Neutrino messengers originate within our own galaxy

The least interactive particles ever observed in the universe are neutrinos. Their neutral charge makes them unaffected by electromagnetism, and their lack of color charge means that they don’t participate in the strong force. The weak force has an exceedingly short range, roughly the diameter of a proton. With those properties, neutrinos can travel through normal matter more or less undetected—every second, a trillion neutrinos from the Sun pass through your hand. (For more on neutrinos, see Physics Today, December 2015, page 16 , and the article by William McDonough, John Learned, and Stephen Dye, Physics Today, March 2012, page 46 .)

The lack of interactions makes neutrinos an ideal messenger of information on various astrophysical processes (see “The elusive Glashow resonance was observed deep within Antarctic ice,” Physics Today online, 8 April 2021 , and the article by Francis Halzen and Spencer Klein, Physics Today, May 2008, page 29 ). High-energy cosmic rays—whether they’re from the Milky Way or some extragalactic source—produce several particles, including charged pions that then decay to make neutrinos. Because neutrinos don’t interact much with other matter, their detection can be used to trace the direction from which they came and to reconstruct the energy of the cosmic-ray interactions that produced them.

Now the IceCube Collaboration has identified a high-energy neutrino flux in the Milky Way that’s focused predominantly in the galactic plane. The finding reinforces previous theoretical hypotheses about the origins of neutrinos and shows how multimessenger astronomy can be incorporated into investigations of our own galaxy.

According to observations of gamma-ray emission by the Large Area Telescope aboard NASA’s Fermi Gamma-Ray Space Telescope, the Milky Way’s center and its dense galactic plane should have many sources of neutrino emission, and those vast areas are best seen in Earth’s southern sky. So near the Amundsen–Scott South Pole Station in Antarctica sits the IceCube Neutrino Observatory. Inside a cubic kilometer of ice are about 5000 spherical optical sensors, which hang on cables drilled and emplaced as deep as 2500 m.

Sometimes incoming neutrinos slam into a water molecule’s nucleus and produce charged particles. If any of them are energetic enough, they travel faster than light does in the ice, which results in the emission of Cherenkov radiation. (The effect is similar to the sonic boom that’s heard when a supersonic jet travels many times faster than the speed of sound.) By measuring the Cherenkov radiation and the momentum transfer from the incoming neutrino, researchers can infer the direction from which the particle arrived and whether cosmic-ray interactions in the Milky Way produced them.

Although hundreds of scientists, engineers, and others have contributed to the collaboration, one critical advance is a hybrid artificial-intelligence technique developed by the collaboration and spearheaded by Mirco Hünnefeld of the Technical University Dortmund and Steve Sclafani of Drexel University. The method, which uses a deep-learning neural network, separates astrophysical neutrinos—characterized by energies in the TeV range and sometimes up to a few PeV—from atmospheric noise. The 10 years of data that the researchers collected yielded 60 000 neutrino events, a sample set that’s 20 times as large as that of previous methods. Compared with earlier analyses, the neural network also improves by a factor of two the angular resolution of the direction of incoming neutrinos.

With those improvements, the collaboration found a signal of diffuse high-energy neutrino emission from the galactic plane, shown in comparison with optical and gamma-ray emissions, that was 4.5 standard deviations above that of background noise. Of the total neutrino flux observed, some 10% of it at 30 TeV is attributable to sources in the Milky Way. Extragalactic neutrinos could come from supermassive black holes (see Physics Today, August 2022, page 14 ), active galactic nuclei (see “IceCube pinpoints an extragalactic neutrino source,” Physics Today online, 12 July 2018 ), and supernovae (see “A supernova for the ages, 30 years later,” Physics Today online, 23 February 2017 ). What ultimately causes the neutrino-producing cosmic rays remains unclear. But to search for sources inside and outside the Milky Way, the team is already at work collecting more data. (IceCube Collaboration, Science 380, 1338, 2023 .)