No more Disagreement with the Standard Model?

The Standard Model has proved to be very successful in describing weak and electromagnetic interactions of elementary particles. However, in order to construct a theory which further unifies these interactions with strong and gravitational forces there is a desperate need to find experimental data that are sensitive to the new physics beyond the Standard Model. Recent progress in highly accurate measurements of parity nonconservation (PNC) in atoms has got to the point where obtaining such data seems to be feasible. Parity nonconservation in the cesium atom was measured in Boulder with an unprecedented accuracy of 0.35%. A recent analysis of this experiment by Bennett and Wieman suggests that the weak charge of the cesium nucleus differs from the prediction of the Standard Model.

Since experiment does not measure the weak nuclear charge directly but rather the product of the weak charge and an electron matrix element, accurate atomic calculations are needed to interpret the results of the measurements. In their analysis, Bennett and Wieman used the most accurate calculations performed in Novosibirsk in 1989 (Dzuba, Sushkov, Flambaum) and at Notre Dame in 1990 (Blundell, Johnson, Sapirstein). It is interesting that they assumed 0.4% accuracy of the calculations contrary to the 1% accuracy claimed in both works. They were able to do so because new measurements performed since 1990 of electromagnetic transition amplitudes and other parameters of cesium resolved major discrepancies between experiment and theory in favour of theory.

The deviation of the cesium experiment from the prediction of the Standard Model claimed by Bennett and Wieman generated many works on the implication of this deviation on the physics of elementary particles. However, this deviation strongly relies on the assumption of higher accuracy of atomic calculations - and this assumption was made by experimentalists! Theorists were rather sceptical about this assumption. In our opinion, it is necessary to perform new calculations before a higher accuracy can be claimed. This also requires the study of many-body and relativistic effects which were previously neglected. Indeed, as it was first demonstrated by Derevianko (ITAMP) and then confirmed by Dzuba and Johnson at Notre Dame, inclusion of the Breit interaction (magnetic interaction between moving electrons) effectively removes the disagreement of the Boulder experiment with the Standard Model.

At first glance, the removal of this discrepancy indicates that the PNC measurement for cesium provides no insight into new physics. However, this measurement does in fact place the most stringent limits on the existence of new particles predicted by certain unification models. Other possibilities to search for new physics in atomic PNC include the study of atoms in which the effect is strongly enhanced (radium, ytterbium) and the study of s-d transitions (Cs, Fr, Ba⁺, Ra⁺) where the incorporation of the experimental data into the many-body calculations can lead to a very accurate interpretation of the PNC measurements.

V. A. Dzuba, V. V. Flambaum, J. S. M. Ginges.