Physicists are close to knowing the mass of the Neutrino

Neutrinos are among the most numerous particles in the known universe, only outnumbered by photons. Yet, we don’t know nearly as much about them as you would imagine because they almost never interact with matter. The neutrino has been baffling physicists for years, but we are finally getting close to understanding enough to change the way we see the cosmos.

About 1,000 trillion of the ghostly particles pass through your body every second, yet they don’t interact with you like photons do. Now we are getting closer to knowing how much the neutrino weighs, which could solve several astrophysics problems.

“The fact that they’re ubiquitous, yet we don’t even know what they weigh, is kind of crazy,” said Deborah Harris, a physicist at the Fermi National Accelerator Laboratory near Chicago and York University in Toronto.

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Finally, in September this year, after 18 years of planning, building and calibrating, the Karlsruhe Tritium Neutrino (KATRIN) experiment in southwestern Germany announced its first results. It found that the neutrino can’t weigh more than 1.1 electron-volts (eV), or about one-five-hundred-thousandth the mass of the electron.

Measuring its mass would help point toward new laws of physics beyond the Standard Model, the remarkably successful yet incomplete description for how the universe’s known particles and forces interact. “Depending on what the mass of the neutrino turns out to be, it may lead to very exciting times in cosmology,” said Diana Parno, a physicist at Carnegie Mellon University and a member of the KATRIN team.

Up until relatively recently, it was assumed that the neutrino was massless. It was first theorised in 1930 and detected for the first time in 1956. Determining the mass of the neutrino will allow us to understand much more about what happened shortly after the Big Bang. So many of these particles were born during the Big Bang that their collective gravity influenced how the matter in the universe clumped into stars, galaxies and even more massive structures.

About a second after the Big Bang, neutrinos were flying around at almost light speed—so fast that they escaped the gravitational pull of other matter. But then they started to slow, which enabled them to help corral atoms, stars and galaxies.

The coming years will be extremely exciting in cosmology and astrophysics as we learn more and more about the origins and evolution of the universe.