Measurements of Strong-Interaction Effects in Kaonic-Helium Isotopes at Sub-eV Precision with X-Ray Microcalorimeters.

We have measured the 3d -> 2p transition x rays of kaonic He-3 and He-4 atoms using superconducting transition-edge-sensor microcalorimeters with an energy resolution better than 6 eV (FWH:M). We determined the energies to be 6224.5 +/- 0.4(stat) +/- 0.2(syst) eV and 6463.7 +/- 0.3(stat) +/- 0.1 (syst) eV, and widths to be 2.5 +/- 1.0(stat) +/- 0.4(syst) eV and 1.0 +/- 0.6(stat) +/- 0.3(stat) eV, for kaonic He-3 and He-4, respectively. These values are nearly 10 times more precise than in previous measurements. Our results exclude the large strong-interaction shifts and widths that are suggested by a coupled-channel approach and agree with calculations based on optical-potential models.