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Neutrinos are fundamental particles characterized by no electric charge and very small masses, which are known to interact with other matter via the weak force or gravity. While these particles have been the focus of numerous research studies, the processes through which they acquire their masses have not yet been elucidated.

One hypothesis is that neutrino masses originate from interactions with ultralight dark matter, a type of dark matter theorized to be made up of particles or fields with extremely small masses below 10 electron volts (eV). Researchers at Shanghai Jiao Tong University and University of Salerno recently set out to test this hypothesis by comparing data collected by the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) experiment to theoretical predictions.

Their findings, in a paper in ÌÇÐÄÊÓƵical Review Letters, suggest that neutrino masses are not likely to have a dark origin.

"The motivation for this work came from one of the most pressing open questions in particle physics: the origin of neutrino masses," Luca Visinelli, senior author of the paper, told ÌÇÐÄÊÓƵ.

"Unlike other fermions, neutrinos are massless in the Standard Model, but experiments have clearly shown that they must possess a tiny mass, as demonstrated by the phenomenon of neutrino oscillations. Because neutrino masses are so small, their origin likely involves new physics beyond the Standard Model, potentially unrelated to the Higgs mechanism that gives mass to other known particles."

In recent years, many physicists have been exploring the possibility that the masses of neutrinos arise from interaction with a so-called "dark sector," or, in other words, with hypothetical fields and forces also connected to dark matter or dark energy. Visinelli and his colleagues Andrew Cheek and Hong-Yi Zhang drew from these earlier works to further test this hypothesis using a combination of theory and experimental observations.

"We developed a framework in which the smallness of neutrino masses results from their interaction with the dark sector, and then rigorously tested if such a connection can be detected using existing neutrino data, including short- and long-baseline neutrino oscillation experiments and solar neutrino measurements," said Visinelli. "Our results suggest that such a dark-sector origin for neutrino masses is not supported by current data."

Overall, the findings derived by the researchers suggest that it is highly unlikely that neutrino masses originate from interactions with a dark sector. On the other hand, it is far more likely for them to be explained by conventional particle physics phenomena. Their study could thus guide future efforts aimed at uncovering the origin of neutrino masses by narrowing down the processes that could explain their emergence.

"To test our hypothesis, we developed a theoretical model in which neutrinos acquire mass through interactions with new particles in a dark sector," explained Visinelli. "Specifically, we considered the case where dark matter is composed of light bosonic fields. These fields behave as coherent waves oscillating in time, with the frequency determined by the mass of the boson.

"In this setup, we carefully analyzed how the frequency of these oscillations compares with the timescales relevant to neutrino experiments, for these oscillations can be either fast or slow depending on the dark matter mass."

As part of their analyses, the researchers also considered the spatial variation of the dark matter field. In other words, they account for the fact that dark matter would have a characteristic oscillation length, which overlaps with the source and detector locations in neutrino experiments, as well as the movements of Earth in our galaxy.

"These considerations lead to a modification of the standard neutrino oscillation probabilities," said Visinelli. "We show that such changes can be tested using data from current neutrino observatories, across a range of dark matter masses.

"Our study provides a testable framework connecting the origin of neutrino masses to interactions with a dark sector. This scenario can be constrained with a systematic analysis of current data from solar and baseline neutrinos, along with a careful modeling of the astrophysical setup."

By comparing theoretical predictions with neutrino observations by the KamLAND experiment, the researchers showed that neutrino masses are more likely to originate from conventional particle physics processes or other new physics unrelated to dark matter. As more neutrino-related data becomes available, Visinelli and his colleagues plan to use them to revisit their framework and test new theoretical predictions.

"Upcoming results from the JUNO and DUNE experiments could allow us to search for subtle time variations in neutrino oscillation parameters, compared with what is expected by a vacuum mass solution," added Visinelli.

"Beyond neutrinos, we're also exploring the possibility of applying similar ideas to other quantum systems. For example, atomic and nuclear systems, like those studied in atomic clocks or precision magnetometers, can also be sensitive to oscillations induced by light bosonic dark matter."

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