Adiabatic Cooling of Antihydrogen

Antihydrogen is now routinely formed by merging antiproton and positron plasmas in the ALPHA experiment. Neutral anti-atoms with energy less than around 0.5 K are trapped in a modified Ioffe-Pritchard magnetic trap. Reducing trapped antihydrogen energy is expected to increase precision in experiments that measure fundamental properties of antihydrogen for precise comparison to hydrogen.

We present a technique to adiabatically cool antihydrogen, by trapping in a small magnetic volume, before axially expanding the trap slowly compared to the axial bounce time of the anti-atoms (approx. 10 ms). Experimental results show that anti-atoms subject to the trap expansion annihilate on average during diagnostic ramp-down when the trap depth is (81 ± 7) mK, compared to (223 ± 9) mK and (173 ± 11) mK for two largely static control trials. We present detailed simulations of the experimental procedure, which qualitatively agree with the experimental annihilation-time data, and show a decrease of (37.9 ± 0.1) % in the mean energy of the population subject to a trap expansion. Two adiabatic models bracket the simulated energy decrease, confirming the simulated cooling is consistent with adiabatic expansion of an antihydrogen population that partially mixes energy between degrees of freedom.