Few-Electron Highly Charged Muonic Ar Atoms Verified by Electronic K X Rays

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar (μAr) by electronic K x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision K x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like μAr. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged μAr.