Neutrinos are elementary particles that are predicted to be massless by the standard model of particle physics, yet their observed oscillations suggest that they do in fact have a mass, which is very low. A further characteristic of these particles is that they only weakly interact with other matter, which makes them very difficult to detect using conventional experimental methods.

The KATRIN (Karlsruhe Tritium Neutrino) experiment is a large-scale research effort aimed at precisely measuring the effective mass of the electron anti-neutrino using advanced instruments located at the Karlsruhe Institute of Technology (KIT) in Germany.

The researchers involved in this experiment recently published the results of a new analysis of data from the second measurement campaign in Physical Review Letters, which set new constraints on interactions involving neutrinos that could arise from unknown physics that is not explained by the standard model, also known as general neutrino interactions.

"We know that beyond standard model (BSM) physics is hiding in the neutrino sector, but we don't know what it looks like yet," Caroline Fengler, lead analyst for this search, told Phys.org. "That's what has motivated us already in the past to look for various BSM physics phenomena with KATRIN, such as light and heavy sterile neutrinos and Lorentz invariance violations.

"The theory work by the group of Werner Rodejohann then gave us the incentive to broaden our search to any possible new neutrino interactions that might contribute to the weak interaction of the beta decay."

The new interactions that the researchers started looking for could hint at the existence of various exciting physical phenomena outside that are not predicted by the standard model of particle physics, but that have been widely explored by theorists. For instance, they could indicate the presence of various hypothetical particles, including right-handed W bosons, charged Higgs bosons, and Leptoquarks.

"The main purpose of the KATRIN experiment is to measure the mass of the neutrino," explained Fengler. "This is done through a highly precise measurement of the energy spectrum of the electrons originating from tritium beta decay, using a high-activity tritium source and a one-of-a-kind electron spectrometer. The shape of the recorded beta spectrum then contains information about the neutrino mass and other BSM physics contributions."

Notably, general neutrino interactions are predicted to prompt characteristic shape deformations of the so-called beta spectrum, which is the distribution of electron energies emitted during a type of radioactive decay known as beta decay. The KATRIN collaboration thus set out to search for these beta spectrum deformations in the data collected as part of the experiment.

"With only a small part (5%) of the final KATRIN dataset, we were already able to set competitive constraints on some of the investigated new neutrino interactions compared to the global constraints from other low-energy experiments," said Fengler. "This shows that the KATRIN experiment is sensitive to these new interactions."

While the KATRIN experiment did not detect signs of general neutrino interactions yet, it set competitive constraints on the strength of these new and elusive interactions, employing a new experimental approach. The KATRIN collaboration hopes that these constraints will contribute to the future search for physics beyond the standard model.

"We are already working on further improving our sensitivity on the general neutrino interactions with KATRIN by extending the data set and fine-tuning our analysis approach," added Fengler. "With the beginning of the upcoming TRISTAN phase at KATRIN in 2026, which is set out to search for keV sterile neutrinos with the help of an upgraded detector, we will gain access to another powerful data set, which promises to greatly improve our sensitivity in the future."

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