Scientists Close the Door on a Leading Explanation for a Neutrino Mystery
Columbia professors contributed to new research that seeks to understand an anomaly that has puzzled particle physics for decades.
An international collaboration of scientists working on the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) in Illinois announced that they have found no evidence for a fourth type of neutrino. The paper was published today in Nature.
Earlier physics experiments saw neutrinos behaving in a way inconsistent with the Standard Model of particle physics. Theorists have suggested that a single sterile neutrino could explain those anomalies. However, with this new result, MicroBooNE has been able to rule out this sterile neutrino explanation with 95% certainty.
The Standard Model is the best theory scientists have for explaining how the universe works. However, it is incomplete.
“We know that the Standard Model does a great job describing a host of phenomena in the natural world,” said Matthew Toups, Fermilab senior scientist and co-spokesperson for MicroBooNE. “And at the same time, we know it’s incomplete. It doesn’t account for dark matter, dark energy or gravity.”
So, physicists are on the hunt for new physics that may shed light on some of the biggest mysteries in the universe.
Ten members of the Columbia Neutrino Group, led jointly by Georgia Karagiorgi, associate professor of physics; Mark Ross-Lonergan, assistant professor of physics; and Mike Shaevitz, professor of physics, were co-authors on the new Nature paper. Ross-Lonergan, the current physics coordinator of MicroBooNE, is helping to guide the direction of physics research, a role he recently took on. He led the “Oscillations and Low Energy Excess” working group as Convener for the majority of the duration of the work outlined in the new paper. Shaevitz was a senior physics advisor for the working group and was a member of editorial board that rigorously reviewed the analysis internally. Karagiorgi’s team contributed to beam simulation and fit validation.
Neutrinos are tantalizing particles when it comes to searches for new physics because so many questions surround these ghostly particles. One mystery in particular has haunted physicists for decades.
According to the Standard Model there are three types, or flavors, of neutrino: muon, electron, and tau. Neutrinos oscillate between these flavors, changing, for instance, from a muon neutrino to an electron neutrino or to a tau neutrino. Scientists have been studying how neutrinos oscillate between these flavors for decades, giving them a strong foundation for understanding how often neutrinos are supposed to change flavor.
The first suggestion that something unexpected may be occurring when neutrinos oscillate was observed by the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in 1995. Fermilab’s MiniBooNE experiment was initiated to verify those results. Both LSND and MiniBooNE made observations suggesting that muon neutrinos were oscillating into electron neutrinos over shorter distances than are possible with only three neutrino flavors.
“They saw flavor change on a length scale that is just not consistent with there only being three neutrinos,” explained Justin Evans, professor at the University of Manchester in the U.K. and co-spokesperson for MicroBooNE. “And the most popular explanation over the past 30 years to explain the anomaly is that there’s a sterile neutrino.”
“With this latest result we've closed one door, but the anomaly still remains unexplained. The simplest answer is now ruled out, which means the real explanation could be far more exciting. At Columbia, we’re racing to explore whether neutrinos might be our portal into an entirely hidden ‘dark sector’, one of the last remaining viable explanations,” Ross-Lonergan said.
For more information, visit Fermilab's website.