X-ray Spectroscopy of Muonic Atoms Isolated in Vacuum with Transition Edge Sensors

High-resolution X-ray spectroscopy of the highly charged muonic atoms/ions isolated in vacuum is an ideal probe to explore quantum electrodynamics under extremely strong electric fields, which is one of the major topic in fundamental atomic physics. A feasibility test measurement with a low-density neon gas target was performed by observing X-rays emitted by muonic neon via the 5→4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$5 \rightarrow 4$$\end{document} transition, ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document} 6.3 keV, using a multi-pixel array of superconducting transition-edge-sensor (TES) microcalorimeters at the J-PARC muon facility. We successfully demonstrated the feasibility of muonic atom X-ray spectroscopy with a gas target at a pressure as low as 0.1 atom using TES array under an intense pulsed muon beam.