A Step Closer to Detecting Ancient Neutrinos

Physics 15, s85
Using radioactive tritium, scientists improve laboratory constraints on the overdensity signal of cosmic relic neutrinos by a factor of 100, an advance that should improve the chances of spotting this elusive particle.

One second after the big bang, so-called cosmic relic neutrino particles scattered out across the Universe. Today, those particles are thought to cluster near galaxies, such as our own Milky Way. When this clustering goes above a certain value, known as the overdensity, researchers predict that the particles should produce a signal detectable on Earth. Though researchers are confident that this signal gets produced, they have yet to capture it in experiments. In a step toward that goal, Thierry Lasserre from the French Alternative Energies and Atomic Energy Commission and colleagues have now updated the constraints on the cosmic relic neutrino overdensity, a result they say should improve the chances of observing the presence of cosmic relic neutrinos [1].

To update the constraint, Lasserre and colleagues analyzed measurements conducted at the Karlsruhe Tritium Neutrino Experiment (KATRIN), Germany. KATRIN consists of a windowless chamber that houses 200𝜇g of gaseous, radioactive tritium and various spectrometers. Relic neutrinos are predicted to interact with tritium, inducing a well-defined electron signal at the edge of the tritium beta-decay spectrum; KATRIN contains the most concentrated source of research tritium in the world.

Beginning in 2019, KATRIN started measuring with high precision the beta-decay spectrum of tritium, and Lasserre and colleagues have now searched that spectrum for a sign of relic neutrino-tritium interactions. While the team have so far found no such signal, they have been able to improve the precision of constraints on the relic neutrino overdensity by a factor of 100. Where previous experiments set the upper bound of this overdensity at ten trillion, Lasserre and colleagues reduce that to a hundred billion.

While Lasserre acknowledges that physicists are still decades away from observing a direct relic neutrino signal, he says that this work represents an important step forward in the search for the holy grail of neutrino physics.

–Allison Gasparini

Allison Gasparini is a freelance science writer based in Santa Cruz, CA.


  1. M. Aker et al. (KATRIN Collaboration), “New constraint on the local relic neutrino background overdensity with the first KATRIN data runs,” Phys. Rev. Lett. 129, 011806 (2022).

Subject Areas

CosmologyParticles and Fields

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