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
π
M1 beam ...line of the Paul Scherrer Institute. X-rays emitted by pionic carbon via the
4
f
→
3
d
transition and the parallel
4
d
→
3
p
transition were observed with a full width at half maximum energy resolution of 6.8 eV at 6.4 keV. The measured X-ray energies are consistent with calculated electromagnetic values which considered the strong interaction effect assessed via the Seki–Masutani potential for the
3
p
energy level, and favor the electronic population of two filled
1
s
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.
The key question of this letter is whether the
K
−
-nucleus optical potential is deep, as it is prefered by the phenomenological fits to kaonic atoms data, or shallow, as it comes out from unitary ...chiral model calculations. The current experimental situation is reviewed.
We study the (γ,p) reaction on 208Pb leading to 207Pb with a bound pion attached to it in the lowest 1s or 2p pionic levels. The reaction can be made recoilless to optimize the production cross ...section but we must choose a bit higher photon energy to overcome the Coulomb barrier in the proton emission. The cross sections obtained are easily measurable and can be larger than 50 per cent of the background from inclusive (γ,p). This makes it a clear case for the detection of the pionic atom signals, converting this reaction into a practical tool to produce deeply bound pionic atoms.
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