On June 7, the Daily Cal reported on the demonstrated trapping of long-lived antihydrogen atoms by the Antihydrogen Laser Physics Apparatus collaboration, which includes members of the Fajans and Wurtele research groups in the UC Physics department. As a follow-up, the graduate students of these groups would like to offer a perspective to explain what this result means and how this ties into the state of the field.
Studies of antimatter have historically been of great importance to research at Berkeley. In 1932, Carl Anderson, using a bubble chamber at our own Lawrence Berkeley National Laboratory, discovered the first observed antimatter particle: the positron. It, oddly enough, behaved like an electron but seemed to carry the opposite charge. Soon afterward, in 1955, Emilio Gino Segre and Owen Chamberlain laid claim to the discovery of the antiproton in the newly constructed Bevatron at LBNL.
Not only do these particles exist, but if a matter and antimatter particle of the same kind overlap, they can annihilate; they disappear as particles and leave behind energetic, lighter particles.
While antimatter has been observed for many decades, the particles have generally not been produced with low enough energy to give us more than a fleeting glimpse of their behavior. What has been determined, however, is that every charged fundamental matter particle has an antimatter equivalent that to the limits of current measurement, which appears to have identical mass and equal but opposite charge.
However, this symmetry creates a problem for physicists, since we observe that the universe is largely made of matter and antimatter particles, and, while not uncommon, are relatively sparse when compared with the quantity of normal matter. If we claim that antimatter behaves the same as matter but with opposite charge, there is no good explanation for why matter became the dominant type of particle in the universe.
With this in mind, physicists today are looking in many places to see if and where the symmetry of matter and antimatter may have been broken. Since any asymmetry is suspected to be very small, it’s usually best to try to measure any differences in very precise, well-established systems, such as atomic clocks which can be measured to better than one part in 100 trillion. In order to make this comparison, however, we have to be able to hold an antimatter atom long enough to use it as a clock.
To understand what we have done, we should clarify what, precisely, was confined in our trap. While holding antimatter may seem like the realm of science fiction, positrons have been captured and confined reliably for over 30 years but are only held as a cloud of free, charged particles. The difference in our trap is that we hold onto antimatter that is in a bound, neutral atomic state. Atomic hydrogen, the simplest element in the periodic table, consists of a positively charged proton and a negatively charged electron which are bound together by electromagnetic forces. If we combine the two antimatter partners, the antiproton and the positron, in the same manner as their matter equivalents, we get antihydrogen. It is this system of antimatter that we have managed to hold for 1,000 seconds.
While this may sound simple in concept, the art is in finding a way to create antihydrogen atoms at low enough energies that they can be reliably trapped. Since antihydrogen is not electrically charged, it is much harder to hold onto once you create it. We rely on the weak magnetic repulsion of antihydrogen to suspend it in the trap. However, since the magnetic moment of the atoms is weak, we are only able to trap very few atoms at a time.
Now that we have been able to get a hold on antihydrogen, we will start performing measurements and learning more about antimatter. There is still much to be studied, and many obstacles will need to be found and surpassed before we get to the final goal of precision; but the recent accomplishments show we are on the right path to get there. These are really exciting times for antimatter.
Marcelo Baquero-Ruiz and Alex Povilus worked with ALPHA.