Measurements of cosmic rays may lead to an understanding of dark matter

Five years' worth of data from the cosmic ray detector installed on the International Space Station challenge the conventional models. The amount of antimatter detected surprises physicists.

By José Tadeu Arantes  |  Agência FAPESP – Dark matter remains one of the most fascinating mysteries in physics, but an answer to the riddle may not be so far off after all, largely because of measurements from the Alpha Magnetic Spectrometer (AMS-02), a multipurpose detector installed on the International Space Station (ISS) in 2011 to measure cosmic rays before they interact with Earth’s atmosphere. The results of its first five years of operation have revealed new and exciting discoveries in this field.

A presentation on the results by Physics Nobel Laureate Samuel Ting, principal investigator for the AMS-02 experiment, was webcast live by CERN, the European Organization for Nuclear Research, on December 8, 2016.

Orbiting approximately 360 km above the Earth’s surface, the AMS-02 has collected data from over 90 billion cosmic rays, to date. The experiment is operated by an international team of researchers from 56 institutions in 16 countries and was developed under US Department of Energy (DOE) sponsorship. 

Manuela Vecchi, a professor at the University of São Paulo’s São Carlos Physics Institute (IFSC-USP) in Brazil, is a member of the team and is supported in this research by a grant from FAPESP.

“In these first five years of observation, we’ve published several articles in Physical Review Letters,” Vecchi told Agência FAPESP. “They include ‘Antiproton flux, antiproton-to-proton flux ratio, and properties of elementary particle fluxes in primary cosmic rays measured with the Alpha Magnetic Spectrometer on the International Space Station’, published in August 2016. The abundance and precision of the data obtained by the AMS-02 are challenging the conventional models of the origin and propagation of cosmic rays.”

Cosmic rays are made up primarily of high-energy particles traveling near the speed of light originating from far beyond our solar system. A small but highly significant proportion of the colossal amount of data collected by the AMS-02 relates to the detection of antimatter: antielectrons (or positrons) and antiprotons.

“Antimatter is a tiny fraction of cosmic radiation – on the order of 1 in every 10,000 particles. Detecting antiparticles is a major challenge due to the huge number of background protons, which make up the overwhelming majority of the recorded data. But by combining different detection techniques, the AMS-02 has succeeded in measuring them with admirable precision. The results are fascinating because antiparticles are vital clues in the indirect search for dark matter,” Vecchi said.

To understand why, it helps to recall a key idea in today’s cosmology. According to the standard Big Bang model, matter and antimatter were equally abundant at the beginning of the universe 14 billion years ago. However, a spontaneous break in the symmetry occurred during a phase transition in the primordial universe, which produced a small excess of matter, on the order of one extra matter particle per billion matter-antimatter particle pairs. It has been calculated that this apparently tiny excess would have been sufficient to produce the entire present universe, with all galaxies, stars, planets and other material objects.

Yoichiro Nambu, Makoto Kobayashi and Toshihide Maskawa won the 2008 Nobel Prize in Physics for discovering the origin of the broken symmetry that contributed to a preponderance of matter over antimatter in the universe.

This breakthrough would explain why antimatter is not currently found in a stable form anywhere in the universe. According to the prevailing cosmic ray model, positrons and antiprotons are produced by interactions between matter particles during their propagation in the interstellar medium. 

However, measurements performed with the AMS-02 have shown that the number of antimatter particles is larger than expected on the basis of conventional astrophysical processes.

“We know cosmic ray propagation in the interstellar medium leads to the production of positrons and antiprotons. We can calculate the expected flux of these antiparticles as a function of energy. Any unpredicted excess requires recourse to other physical phenomena to explain the difference,” Vecchi said. “This excess had already been detected by the PAMELA satellite-borne experiment in 2008, and, it’s been confirmed with great precision by the AMS-02.”

This is where dark matter comes in. One plausible hypothesis is that particle-antiparticle pairs are being produced in large numbers by the annihilation of dark matter. According to current calculations, the universe consists of 73% dark energy, 23% dark matter, and only 4% known matter. “I wouldn’t say dark matter is the only source of the antimatter in cosmic rays, but it undoubtedly should be considered as one of the possible sources,” Vecchi said.

A strong candidate for dark matter is the WIMP, short for weakly interacting massive particle. “The WIMP is part of the standard model, but it’s predicted in extensions of the model. It’s thought to be a stable particle and hence not subject to decay. Its annihilation would give rise to a particle-antiparticle pair,” Vecchi explained.

In addition to dark matter, other possible sources for the excess of antiparticles are more conventional astrophysical objects such as pulsars or the remains of supernovae. 

The excess of antiparticles is not the only finding from the data collected by the AMS-02 that challenges the prevailing model. Other measurements also appear to require a radical revision. Protons – ionized hydrogen nuclei – make up 90% of the composition of cosmic rays. Another 8% consists of alpha particles – helium nuclei with two protons and two neutrons. Next, in smaller proportions, come electrons, other nuclei produced in the sources (carbon, nitrogen, oxygen and other elements including iron), nuclei resulting from the fragmentation of other cosmic rays during propagation through the interstellar medium (lithium, beryllium and boron) – and antiparticles, comprising a small fraction. “The data obtained so far doesn’t match what the conventional model leads us to expect,” Vecchi said.

Another surprising result, albeit still preliminary, is the possible detection of antihelium. “In five years, the AMS-02 detected 3.7 billion helium events and only a very few antihelium events. Even so, this finding could have major phenomenological implications: if confirmed, it would be the first detection of antinuclei in cosmic rays,” Vecchi said.

Dr. Manuela Vecchi is a professor at IFSC-USP and an active scientific collaborator with CERN. Her Young Investigator Research Grant from FAPESP was awarded in 2015 and will last until 2019.