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Detecting Neutrinos

Detectors in high energy physics use the particle's electrical properties to detect and identify them. Neutrinos, or ``little neutral ones,'' are difficult to observe because they lack any electrical charge.

The presence of neutrinos can only be inferred by the detection of the charged particles they leave behind when they collide and interact with matter. By identifying each of the resulting products, physicists can trace back to identify the presence of a neutrino and distinguish between the three different kinds of neutrinos.

In any search for neutrinos oscillations, it is essential that neutrinos of different types be identified. For BooNE, this is achieved by investigating neutrino interactions within the 800 tons of mineral oil in the MiniBooNE detector. While electron-type neutrinos interact leaving an electron amongst its resulting products, muon-type neutrinos produce heavier muon particles. Both the electron, and its more massive cousin, the muon are charged and will therefore be detected. However, their detection signatures are distinctly different, allowing the flavor of the initial neutrino to be determined.

Shockwaves of light are at the heart of MiniBooNE's ability to distinguish between the different types of neutrinos. 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 oil. Another example of a wake or shockwave is the sonic boom that is heard whenever a airplane breaks the sound barrier. In a similar way, light shockwaves are produced whenever charged particles break the ``light barrier.'' 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 work like light bulbs in reverse - producing an electrical signal whenever light strikes them. When the light cone strikes the walls, a ring of PMTs lights up, representing the detection of either an electron or muon. The shape and sharpness of the ring is used to separate the electrons from the muons, and hence, the electron neutrinos from the muon neutrinos.