Basu R. Lamichhane

We have measured multiple differential cross sections for dissociative capture in collisions of 75-keV protons with D2. Data were analyzed for kinetic energy releases (KERs) ranging from 0 to 2 eV, where the dissociation is primarily caused by excitation of the nuclear motion to a vibrational continuum state. Pronounced signatures of two-center interference effects were observed. The data were compared to previously published cross sections for dissociative capture in 𝑝+H2 collisions, for which we reported a scattering angle dependent phase shift in the interference term. The D2 data reveal qualitative differences in the interference structure to the H2 case. Some aspects of the present results appear to support a hypothetical explanation which we offered earlier for the H2 data, but other aspects are not fully understood yet.

We present a theoretical study on the energy dispersion of an ultrathin film of periodically-aligned single-walled carbon nanotubes (SWCNTs) with the help of the Bogoliubov–Valatin transformation. The Hamiltonian of the film was derived using the many-particle green function technique in the Matsubara frequency formalism. The periodic array of SWCNTs was embedded in a dielectric with comparatively higher permittivity than the substrate and the superstrate such that the SWCNT film became independent with the axis of quantization but keeps the thickness as the variable parameter, making the film neither two-dimensional nor three-dimensional, but transdimensional. It was revealed that the energy dispersion of the SWCNT film is thickness dependent.

We performed a fully differential experimental and theoretical study on ionization of He in intermediate-energy collisions with protons for a small projectile coherence length. Data were taken for an ejected electron energy corresponding to a speed close to the projectile speed (velocity matching). In the fully differential angular electron distributions, a pronounced double-peak structure, observed previously for a coherence length much larger than the atomic size, is much less pronounced in the current data. This observation is interpreted in terms of interference between first-and higher-order transition amplitudes. Although there is large quantitative disagreement between experiment and theory, the qualitative agreement supports this interpretation.

We have measured and calculated fully differential cross sections for vibrational dissociation following capture in 75-keV p + H-2 collisions. For a molecular orientation perpendicular to the projectile beam axis and parallel to the transverse momentum transfer we observe a pronounced interference structure. The positions of the interference extrema suggest that the interference term is afflicted with a phase shift which depends on the projectile scattering angle. However, no significant dependence on the kinetic-energy release was observed. Considerable discrepancies between our calculations and experimental data were found.

We describe a project that brings together researchers from atomic physics, nuclear physics and sub-atomic particle physics, to develop a high-precision laboratory-scale experiment able to search for very weakly coupled sterile neutrinos in the mass range extending from 5-10 keV/c(2) to several 100 keV/c(2). Observed neutrino flavor eigenstates are known to be quantum mixtures of at least three sub-eV/c(2) mass eigenstates. There is a strong theoretical belief that there may exist further neutrino mass eigenstates at higher mass levels, and which, if in the keV/c(2) mass range, might form all or part of the galactic dark matter. This has led to many searches for anomalous events in both astrophysical and particle physics experiments, and searches for distortions in beta decay spectra. The present experiment will utilize K-capture events in a population of Cs-131 atoms suspended in vacuum by a magneto-optical trap (MOT). Using AMO and nuclear physics techniques, individual events will be fully reconstructed kinematically. Normally each event would be consistent with an emitted neutrino mass close to zero, but the existence of a sterile neutrino of keV/c(2) mass that mixes with the electron type neutrino produced in the decay would result in a separated population of events with non-zero reconstructed missing mass (up to the Q = 352 keV available energy of the reaction). Detailed calculations and simulations of all significant background processes have been made, in particular for scattering in the source itself, radiative K-capture, local radioactivity, cosmic ray muons, and knock-out of electrons by x-rays. A phase 1 of the experiment, under construction with funding from the W M Keck Foundation, has the potential to reach sterile neutrino mixing angles down to sin(2) theta similar to 10(-4). With further upgrades this technique could be progressively improved to eventually reach much lower coupling levels similar to 10(-10), in particular reaching the level needed to be consistent with galactic dark matter below the astrophysical x-ray limits.

We have measured fully momentum-analyzed recoiling target ions and scattered projectiles, produced in ionization of He and H-2 by 75 keV proton impact, in coincidence. The momentum of the ejected electrons was deduced from momentum conservation. From the data we extracted fully differential ionization cross sections as a function of the polar electron emission angle (for fixed electron energies) and as a function of the electron energy (for fixed electron emission angles). Comparison between experiment and various distorted wave calculations confirms that under kinematic conditions where the post-collision interaction plays an important role, the few-body dynamics underlying the ionization process are still poorly understood.

We have measured differential yields for double capture and double capture accompanied by ionization in 75 keV p + Ar collisions. Data were taken for two different transverse projectile coherence lengths. A small effect of the projectile coherence properties on the yields were found for double capture, but not for double capture plus ionization. The results suggest that multiple projectile-target interactions can lead to a significant weakening of projectile coherence effects.