Good news, universe! Scientists are one step closer to finally understanding dark matter

Dark matter is made up of axions, elementary particles that are full of suspense.
Rupendra Brahambhatt
Massive black holes in nebula.sakkmes erke/iStock

About 85 percent of our universe is believed to be composed of dark matter, a hypothetical material that does not interact with light. So it neither reflects nor emits nor absorbs any light rays, and therefore, we can not see this unusual form of the matter directly. However, to understand and explain the nature of dark matter, scientists have created various models.

Surprisingly, a new study has ruled out one such popular explanation of the dark matter, called the axion-like particle (ALP) cogenesis model. The exclusion of ALP means that scientists will now have to consider fewer models while conducting dark matter research. This would increase both the speed and accuracy of their research works and bring us one step closer to understanding the most strange phenomenon of the universe. 

Why do scientists integrate the ALP cogenesis model?

Good news, universe! Scientists are one step closer to finally understanding dark matter
Source: Bluemount_Score/Pixabay

Before going deep into the ALP model you first need to understand axions. Since the Standard Model of Physics does not give any details about neutrons, anti-matter, and dark matter, a team of particle physicists suggested the presence of axions. supposed elementary particles that were first mentioned for the first time in 1977’s Peccei–Quinn theory.

Although, axions were originally proposed to explain the charge conjugation and parity-related properties of neutrons. Later researchers discovered that these hypothetical particles could also play an important role in explaining dark matter and antimatter (both are different). It is believed that axions collectively form the dark matter in our universe. 

Further explaining the elementary dark matter particles, the researchers wrote in their research paper, “axions are hypothetical, massive spin-0 particles that were first postulated as a result of an elegant solution to the strong charge-parity (CP) problem in quantum chromodynamics (QCD). The weakly interacting nature of axions, combined with theoretical predictions and early-universe production mechanisms before or after inflation, simultaneously makes them a compelling dark matter candidate.”

Studies in the past have revealed that the density of dark matter and baryons (particle-like protons and neutrons that form the regular matter) can be explained only after applying the ALP cogenesis model to the Standard Model of Physics. This can be done by coupling axions with other subatomic particles such as electrons and nucleons.

In order to integrate axions to overcome the limitations of the standard model, physicists employ a widely accepted SMASH model (standard model axion seesaw Higgs portal inflation). The SMASH model suggests that the mass of each axion should range between 50 to 200 micro-electron volts (μeV). 

The reason behind the exclusion of the ALP cogenesis model

Good news, universe! Scientists are one step closer to finally understanding dark matter
The haloscope detector being used in the ORGAN experiment. Source: Quantum Technology and Dark Matter Laboratory

The recent study that rules out the APL model is based on the initial results obtained during the ORGAN (Oscillating Resonant Group AxioN) experiment. ORGAN is a project run by the University of Western Australia, it is a haloscope search program that aims to discover axions having large masses. The haloscope detection method involved the use of a uniform but powerful and static magnetic field inside a resonant cavity

Many experiments including ORGAN consider the mass of axions to fall between 62 and 207 μeV and the ALP model focuses on axions existing at a low mass range i.e. between 63 to 67 μeV. Interestingly, during the study, scientists didn’t find any axions in the range of 63 to 67 μeV suggesting the absence of dark matter.

Based on these findings, the researchers concluded that since the mass range studied under the APL cogenesis model indicates the absence of axions, it can no longer be used to explain the nature of dark matter. These results will play a significant role in narrowing and simplifying future dark matter research.  

The study is published in the journal Science Advances.

Abstract:

The standard model axion seesaw Higgs portal inflation (SMASH) model is a well-motivated, self-contained description of particle physics that predicts axion dark matter particles to exist within the mass range of 50 to 200 micro–electron volts. Scanning these masses requires an axion haloscope to operate under a constant magnetic field between 12 and 48 gigahertz. The ORGAN (Oscillating Resonant Group AxioN) experiment (in Perth, Australia) is a microwave cavity axion haloscope that aims to search the majority of the mass range predicted by the SMASH model. Our initial phase 1a scan sets an upper limit on the coupling of axions to two photons of ∣ga∣ ≤ 3 × 10−12 per giga–electron volts over the mass range of 63.2 to 67.1 micro–electron volts with 95% confidence interval. This highly sensitive result is sufficient to exclude the well-motivated axion-like particle cogenesis model for dark matter in the searched region.

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