On the Dynamics of Adiabatically Cooled Antihydrogen in an Octupole-Based Ioffe-Pritchard Magnetic Trap

Abstract

Antihydrogen is now routinely formed in ALPHA by combination of antiproton and positron plasmas. Formed anti-atoms with energy less than around 0.5 K are trapped in an octupole-based Ioffe-Pritchard magnetic trap. Reducing trapped antihydrogen energy is expected to increase precision in experiments that measure fundamental antihydrogen properties for precise comparison to hydrogen. Cooling is expected to permit confinement in a shallower magnetic trap, hence reducing magnetic field errors in gravity experiments and increasing sensitivity in experiments that measure the hypothetical antihydrogen charge. In antihydrogen spectroscopy experiments, cooling is expected to increase laser interaction time, thus narrowing spectral linewidth. A technique to adiabatically cool antihydrogen is presented, which involves trapping in a short magnetic well, before slowly (compared to the antihydrogen speed) expanding the trap volume along the trap axis. During magnetic release, lower energy anti-atoms are expected to escape and annihilate when the trap depth is lower which happens later in time. Adiabatically cooled (Trial A) annihilation times are compared to control samples held in the small (Trial B) and large (Trial C) magnetic volumes, showing Trial A anti-atoms are lost on average when the trap depth is 0.081 +/-0.007 K, compared to 0.223 +/- 0.009 K and 0.173 +/- 0.011 K for Trials B and C respectively. Results of detailed Monte Carlo simulations are presented, which show that Trial A anti-atoms have average energies 1.616 +/- 0.002 and 1.558 +/- 0.002 times lower than Trials B and C respectively, corresponding to a 37.9 +/- 0.1 % energy decrease during expansion and achieving mean final energy 0.2226 +/- 0.0002 K. The magnetic trap expands predominantly in the axial direction, giving rise to a 68.1 +/- 0.2 % axial and 14.2 +/- 0.2 % transverse energy decrease resulting from non-trivial energy mixing dynamics. Two adiabatic models that make limiting assumptions about the energy mixing dynamics are shown to bound the simulation results, confirming the cooling is consistent with an adiabatic process. A possible factor of around 10 increase in precision of antihydrogen charge measurements is presented and a potential issue for the use of axial adiabatic cooling in ALPHA antihydrogen spectroscopy experiments is raised, that a significant fraction of adiabatically cooled anti-atoms tend not to occupy the spatial region of the 1S-2S spectroscopy laser beam.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Appleby (Supervisor) & William Bertsche (Supervisor)