Neutrinos behave like waves as they travel through matter and space (Curious? Search on wave-particle duality.) There are three kinds of neutrinos – electron neutrinos, muon neutrinos and tau neutrinos. Interference between the waves associated with these three types of neutrinos produce neutrino mixing in which individual neutrinos become mixtures of the three types. This leads to a phenomenon called neutrino oscillation in which neutrinos change from one kind to another as they travel.
Scientists can measure these oscillations by recording the interactions of neutrinos with matter. When a neutrino interacts with matter it produces an electron, a muon or a tau particle, depending on where it is in its oscillation cycle at that instant. A neutrino caught at one moment might produce an electron. A moment later, it might produce a muon or a tau! The problem is catching them – most neutrinos sail right through matter. So the trick is to get enough neutrinos to make a path through a massive enough detector that some reasonable number of neutrinos will interact with the target material (within some reasonable time period) and create detectable signals.
The Long-Baseline Neutrino Experiment is designed to make precision measurements of parameters called mixing angles, that describe the propagation and interference of the three neutrino types. This will enable LBNE to observe CP violation – the broken symmetry between matter and antimatter – among neutrinos, if it exists, and compare it to CP violation observed in quarks and antiquarks. By sending an intense beam of neutrinos through the Earth's mantle to a massive detector hundreds of miles away – a distance long enough to allow time for oscillations to occur – scientists hope to record the interactions of many neutrinos in the detector – enough to get a sufficient statistical sampling over a ten-to-twenty-year period.
The LBNE beamline is designed to extract protons from the Fermilab Main Injector and transport them to a target area where the collisions generate a secondary beam of charged particles. This secondary beam, aimed toward the far detector, is followed by a decay-pipe in which the particles of the secondary beam decay to generate the neutrino beam. A near detector, a couple of hundred meters downstream of the beamline, will help the experiment get a good picture of the neutrino beam before the neutrinos have a chance to oscillate. The far detector, described on the Neutrino Detectors page, will measure interactions from the oscillated beam.
If the far detector is located deep underground to shield it from cosmic rays, its potential use widens. Scientists could use it to search for proton decay and to study the oscillation of neutrinos produced in the Earth's atmosphere. In addition, they might be able to detect neutrinos from a supernova in our galaxy, which astronomers predict occurs about every 40 years, and detect relic neutrinos, those neutrinos left over from the myriad supernovae since the beginning of time.