For many years, atomic spectroscopy allowed the determination of the composition of an element by its electromagnetic spectrum. Every chemical element has a unique `signature' which can be revealed by analyzing the light it gives off. Measuring how elements emit or absorb light allows scientists to characterize atoms and molecules and thus to delve deeper in the study of the microcosm. The development of spectroscopy opened a new window for studying the structure of the ordinary matter at its tiniest scale while it is also used for studying astronomical objects and other galaxies. Today we learn about the composition of distant stars by studying the spectrum of the light they emit. In fact, we can learn a lot, not only about the chemical elements present, but also about physical conditions.

Since spectroscopy has given us a wealth of knowledge about matter, scientists thought to apply the same technique in the study of antimatter. Ever since the existence of antimatter was proposed in the early 20th century, scientists have sought to understand how relates to normal matter, and why there is an apparent imbalance between the two in the Universe.

Group photo with members of the ALPHA collaboration just after the end of the 2016 run. (Image Credit: CERN, ALPHA collaboration) 

Studying antimatter spectroscopically presents certain difficulties as any atom of antihydrogen produced in the laboratory typically lasts for a short time before it gets annihilated. In a paper published last December in Nature, the ALPHA collaboration reported the first measurement of the optical spectrum of an antimatter atom. The new result allows the light spectrum of matter and antimatter to be compared for the first time.

ALPHA experiment is able to produce antihydrogen atoms and hold them in a specially-designed magnetic trap, manipulating antiatoms a few at a time. Trapping antihydrogen atoms allows them to be studied using lasers or other radiation sources.

Artist's impression of a cloud of trapped antihydrogen atoms (Chukman So)

“Moving and trapping antiprotons or positrons is easy because they are charged particles,” said the Spokesperson, Jeffry Hangst. “But when you combine the two you get neutral antihydrogen, which is far more difficult to trap, so we have designed a very special magnetic trap that relies on the fact that antihydrogen is a little bit magnetic.”

Antihydrogen is made by mixing plasmas of about 90,000 antiprotons from the Antiproton Decelerator with positrons, resulting in the production of about 25,000 antihydrogen atoms per attempt. Antihydrogen atoms can be trapped if they are moving slowly enough when they are created. Using a new technique in which the collaboration stacks anti-atoms resulting from two successive mixing cycles, it is possible to trap on average 14 anti-atoms per trial, compared to just 1.2 with earlier methods. By illuminating the trapped atoms with a laser beam at a precisely tuned frequency, scientists can observe the interaction of the beam with the internal states of antihydrogen. The measurement was done by observing the so-called 1S-2S transition. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement.  

Within experimental limits there was no difference found between the spectrum of anti-hydrogen and the equivalent spectral lines in hydrogen. These results show that antimatter’s behaviour – vis a vis its spectrographic characteristics – are consistent with the Standard Model. Specifically, they are consistent with what is known as Charge-Parity-Time (CPT) symmetry.

This symmetry theory, which is fundamental to established physics, predicts that energy levels in matter and antimatter would be the same. As the team explained in their study:

“We have performed the first laser-spectroscopic measurement on an atom of antimatter. This has long been a sought-after achievement in low-energy antimatter physics. It marks a turning point from proof-of-principle experiments to serious metrology and precision CPT comparisons using the optical spectrum of an anti-atom. The current result… demonstrate that tests of fundamental symmetries with antimatter at the AD are maturing rapidly.”

The confirmation that matter and antimatter have similar spectral characteristics is yet another indication that the Standard Model holds up. It also demonstrates the effectiveness of the ALPHA experiment at trapping antimatter particles, which will have benefits for other antihydrogen experiments.

This achievement features technological developments that open up a completely new era in high-precision antimatter research. The measurement requires that the particles that constitute antihydrogen – antiprotons and positrons (anti-electrons) – be captured and cooled so that they may come together. In addition, it is then necessary to maintain these particles long enough to observe their behaviour, before they make contact with normal matter and annihilate.

The measurement of anti-hydrogen’s spectrum is the result of over 20 years of work by the CERN antimatter community. The ALPHA collaboration expects to improve the precision of its measurements in the future. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether antimatter behaves differently from matter and thus to further test the robustness of the Standard Model.

The current result, along with recent limits on the ratio of the antiproton-electron mass established by the ASACUSA collaboration, and antiproton charge-to-mass ratio determined by the BASE collaboration, demonstrate that tests of fundamental symmetries with antimatter at CERN are maturing rapidly.