In a groundbreaking experiment at the Brookhaven National Laboratory in the United States, an international team of physicists has achieved a remarkable milestone. They have successfully detected the heaviest "anti-nuclei" ever observed. These tiny, short-lived objects are composed of exotic antimatter particles.
The measurements of how these entities are produced and their properties not only confirm our current understanding of the nature of antimatter but also provide valuable insights into the search for another mysterious kind of particles—dark matter—in the vast expanse of deep space.
The results of this significant research have been published in the prestigious scientific journal Nature.
A Missing Mirror World
The concept of antimatter is relatively recent, dating back less than a century. In 1928, British physicist Paul Dirac developed a highly accurate theory to describe the behavior of electrons. However, this theory made a startling prediction: the existence of electrons with negative energy, which would have rendered our stable universe impossible.
Fortunately, scientists found an alternative explanation for these "negative energy" states: antielectrons, or the counterparts of electrons with opposite electric charges. Antielectrons were subsequently discovered in experiments in 1932. Since then, scientists have determined that all fundamental particles have their own antimatter equivalents.
This discovery raised another intriguing question. Antielectrons, antiprotons, and antineutrons should be capable of combining to form entire antiatoms, and even antiplanets and antigalaxies. Moreover, our theories of the Big Bang suggest that equal quantities of matter and antimatter must have been created at the beginning of the universe.
However, everywhere we look, we observe matter—and only insignificant amounts of antimatter. The question of where the antimatter went has puzzled scientists for nearly a century.
Fragments of Smashed Atoms
The latest findings come from the STAR experiment, located at the Relativistic Heavy Ion Collider at Brookhaven National Lab.
The experiment operates by colliding the cores of heavy elements, such as uranium, at extraordinarily high speeds. These collisions create tiny, intense fireballs that briefly replicate the conditions of the universe in the first few milliseconds after the Big Bang.
Each collision produces hundreds of new particles, which the STAR experiment can detect. Most of these particles are short-lived, unstable entities called pions, but occasionally, something more intriguing appears.
In the STAR detector, particles travel through a large container filled with gas within a magnetic field, leaving visible trails in their wake. By measuring the "thickness" of these trails and how much they bend in the magnetic field, scientists can determine the type of particle that produced them. Matter and antimatter have opposite charges, so their paths will bend in opposite directions within the magnetic field.
'Antihyperhydrogen'
In nature, the nuclei of atoms are composed of protons and neutrons. However, it's possible to create a "hypernucleus," where one of the neutrons is replaced by a hyperon—a slightly heavier version of the neutron.
The STAR experiment detected an antihypernucleus—a hypernucleus made of antimatter. This was the heaviest and most exotic antimatter nucleus ever observed. Specifically, it consists of one antiproton, two antineutrons, and an antihyperon, and is named antihyperhydrogen-4. Among the billions of pions produced, the STAR researchers identified just 16 antihyperhydrogen-4 nuclei.
Results Confirm Predictions
The new paper compares these newly discovered antinuclei and other lighter antinuclei to their counterparts in normal matter. All hypernuclei are unstable and decay after about a tenth of a nanosecond.
By comparing hypernuclei with their corresponding antihypernuclei, we observe that they have identical lifetimes and masses—which is precisely what we would expect based on Dirac's theory.
Existing theories also accurately predict the more frequent production of lighter antihypernuclei and the less frequent production of heavier ones.
A Shadow World as Well?
Antimatter also has fascinating connections to another exotic substance, dark matter. Observations indicate that dark matter permeates the universe and is five times more prevalent than normal matter, yet we have never been able to detect it directly.
Some theories of dark matter predict that when two dark matter particles collide, they will annihilate each other, producing a burst of matter and antimatter particles. This would then result in the creation of antihydrogen and antihelium—which an experiment called the Alpha Magnetic Spectrometer aboard the International Space Station is searching for.
If we were to observe antihelium in space, how would we know if it originated from dark matter or normal matter? Measurements like the ones from STAR allow us to calibrate our theoretical models for how much antimatter is produced in collisions of normal matter. This latest research provides a wealth of data for such calibration.
Basic Questions Remain
Despite significant advancements in understanding antimatter over the past century, we are still no closer to answering the fundamental question of why we see so little of it in the universe.
The STAR experiment is not the only one dedicated to unraveling the nature of antimatter and its whereabouts. Work at experiments such as LHCb and Alice at the Large Hadron Collider in Switzerland is contributing to our understanding by searching for signs of differences in behavior between matter and antimatter.
Perhaps by 2032, when we mark the centennial of the initial discovery of antimatter, we will have made substantial progress in understanding the role of this curious mirror matter in the universe and its connection to the enigmatic dark matter.
Sources:
Published 21 August 2024 in Nature; Observation of the antimatter hypernucleus
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