Pionic Deuterium (Vol. 42, No. 5)

A new precise measurement of the pD(3p-1s) X-ray transition in the pionic deuterium atom has been performed at the PSI accelerator in Switzerland. The pionic deuterium is a short lifetime atom, where the negative pion (p^-) replaces the electron, resulting in an atomic size scaled down by the ratio of the pion mass over electron mass, a factor of about 270.

The experiment makes use of a high intensity decelerated beam of ^- stopping in a cooled deuterium gas target where the p- is captured. Following the capture an atomic de-excitation quantum cascade of 0.1ns duration takes place and the atom ends up in the 1s ground state as shown in the Figure. A Bragg spectrometer equipped with a bent Silicon crystal and pixel semiconductor detectors provides the precise X-ray detection in the appropriate keV region.

The measurement of the energy of the X-ray emitted in the pD(3p-1s) transition leads to a new value of 3075.583 ± 0.030 eV. A new and updated calculation of this transition energy assuming a pure electromagnetic system (pure QED - no strong interaction) leads to a value of 3077.939 ± 0.008 eV. The difference between these two quantities gives exactly the hadronic shift e_1s = -2.336 ± 0.031 eV. The line-shape has been analysed, providing a new and precise value of the hadronic broadening G_1s = 1.171+0.023 -0.049 eV.

The accuracy of 1.3% achieved for the shift e1s leads to a more precise determination of the isoscalar scattering length a+ (pD being an isoscaler object). The new precise value obtained for the hadronic broadening G1s leads to a new determination of the threshold parameter a, the transition strenght for a S-wave pion, with unprecedented accuracy.