For decades, Astrophysicists have theorized that the majority of matter in our universe is made up of a mysterious invisible mass known as dark matter (DM). Although scientists have yet to find direct evidence of this invisible mass or confirm what it looks like, there are several possible ways to search for it soon.
One theory is that dark matter particles could collide and annihilate to produce cosmic rays that proliferate throughout our galaxy, much like cosmic ray collisions with the interstellar medium (ISM) do.
This theory could be tested soon, thanks to research carried out using the ALICE experiment (A Large Ion Collider Experiment), one of the many detection experiments at CERN’s Large Hadron Collider (LHC).
ALICE is optimized to study the results of collisions between nuclei moving very close to the speed of light (ultra-relativistic velocities). According to new research from the ALICE collaboration, dedicated instruments could detect anti-helium-3 nuclei (the anti-matter homolog of He3) as they reach Earth’s atmosphere, providing evidence of DM.
How the search for dark matter began
Dark matter theory emerged in the 1960s when astronomers performed observational tests of general relativity (GR) using distant galaxies and galaxy clusters.
A key prediction of GR is that the curvature of spacetime is altered in the presence of gravitational fields caused by massive objects. This can be seen with gravitational lensing, a phenomenon where light from a distant source is distorted and amplified (leading to Einstein’s rings, crosses and arcs). However, when observing large structures in the Universe, astronomers noted that the curvature they observed was much larger than expected.
This suggested two possibilities: either Einstein was wrong (despite all the tests that proved him to be correct), or there must be some mass in the universe that we cannot see. Since then, the challenge for astrophysicists and cosmologists has been to find direct evidence of this elusive dark matter.
How to detect mystery matter
As they indicated in their study recently published in the journal Natural Physics, antinuclei produced by DM annihilations could be detected (depending on the nature of the DM itself). In this case, the ALICE collaboration used the main theoretical profile known as weakly interacting massive particles (WIMP).
According to the WIMP theory, DM consists of particles that neither emit nor absorb light and interact with other particles only through the weak nuclear force. This same theory also states that the interaction between these particles causes them to annihilate each other and produce anti-He3 nuclei, composed of two antiprotons and one antineutron.
These antinuclei would travel throughout our galaxy and could be measured as cosmic rays, high-energy particles that originate from beyond our solar system and collide with our atmosphere (producing “rains” of elementary particles).
However, other types of cosmic rays (protons from helium nuclei) can also collide with the interstellar medium (ISM) to create anti-He3 nuclei. Since this source of antinuclei is unrelated to DM, it would form the background of DM research. As Laura Serksnyte, a researcher at the Technische Universitat Munich and one of the study’s experts, said. Universe today by email:
“The expected number of low-energy antihelium-3 nuclei from dark matter annihilation should be much larger than the background contribution. Thus, the detection of even a few low-energy antihelium-3 nuclei in cosmic rays would provide a smoldering signal for dark matter, which means antihelium-3 is a very “clean” probe for dark matter searches.
However, this smoking gun could be difficult to track down, as anti-He3 nuclei could also interact with gas in the ISM as they propagate through the Milky Way. This inelastic interaction would cause the anti-He3 nuclei to disappear before they reach the Earth’s atmosphere, where dedicated instruments could detect them.
On Earth, the only way to produce and study antinuclei with high precision is to create them in high-energy particle accelerators. This, Serksnyte said, is where the LHC and the ALICE instrument came in:
“Our experiment investigated the inelastic interactions of antihelium-3 (produced in LHC collisions) with matter, where the ALICE detector itself is used as a target. Our work thus provided the first-ever measurement of the inelastic cross section of antihelium-3, which limits the probability that antihelium-3 will disappear if it collides with matter.
After measuring the anti-He3 produced in the LHC, the team then applied their measurements to see how these antinuclei would interact with gas in the ISM, either as a result of DM annihilation or as a result ordinary collisions of cosmic rays with ISM gas.
By calculating the level of antinuclei that disappear while traveling from their point of origin to detectors in the Earth’s atmosphere, they were able to estimate the fraction that would be detectable by our instruments. The results, Serksnyte said, were quite encouraging:
“Our results show that the transparency of our galaxy to the passage of cosmic rays from antihelium-3 is high, and therefore such antinuclei could well reach Earth and be measured by dedicated experiments. Thus confirming that antihelium-3 is a promising candidate for dark matter research. Our measure of the vanishing probability of antihelium-3 nuclei interacting with matter will also be used by scientists to understand antihelium-3 cosmic ray fluxes once measured and to constrain dark matter models.
By placing tighter constraints on what scientists might search for, future investigations will help solve one of the most pressing mysteries in astrophysics today. Detecting dark matter would not only confirm where 85% of the matter in the universe is hiding.
It would also validate an essential part of the most widely accepted theory of cosmology – the Lambda-Cold Dark Matter Model (LCDM) – and confirm that general relativity (a basic building block of modern physics) is correct. Although this is not the end of the cosmological mysteries, it will lead to a better understanding of everything.
This article was originally published on Universe today by MATT WILLIAMS. Read the original article here.