A team of researchers has indicated that observations of antihelium in cosmic rays could indicate the presence of WIMPs, theoretical particles that could comprise dark matter. The study, which has reignited interest in this elusive matter, challenges existing astrophysical models and suggests the need for new, more exotic theories of particle physics.
Unraveling the mysteries of dark matter
One of the great challenges of modern cosmology is to reveal the nature of dark matter. Although we know it exists (it makes up over 85% of the matter in the Universe), we have never directly observed it and remain uncertain about its composition. A recent study published in the Journal of Cosmology and Astroparticle Physics investigated traces of antimatter in the cosmos that may indicate a new class of previously undetected particles known as WIMPs (Weakly Interacting Massive Particles), which are potential constituents of dark matter.
The study indicates that recent observations of “antinuclei” in cosmic rays align with the existence of WIMPs, suggesting that these particles may be even stranger than we imagined.
“WIMPs are particles theorized but never observed, and could be the ideal candidate for dark matter,” explains Pedro De la Torre Luque, physicist at the Institute of Theoretical Physics in Madrid, which interacts with other particles only through gravity and gravity. weak interaction force, one of the four fundamental forces that acts only at very close distances.
A few years ago, the scientific community hailed a “miracle”: WIMPs seemed to satisfy all the requirements of dark matter, and it was thought, once “imagined” what they could be and how they could be detected, that within a few years we would have the first direct proof of their existence.
On the contrary, research in recent years has led to the exclusion of entire classes of these particles, on the basis of their peculiar emissions. Today, although their existence has not been completely ruled out, the range of possible types of WIMPs has narrowed considerably, along with the methodologies for trying to detect them.
“Of the numerous most substantiated proposed models, most have been ruled out today and only a few of them survive today,” says De la Torre Luque.
A recent discovery, however, seems to have reopened the case. “These are some observations from the AMS-02 experiment,” explains De la Torre Luque. AMS-02 (Alpha Magnetic Spectrometer) is a scientific experiment on board the International Space Station that studies cosmic rays. “The project managers revealed that they had detected traces of antinuclei in cosmic rays, in particular antihelium, which no one expected.”
To understand why these antinuclei are important for WIMPs and dark matter, we must first understand what antimatter is.
Antimatter is a form of matter with an electrical charge opposite to that of “normal” matter particles. If you followed physics lessons at school, you will know that ordinary matter, the one that surrounds us, is composed of particles with a negative electrical charge, such as electrons, positive charge (protons) or neutral charge. Antimatter is composed of “mirror” particles with opposite charges (a “positive” electron, the positron, a “negative” proton, etc.). When matter and antimatter meet, they annihilate each other, emitting strong gamma radiation.
In our Universe, composed overwhelmingly of normal matter, there is a small amount of antimatter, sometimes closer than one might think, given that positrons are used as contrast agents for PET, the diagnostic test for images that some of you may have subjected yourself to.
Some of this antimatter was formed, according to scientists, during the Big Bang, but more is constantly being created by specific events, making it very significant to observe. “If you see the production of antiparticles in the interstellar medium, where you expect very little, it means that something unusual is happening,” explains De la Torre Luque. “That’s why observing antihelium was so exciting.”
What produces the antihelium nuclei observed by AMS-02 may actually be WIMPs. According to the theory, when two WIMP particles meet, in some cases they annihilate, that is, they destroy each other, emitting energy and producing both matter and antimatter particles. De la Torre Luque and his colleagues tested some of the WIMP models to see if they are compatible with the observations.
The study confirmed that some antihelium observations are difficult to explain by known astrophysical phenomena. “Theoretical predictions suggested that, although cosmic rays can produce antiparticles through interactions with gas in the interstellar medium, the amount of antinuclei, particularly antihelium, should be extremely low,” explains De la Torre Luque.
“We expected to detect an antihelium event every few dozen years, but the ten or so antihelium events observed by AMS-02 are many orders of magnitude higher than predictions based on standard cosmic ray interactions. This is why these antinuclears are a plausible indication of the annihilation of the WIMPs.”
However, there could be more. The antihelium nuclei observed by AMS-02 are of two distinct isotopes (the same element, but with a variable number of neutrons in the nucleus), antihelium-3 and antihelium-4. Antihelium-4, in particular, is much heavier and also much rarer.
We know that the production of heavier nuclei becomes increasingly unlikely as their mass increases, especially through natural processes involving cosmic rays, which is why seeing so many of them is a warning sign. “Even in the most optimistic models, WIMPs could only explain the amount of antihelium-3 detected, but not of antihelium-4,” continues De la Torre Luque, and this would require imagining a particle (or a class of particles) even more strangest of the WIMPs proposed so far or, in technical jargon, even more “exotic”.
Therefore, the study by De la Torre Luque and his colleagues indicates that the road to WIMPs is not yet closed. Many more precise observations are needed and we may have to expand or adapt the theoretical model, perhaps introducing a new dark sector into the standard model of particles known to date, with new “exotic” elements.
The research was published in Journal of Cosmology and Astroparticle Physics.
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