The first-ever observation of the Milky Way galaxy using neutrino particles has been made possible by data collected by an observatory in Antarctica. Unlike previous observations that relied on different wavelengths of light, this new view provides researchers with a fresh perspective on the cosmos. Neutrinos are believed to be emitted when high-energy cosmic rays interact with other matter. Since our knowledge of cosmic rays is limited due to the constraints of our detection equipment, studying neutrinos offers another avenue for understanding them.
Throughout history, it has been speculated that the Milky Way, as seen in the night sky, consists of stars similar to our Sun. In the 18th century, astronomers realized that it is actually a flattened collection of stars viewed from within. Only a century ago, it was discovered that the Milky Way is a galaxy, or an “island universe,” among countless others. This revelation, brought about by the work of American astronomer Edwin Hubble and Henrietta Swan Leavitt, completely transformed our understanding of our place in the universe.
Over the years, advancements in astronomy have allowed us to observe our galactic home using different wavelengths of light, such as radio waves, infrared, X-rays, and gamma-rays. Now, with the groundbreaking detection of neutrinos, we can add another dimension to our understanding. Neutrinos, often referred to as “ghost particles,” have extremely low mass and interact very weakly with other matter. They are emitted from our galaxy when cosmic rays collide with interstellar matter. However, they can also be produced by stars like the Sun, supernovas, gamma-ray bursts, quasars, and other high-energy phenomena in the universe. This gives us an unprecedented insight into the energetic processes taking place in our galaxy.
The pioneering detection of neutrinos required the use of a unique “telescope” buried deep, several kilometers beneath the Antarctic ice cap at the South Pole. The IceCube Neutrino Observatory utilizes a massive amount of ultra-transparent ice, subjected to enormous pressures, to detect Cherenkov radiation, a form of energy. Ice allows charged particles to travel faster than light (though not in a vacuum), emitting faint radiation. These particles are created by incoming neutrinos colliding with atoms in the ice.
Cosmic rays, consisting mainly of proton particles, along with a few heavy nuclei and electrons, shower the Earth uniformly from all directions. However, their sources are still not definitively known due to the influence of magnetic fields in space that scramble their travel directions.
Neutrinos can serve as unique indicators of cosmic ray interactions within the Milky Way. However, they are also generated when cosmic rays collide with Earth’s atmosphere. To differentiate between these astrophysical neutrinos and those created within our atmosphere, researchers focused on a specific type of neutrino interaction called a cascade. These interactions result in spherical showers of light and provide a higher level of sensitivity to astrophysical neutrinos. Although reconstructing cascades may be challenging, they offer a better measurement of a neutrino’s energy.
By analyzing ten years of data from IceCube using advanced machine learning techniques, nearly 60,000 neutrino events with an energy above 500 gigaelectronvolts (GeV) were discovered. Of these, only about 7% were determined to be of astrophysical origin, while the remaining events were attributed to neutrinos generated in Earth’s atmosphere. The hypothesis that all neutrino events were caused by cosmic rays hitting the Earth’s atmosphere was definitively rejected with a statistical significance of 4.5 sigma, indicating that our result is highly unlikely to be a chance occurrence.
Although this falls slightly short of the conventional 5 sigma standard used in particle physics to claim a discovery, the emission of neutrinos from the Milky Way is theoretically expected. As the IceCube experiment expands with the construction of IceCube-Gen2, which will be ten times larger, we can look forward to many more neutrino events and a more detailed view of our galaxy than ever before.
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