Pedagoguery

Neutrinos are ephemeral things. Nearly massless, they interact with other matter only through the weak nuclear force and gravity. This gives them the ability of passing through other matter nearly unscathed. If we could see neutrinos with our own eyes, for example, the Sun would appear vastly different, as we would see the core where the nuclear reactions occur, not the photosphere where photons finally can escape the sun's matter. Thus, neutrinos can provide us with information not attainable through other means. However, it is this very property that makes detecting neutrinos so difficult. How do you build a detector for something can can pass through the entire Earth without a trace?

In order to detect neutrinos, you need large amounts of matter to detect the infrequent interaction of a neutrino with ordinary matter. One of the first large scale neutrino observatories was in the Homestake Mine in South Dakota. It consisted of a large tank of tetrachloroethylene (C2Cl4). It was built underground to shield it, somewhat, from the neutrinos that are generated when cosmic rays strike the upper atmosphere, since it's primary purpose was to observe neutrinos coming from the sun. If a neutrino of a sufficiently high energy hits a nucleus of chlorine 37, the chlorine atom can absorb it, spitting out an electron and transforming into argon 37, which can be periodically flushed out of the tank and measured.

The problem with this type of observatory, however, is twofold. First, it only detects relatively high energy neutrinos. The bulk of the neutrinos produced by the sun are produced during the proton-proton reaction when two protons interact and one of them undergoes reverse beta decay to become a neutron, spitting out a positron and a neutrino. Those neutrinos have much too low an energy to cause chlorine 37 to transform into argon 37. The second issue is that it only detects electron neutrinos. Neutrinos come in three “flavors”, each paired with one of the three leptons in the standard model. So, you have the electron neutrino, the muon neutrino, and the tau neutrino. Add on top of that the strange ability of neutrinos to morph into other neutrino flavors. In addition, a neutrino can have one of three mass states, labeled 1, 2, and 3. The really weird thing, however, is that a given flavor does not imply a given mass, or vice versa. The mass determines how it propagates through space and the flavor determines how it interacts with matter. The two states interact as the neutrino travels, causing the neutrino to assume different flavors as it travels. When it finally arrives at a detector, the ratio of neutrino flavors detected will tell the investigators something of what was originally produced. For example, simple beta decay, which produces only electron neutrinos, will be detected as a 5:2:2 ratio of electron, muon, and tau neutrinos.

Modern detectors get around the problems of the Homestake Mine detector by using ordinary matter, usually water, and photodetectors to detect the interactions of neutrinos with ordinary matter. The first of these, the Super-Kamiokande, situated in an old zinc mine north of Nagoya, Japan, is a giant spherical tank filled with 50,000 cubic meters of ultra pure water and surrounded by photodetectors. When a neutrino strikes a nucleus in the water, it will knock free a lepton that corresponds with its flavor. That particle will typically be traveling faster than the speed of light in water (but less than the speed of light in a vacuum), which produces Cherenkov radiation. The pattern of the Cherenkov radiation provides a clue to the identity of the particle that causes it.

A more recent, and more ambitious effort is called Icecube. It involves lowering strings of detectors deep into Antarctic ice over the space of a square kilometer. Overall, the detector volume is one cubic kilometer. Like Super-Kamiokande, it is the pattern of the Cherenkov radiation that tells us the type of particle that causes it, and thus the type of neutrino that was detected. Electrons produce light over a nearly spherical area. Muons, by contrast, interact less with their surrounding matter and produce a cone-shaped pattern over a much larger area. Tau particles are massive and highly unstable, so they produce two spheres of light – one where they are produced, and another where they decay.

These and other observatories in operation or planned over the globe are starting to give us a glimpse of the neutrino universe. They will undoubtedly provide information we cannot see in any other way, and the possibility of surprising results.

Next time, how Earth's history can help us identify extrasolar Earth-like planets.