The first view of the Milky Way seen through the lens of neutrino particles – JARA News

Data collected by an Antarctic observatory has produced our first view of the Milky Way galaxy through the lens of neutrino particles. This is the first time that our galaxy has been “colored” by a particle rather than different wavelengths of light.

The result, published in Science, gives researchers a new window into space. Neutrinos are thought to be produced in part by high-energy, charged particles called cosmic rays colliding with other matter. Because of the limitations of our detection equipment, we still don’t know much about cosmic rays. So neutrinos are another way to study them.

The Milky Way we see in the night sky has long been believed to be made up of stars like our Sun. In the 18th century, it was recognized that it is a flattened plate of stars that we see from the inside. It’s only been 100 years since we learned that the Milky Way is actually a galaxy, or “island universe,” one of a hundred billion others.

In 1923, American astronomer Edwin Hubble identified a type of pulsating star called a “Cepheid variable” in what was then known as the Andromeda “nebula” (a huge cloud of dust and gas). Thanks to previous work by Henrietta Swan Leavitt, it provided a measurement of the distance from Earth to Andromeda.

It showed that Andromeda is a distant galaxy like our own, settling a long-standing debate and completely reshaping our understanding of our place in the universe.

Openable windows

As new astronomical windows have opened in the sky, we have seen our galactic home in many different wavelengths of light—radio waves, various infrared bands, X-rays, and gamma rays. We can now see our cosmic home in neutrino particles, which have very little mass and interact only very weakly with other matter, hence their nickname “ghost particles”.

Neutrinos are emitted from our galaxy when cosmic rays collide with interstellar matter. However, neutrinos are also produced by stars like the Sun, some exploding stars or supernovae, and probably most of the high-energy phenomena we observe in the universe, such as gamma-ray bursts and quasars. As such, they can give us unprecedented insight into the highly energetic processes in our galaxy, a view we can’t get using light alone.

The new breakthrough required a rather strange “telescope” buried several kilometers deep in the Antarctic ice cap under the South Pole. The IceCube Neutrino Observatory uses a gigaton of ultra-transparent ice under enormous pressure to detect a type of energy called Cherenkov radiation.

This weak radiation is emitted by charged particles that can travel faster than light in ice (but not in a vacuum). The particles are created by incoming neutrinos that come from cosmic ray collisions in the galaxy hitting icy atoms.

Cosmic rays are mainly proton particles (they, along with neutrons, make up the nucleus of an atom) along with some heavy nuclei and electrons. About a century ago, it was discovered that they rain evenly on the Earth from all directions. We do not yet know definitively all of their sources, as their directions of travel are confused by the magnetic fields that exist in interstellar space.

Deep in the ice

Neutrinos can act as unique tracers of cosmic ray interactions deep in the Milky Way. However, ghost particles also occur when cosmic rays hit the Earth’s atmosphere. So the researchers using the IceCube data needed a way to distinguish neutrinos of “astrophysical” origin – those originating from extraterrestrial sources – from those produced by cosmic ray collisions in our atmosphere.

The researchers focused on a type of neutrino interaction in ice called a cascade. They produce roughly spherical showers of light and give researchers a better level of sensitivity to astrophysical neutrinos in the Milky Way. This is because the cascade provides a better measure of neutrino energy than other types of interactions, even though they are more difficult to reconstruct.

Analysis of ten years of IceCube data using sophisticated machine learning techniques produced nearly 60,000 neutrino events with energies above 500 gigaelectronvolts (GeV). Of these, only about 7% were of astrophysical origin, with the rest attributed to a neutrino “background” source that originates in the Earth’s atmosphere.

The hypothesis that all neutrino events could be due to cosmic rays hitting the Earth’s atmosphere was conclusively rejected at a level of statistical significance known as 4.5 sigma. In other words, the probability of our result is only about 1 in 150,000.

This is a little short of the usual 5 sigma standard for claiming a discovery in particle physics. However, such Milky Way emission is expected on sound astrophysical grounds.

With the upcoming expansion of the experiment – ​​IceCube-Gen2 will be ten times larger – we’ll get many more neutrino events, and the current blurry picture will turn into a detailed view of our galaxy like we’ve never had before.

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