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Antimatter is a fascinating kind of matter made up of antiparticles, which have a mass equivalent to that of their normal matter counterparts, yet they exhibit an opposite charge and distinct quantum properties.

While normal matter has been extensively studied over the past decades and is now well understood, investigating the properties of antimatter with the same precision using existing experimental techniques has proved more challenging.

The ALPHA experiment, a research effort involving researchers at different institutes worldwide, has been trying to unveil the physical properties of antimatter using the Antiproton Decelerator at CERN, specifically those of anti-hydrogen atoms confined in a magnetic trap.

Their most recent , published in Nature Â鶹ÒùÔºics, presents new measurements of the properties of antihydrogen's 1S–2S transition, which is an electronic energy transition in antihydrogen atoms.

"This is the latest in a long line of results of studies exploring the internal structure of antihydrogen," Jeffrey Scott Hangst, spokesperson for the ALPHA antihydrogen experiment, told Â鶹ÒùÔº.

"The ALPHA collaboration is unique in the world in the ability to produce, confine and then study antihydrogen atoms, with studies taking two forms primarily. One is the spectroscopy of the internal structure, the internal energy levels of the anti-hydrogen atom to compare them to hydrogen, which is something we understand very well. The other entails understanding how antimatter behaves in a gravitational field, and we're the only ones who have published anything about this so far."

The structure and properties of hydrogen atoms have been extensively studied at high precision and are thus now well-understood. The key objective of the ALPHA experiment is to collect the same precise measurements for its antimatter counterpart, antihydrogen.

These measurements could help to shed light on the extent to which matter and antimatter obey the same physics laws, as is required by the Standard Model. To achieve this, the ALPHA collaboration is using a combination of techniques, including a new laser cooling method.

"Our new technique of laser cooling trapped antihydrogen atoms can make the spectrum narrower," said Hangst.

"In the end, you're trying to find one frequency, which is the center line of this spectral line that's perturbed in many ways: by magnetic fields, by the motion of the atoms and so on. Our paper demonstrates the capability of the modern experiment that we now have that traps anti-hydrogen and then cools it to very low temperatures.

"We're now making great strides in trying to understand something in antimatter that is known to a precision of 10^-15 in normal hydrogen."

The recent work by Hangst and his colleagues yielded some of the most precise measurements of antihydrogen reported to date. Specifically, the researchers simultaneously observed both accessible hyperfine components of the 1S–2S transition in trapped hydrogen atoms.

"Hydrogen has quantum levels, principal quantum levels (i.e., n=1, n=2, n=3, etc.), which are essentially the orbits in the cartoon model of the hydrogen atom," explained Hangst.

"The one that we're studying ranges from n=1 to n=2. When you're in a magnetic field, however, this line splits into multiple levels and we're able to probe the two levels that are accessible in our trap. In fact, we're able to examine them both in a single run of the experiment."

The two separate frequencies examined by the ALPHA collaboration offer valuable information about the internal structure of antihydrogen atoms. A key milestone of their paper is the new laser cooling-based technique they employed, which allowed them to investigate both these frequencies at low temperatures.

"One of the frequencies is more susceptible than the other to the laser cooling, but we mastered the technique of accumulating a lot of antihydrogen atoms and this allows us to measure this spectrum in about an hour," said Hangst. "We can now accumulate our anti atoms overnight and then measure in a very short time."

When they conducted a similar experiment back in 2017, the researchers were only able to collect measurements over a period of 10 weeks. With the new techniques they employ, they can now routinely collect the same measurements in just one day.

"The notable achievement is not just that we can do it fast, but also that we can study small changes in the equipment (i.e., systematic effects on the measurement)," said Hangst. "The more measurements you can repeat, the better an idea you have of how stable your apparatus is, for example, how reproducible your lasers are."

For Hangst and his colleagues, the speed and hyperfine sensitivity they achieved in their most recent experimental runs represent a paradigm change in the study of anti-hydrogen. Their recent findings highlight the potential of the methods they devised, showing that they will soon yield even more precise measurements.

"For us, this is a notable step forward in our ability to study this very exotic physics," said Hangst. "This article focuses more on the techniques we employ, showing what's possible. Since this data was collected, we've already made better progress, which we plan to publish in future papers. Our next steps will be to perform the very careful measurements we need to extract a precise number."

Later this year, the ALPHA collaboration could publish even more precise measurements associated with the internal structure and physical properties of anti-hydrogen. The researchers' long-term goal will be to reach the same levels of precision as those attained for regular hydrogen atoms, to closely compare matter and antimatter.

These equivalent measurements could help to settle long-standing theoretical debates. Specifically, if anti-hydrogen measurements agree with those of hydrogen, this would confirm that the physics of antimatter is compatible with the Standard Model, which is the basis of the present understanding of elementary particles and their interactions.

"The next step for us will be to exploit everything that we've learned to make the ultimate measurements and get closer and closer to the precision with which we understand hydrogen," added Hangst. "To put this in context, hydrogen precision is parts in 10¹⁵, or one in a thousand trillion. Our best published result is about a few parts in a trillion and the next article will be much, much better."

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