  High-resolution pionic-atom x-ray spectroscopy was performed with an x-ray
spectrometer based on a 240-pixel array of superconducting
transition-edge-sensor (TES) microcalorimeters at the piM1 beam line of the
Paul Scherrer Institute. X-rays emitted by pionic carbon via the 4f->3d
transition and the parallel 4d->3p transition were observed with a
full-width-at-half-maximum energy resolution of 6.8 eV at 6.4 keV. Measured
x-ray energies are consistent with calculated electromagnetic values which
considered the strong-interaction effect assessed via the Seki-Masutani
potential for the 3p energy level, and favor the electronic population of two
filled 1s electrons in the K-shell. Absolute energy calibration with an
uncertainty of 0.1 eV was demonstrated under a high-rate hadron beam condition
of 1.45 MHz. This is the first application of a TES spectrometer to
hadronic-atom x-ray spectroscopy and is an important milestone towards
next-generation high-resolution kaonic-atom x-ray spectroscopy.
