M ar 2 01 1 Photon-photon polarization correlations as a tool for studying parity non-conservation in heliumlike Uranium

Parity non-conservation (PNC) had been at first theoretically proposed by Lee and Yang in 1956 in order to find a way out of the so–called “τ −γ puzzle” [1, 2]. The next year, Wu and collaborators observed an asymmetry in nuclear beta decay ascribed to parity non-conservation in weak processes [3], and, subsequently, Lee and Yang have been forthwith awarded with the Nobel Prize. Many later experiments in nuclear and high energy physics confirmed parity violation in weak interactions and precisely recorded weak charge and other related parameters [4–7]. Although with some initial controversies, the “τ − γ puzzle” was also solved out by understanding that both τ and γ were two decay channels of the same parent particle, known today as the charged kaon K [8, 9]. In contrast to nuclear and high energy physics, fewer experiments have been carried out in atomic physics to measure weak interaction’s properties. In fact, the conflicting results of the early Bismuth experiments in the ’70 [10–13] spread the conviction that nothing fundamentally useful could have ever been extracted from atomic physics experiments. Nonetheless, renewed interest on the subject rose again in the late ’80 and ’90 and led to the successful measurements of the weak charge Qw and related parameters in atomic Cesium [14–19], Thallium [20], Lead [21] and Ytterium [22]. On the theoretical side, starting from the early work of Curtis-Michel [23], several investigations of PNC have been made in the context of neutral atoms [24], few–electron ions [25, 26] and muonic atoms [27, 28]. In all the proposed studies, the little role played by PNC effects together with the need of precise measurements have been highlighted.