F. W. Hersman

Measurement of the Neutral Weak Form Factors of the Proton

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [{l_angle}{theta}{sub lab }{r_angle}=12.3{degree} and {l_angle}Q{sup 2}{r_angle}=0.48 (GeV /c){sup 2} ] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G{sup s}{sub E} . The result, A={minus}14.5{plus_minus}2.2 ppm , is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies G{sup s}{sub E}+0.39G{sup s}{sub M}=0.023 {plus_minus}0.034(stat){plus_minus}0.022( syst){plus_minus}0.026({delta}G{sup n}{sub E}) , where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor. {copyright} {ital 1999} {ital The American Physical Society}

The time-of-flight system for CLAS

The time-of-flight system for the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility is described. The system, covering an area of 206-square-meters, is composed of scintillation counters 5.08 cm thick, 15 and 22 cm wide, and lengths which vary from 32 cm at the most forward angle to 450 cm at larger angles. All of the components of the system have been designed to optimize the time resolution. Event timing, achieved by leading-edge discrimination with time-walk correction, has been measured with cosmic rays, a laser pulser, and known particle interactions. The intrinsic time resolution varies from about 80 ps for the short counters to 160 ps for the longer counters. Reconstruction of interacting particles during the first period of operation yields an average time resolution for electrons of 163 ps.

Nab: Measurement principles, apparatus and uncertainties

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{sup -3} accuracy level in {Delta}a/a, and will provide an independent measurement of {lambda} = G{sub A}/G{sub 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.

Neutron Beta Decay Studies with Nab

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.

Upper Bounds on Parity Violating Gamma-Ray Asymmetries in Compound Nuclei from Polarized Cold Neutron Capture.

Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states. Using a statistical approach, we estimate the root-mean-square of the distribution of expected parity-odd correlations s(vector sign){sub n}{center_dot}k(vector sign){sub {gamma}}, where s(vector sign){sub n} is the neutron spin and k(vector sign){sub {gamma}} is the momentum of the {gamma}, in the integrated {gamma} spectrum from the capture of cold polarized neutrons on Al, Cu, and In. We present measurements of the asymmetries in these and other nuclei. Based on our calculations, large enhancements of asymmetries were not predicted for the studied nuclei and the statistical estimates are consistent with our measured upper bounds on the asymmetries.

Analysis of sparsely distributed data: an application of the extended maximum-likelihood method

Abstract Experimental measurements of quantities depending on several degrees of freedom are challenging to conventional fitting techniques. To address this problem we have developed and implemented a technique for fitting distribution functions to low-statistics data using the Poisson probability distribution function. We present a derivation of the likelihood function, which parallels χ 2 , and discuss its properties. Finally, we apply this technique to the extraction of a cross section from a recent 12 C(e, e′pp) triple-coincidence experiment. From a data set dominated by random-coincidences containing only a few hundred events, we were able to determine the number of true triple-coincidence events and its experimental uncertainty.

NPDGamma: A measurement of the parity-violating gamma asymmetry A γ in n-p capture

The NPDGamma Experiment measures the parity-violating correlation Aγ between neutron spin and photon momentum in the reaction + p → d + γ. Knowledge of Aγ and other parity-violating observables in few-body nuclear systems will provide constraints for a parameterized description of ΔS = 0 parity-violating phenomena free from complications of nuclear structure. The NPDGamma experiment uses a polarized cold pulsed neutron beam, a liquid parahydrogen target, and a cylindrical array of 48 CsI(Tl) scintillation detectors operated in current mode to search for the asymmetry. NPDGamma recently completed the first phase of the program to measure Aγ at the Los Alamos Neutron Science Center with the preliminary result Aγ = (−1.2 ± 2.1(stat.) ± 0.1(sys.)) × 10−7, reproducing the previous upper limit from a measurement at a reactor facility. We discuss the theoretical background and experimental method and report on preliminary analysis of the LANSCE data. The second phase of the program to measure Aγ is in progress at the Spallation Neutron Source at Oak Ridge National Laboratory.

Nuclear charge densities in the transition region: 192Os

Abstract Data from the first electron-scattering study of an Os-Pt transition region nucleus, 192 Os, are analyzed in combination with moonic-atom data by using the model-independent Fourier-Bessel method. The results are discussed in terms of available IBA and BET model calculations. New Hartree-Fock (DME) calculations are presented as an example of the agreement achieved with microscopic-based theories.

Neutron Decay Correlations in the Nab Experiment

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.

Measurement of Parity‐Violating Gamma‐ray Asymmetry in Compound Nuclei with Cold Neutrons

The NPDGamma collaboration has constructed and commissioned an apparatus on flight path 12 at LANSCE to measure with a high precision, 5×10−9, the small parity‐violating gamma‐ray asymmetry, Aγ, in polarized neutron capture on protons. This asymmetry can be determined unambiguously the weak pion‐nucleon coupling constant. To study the hadronic weak interaction at low energy, the collaboration has used the NPDGamma apparatus to measure parity‐violating gamma‐ray asymmetries in compound nuclei with cold neutrons. Using the statistical model of compound nuclei and spectroscopic information of the target nuclei, we can set upper limit on the spreading width of the hadronic weak interaction for intermediate‐mass nuclei. We describe the experiment and the preliminary results of measured gamma‐ray asymmetries of Al, Sc, Ti, Mn, and Co.