H. O. Meyer

Role of heavy-meson exchange in pion production near threshold

Recent calculations of s-wave pion production have severely underestimated the accurately known pp → pp� 0 total cross section near threshold. In these calculations, only the single-nucleon axial-charge operator is considered. We have calculated, in addition to the one-body term, the two-body contributions to this reaction that arise from the exchange of mesons. We find that the inclusion of the scalar �-meson exchange current (and lesser contributions from other mesons) increases the cross section by about a factor of five, and leads to excellent agreement with the data. The results are neither very sensitive to changes in the distorting potential that generates the NN wave function, nor to different choices for the meson-nucleon form factors. We argue that pp → pp� 0 data provide direct experimental evidence for meson

An internal microparticle target for a storage ring with electron cooling

The use of microparticles for internal targets in storage rings is investigated. The design and construction of a specific microparticle target are described in detail. In the scheme used here, the particulate material is suspended in a carrier gas and accelerated in the flow of the gas through a capillary. The resulting microparticle beam is crossed with the stored ion beam. The target has been tested in a stored, cooled proton beam at the Indiana Cooler. Target thicknesses achieved range from 3×1013 to 2×1015 atoms/cm2.

Polarizing stored beams by interaction with polarized electrons

A polarized, internal electron target gradually polarizes a proton beam in a storage ring. Here we derive the spin-transfer cross section for [ital [rvec e]]([ital p],[ital [rvec p]])[ital e] scattering. A recent measurement of the polarizing effect of a polarized atomic hydrogen target is explained when the effect of the atomic electrons is included. We also consider the interaction of a stored beam with a pure electron target which can be realized either by a comoving electron beam or by trapping of electrons in a potential well. In the future, this could provide a practical way to polarize antiprotons.

On the mean free path of medium-energy nucleons in nuclear matter☆

Abstract We show that the “experimental” value of the mean free path of nucleons in nuclear matter deduced from elastic nucleon—nucleus scattering is ambiguous and depends critically on the parametrization chosen for the radial shape of the optical potentials. We find that a realistic choice of radial potential shapes, based on recent theoretical advances in nucleon optical models, leads to derived values for the mean free path at medium energies which are compatible with a simple microscopic expectation.

Facility for studying spin dependence in pion production near threshold

Abstract We describe an experimental setup for the measurement of polarization observables in pion production near threshold. Experiments carried out with this facility use a polarized proton beam in the Indiana University Cooler storage ring, and an internal, polarized hydrogen target. The detector system measures energy, direction and velocity of multiple outgoing charged particles that are within a forward cone of about 32° opening angle. An array of scintillators also allows the detection of neutrons. In addition to the technical details of the apparatus, we describe the procedure for data acquisition, as well as some aspects of the analysis.

Faddeev Calculations of Breakup Reactions with Realistic Experimental Constraints

Abstract.We present a method to integrate predictions from a theoretical model of a reaction with three bodies in the final state over the region of phase space covered by a given experiment. The method takes into account the true experimental acceptance, as well as variations of detector efficiency, and eliminates the need for a Monte-Carlo simulation of the detector setup. The method is applicable to kinematically complete experiments. Examples for the use of this method include several polarization observables in dp breakup at 270 MeV. The calculations are carried out in the Faddeev framework with the CD Bonn nucleon-nucleon interaction, with or without the inclusion of an additional three-nucleon force.

Pion production near threshold

The production of pions in nucleon-nucleon collisions has recently attracted renewed attention because of the advent of highly precise, near-threshold total cross section data in three different channels, and because of the apparent failure of conventional theory to reproduce the magnitude of the s-wave production cross section, leading to the speculation that we might learn something new about the pion field of a nucleon. The onset of higher partial waves is observed in differential cross section and analyzing power measurements. The measurement of spin correlation coefficients close to threshold will become possible in the near future in storage rings with electron cooling and internal targets. Recent data on pion production in the three-nucleon system support the importance of a quasi-free production of pions from nuclei.

Gas jet density profile measured by electron scattering

Abstract Small-angle (1°) elastic scattering of 25 keV electrons has been utilized to obtain calibrated density profiles of supersonic, low-density H2 and N2 gas jets to be used as internal nuclear targets in a storage ring.

Experimental search for evidence of the three‐nucleon force and a new analysis method

A research program with the aim of investigating the spin dependence of the three nucelon continuum in p d collisions at intermediate energies was carried out at IUCF using the Polarized INternal Target Experiments (PINTEX) facility. In the elastic scattering experiment at 135 and 200 MeV proton beam energies a total of 15 independent spin observables were obtained. The breakup experiment was done with a vector and tensor polarized deuteron beam of 270 MeV and an internal polarized hydrogen gas target. We developed a novel technique for the analysis of the breakup observables, the sampling method. The new approach takes into account acceptance and non‐uniformities of detection efficiencies and is suitable for any kinematically complete experiment with three particles in the final state.

Use of microparticles as internal targets for nuclear physics with storage rings

Abstract We report on the development of ultrathin (1014 to 1016 at/cm2) internal targets for storage rings using microparticles. A “dust beam” is created by a gas-particle mixture flowing through a capillary into vacuum. In a laminar flow, the viscous drag accelerates the particles in the direction of the gas flow, while the Bernoulli force concentrates them near the axis of the tube. At the exit of the tube the gas diffuses, but the particles, due to their inertia, continue with small divergence. This property will allow us to differentially pump the carrier gas along the dust beam axis before the microparticles enter the high vacuum of the storage ring.