New Physics Discovered by MiniBooNE?


There’s exciting news coming from an experiment at Fermilab aimed at studying neutrino oscillations: a potential fourth “flavor” of neutrino, which would require altering the current Standard Model of particle physics.

The experimental results — announced at a conference earlier this year and now being published in Physical Review Lettersalso hint at a possible clue for the matter/antimatter asymmetry that existed in the earliest moments of the birth of our universe.

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The experiment is called MiniBooNE (short for Mini Booster Neutrino Experiment). It’s much like every other particle collider in principle. A bunch of protons smash into a fixed target, creating a horde of scattered mesons, which last for fractions of a second before decaying into neutrinos. The detector picks up the telltale “signatures” of these decay patterns and records them, and the data is then analyzed by scientists.

A bit of background is helpful to understand the significance of these results. The Standard Model calls for three different kinds of neutrinos (electron, muon and tau); the most common are the solar (tau) neutrinos that come from the nuclear processes taking place at the core of the Sun.

When a neutron inside an atom decays, it produces a proton, an electron, and a neutrino. This occurs hundreds of billions of times every second in the core of stars like our sun, as hydrogen is converted into heavier elements like helium, releasing huge amounts of energy in the process. Trillions of neutrinos are produced by the sun every day.

Solar neutrinos have an interesting feature: they can change into another kind of neutrino on their way to Earth — a phenomenon called “neutrino oscillation.” Why does this happen? It has to do with the wavelike nature of neutrinos. Waves oscillate back and forth. Add two waves together and you get a new composite wave.

For instance, when two very similar musical notes are played together, there’s an interference effect that causes the sound to wobble between loud and soft, producing “beats.” Similarly, oscillating neutrinos are comprised of three different waves that combine in different ways as they travel through space. The “beats” are caused by small physical differences in mass that lead to those telltale interference effects.

About 10 years ago, an experiment at Los Alamos National Laboratory threw another unexpected wrinkle into the mix: the possibility of a fourth kind of neutrino, a “sterile” neutrino that would only interact through gravity. MiniBooNE was conceived to test the results of that earlier Liquid Scintillator Neutrino Detector (LSND) experiment.

Back in 2004, MiniBooNe scientists presented results that seemed to contradict the LSND findings, but this time around, there are some striking similarities. Specifically, the experiment detected more oscillations than would be possible if, indeed, there were only three neutrino flavors. “These results imply that there are either new particles or forces we had not previously imagined,” Byron Roe, one of the paper’s co-authors, told “The simplest explanation involves adding new neutrino-like particles, or sterile neutrinos.”

What made the difference between these two runs? Well, the initial MiniBooNe experimental run used a muon neutrino beam, whereas the original LSND experiment used a muon antineutrino beam; this latest MiniBooNe experiment also used an antineutrino beam.

Frankly, it’s weird that this should make a difference in the results, but it hints at the possibility of radically new physics, although scientists are reluctant to speculate as to what the link might be. That’s probably because they really don’t know yet and are waiting for a bit more data, but the prospect is certainly “IN-ter-esting,” as Richard Feynman might say.

Alas, the reality is that the far more likely explanation is a misreading in the data analysis. The newest results are classified as a “three-sigma” result — enough to claim “evidence,” but falling short of the five-sigma signal that would allow MiniBooNe to claim “discovery.” It’s an important distinction in particle physics. A whopping 99% of three-sigma results turn out to be wrong. That’s because analyzing this kind of data is mind-bogglingly difficult. It’s very easy to confuse the signatures of the events of interest with background noise or a different kind of event altogether.

Joseph Lykken, a Fermilab physicist, points out that interesting they may be, MiniBooNe’s results are “still not a great match” to the LSND results, and says that many particle physicists remain skeptical. “These experiments are very tricky and are plagued by nasty background — ordinary physics that mimics the signal you are looking for,” he says, a problem that is exacerbated by the fact that MiniBooNe only has a single detector. (A second detector in a different location would help distinguish background noise from a genuine signature.)

“Which is more likely? Radically new physics or incompletely-understood detector background? Always the latter,” says a physicist who asked to be identified only as “Deep Neutrino” when speaking with Discovery News. “We may be stuck with inconclusive results for a long time to come. The LSND signal may become something much worse than sterile neutrinos: zombie neutrinos that can’t be brought to life or killed.”

Neutrinos of the Living Dead — it as a nice ring to it.

Image: A close-up of the interior of the MiniBooNE tank, before it was filled with ultraclean mineral oil (Courtesy Fermilab Visual Media Services)

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