Neutrino Oscillations
The neutrino is included in the Standard Model of particle interactions as a completely masseless particle; however, over the past 10 years, compelling evidence has emerged indicating that the neutrino is not massless.
There are three flavors of neutrinos, the electron neutrino, the muon neutrino and the tau neutrino. When a neutrino is produced via the weak interaction, it is produced in one of these flavors. When the neutrino interacts in a detector, it must again be detected as one of these flavors. Recent neutrino experiments have discovered that a neutrino produced as a definite flavor does not remain that flavor as it travels. For example, a neutrino produced in the fusion reactions of the sun is born as an electron neutrino, but by the time it has travelled to the earth, it may not be detected as an electron neutrino. This phenomenon is known as neutrino oscillation, and it can only happen if neutrinos are massive particles.
To understand oscillations, imagine that each of the three neutrino flavors are made up of three underlying mass states. In quantum mechanics, we would call these mass states the eigenvectors of the free particle Hamiltonian; they are the states that propagate with a definite energy value. If the masses associated with the different mass states are not the same, the energy of the particles they represent will be slightly different, and the neutrino mass states will evolve differently with time. After traveling many hundreds to thousands of kilometers, depending on the neutrino's energy, the relative phase of the three mass states that make up the neutrino will have changed. When interacting in a detector, the neutrino is forced back into a flavor state, but since the relative composition of the mass states has changed relative to what it was at the outset, the flavor of the detected neutrino may not be the same as the flavor of the neutrino at the start of the journey. This idea is illustrated in the following figure:
Neutrino oscillation experiments measure the probability for a neutrino to change flavor as a function of the distance the neutrino travels and the energy of the neutrino. By doing so, these experiments are able to measure the difference in the (squared) masses of the different mass states. While oscillation experiments can not measure the absolute mass of any of the states, observing flavor transistions proves that neutrinos must have mass.
The MINOS Experiment
The MINOS experiment is a currently running neutrino oscillation experiment. The neutrinos it studies are produced by an accelerator at the Fermi National Accelerator Laboratory, near Chicago, Il. The neutrinos studied are produced primarily as muon type neutrinos. The number of neutrinos produced, their type, and their energy spectrum are measured in a Near Detector located close to the production point of the neutrinos. The MINOS experiment uses another detector, located 735 kilometers away, in Northern Minnesota, to measure the beam of neutrinos again. It then compares the number of muon neutrinos recorded in the Far Detector to the number in the Near Detector as a function of neutrino energy. Differences in the number of neutrinos in the Far Detector, relative to what one would expect based on the Near Detector measurement are evidence of neutrino oscillations. The next figure shows a cartoon of what the MINOS experiment should see:
The plot to the left shows a sample of the number of muon type neutrinos MINOS should count in the Far Detector as a function of the energy of the neutrino. The solid line shows what one would expect if neutrinos do not oscillate. The points show what one would expect if neutrinos do oscillate. With oscillations, the number of muon type neutrinos is suppressed, and the magnitude of the suppression changes as a function of energy. The right hand plot shows the ratio of the number of muon neutrinos seen in the Far Detector with oscillations to that expected without oscillations. The energy at which the maximum suppression of events occurs is related to the differences in the (squared) masses of the neutrino mass states, and the magnitude of the suppression is related to how different the mass states are from the flavor states.
MINOS is currently in its third year of data taking. At this point, the experiment has confirmed that neutrinos change flavor between the Near and Far Detectors. Further data taking will allow an increasingly precise measurement of the difference of the squared masses of the mass states (delta m^2) and how much they mix (sin^2(2theta). Follow the link to find out more about MINOS.