the various ingredients in this model yield the measured behavior, including discussions of when dipole form factors are to be expected or not, of which mesons are the major contributors, for instance, at low-\\(Q^2\\) or large distances, and of what effects are predicted from coupling to the continuum. Such insights are first discussed in momentum space, followed by an analysis of how different and potentially useful information emerges when both the experimental and theoretical electric form factors are Fourier transformed to coordinate space. While these Fourier transforms should not be interpreted as \"charge distributions\", nevertheless the roles played by the various mesons, especially which are dominant at large or small distance scales, can be explored via such experiment--theory comparisons.

The Nab collaboration will perform a precise measurement of 'a', the electron-neutrino correlation parameter, and 'b', the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will provide an independent measurement of lambda = G_A/G_V, the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of 'b' in neutron decay, which will provide an independent limit on the tensor weak coupling.

We are developing an experiment to measure the correlations a, A, and B, and the Fierz interference term b in neutron decay, with a precision of approximately 10-4. The experiment uses an electromagnetic spectrometer in combination with two large-area segmented silicon detectors to detect the proton and electron from the decay in coincidence, with 4pi acceptance for both particles. For the neutron-polarization-dependent observables A and B, precision neutron polarimetry is achieved through the combination of a pulsed neutron beam, under construction at the SNS, and a polarized 3He neutron polarizer. Measuring a and A in the same apparatus provides a redundant determination of lambda = gA/gv. Uncertainty in 2 dominates the uncertainty of CKM unitarity tests.

We report a measurement of two energy-weighted gamma cascade angular distributions from polarized slow neutron capture on the \\({}^{35}\\)Cl nucleus, one parity-odd correlation proportional to \\(\\vec{s_{n}} \\cdot \\vec{k_{\\gamma}}\\) and one parity-even correlation proportional to \\(\\vec{s_{n}} \\cdot \\vec{k_{n}} \\times \\vec{k_{\\gamma}}\\). A parity violating asymmetry can appear in this reaction due to the weak nucleon-nucleon (NN) interaction which mixes opposite parity S and P-wave levels in the excited compound \\(^{36}\\)Cl nucleus formed upon slow neutron capture. If parity-violating (PV) and parity-conserving (PC) terms both exist, the measured differential cross section can be related to them via \\(\\frac{d\\sigma}{d\\Omega}\\propto1+A_{\\gamma, PV}\\cos\\theta+A_{\\gamma,PC}\\sin\\theta\\). The PV and PC asymmetries for energy-weighted gamma cascade angular distributions for polarized slow neutron capture on \\(^{35}\\)Cl averaged over the neutron energies from 2.27~meV to 9.53~meV were measured to be \\(A_{\\gamma,PV}=(-23.9\\pm0.7)\\times 10^{-6}\\) and \\(A_{\\gamma,PC}=(0.1\\pm0.7)\\times 10^{-6}\\). These results are consistent with previous experimental results. Systematic errors were quantified and shown to be small compared to the statistical error. These asymmetries in the angular distributions of the gamma rays emitted from the capture of polarized neutrons in \\(^{35}\\)Cl were used to verify the operation and data analysis procedures for the NPDGamma experiment which measured the parity-odd asymmetry in the angular distribution of gammas from polarized slow neutron capture on protons.

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.