Studies of mesic atoms and nuclei

TL;DR

This paper reviews recent progress in understanding $K^-$ mesic atoms and nuclei, focusing on theoretical models, optical potentials, and the feasibility of observing deeply bound states, concluding that such states are likely unobservable except in very light systems.

Contribution

It introduces a self-consistent method for transforming free-space scattering amplitudes into in-medium optical potentials and evaluates their ability to reproduce experimental data and predict bound states.

Findings

Only two models fit kaonic atom data and absorption fractions.

Deeply bound $K^-$ states have widths above 100 MeV, making them unobservable.

Observable $K^-$ bound states are limited to very light systems like $K^-pp$.

Abstract

$K^-$ mesons offer a unique setting where mesic atoms have been studied both experimentally and theoretically, thereby placing constraints on the possible existence and properties of meson-nuclear quasibound states. Here we review progress in this field made recently by the Jerusalem--Prague Collaboration using near-threshold $K^-N$ scattering amplitudes generated in several meson--baryon coupled channels models inspired by a chiral EFT approach. Our own procedure of handling subthreshold kinematics self consistently is used to transform these free-space energy dependent amplitudes to in-medium density dependent amplitudes from which $K^-$ optical potentials are derived. To fit the world data of kaonic atoms, these single-nucleon optical potentials are augmented by multi-nucleon terms. It is found that only two of the studied models reproduce also the single-nucleon absorption fractions available from old bubble chamber experiments. These two models are then checked for possible $K^-$ nuclear quasibound states, despite realizing that $K^-$ optical potentials are not constrained by kaonic atom data at densities exceeding half nuclear-matter density. We find that when such states exist, their widths are invariably above 100 MeV, forbiddingly large to allow observation. Multi-nucleon absorption is found to be substantial in this respect. This suggests that observable strongly bound $K^-$ mesons are limited to the very light systems, such as $K^-pp$.