A strange ‘neutrino force’ helped close a crack in particle physics

A strange force that circumvents traditional standards of physics has helped erase a stain on the foundations of particle physics.

This is a phenomenon called the “neutrino force,” whereby neutrinos could theoretically combine to transmit influences between particles. Two objects would feel this subtle force when they exchange pairs of neutrinos. This effect is not limited to neutrinos either. Electrons and other particles could transmit similar forces. This is a counterintuitive concept, because these particles are not the type commonly associated with the transmission of forces, according to particle physicists’ well-established theory, called standard model.

Today, physicists report that experiments already appear to be influenced by such effects. Include these forces in theoretical predictions resolves a subtle shift or tensionwith experiments, physicists report in a paper submitted in February to arXiv.org. Once the forces were taken into account, “the tension completely disappeared,” says theoretical physicist Victor Flambaum of the University of New South Wales in Sydney, co-author of the paper.

The neutrino force and its relatives have long been neglected, considered unimportant. But “it’s a bigger effect than anyone imagined,” says physicist John Behr of the Canadian particle accelerator center TRIUMF, who was not involved in the work. “If you take that into account, you get a better deal. I think everyone would agree that it’s interesting.”

According to the standard model, forces are transmitted by a class of particles called bosons. For example, particles of light, or photons, are bosons that transmit electromagnetic forces. Another class of particles, called fermions, make up matter: think of electrons.

Neutrinos are fermions, so they are not expected to transmit forces. But two fermions can combine to act like a boson. In the 1960s, scientists realized that this implied the possibility of a neutrino force.

Neutrinos are part of natural elements most ethereal particles. They rarely interact with other materials. They have no electrical charge and limited mass. Detecting even a single neutrino is a challenge, let alone a rare force produced by two of them. “Ultimately, this force is so small that until now we have never been able to see it,” explains theoretical physicist Yuval Grossman of Cornell University.

But perhaps the force of neutrinos could have an unknown influence on certain very specific experiencesGrossman and colleagues reported December 2025 in Physical Examination Letters.

The team proposed that neutrino strength could have an effect on measurements of parity violation in atoms. This is a phenomenon in which systems that are mirror images of each other do not behave identically. It’s like a clock designed to run clockwise behaves differently than a clock designed to run counterclockwise. Parity violation is known to occur through the weak interaction, one of the four fundamental forces of nature. This is how neutrinos interact, so it made sense to think that neutrino strength might be relevant.

Notably, the results of parity violation experiments on cesium atoms differed slightly from the Standard Model predictions. Physicists are constantly on the lookout for these minor deviations. It is thought that the standard model must break down somewhere, due to enduring mysteries such as the nature of dark matter, an invisible substance that apparently permeates the cosmos. In the case of a cesium parity violation, the difference was small enough to be due to chance. But given the stakes, even a hint of discrepancy can attract attention.

Grossman and his colleagues pointed out that the force of neutrinos could explain the discrepancy, but they did not fully calculate the impact of the force on the cesium experiments. Flambaum and a colleague, in the new paper, fleshed out the calculation and found that including the neglected forces resolved the discrepancy. But it turned out that neutrinos were only part of the story. Similar forces carried by pairs of quarks, electrons and other particles were responsible for most of the movement.

This new harmony is good news for physics. “It’s very good that theorists are getting better and better, and that’s important,” says physicist Dmitry Budker of the Johannes Gutenberg University in Mainz, Germany. “The story continues and it’s fun to watch.”