Investigating Δm = ±1 transitions in a caesium fountain clock -Challenges in precision measurements of the g-factor ratio

Theoretical calculations of atomic structure are constantly being refined and are now reaching a degree of accuracy where the final discrepancies to tabulated measurements (see e.g. [1]) might be due to the experimental values [2,3]. This uncertainty also exists for caesium, where the difference in the measured nuclear magnetic moments for free caesium atoms [4], free caesium ions [5] and ions in solution [6] do not agree with calculated shielding factors [7]. The best known ratio of the nuclear and electronic g-factors in free caesium atoms is given in [4] as g_i/g_j= -1.9917400(26) ×10^-4. We aim to remeasure this ratio by investigating a set of hyperfine transition frequencies using the two available caesium fountains CSF1 and CSF2 at PTB. In this report we describe the results of the first tests and the challenges they show for a precision measurement.