A discovery of the Large Hadron Collider could point the way to dark matter

For decades, Astrophysicists have hypothesized that the majority of matter in our universe consists of a mysterious invisible mass known as dark matter (DM). Although scientists have yet to find any direct evidence of this invisible mass or confirm what it looks like, there are several possible ways we could be looking for it soon.

One theory is that dark matter particles could collide and annihilate each other to produce cosmic rays that propagate throughout our galaxy—similar to how cosmic rays collide with the interstellar medium (ISM).

This theory could soon be tested, thanks to research conducted using the A Large Ion Collider Experiment (ALICE), one of several detector experiments at CERN’s Large Hadron Collider (LHC).

ALICE is optimized to study the effects of collisions between nuclei traveling at very close to the speed of light (superrelativistic velocities). According to new research by the ALICE Collaboration, special instruments could detect anti-helium-3 nuclei (the antimatter equivalent of He3) as they reach Earth’s atmosphere, thus providing evidence of the DM.

How the search for dark matter began

The dark matter theory emerged in the 1960s when astronomers conducted observational tests of general relativity (GR) using distant galaxies and galaxy clusters.

A key prediction of GR is that the curvature of spacetime changes 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 Bows). However, when observing large structures in the Universe, astronomers noticed that the curvature they observed was much greater than expected.

This suggested two possibilities: either Einstein was wrong (despite all the tests that proved him right), or there must be mass in the universe that we cannot see. The challenge for astrophysicists and cosmologists ever since has been to find direct evidence of this elusive dark matter.

How to detect the mysterious matter

As they reported in their study, which was recently published in the journal Physics of Nature, antinuclei produced by DM annihilations could be detected (depending on the nature of the DM itself). In this case, the ALICE Collaboration used the leading theoretical profile known as Weakly-Interacting Massive Particles (WIMPs).

According to the theory of WIMPs, DM consists of particles that neither emit nor absorb light and only interact with other particles 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, which consist of two antiprotons and one antineutron.

These antinuclei would travel throughout our galaxy and could be measured as cosmic rays, high-energy particles that come from beyond our solar system and collide with our atmosphere (creating a “shower” of elementary particles).

However, other types of cosmic rays (protons of helium nuclei) can also collide with the interstellar medium (ISM) to create anti-He3 nuclei. Since this antikernel source is not related to DM, it would form the background for DM searches. As Laura Serksnyte – a researcher at the Technische Universitat Munich and one of the experts in the study – said Universe today via email:

“The expected number of low-energy antisolar-3 nuclei from dark matter annihilation is expected to be much larger than from the background contribution. Thus, the detection of even a few low-energy antisolar-3 nuclei in cosmic rays would provide a smoke signal for dark matter, meaning that antisolar-3 is a very ‘clean’ detector for dark matter surveys.”

Recent research suggests that the antisolar could help scientists find dark matter.Shutterstock

However, this smoking gun could be difficult to detect, as anti-He3 nuclei may also interact with gas in the ISM as they propagate throughout the Galaxy. This inelastic interaction would cause the anti-He3 nuclei to disappear before reaching the Earth’s atmosphere, where special 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 into play:

“Our experiment studied the inelastic interactions of antisolar-3 (produced in collisions at the LHC) with matter, where the ALICE probe itself is used as a target. Thus, our work has given us the first measurement of the inelastic cross section of antihelium-3, which constrains how likely antihelium-3 is to disappear if it collides with matter.”

After measuring the anti-He3 produced at the LHC, the team applied their measurements to see how these antinuclei would interact with the gas in the ISM—either as a result of DM annihilation or from ordinary cosmic-ray collisions with the ISM gas .

By calculating the level of antinuclei that disappear as they travel from their point of origin to detectors in Earth’s atmosphere, they were able to estimate the fraction that would be detectable in our instruments. The results, Serksnyte said, were quite encouraging:

“Our results show that the transparency of our galaxy to the passage of antisolar-3 cosmic rays is high, and so such antinuclei could indeed reach Earth and be measured by special experiments. Thus confirming that antihelium-3 is a promising candidate for dark matter searches. Our measurement of the extinction probability of antisolar-3 nuclei interacting with matter will also be used by scientists to understand the cosmic ray fluxes of antisolar-3 once measured and to put constraints on dark matter models.”

The Hubble Space Telescope offers a cosmic spider web of galaxies and dark matter in the Abell 611 cluster. Credits: ESA/Hubble, NASA, P. Kelly, M. Postman, J. Richard, S. Allen

By placing tighter limits on what scientists could look for, future investigations will help solve one of the most pressing mysteries in astrophysics today. Detecting dark matter would not only confirm where 85 percent of the matter in the Universe is hidden.

It would also validate a vital part of the most widely accepted theory of cosmology – the Lambda-Cold Dark Matter (LCDM) model – and confirm that general relativity (a staple of modern physics) is correct. While this will not be the end of the cosmological mysteries, it will lead to a greater understanding of everything.

This article was originally published on Universe today by MATT WILLIAMS. Read the original article here.

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