Implementation of Local Chiral Interactions in the Hyperspherical Harmonics Formalism

With the goal of using chiral interactions at various orders to explore the properties of the few-body nuclear systems, we write the recently developed local chiral interactions as spherical irreducible tensors and implement them in the hyperspherical harmonics expansion method. We devote particular attention to three-body forces at next-to-next-to leading order, which play an important role in reproducing experimental data. We check our implementation by benchmarking the ground-state properties of 3H, 3He, and 4He against the available Monte Carlo calculations. We then confirm their order-by-order truncation error estimates and further investigate uncertainties in the charge radii obtained by using the precise muonic atom data for single-nucleon radii. Having local chiral Hamiltonians at various orders implemented in our hyperspherical harmonics suites of codes opens up the possibility to test such interactions on other light-nuclei properties, such as electromagnetic reactions.

Using the Emission of Muonic X-rays as a Spectroscopic Tool for the Investigation of the Local Chemistry of Elements

There are several techniques providing quantitative elemental analysis, but very few capable of identifying both the concentration and chemical state of elements. This study presents a systematic investigation of the properties of the X-rays emitted after the atomic capture of negatively charged muons. The probability rates of the muonic transitions possess sensitivity to the electronic structure of materials, thus making the muonic X-ray Emission Spectroscopy complementary to the X-ray Absorption and Emission techniques for the study of the chemistry of elements, and able of unparalleled analysis in case of elements bearing low atomic numbers. This qualitative method is applied to the characterization of light elements-based, energy-relevant materials involved in the reaction of hydrogen desorption from the reactive hydride composite Ca(BH4)2-Mg2NiH4. The origin of the influence of the band-structure on the muonic atom is discussed and the observed effects are attributed to the contribution of the electronic structure to the screening and to the momentum distribution in the muon cascade.

Systematic study of the muon-nucleus overlap integrals for various processes in muonic atoms

As it is known, the bound muon of a muonic atom can participate in many electroweak processes as the allowed channels of the ordinary ^""-capture by the atomic nucleus, μ~ •+- (A, Z) -» (A,Z — l) + νβ, and the muon decay in orbit, μ~ -» e~ + υμ ·+- ve, as well as the exotic channels of the muon-to-electron, μ~ + (A, Z) —> (.A, Z)* + e~, and muon-to-positron, μ~ + (A, Ζ) -» (A, Ζ — 2) + e+, conversions. The latter reactions have not been seen by experiments up to now, but they are predicted by various extensions of the standard model (they violate the flavor and/or lepton quantum numbers). For all the above muonic processes, the muon-nucleus overlap integrals are necessary in order to calculate the relevant rates. These integrals can be evaluated if, in addition to the nuclear states, the wave function of the bound muon (also that of the outgoinglepton) are known. In the present work, we perform precise calculations of the muon (and electron/positron) wave functions for both the Schrödinger and Dirac equations. We use modern neural network techniques to overcome the difficulties arising from the finite size of the nucléon and nuclear Coulomb potential. As some applications, the obtained muonnucleus integrals for various muonic atoms are going to be used for evaluating exclusive muon-capture rates and muon to electron/positron conversion branching ratios.

Nuclear structure with radioactive muonic atoms

Muonic atoms have been used to extract the most accurate nuclear charge radii based on the detection of X-rays from the muonic cascades. Most stable and a few unstable isotopes have been investigated with muonic atom spectroscopy techniques. A new research project recently started at the Paul Scherrer Institut aims to extend the highresolution muonic atom spectroscopy for the precise determination of nuclear charge radii and other nuclear structure properties of radioactive isotopes. The challenge to combine the high-energy muon beam with small quantity of stopping mass is being addressed by developing the concept of stopping the muon in a high-density, a high-pressure hydrogen cell and subsequent transfer of the muon to the element of interest. Status and perspectives of the project will be presented.