S. A. Page

Hadronic parity violation: A New view through the looking glass

Studies of the strangeness-changing hadronic weak interaction have produced a number of puzzles that have evaded a complete explanation within the Standard Model. Their origin may lie either in dynamics peculiar to weak interactions involving strange quarks or in more general aspects of the interplay between strong and weak interactions. In principle, studies of the strangeness-conserving hadronic weak interaction using parity-violating hadronic and nuclear observables provide a complementary window to this question. However, progress in this direction has been hampered by the lack of a suitable theoretical framework for interpreting hadronic parity violation measurements in a model-independent way. Recent work involving effective field theory ideas has led to the formulation of such a framework, simultaneously motivating the development of a number of new hadronic parity violation experiments in few-body systems. We review these recent developments and discuss the prospects and opportunities for further experimental and theoretical progress.

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.

Search for double-Λ hypernuclei and the H-dibaryon in the (K−,K+) reaction on 12C

Abstract A search for double- Λ hypernuclei ( 12 ΛΛ Be) and H -dibaryons using the 12 C( K − , K + ) reaction was performed at the BNL-AGS using a high-intensity 1.8 GeV/ c K − beam. A missing-mass analysis below the end point of the quasi-free Ξ − production was used to investigate these S =−2 systems. The upper limit obtained for the forward-angle cross section of 12 ΛΛ Be production is 6 to 10 nb/sr. This is the first search for the direct production of double- Λ hypernuclei to reach the sensitivity required to observe the signal predicted by theoretical calculations. For the H -production cross section, we have obtained an upper limit in the range of a few nb/sr to 10 nb/sr for the H mass below 2100 MeV/ c 2 . This upper limit is the most sensitive H search result to date for a tightly bound H .

A time-of-flight array for 1 to 2 GeV/c particles

A time-of-flight detector array has been developed for an experiment searching for the strangeness −2 H-particle. The array consists of 40 logs of plastic scintillator with dimensions 2.00 × 0.085 × 0.050 m3. The photomultiplier tubes are coupled to the scintillators without the use of light guides, and the mounting of the bars is designed for easy adjustment and servicing. The average intrinsic time resolution was found to be 110 ps σ.

A measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on 35Cl, 113Cd, and 139La

Abstract An apparatus for measuring parity-violating asymmetries in gamma-ray emission following polarized cold neutron capture was constructed as a 1/10th scale test of the design for the forthcoming n → + p → d + γ experiment at LANSCE. The elements of the polarized neutron beam, including a polarized 3 He neutron spin filter and a radio frequency neutron spin rotator, are described. Using CsI(Tl) detectors and photodiode current mode readout, measurements were made of asymmetries in gamma-ray emission following neutron capture on 35 Cl , 113 Cd , and 139 La targets. Upper limits on the parity-allowed asymmetry s n ·( k γ × k n ) were set at the level of 7×10 −6 for all three targets. Parity-violating asymmetries s n · k γ were observed in 35 Cl , A γ =(−29.1±6.7)×10 −6 , and 139 La , A γ =(−15.5±7.1)×10 −6 , values consistent with previous measurements.

Transverse Beam Spin Asymmetries at Backward Angles in Elastic Electron-Proton and Quasielastic Electron-Deuteron Scattering

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasielastic scattering on the deuteron, at backward angles (lab scattering angle of 108°) for Q² = 0.22 GeV²/c² and 0.63 GeV²/c² at beam energies of 362 and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single-photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (πN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasistatic deuterium approximation, and is also in agreement with theory.

A beam intensity profile monitor based on secondary electron emission

Two dual function intensity profile monitors have been designed for a measurement of parity violation in p-p scattering at about 230 MeV using longitudinally polarized protons. Each device contains a set of split secondary electron emission (SEM) foils to determine the median of the beam current distribution (in x and y). The split foils, coupled through servoamplifiers and operational amplifiers to upstream aircore steering magnets, have demonstrated the ability to hold the beam position stable to ±5 μm for beam position fluctuations up to 1000 Hz. These monitors also contain a set of foil strip planes giving information on the intensity distribution projected onto the two orthogonal axes, x and y. Data were acquired using 0.008 mm thick, 0.90 mm wide aluminum foil strips at 1.00 mm centers. The foil strip planes were able to determine the beam centroid to within ±3 μm after one hour of data taking with a 100 nA, 15 mm FWHM Gaussian beam.

Searching for the H-Dibaryon at Brookhaven

At the Brookhaven AGS several experiments are searching for the unique strangeness S = −2 H-dibaryon with the quark composition (uuddss). The E813/E836 collaboration, in particular, is using a high-intensity, separated 1.8 GeV/c K − beam and two different target configurations. In E836 the reaction K − + 3He→K + + H + n is used to search for a relatively deeply-bound H. Complementary to E836 the reactions K − + p → Ξ − + K +, followed by (Ξ −, d)atom → H + n are used to search near twice the A mass. The status of these two experiments is summarized, and other H-dibaryon searches are briefly reviewed.

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.

Parity violation in P‐P scattering at TRIUMF

The TRIUMPF experiment 497 which will measure the (weak) parity violating component of the nucleon‐nucleon interaction with proton‐proton quasi‐elastic scattering at 223 MeV is described. The longitudinal analyzing power Az=(σ+−σ−)/(σ++σ−) where σ+ and σ− are the scattering cross sections for positive and negative beam helicity, respectively, with an expected precision of ±2×10−8. (AIP)