Robert Golub

Neutron electric-dipole moment, ultracold neutrons and polarized 3He

Abstract A brief review of the history of the experimental search for the neutron electric-dipole moment (EDM) is presented, followed by a discussion of the “state of the art” experimental techniques based on the storage of ultracold neutrons. Also discussed is the recent work on the construction of an improved experiment incorporating a 199 Hg magnetometer within the ultracold neutron storage volume. We then review a number of well-known experimental and theoretical results and propose an entirely new experimental technique to search for the neutron EDM based on storing together, in superfluid 4 He, polarized ultracold neutrons and a polarized gas of 3 He atoms; this forms a unique system of two spins interacting by means of a spin-dependent mutual absorption. Such a system appears to be ideally suited for use in a neutron EDM search. Following a brief description of the method, we present an analysis of the dynamics of such a system and calculate the statistical uncertainties to be expected in an EDM search. We show that, in principle, improvement by a factor of over 1000 in the experimental limit is possible. This limit would be more than sufficient to determine whether the known CP violation leads to the observed cosmological baryon asymmetry and, in addition, would set very strict limits on the supersymmetric, multi-Higgs, and left-right-symmetric models of CP violation. We conclude with a discussion of some technical questions related to the proposed experimental technique.

Magnetic trapping of neutrons

Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force and the creation of matter during the Big Bang. Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral-particle and charged-particle traps are widely used for studying both composite and elementary particles, because they allow long interaction times and isolation of particles from perturbing environments. Here we report the magnetic trapping of neutrons. The trapping region is filled with superfluid 4He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons in the trap have a lifetime of 750+330-200 seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement.

Magnetically stabilized luminescent excitations in hexagonal boron nitride

Magnetically stabilized luminescence is observed in hexagonal boron nitride. The luminescence is induced by absorption of cold neutrons and is in the visible region. In the absence of a magnetic field, the photon emission level is observed to decay over several hundred seconds. A fraction of this luminescence can be suppressed if the temperature is T . 0.6 K and the magnetic field is B & 1.0 T. Subsequent to irradiation and suppression, luminescence can be induced by an increase in T or lowering of B. Possible explanations include stabilization of triplet states or the localization and stabilization of excitons.

Detection of neutron-spin rotation in neutron scattering by 204Pb

The effect of neutron-spin rotation due to the parity-nonconserving interaction of neutrons with nuclei in a lead sample enriched in the 204Pb isotope had been measured up to the end of 1999. The problem that initiated the experiment was the earlier observed effect of neutron-spin rotation in natural lead and futile attempts at discovering this effect in isotopes constituting natural lead. At present, the final data processing has been completed. A simple model of the experiment is proposed and considered. After a careful consideration, some possibility of evaluating the instrumental error is revealed and successfully used in the array of the data obtained. The result obtained for the neutron-spin-rotation angle in a lead sample is (8±2)×10−5 rad/cm for lead containing 100% 204Pb. This value corresponds to the proposition that the presence of 204Pb is responsible for the observed effect of neutron-spin rotation in natural lead.

A new experiment to measure the electric dipole moment of the neutron

For nearly fifty years, the limits on the electric dipole moment of the neutron have provided information of great importance in our understanding of the fundamental symmetries of nature. Current experiments using bottled Ultra Cold Neutrons (UCN) provide the best experimental limits on the neutron EDM. While modest improvements may be expected by extension of current methods, major reductions in the experimental error appear unlikely due to statistical sensitivity and systematic effects. This situation is unfortunate as several theoretical notions (supersymmetry and the origin of the baryon asymmetry) suggest a magnitude for the neutron EDM which may be only one or two orders of magnitude below the current limit. Recently, Golub and Lamoreaux (1) have suggested a new method for the measurement of the neutron EDM that uses a novel feature of the interaction between low energy neutron and superfluid 4He to provide a very high density of UCN in an experimental volume. The proposed method also promises a sig...