An article published in the journal “Nature” describes the measurement of a spectral line of an antihydrogen atom. The ALPHA experiment at CERN, which is specifically intended to conduct experiments on anti-hydrogen to better understand antimatter’s characteristics, managed to trap an anti-atom to examine it with a laser and to establish that its spectral characteristics are identical to those of hydrogen.
The examin of an atom can seem a trivial thing but the exam of an anti-atom is the culmination of 20 years of work of many people at CERN. All this time was necessary to solve a number of technological problems to produce anti-hydrogen atoms and trap them so that they remained isolated from particles of ordinary matter long enough to study them.
The asymmetry between matter and anti-matter hasn’t yet been clarified and being able to study anti-atoms would be very useful to try to understand its cause. The creation of particles of antimatter is the easiest part, in fact that normally happens in particle accelerators but when one of these particles touches one of common matter they disintegrate. In a universe composed almost exclusively of matter, a contact is very likely so the formation of anti-atoms, even very simple ones, is already a big problem.
The most common isotope of hydrogen is simply formed by a proton and an electron and is the most common atom in the universe. For this reason, its features, also at spectral level, are known with extreme precision. The ALPHA experiment has the purpose to work with anti-hydrogen atoms, formed by an anti-proton and a positron, the common name of the anti-electron. It’s one of the experiments of the Antiproton Decelerator (AD) a storage ring, a type of particle accelerator, at CERN.
Over the years the ALPHA experiment has developed the technologies to produce anti-hydrogen atoms and to keep them in specifically designed magnetic traps. The manipulation of anti-atoms has become very precise and now allows to manipulate a few of them at a time. Until a short time ago, scientists were able to obtain 25,000 atoms of anti-hydrogen from the plasma present in the AD per attempt and of these an average of 1.2 got trapped. Now the amount of anti-atoms trapped increased to 14.
Illuminating the anti-atoms that got trapped with a laser beam at a specific frequency, the scientists were able to observe the beam interactions with the internal states of anti-hydrogen. The measurements were made by observing what is called 1S-2S transition, meaning the transition from the ground state to the second excited state thanks to the energy that is absorbed by the positron. In atomic hydrogen, the S2 state generated by the energy absorbed by an electron generates a narrow spectral line which is suitable to high-precision measurements.
The lack of differences between the spectral lines of the anti-hydrogen and the hydrogen atoms is consistent with the Standard Model, the theory which today offers the best description of the existing particles and of forces. Essentially a symmetry between the energy levels of hydrogen and anti-hydrogen was confirmed, what is called CPT symmetry (Charge, Parity, Time).
This result represents a milestone in antimatter research, however there are still many more steps before understanding the asymmetry with matter. The next stage will consist in the study of anti-hydrogen with the laser at a wide range of energies to test CPT symmetry more rigorously.