Tests of Fundamental Interactions with Heavy Ions and Atoms

QED calculations of heavy two-electron ions and their applications to a feasible PNC experiment are discussed. Recent progress on calculations of the PNC effects in neutral alkali-metal atoms is also reported. Heliumlike ions, being the simplest many-electron systems, serve as an important testing ground for investigations of few-electron QED effects in strong fields. In Ref. [1], all second-order two-electron QED corrections to the energy levels of the n = 1 and n = 2 states of He-like ions with the nuclear charge number within the range Z = 12100 were calculated. As a result, a significant improvement in the accuracy of the theoretical predictions was achieved, especially in the high-Z region. To date, the most accurate experiment with heavy He-like ions was performed at GSI [2]. In this experiment the two-electron contribution to the ground state energy of He-like uranium has been measured. The experimental value of this contribution was found to be 2248(9) eV, while the corresponding theoretical result from Ref. [1] is 2245.89(18) eV. Investigations of parity non-conservation (PNC) effects in atomic systems play a prominent role in tests of the Standard Model (SM). In Refs. [3, 4], the full gauge-invariant set of one-loop QED corrections to the PNC transition amplitudes in neutral cesium and francium was calculated. This resulted in a significant improvement of the corresponding PNC amplitudes. Compared with the most precise experiment for cesium [5], these calculations provided a new value for the weak charge of Cs which deviates by 1.1 σ from the SM prediction. Further progress on tests of the Standard Model with neutral atoms from the theoretical side is mainly limited by difficulties with accurate calculations of the electron-correlation effects. For this reason PNC experiments with heavy few-electron ions, where the correlation effects can be evaluated to a high accuracy, seem highly desirable. A promising situation for PNC tests occurs in heavy Helike ions with the nuclear charge number near Z=64 and Z=90, where the two levels 2S0 and 2P0 with opposite parity come close to each other thus enhancing the PNC effects significantly. Experimental identification of the PNC effects will require precise knowledge of the 2P0 − 2S0 interval in He-like ions with nuclear charge numbers near these values of Z. The results of the calculations of this interval from Ref. [1] are shown in Table 1 . For comparison, the results obtained in other works are also presented in the table. Table 1: The 2 P0 − 2 S0 transition energy, in eV. Ref. Z = 63 Z = 64 Z = 65 Z = 66 [1] −0.224(69) 0.004(74) 0.32(12) 0.495(84) [6] −0.591 −0.389 −0.153 0.016 [7] −0.170 0.341 [8] −0.168 0.067 0.328 0.614 Ref. Z = 89 Z = 90 Z = 91 Z = 92 [1] 1.61(46) 0.61(46) −0.31(55) −2.64(28) [6] −1.971 −4.511 [7] −0.095 −2.639 [8] 1.731 0.718 −0.209 −1.816 A feasible PNC experiment with heavy ions was suggested by Labzowsky et al. [9]. It was proposed to study a one-photon 2S0−1S0 transition in polarized He-like Eu. The total amplitude of the one-photon 2S0 − 1S0 transition is a mixture of the basic hyperfine-quenched (hfq) M1 and the additional hyperfineand weak-quenched E1 amplitude. The probability for the emission of a photon in direction n is given by [9] dW (n) = WM1 4π [1 + ε(ν · n)]dΩ , (1) where WM1 is the total probability of the hfq M1 2S0 − 1S0 transition, ν is the unit vector directed along the ion polarization, and ε is the asymmetry parameter caused by the PNC effects. Our calculation for Eu employing the transition energies from Ref. [1] (see Table 1 ) yields ε ≈ 0.0004λ0, where λ0 is the degree of the ion polarization. This work has been supported partly by INTAS-GSI (Grant No. 03-54-3604).