Maria Gonçalves
Be+-assisted antihydrogen synthesis and trapping in the ALPHA apparatus
Antihydrogen, the bound state of a positron and an antiproton, is a uniquely well-suited system for testing fundamental symmetries between matter and antimatter. Experiments by the ALPHA collaboration have measured the antihydrogen 1S-2S transition, hyperfine structure, and the interaction between this antiatom and gravity. The precision of these studies is ultimately limited by the antihydrogen trapping rate. At ALPHA, antihydrogen atoms are synthesised by slowly merging cold non-neutral positron and antiproton plasmas in a Penning-Malmberg trap. Under these conditions, antihydrogen is predominantly formed through three-body recombination, a process in which the positron temperature is a key parameter governing production and trapping rates: colder positron plasmas result in a higher antihydrogen trapped fraction.
Positrons cool through cyclotron radiation emission, reaching a lower temperature limit of approximately 15 K in the ALPHA-2 apparatus. To reduce this temperature further, the positrons were sympathetically cooled using laser-cooled Be+ ion plasmas, decreasing their temperature to the sub-10 K regime. This talk will present the development and implementation of Be+-assisted antihydrogen synthesis in the ALPHA experiment. The development of this technique required overcoming a number of experimental challenges, including the preparation of reproducible ion plasmas, long-term optical alignment through active beam stabilisation, and understanding mixed-species plasma dynamics in the presence of a radially asymmetric magnetic field. These developments culminated in the first demonstration of Be+-assisted antihydrogen synthesis and trapping. Subsequent optimisation of the technique resulted in a twentyfold increase in the antihydrogen trapped fraction, with a record 30,000 antihydrogen atoms accumulated in eight hours. The resulting increase in available antihydrogen has significantly enhanced the statistical reach of ALPHA’s precision measurement programme.