Ghostly particles might just break our understanding of the universe

Infamous ghost particles called neutrinos may have opened a hole in our understanding of all the particles and forces in the universe.

standard model of particle physicswhich catalogs every particle and force we know is one of the greatest successes of modern physics, but physicists have also spent decades trying to destroy it. That's because it has enough flaws (notably, it doesn't relate gravity to any of the other three fundamental forces) for researchers to suspect that they need to formulate a different, better model.

If the standard model cracks in a stress test, that will indicate where we should start building the next model. Francesca Dordey from the Italian National Institute of Nuclear Physics (INFN) in Cagliari and her colleagues identified one possible crack by studying mysterious neutrino.

“In all checks [of the standard model] What we've done over the past two decades has been to stubbornly confirm the standard model, which means we'll have to move toward even more accurate results. In this sense, neutrinos are special particles,” says Dordey.

First, neutrinos have incredibly low mass—so small that physicists once thought they were massless. Moreover, they interact weakly, which means they pass through objects and bodies undetected, like tiny ghosts. However, careful study has revealed some small electromagnetic interactions involving neutrinos, which can be quantified using a number called charge radius. Neutrinos can also interact with other particles through the weak nuclear force.

Dordey and her colleagues have explored the details of this interaction and the neutrino's charge radius in numerous experiments searching for signatures of these elusive particles in recent years. For example, they combined observations of neutrinos created in nuclear reactorsparticle accelerators and thermonuclear processes inside the Sun. The team also took advantage of the fact that some detectors designed for dark matter – a mysterious substance that permeates space – is also sensitive to neutrinos.

Team Member Nicola Cargiolialso from INFN, says that putting all this data together was not easy, but it provided a powerful overview of everything we know about neutrinos. “We used almost all the data [there is]”, – speaks Christopher Ternes at the Gran Sasso Scientific Institute in L'Aquila, Italy, who also worked on the project.

The value of the neutrino's charge radius did not deviate from the predictions of the standard model, but the researchers discovered something more interesting when they looked at weak particle interactions. Here they identified “mathematical degeneracy,” which means that both the standard model and a slightly different model could produce the same observations. Amazingly, further analysis revealed that this alternative to the standard model may fit the data slightly better, perhaps indicating a welcome crack in our current understanding of particle physics.

The new analysis falls statistically short of a definitive discovery, and the researchers see it as the first step in stress testing the standard model with neutrinos. They hope to obtain more data that could add weight to (or refute) their current results as new detectors become available over the next few years. However, if this crack persists in the future, it could have serious consequences.

“If we find a crack, then we may have to rethink everything,” Cargioli says. For example, a new model that goes beyond the standard may include some completely new types of particles whose interaction with neutrinos would be consistent with the team's research analysis.

Omar Miranda The Center for Research and Advanced Studies at the National Polytechnic Institute of Mexico says that measuring neutrino interactions, especially at very low energies, as is the case with much of the new study's data, is very challenging and has only recently become possible thanks to advances in detector technology, including dark matter detectors. This really underscored the relevance of neutrino detection as a test of the standard model, he said.

The new analysis represents a call for particle physicists to conduct more ultra-precise experiments with neutrinos under different conditions in the future, he says. Jose Valle at the University of Valencia in Spain. Better measurements of the electromagnetic properties of neutrinos are still needed, he said, as they could shed light on, for example, the internal structure of neutrinos.

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