R. Prieels

Positron-annihilation in Liquid-helium and Liquid Argon Under An Electric-field

We have made new measurements of the lifetimes at 4.29 K and 2.92 K in He I, and at 2.14 K and 1.67 K in He II, without and with an electric field. At 4.19 K we have a new value of the lifetime, 2.01 +/- 0.002 ns, leading to a Z(eff) of 3.53 at zero field. In contrast to earlier reports, Z(eff) decreases very slightly as the temperature is lowered and the density increases. The orthopositronium lifetime at 4.19 K is 98.1 +/- 0.2 ns, at zero field, independent of field up to 18 kV cm(-1). At 4.19 K the free positron decay rate falls exponentially with increasing field to a value 5% below its zero-field value, the drop-off rate being 0.36 +/- 0.04 cm amagat V-1. From 28 through 50 V cm(-1) amagat(-1) the decay rate of the free component rises slowly as expected because of the increasing positronium fraction over the same range of electric field. In liquid argon there is positronium enhancement beginning at or below 3 V cm(-1) amagat(-1), which contrasts with 160 V cm(-1) amagat(-1) for argon gas. At least two lifetimes make up the long triplet component, suggesting different degrees of thermalization, or different states of positronium within the liquid. The effect of the electric field is to enhance the longer-lived, or more thermal component, in strong contrast with the findings of Charlton et al for the gas. The argon results suggest positron behaviour similar to that of electrons in the liquid phase, namely, such particles move in a conduction band subject to small effective scattering and diffusion cross sections. An analytic model in which the scattering length is an adjustable parameter yields the onset of positronium enhancement just below 3 V cm(-1) amagat(-1) when a(s) = -0.043 a(0), corresponding to an elastic scattering and diffusion cross section of 0.034 pi a(0)(2). Consequences for positron thermalization are discussed. A model of positronium losing energy slowly in the liquid without bubble formation is suggested.

A novel approach for measuring the beta-neutrino angular correlation in nuclear beta decay

The experiment described here will search for deviations from the V-A structure of the standard electroweak model. It is based on measuring the recoil energy spectrum in nuclear beta decay which is determined by the electron-neutrino angular correlation. For pure Fermi decays this is exactly known in the standard model and any deviation will point to additional scalar interaction. The experiment consists of a Penning trap coupled to a retardation spectrometer to measure the energy of the recoiling daughter nuclei. The current status will be presented.

Search for physics beyond the standard model via a polarization-asymmetry correlation experiment on 107In

We report on a new precision measurement of the longitudinal polarization of positrons emitted by polarized 107In nuclei. Preliminary results yield a lower limit of 303 GeV/c2 for the mass of a possible, predominantly right-handed, W gauge boson, if interpreted in the framework of the manifest, left-right symmetric model. In more general left-right symmetric models our result is complementary to those from muon decay experiments and to results from searches for a similar particle in collider experiments. Our data also provide new results on possible tensor contributions to the weak interaction.