Ghostly particles might just break our understanding of the universe

Notoriously ghostly particles called neutrinos may have revealed a crack in our understanding of all the particles and forces in the universe.

The standard model of particle physics, which catalogues all the particles and forces we know to exist, is one of the biggest successes of modern physics, but physicists have also spent decades trying to break it. That is because it has enough flaws – notably, it doesn’t connect gravity to any of the three other fundamental forces – for researchers to suspect that they must formulate another, better model.

If the standard model cracks under a stress test, that would point to where we should start building this next model. Francesca Dordei at the Italian National Institute for Nuclear Physics (INFN) in Cagliari and her colleagues have now identified one possible crack by studying the enigmatic neutrino.

“In all the checks [of the standard model] that we did in the last two decades, every time, stubbornly, they confirmed the standard model, which means that we have to go to even more precise results. In this sense, neutrinos are special particles,” says Dordei.

For one, neutrinos have incredibly small masses – so small that physicists once thought they were massless. What’s more, they are weakly interacting, which means they pass through objects and bodies undetected, like tiny ghosts. Yet careful study has pinpointed some small electromagnetic interactions that neutrinos take part in, which can be quantified through a number called charge radius. Neutrinos can also interact with other particles through the weak nuclear force.

Dordei and her colleagues examined the details of that interaction and of the neutrinos’ charge radius across the many experiments that have looked for signs of these elusive particles in recent years. For instance, they combined data from observations of neutrinos created in nuclear reactors, particle accelerators and fusion processes inside the sun. The team also took advantage of the fact that some detectors built for dark matter – the mysterious substance that permeates the cosmos – are sensitive to neutrinos as well.

Team member Nicola Cargioli, also at INFN, says putting all this data together was challenging, but provided a powerful overview of everything we know about neutrinos. “We have used basically all of the data [there is],” says Christoph Ternes at the Gran Sasso Science Institute in L’Aquila, Italy, who also worked on the project.

The value of the neutrinos’ charge radius didn’t deviate from the predictions of the standard model, but the researchers found something more exciting when they looked at the particle’s weak interactions. Here, they identified a “mathematical degeneracy”, which means that both the standard model and a slightly different model could have produced the same observations. Strikingly, further analysis showed that this alternative to the standard model may fit the data slightly better, possibly indicating the long-sought-after crack in our current understanding of particle physics.

The new analysis doesn’t statistically reach the level of an unambiguous discovery, and the researchers see it as a first step in stress-testing the standard model with neutrinos. They hope to acquire more data that could add weight to – or dispel – their current results as new detectors come online in the next few years. However, if this crack persists in the future, it could have serious ramifications.

“If we have found a crack, then we may have to rethink everything,” says Cargioli. For example, a new model that went beyond the standard model could include some completely new kinds of particles whose interaction with neutrinos would match up with the analysis in the team’s study.

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

The new analysis presents a plea to particle physicists for more ultra-precise experiments with neutrinos in different settings in the future, says José Valle at the University of Valencia in Spain. Better measurements of neutrinos’ electromagnetic properties are also still necessary, as they would shed light on, for instance, neutrinos’ internal structure, he says.

CERN and Mont Blanc, dark and frozen matter: Switzerland and France

Prepare to have your mind blown by CERN, Europe’s particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.