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

A weird force that skirts traditional physics norms has helped erase a smudge on the foundations of particle physics.

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

Now, physicists report that experiments seem to already be influenced by such effects. Including these forces in theoretical predictions solves a subtle mismatch, or tension, with experiments, physicists report in a paper submitted in February to arXiv.org. Once the forces were considered, “the tension disappeared completely,” says theoretical physicist Victor Flambaum of University of New South Wales in Sydney, a coauthor of the paper.

The neutrino force and its relatives were long neglected, considered unimportant. But, “it’s a bigger effect than anybody had guessed,” says physicist John Behr of the Canadian particle accelerator center TRIUMF, who was not involved with the work. “You take this into account, you get better agreement. I think everyone would agree that’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, makes up matter — think electrons.

Neutrinos are fermions, so they wouldn’t be expected to transmit forces. But two fermions can team up to act as a boson. In the 1960s, scientists realized that implied the possibility of a neutrino force.

Neutrinos are some of nature’s most ethereal particles. They rarely interact with other matter. They have no electric charge and scant mass. Detecting even a single neutrino is challenging, let alone a rare force produced by pairs of them. “At the end of the day, this force is so, so small that so far we were never able to see it,” says theoretical physicist Yuval Grossman of Cornell University.

But perhaps the neutrino force could have an unrecognized influence on certain highly precise experiments, Grossman and colleagues reported December 2025 in Physical Review Letters.

The team proposed that the neutrino force might have an effect on measurements of parity violation in atoms. That’s a phenomenon in which systems that are mirror images of one another don’t behave identically. It’s as if a clock built to tick clockwise behaved differently than one built to tick counterclockwise. Parity violation is known to occur through the weak interaction, one of nature’s four fundamental forces. That’s how neutrinos interact, so it made sense to think that the neutrino force might be relevant.

Notably, the results of parity violation experiments in cesium atoms differed slightly from predictions of the standard model. Physicists are constantly on the lookout for such minor deviations. It’s thought that the standard model must break down somewhere, because of persistent mysteries such as the nature of dark matter, an invisible substance that apparently pervades the cosmos. In the case of parity violation in cesium, the difference was small enough to be due to random chance. But given the stakes, even a hint of a mismatch can draw attention.

Grossman and colleagues pointed out that the neutrino force might explain the discrepancy, but they didn’t 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 neutrinos were only part of the story. Similar forces carried by pairs of quarks, electrons and other particles were responsible for most of the shift.

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