More comprehensive search for sterile neutrinos comes up empty

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Particle physicists have been looking for so-called "sterile neutrinos" for a few decades now. They are a hypothesized particle that would have a tiny mass like the three known neutrinos but would not interact by the weak force or any other Standard Model force, only through gravitational interactions.

Its existence—or their existence—would solve some anomalies seen in neutrino experiments, help answer questions beyond the Standard Model of particle physics, and, if massive enough, could explain cold dark matter or warm dark matter.

But sterile neutrinos have not been seen in any particle experiments, despite many attempts. Now an experiment by the IceCube Collaboration has used 10.7 years of data from their detector near the Amundsen-Scott South Pole Station to lower the probability that at least one sterile neutrino does not exist. Their paper appears in Physical Review Letters.

"With IceCube, we have advanced the search for a fourth type of neutrino, the sterile neutrino," said Alfonso García Soto, a researcher at the Instituto de Física Corpuscular (IFIC) in Spain, a lead analyzer for the group. "This work was possible because of improved models for our data and artificial intelligence."

Neutrinos themselves are still largely mysterious particles. Three are known, coming in three lepton flavors—the electron neutrino, the muon neutrino and the tau neutrino—are uncharged, have spin ½ and while known to be massive, their individual masses have remained undetermined. They're known to oscillate flavors as they travel, from one lepton flavor to another. They interact only via the weak force and, because they are not massless, exert a meager gravitational force as well.

IceCube is a neutrino observatory near the South Pole occupying a cubic kilometer underground that detects neutrinos produced by cosmic ray (mainly protons) collisions in the upper atmosphere. Eighty-six holes were drilled in the extremely clear Antarctic ice to a depth of 2.5 km, containing 5,160 sensors known as Digital Optical Modules (DOMs) containing photomultiplier tubes.

These are sunk into the holes between 1,450 m and 2,450 m, 125 meters apart. A denser configuration at the center of the detector allows neutrino energies of 10 to 100 GeV to be detected, making it possible for physicists to study neutrino oscillations. IceCube is the largest neutrino observatory in the world as well as the globe's largest particle detector.

When an atmospheric neutrino interacts with the ice within the detector, it creates a shower of other particles such as muons, the heavier version of the electron. These secondary particles travel at nearly the speed of light, and faster than the speed of light in ice, so they emit Cerenkov radiation.

This light will set off many detectors within the array, and reconstructing the patterns of the signaled DOMs can determine the particle's direction and energy. As atmospheric muons are also produced by cosmic rays, IceCube eliminates them by only looking at up-going tracks in their detector, which weeds out the muons entering from the top side of the Earth.

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If at least a fourth neutrino flavor exists, it will have no direct interactions with the ice so cannot be detected via IceCube's traditional channels. But a sterile neutrino would still leave an indirect (and measurable) signal if neutrinos are able to oscillate into ("mix with") a sterile neutrino and disappear in the detector, or if a disappearance or gap is seen if a sterile neutrino can oscillate into one of the three standard neutrinos.

IceCube has published a series of studies over the years, as have other groups like MicroBoone, but all show no evidence of sterile neutrinos.

Earlier this year, the IceCube Collaboration again found no sterile neutrinos using 7.5 years of data from IceCube's inner detector core, known as DeepCore, a result compatible with the lack of any mixing between active and sterile neutrino states. The best-fit point was consistent with the standard three-neutrino hypothesis (meaning no sterile neutrino) at a p-value of 8%.

In their most comprehensive search, the group has now looked at 10.7 years of data while increasing the upper range of muon neutrino energies from 10 TeV to 100 TeV. They also incorporated significant improvements in modeling the neutrino flux and the detector response compared to earlier studies. Their results are again consistent with the absence of a sterile neutrino, but now with a lower probability of 3.1%.

"Advancing this search was made possible because of the IceCube Collaboration's international efforts in operating the detector, preparing the data, and harnessing the data to study the physics of neutrinos," said Ignacio Taboada of the Georgia Institute of Technology in the US and IceCube's spokesperson.

The present paper has 420 authors from 58 institutions from 14 countries.

More information: R. Abbasi et al, Search for an eV-Scale Sterile Neutrino Using Improved High-Energy νμ Event Reconstruction in IceCube, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.201804

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