MiniBooNE serves up a slice of π

Imagine you toss a football to your friend, but when it gets there, it's a baseball. Bizarre, right? Well, every time a neutrino of a particular flavor (electron, muon or tau) flies through space, there is a chance that it won't be the same flavor at the end of its journey. Studying this phenomenon, known as neutrino oscillation, yields new insight into the nature of nature.

In particular, measuring the probability that a muon neutrino will transition to an electron neutrino may help resolve the long-standing question of why there is far more matter than antimatter in the universe. This probability is measured by sending a beam of muon neutrinos to a detector and counting how many electron neutrinos appear. When electron neutrinos interact in the detector they typically produce a single electron, which starts an electromagnetic shower of photons, electrons and positrons. This shower is the smoking gun that indicates the presence of an electron neutrino.

However, one has to watch out for other particle interactions that can fake this signal. MiniBooNE is the first experiment to carefully determine the kinematics of one of the major background processes, π⁰ production. The measurement utilized the world’s largest sample of neutrino-induced π⁰ events ever recorded.

Neutrinos of any flavor can interact in the MiniBooNE detector and produce a π⁰ through neutral current interactions. The π⁰ decays very quickly to two photons, which each produce electromagnetic showers, much like the electron. If one of the photons goes undetected, the event is indistinguishable from an electron-neutrino signature, creating a rather pernicious background.

The task of counting electron neutrinos, as Colin Anderson, a graduate student at Yale University and lead on the π⁰ analysis puts it, "becomes akin to being color blind and having to count the number of red jelly beans in a jar full of all colors."

The MiniBooNE collaboration measured the rate of π⁰ production by searching through MiniBooNE's enormous data set of neutrino interactions for the π⁰ tell-tale signature: two photons with an invariant mass matching the π⁰ mass. They measured the cross section (probability) for muon neutrinos and antineutrinos to produce a π⁰ by neutral current interactions.

With this data, MiniBooNE can better predict the number of electron-like events due to π⁰ production — the green jelly beans in the jar. While this result has been used internally to improve the precision of the neutrino oscillation search at MiniBooNE, with this new publication the data is now available for others to use.

The π⁰ measurement gives theorists a new window into this important process and will allow other experiments to better constrain π⁰ backgrounds as they aim to make even more precise measurements of ν_μ→ν_e oscillations.

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