In the last decade, physicists, including a group from Berkeley, have been able to study the antimatter counterpart to hydrogen. This counterpart, antihydrogen, consists of a positively charged electron, called a positron, and a negatively charged proton, called an antiproton. As we currently understand them, the laws of physics demand that aside from the changes in sign, hydrogen and antihydrogen have identical properties. Any differences would break these laws and would revolutionize physics. These laws require CPT (charge-parity-time) invariance, which specifically demands that the spectra of hydrogen and antihydrogen be identical. Further, Einstein’s weak equivalence principle demands that hydrogen and antihydrogen interact with gravity identically. These laws are very well tested, but physicists across many fields are exploring new ways to test them. The surprising scarcity of antimatter in the universe hints that these laws might just be broken.
The ALPHA experiments at CERN traps antihydrogen atoms with an energy close to absolute zero (0.5K). Still, both spectral measurements and gravity measurements would be more precise if the antiatom energies could be further reduced. In a recent cover article in Nature, ALPHA demonstrated laser cooling of these atoms. Laser cooling is a widely used technique to cool individual atoms, but it is very difficult to use with hydrogen or antihydrogen because it requires 121nm laser light. This light is far into the ultraviolet, and is very difficult to generate. Berkeley Professors Joel Fajans and Jonathan Wurtele, along with current Physics graduate student Andrew Christensen and recently minted Berkeley Ph.D.s Celeste Carruth and Eric Hunter, are co-authors of the Nature article Laser Cooling of Antihydrogen, 592, p 35 (2021).
Generations of graduate students and over two dozen undergraduates have contributed to Berkeley’s antihydrogen research program at CERN, specializing in the optimization of the positron and antimatter plasmas from which antihydrogen is created, measurements of the charge of antihydrogen, preliminary measurements of how antihydrogen interacts with gravity, and instrumentation and control of the ALPHA experiment. This summer both faculty members, two graduate students and four undergraduates will head to CERN to work on an experiment to determine which way antihydrogen falls; it is extremely unlikely that antihydrogen falls upwards but the direction of fall has never been measured directly. This experiment, which was conceived of at Berkeley, may yield results this fall.