The effect of positron temperature and density on antihydrogen synthesis and trapping

The ALPHA experiment has long been at the forefront of antimatter research, using antihydrogen for precision tests of fundamental physics. Antihydrogen is synthesised by merging cold antiproton and positron plasmas in a Penning-Malmberg trap. Under the high-density, low-temperature conditions of the positron plasma, three-body recombination dominates antihydrogen formation: two positrons collide near an antiproton, one becomes bound and the other carries away excess energy.

Simulations suggest that lower positron temperatures strongly enhance the antihydrogen trapping rate [1], motivating efforts to cool the positron plasma as much as possible during synthesis.

A new synthesis method was developed where positrons are cooled to cryogenic temperatures via Coulomb collisions with laser-cooled Be+ ions, reducing the positron temperature by a factor of ∼ 1.5 and leading to a near 8-fold increase in the antihydrogen trapping rate in ALPHA-2.

Beyond improving trapping efficiency, this technique enabled the first independent control of positron temperature and density during antihydrogen synthesis, allowing for the first systematic study of the effect of these parameters in the antihydrogen trapping fraction. We present the results of these studies and the first quantitative comparison between experimental data and simulations, paving the way for understanding the dynamics of antihydrogen synthesis and trapping.

[1] S. Jonsell and M. Charlton, New J. Phys. 20, 043049 (2018)