A new determination of the π-H and π-D 2P-1s strong interaction shifts using crystal diffraction

The 2P-1S X-ray transition energy in pionic hydrogen (π ^-H) and pionic deuterium (π ^-D) has been measured using a curved graphite crystal spectrometer. E _{2 P-1 S}(π ^-H) = 2434.5 ± 0.4( stat) ± 0.3 (syst) eV, E _{2 P-1 S}(π ^-D) = 2592.1 ± 0.8 ( stat) ± 0.5 (syst) eV. The deduced strong interaction shifts ΔE(π ^-H) = -4.9 ± 0.4 ( stat) ± 0.3 (syst) eV, ΔE(π ^-D) = +5.5 ± 0.8 ( stat) ± 0.5 (syst) eV, corresponding to the S-wave scattering lengths Rea _{π- p}⁰ = +0.084 ±0.009 fm, Rea _{π- d}⁰ = -0.083 ± 0.014 fm, are compared with recent calculations. The X-ray yield is found to be 0.04 ± 0.02 2P-1S X-rays per stopped π^- in saturated hydrogen vapour of 3 atm pressure.