Microwave spectroscopic study of the hyperfine structure of antiprotonic 3He

In this work, we describe the latest results for the measurements of the hyperfine structure of antiprotonic He-3. Two out of four measurable super-super-hyperfine (SSHF) transition lines of the (n, L) = (36, 34) state of antiprotonic He-3 were observed. The measured frequencies of the individual transitions are 11.125 48(08) GHz and 11.157 93(13) GHz, with the increased precisions of about 43% and 25%, respectively, compared to our first measurements with antiprotonic He-3 (Friedreich et al 2011 Phys. Lett. B 700 1-6). They are less than 0.5 MHz higher with respect to the most recent theoretical values, still within their estimated errors. Although the experimental uncertainty for the difference of 0.032 45(15) GHz between these frequencies is large as compared to that of theory, its measured value also agrees with theoretical calculations. The rates for collisions between antiprotonic helium and helium atoms have been assessed through comparison with simulations, resulting in an elastic collision rate of gamma(e) = 3.41 +/- 0.62 MHz and an inelastic collision rate of gamma(i) = 0.51 +/- 0.07 MHz.