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We live in a universe made of matter. But at the moment of the Big Bang, matter and antimatter existed in equal amounts. That antimatter has all but disappeared suggests that nature, for some reason, has a strong preference for matter. Physicists want to know why matter has replaced its antimatter twin, and this week the ALPHA collaboration at CERN got a step closer to unraveling the mystery.
ALPHA, an international collaborative experiment established in 2005, was designed to trap and measure antihydrogen particles with a specially designed experiment. It’s picking up where its antimatter-searching predecessor, ATHENA, left off. The focus is on antihydrogen because hydrogen is the most prevalent element in the universe and its structure is extremely well known to scientists.
Each hydrogen atom has one electron orbiting its nucleus. Firing light at the atoms excites the electron, causing it to jump into an orbit further away from the nucleus before it relaxes and returns to its resting orbit emitting light in the process. The frequency distribution of this emitted light is known; it has been precisely measured and, in our universe made of matter, is unique to hydrogen.
Basic physics dictates that hydrogen’s antimatter twin, antihydrogen, should be equally recognizable by having an identical spectrum. That is, if everything we know about particle physics is right. Capturing and measuring antihydrogen’s spectrum is the main goal of the ALPHA group.
ALPHA has taken the first modest measurements of antihydrogen. In the ALPHA apparatus, antihydrogen is trapped by an arrangement of magnets that affect the magnetic field of the atoms. Microwaves tuned to a specific frequency aimed on these antihydrogen atoms flips their magnetic orientation, liberating them. The freed antihydrogen meets hydrogen as it escapes and the two annihilate one another, leaving a well known pattern in particle detectors surrounding the apparatus.
The apparatus captured evidence of the electron jumping orbits in an antihydrogen atom after microwave radiation changed its internal state. The result further proves the validity of ALPHA’s approach, demonstrating that the apparatus has enough control and sensitivity to successfully carry out the experiment it was designed for. In the future, ALPHA will focus on improving the precision of its microwave measurements to uncover the antihydrogen spectrum using lasers.
The exciting results were hard to come by as antihydrogen does not exist in nature. It’s made in the ALPHA apparatus from antiprotons that are themselves made in the Antiproton Decelerator and positrons from a radioactive source. And it has to have a low enough energy level to stay trapped for measurements. But it’s working, and it just might give physicists the key they need to understand the mystery of the early universe.