Monday, December 19, 2016

Shining a Light on Antimatter: The First Spectroscopic Measurement of an Antimatter Atom

Antimatter doesn’t just fuel science fiction, it fuels cutting-edge physics research into the heart of our very existence. In a paper published today in the journal Nature, a 54-member team of researchers from the ALPHA experiment at CERN announced an exciting achievement in antimatter research. For the first time, scientists have measured the spectrum of light given off by a particle of antimatter.

ALPHA scientist and spokesperson Jeffrey Hangst with the ALPHA experiment.
Image Credit: Photo by Maximilien Brice, CERN.

Like a regular atom, an atom of antimatter can absorb laser light and jump up to an excited state. When the excitement fades, the particle drops back down to its ground state and releases light. (Click here for a chemistry refresher.) According to the Standard Model, a well-developed, well-accepted framework that describes the particles that make up our world and how they interact, the light spectrum of hydrogen and antihydrogen related to the transition from the ground state to the excited state should be identical.

The problem is, the Standard Model also predicts that the universe should contain equal amounts of matter and antimatter. Lucky for us that’s not the case. When antimatter and matter collide, they can annihilate, leaving behind radiation where there was once matter (and antimatter). The question of why the universe is so dominated by regular matter, and why we exist at all, is one of the biggest unanswered questions in physics.

So, is there something wrong with the Standard Model? Are the laws that govern antiparticles a little bit different than those that govern regular particles? In search of answers to these questions, scientists have been designing unique experiments to create and trap antimatter so that they can study its properties and compare antimatter to regular matter.

As you might imagine, creating and trapping something that annihilates on contact with matter is no easy task. Then add to that the ability to shine a laser on the antiparticle and measure the outcome, and you have a pretty ambitious undertaking. The ALPHA collaboration achieved this by using magnetic fields to trap antihydrogen atoms, about 14 at a time, in a vacuum chamber. Shaped like a cylinder, the chamber is about 11 inches long and 1.7 inches in diameter. Ultraviolet laser light shines on antihydrogen through windows in the chamber, exciting them and stimulating the transition.

The team’s results show a spectrum consistent with that of hydrogen. However, scientists have measured the transition spectrum of hydrogen very precisely, to a precision of a few parts in 1015! That is more precise than this first measurement of the transition spectrum of antihydrogen. As scientists refine their technique and make more precise measurements, this comparison will become a very sensitive test of the symmetry between matter and antimatter.

The ALPHA team first synthesized antihydrogen in 2010 and has been studying it ever since. Their goal is to study the differences between hydrogen and antihydrogen in order to better understand the differences between matter and antimatter more generally. Analyzing the spectra of antihydrogen is an important part of this process. No matter what they uncover along the way, the team’s results are sure to spark the imagination of science fiction authors right along with the rest of us.

Kendra Redmond

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