ttch asked in another thread about neutrino oscillation.
Basically, neutrino oscillation is the idea that neutrinos change flavor in flight. According to the standard model of particle physics, there are three pairs of leptons. The lightest and most familiar are the electron and its neutrino, then the muon and its neutrino, and finally the heavy tau and its neutrino. Electron neutrinos are produced as a byproduct of beta decay (which also produces an electron) and muons and mu neutrinos are produced in cosmic rays.
Oscillation means that a mu neutrino, for example, might shift flavor into an electron neutrino and back again. This phenomenon might help explain why too few electron neutrinos are detected from the sun; some of them turned into something else.
It turns out that neutrino oscillation implies a non-zero mass for at least some neutrinos. By any other experiment, the neutrino mass is too small to measure and possibly zero.
How does this work? Well, trying not to get too far into the math, imagine two possible states a neutrino can be in, call them e and m for electron neutrino and muon neutrino. In quantum mechanics, you can form mixed states by coming them. For example, (1/sqrt(2) e + 1/sqrt(2) m) is a mixture in equal proportions of both states. The probability of measuring either state is the square of its coefficient, 1/2 in both cases in this example. If a neutrino is in this state, and you are looking for muon neutrinos, you have only a 50% chance of seeing this neutrino even if you normally detect every muon neutrino.
Particles have frequencies and wavelengths associated with them. These are related to how much energy the particle has. (Mathematically, this is represented by including a complex factor in the coefficient of a state that oscillates.)
If one type of neutrino has a bit more mass than the other, it'll have a slightly different wavelength for a given momentum (or, alternately, if it has a given energy, it'll have a slightly different leftover kinetic energy and therefore a slightly different momentum).
Now, most of the time, fixed energy states are compatible with fixed particle flavor states. However, if you can't have a fixed flavor state and a fixed energy state at the same time, you're going to have a state that necessarily has a mix of flavors... and there's a slight difference in wavelength. That leads to something analogous to constructive and destructive interference.
As a result, the mix of particle types leads to probabilities that vary as the two parts of the wave go in and out of phase. One flavor will peak in probability while the other hits its trough.
Two conditions must be met for this to happen: the two flavor states have to have different masses, and fixed energy states have to produce mixed flavor states.
How do you test this experimentally? Well, basically, you produce some neutrinos, and then sample the beam over varying distance to see what the probability of finding a neutrino of a given type is.
In SuperK this is done by measuring muon neutrinos produced in the upper atmosphere by cosmic rays. Neutrinos produced directly overhead travel a short distance, neutrinos on the horizon travel a long distance (and thus further along the wave of varying probability). The pattern and amount of light in the SuperK detector allow estimating both direction and energy, and it turns out that indeed the number of neutrinos falls off with distance, and the curve is compatible with a mass difference of 10-3 eV.
Basically, neutrino oscillation is the idea that neutrinos change flavor in flight. According to the standard model of particle physics, there are three pairs of leptons. The lightest and most familiar are the electron and its neutrino, then the muon and its neutrino, and finally the heavy tau and its neutrino. Electron neutrinos are produced as a byproduct of beta decay (which also produces an electron) and muons and mu neutrinos are produced in cosmic rays.
Oscillation means that a mu neutrino, for example, might shift flavor into an electron neutrino and back again. This phenomenon might help explain why too few electron neutrinos are detected from the sun; some of them turned into something else.
It turns out that neutrino oscillation implies a non-zero mass for at least some neutrinos. By any other experiment, the neutrino mass is too small to measure and possibly zero.
How does this work? Well, trying not to get too far into the math, imagine two possible states a neutrino can be in, call them e and m for electron neutrino and muon neutrino. In quantum mechanics, you can form mixed states by coming them. For example, (1/sqrt(2) e + 1/sqrt(2) m) is a mixture in equal proportions of both states. The probability of measuring either state is the square of its coefficient, 1/2 in both cases in this example. If a neutrino is in this state, and you are looking for muon neutrinos, you have only a 50% chance of seeing this neutrino even if you normally detect every muon neutrino.
Particles have frequencies and wavelengths associated with them. These are related to how much energy the particle has. (Mathematically, this is represented by including a complex factor in the coefficient of a state that oscillates.)
If one type of neutrino has a bit more mass than the other, it'll have a slightly different wavelength for a given momentum (or, alternately, if it has a given energy, it'll have a slightly different leftover kinetic energy and therefore a slightly different momentum).
Now, most of the time, fixed energy states are compatible with fixed particle flavor states. However, if you can't have a fixed flavor state and a fixed energy state at the same time, you're going to have a state that necessarily has a mix of flavors... and there's a slight difference in wavelength. That leads to something analogous to constructive and destructive interference.
As a result, the mix of particle types leads to probabilities that vary as the two parts of the wave go in and out of phase. One flavor will peak in probability while the other hits its trough.
Two conditions must be met for this to happen: the two flavor states have to have different masses, and fixed energy states have to produce mixed flavor states.
How do you test this experimentally? Well, basically, you produce some neutrinos, and then sample the beam over varying distance to see what the probability of finding a neutrino of a given type is.
In SuperK this is done by measuring muon neutrinos produced in the upper atmosphere by cosmic rays. Neutrinos produced directly overhead travel a short distance, neutrinos on the horizon travel a long distance (and thus further along the wave of varying probability). The pattern and amount of light in the SuperK detector allow estimating both direction and energy, and it turns out that indeed the number of neutrinos falls off with distance, and the curve is compatible with a mass difference of 10-3 eV.
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