New Limits in the Search for Sterile Neutrinos

New searches for sterile neutrinos, hypothetical particles that do not directly “talk to” any particles in the Standard Model, significantly narrow the remaining regions where these particles may be hiding.

Sterile Neutrinos
Detectors from the Daya Bay and MINOS neutrino experiments, in China and the United States, respectively. Image courtesy of the Daya Bay and MINOS Experiment

The Science

Travelling at near the speed of light, particles smaller than an atom zoom through you, me, and, in fact, all matter. In a revelation that earned the Nobel Prize in Physics, scientists found that the particles, called neutrinos, change forms. They oscillate, or change, between three different “flavors:” electron, muon, and tau. What if that wasn’t all? A fourth, undetectable, or “sterile,” flavor could potentially be a component of dark matter, which makes up about a quarter of the universe, and would have other profound effects on scientists’ understanding of the Standard Model of particle physics. New measurements have narrowed down the search for these sterile neutrinos by looking for signs of their oscillations into the other known neutrino flavors.

The Impact

The undetectable, or light, sterile neutrinos could be components of the mysterious “dark world,” including the dark matter that makes up about a quarter of the universe, but has never been directly detected. True evidence that sterile neutrinos exist would profoundly change our understanding of the Standard Model and the universe. This study narrows down the search for these particles by eliminating available space in neutrino oscillation amplitude and frequency.

Summary

The detection of neutrino oscillations, a discovery that garnered the 2015 Nobel Prize in Physics, is a proof that neutrinos have mass – proving that the Standard Model, in which neutrinos are described as massless, is incomplete. The fact that neutrinos have mass is a strong hint that we may discover other things we don’t know about these particles. If a fourth type, a sterile neutrino, really existed, it would be the first discovery of an unexpected fundamental particle since the Standard Model was established.

The search specifically for sterile neutrinos as a possible missing component of the Standard Model is motivated by theoretical ideas and hints from an earlier experiment at Los Alamos National Laboratory that may have observed a new kind of neutrino oscillation. This new oscillation is approximately described as a two-step shift that transforms existing muon neutrinos into neutrinos of an undetectable, or “sterile,” flavor, which then oscillate again to become electron neutrinos. The Los Alamos team measured the rate of this proposed two-step transformation through a possible sterile flavor.

The new results from the Main Injector Neutrino Oscillation Search (MINOS) collaboration experiment at Fermi National Accelerator Laboratory (Fermilab) in the central United States and the Daya Bay experiments at a nuclear reactor complex in southern China test this observation by measuring the rates of the individual steps. Multiplying the rates measured for each of the two individual steps that include the proposed sterile neutrino should equal the rate measured by the Los Alamos team—if the sterile neutrino indeed exists. The recent results from Daya Bay and MINOS, described in the three papers published inPhysical Review Letters (PRL), like results recently announced by another experiment known as IceCube, greatly constrain the allowed properties of a sterile neutrino and narrow the region where scientists must hunt.

The October 7, 2016, issue of PRL featured a significantly better sterile neutrino search limit with a factor of two improvement by the Daya Bay collaboration. They obtained the new limit through the search for anomalous electron antineutrino disappearance in Daya Bay with its full eight-detector configuration. Simultaneously, a significantly improved sterile neutrino search limit was published in PRL by the MINOS collaboration with the search for anomalous accelerator muon neutrino disappearance. Researchers combined the results from these two experiments with a reanalysis of the Bugey-3 experiment’s search for anomalous electron antineutrino disappearance to set a new limit on the active-to-sterile neutrino oscillation in a third PRL paper.

Although these search results are negative, they significantly constrain the properties of a sterile neutrino and set an excellent example of how to properly combine results from multiple experiments. The remaining allowed region will be searched by future experiments at Fermilab and other places to obtain a definite answer to the question of whether light sterile neutrinos exist.