Speaker
Description
The imbalance of observed antimatter in the Universe remains an open problem in modern physics. Simple anti-atomic systems provide unique platforms to test the fundamental symmetries that predict equal amounts of matter and antimatter. Antihydrogen is an attractive candidate owing to its simple composition and the extensive study of its matter counterpart. The ALPHA (Antihydrogen Laser PHysics Apparatus) collaboration produces and magnetically traps neutral antihydrogen for experimental investigation. To date, ALPHA has performed spectroscopy of the 1S-2S transition [1] and ground-state hyperfine splitting [2], demonstrated laser cooling of antihydrogen [3], and measured gravity’s effect on antimatter [4]. The spectroscopic studies test Charge–Parity–Time (CPT) symmetry, while gravitational measurements probe the Weak Equivalence Principle (WEP). More stringent tests can be achieved through reduced systematic uncertainties, new diagnostic tools (particularly for sub-millikelvin populations), and benchmarking of simulations used to interpret spectroscopic line shapes or extract fundamental physics parameters. Central to these goals is understanding trapped anti-atom energy distributions and dynamics.
Here we present a technique, novel to anti-atom studies, that selectively probes antihydrogen in an energy-dependent manner. Anti-atoms are exposed to microwave radiation resonant with a positron spin flip transition to an untrappable state, leading to ejection and annihilation on the surrounding apparatus. Since the frequency required to induce the transition is a roughly linear function of the magnetic field, it can be tuned to eject the subset of anti-atoms with sufficient energy to reach the resonant field region. The described technique has been used to perform the first experimental study of energy exchange between motional degrees of freedom in magnetically trapped anti-atoms. This study directly benchmarks intriguing simulation predictions about the dynamics of trapped antihydrogen [5], which impact many ALPHA experiments. Furthermore, the energy-dependent ejection technique enables truncation of energy distributions, which can be used to reduce mean energies or to characterize populations beyond the photon recoil limit of laser-induced ejection with time-of-flight detection [3]. The energy-selective positron spin flip method therefore provides a powerful tool to benchmark simulations, reduce systematic uncertainties, and increase precision in future antihydrogen measurements, enabling more stringent tests of CPT symmetry and the WEP.
[1] Baker, C.J. et al., Nat. Phys. 21, 201-207 (2025).
[2] Ahmadi, M. et al., Nature 548, 66-69 (2017).
[3] Baker, C.J. et al., Nature 592, 35-42 (2021).
[4] Anderson, E.K. et al., Nature 621, 716-722 (2023).
[5] Zhong, A. et al., New J. Phys. 20, 053003 (2018).