Detecting Neutrinos

Detectors in high energy physics use a particle's electrical properties to detect and identify it. Neutrinos, or ``little neutral ones,'' are impossible to track directly because, as the term "neutral" implies, they lack any electrical charge.

The presence of neutrinos can only be inferred by detecting the charged particles they produce when they collide and interact with matter. By characterizing the resulting product particles (their charge and energy), physicists can trace back and characterize the incoming neutrino. Some neutrino interactions produce the neutrino's namesake lepton (electron, muon, or tau); this allows the type of neutrino to be tagged.

Since neutrino oscillation searches need to explicitly categorize neutrinos by type, it is essential that clear signatures for the namesake leptons be determined. Shockwaves of light are at the heart of MiniBooNE's ability to distinguish between the different types of namesake leptons. Just as a boat leaves a wake as it speeds across a lake, charged particles leave electromagnetic wakes (in the form of light) as they pass through the ultra-clear mineral oil in the MiniBooNE detector. These "photonic flashes" (similar to the sonic booms heard whenever a airplane breaks the sound barrier) or light shockwaves are produced whenever charged particles break the ``light barrier'' in the oil. Of course, nothing can travel faster than c, the speed of light in a vacuum, but for MiniBooNE light is traveling through oil. The oil slows down the light, leaving an opportunity for particles to travel faster than light in oil while remaining below the universal speed limit of c.

The high energies of the incoming neutrinos ensure that the electrons and muons that are produced in the neutrino collisions travel fast enough to leave wakes in their path. These shockwaves of light are conical in shape, spreading out from the collision point, and striking the walls of the MiniBooNE detector. As the cone-like wake strikes the walls, a circular ring of light appears.

The eyes of the MiniBooNE detector are the 1520 phototubes (PMTs). These individual detectors are exquisitely sensitive light sensors, sending out an electrical signal when hit by a particle of light. When the shockwave light cone intercepts the walls, a ring of PMTs are triggered, whose shape and sharpness are used to identify the particle. This allows MiniBooNE to separate electrons from muons, and thence electron neutrinos from muon neutrinos.