For neutrino and neutrino oscillation, the Wikipedia article “Neutrino oscillation” (at http://en.wikipedia.org/wiki/Neutrino_oscillation ) has provided a good description, and thus, I will not repeat them here.
Now, neutrino oscillation is an observed fact, and thus, it demands that neutrino(s) are having “rest” mass. This demand puts Standard Model (SM) in jeopardy as there is no way to give neutrino a rest mass in SM’s framework. The Higgs mechanism which gives quarks and electron rest masses will not work for neutrino as no right-handed neutrino is predicted (in SM) or observed.
Then, the LSND (Liquid Scintillator Neutrino Detector) anomaly was announced and it hinted the existence of the fourth (a sterile) neutrino. However, this LSND anomaly was ruled out later. Yet, recently, a new hint for a different kind of sterile neutrino (perhaps, a Majorana type) was observed, and a search program for it is ongoing (see the article “MINOS Search for Sterile Neutrinos” at http://arxiv.org/abs/1110.3455 ). If there is a sterile neutrino (whatever kind it is), a seesaw mechanism can give flavor neutrinos their rest masses via this sterile neutrino. If Nature indeed did this, we should send an email to that Nature engineer a better design. Why should we invent two mass giving mechanisms? one for neutrino(s), another for all other particles (both hadrons and leptons [excluding neutrino leptons]).
All above issues arose from the fact of neutrino oscillation. Thus, if the neutrino oscillation mechanism is fully understood, then all those issues will be resolved. The nutshell of neutrino oscillation is that whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor. We can envision this process with three cases analogy.
Case 1 --- Mr. A (one person) has multiple personalities.
A1 --- a nice guy.
A2 --- a murder.
A3 --- a cry baby.
Case 2 --- there are “three” gentlemen.
Mr. B --- a nice guy.
Mr. C --- a murder.
Mr. D --- a cry baby.
It is not too hard for us to imagine the case 1 oscillation (Os1). It is a bit difficult for us to imagine the case 2 oscillation (Os2). Case 2 can happen only if there is an “Uncertainty force”( UF) which makes the case 1 and case 2 to be undistinguishable, the Os3. However, the known “uncertainty principle” is, seemingly, not the force for the above UF. Thus, if case 2 can happen, we need a new UF. Which will be the case (among those three oscillations) for neutrinos? We should review the equation which predicted the neutrino oscillation.
(Fe, Fm, Ft) = MM * (Me, Mm, Mt)
(Fe, Fm, Ft) is the flavor vector (or matrix) for three flavor neutrinos.
(Me, Mm, Mt) is the rest mass vector (or matrix) for three flavor neutrinos.
MM is the mixing matrix which connects the two vectors above.
The above equation is identical to a transformation between two reference frames, such as,
(X, Y, Z) = TM * (U, V, W)
(X, Y, Z) and (U, V, W) are two different reference frames, and TM is transformation matrix.
With this understanding, Fe (electron neutrino), Fm (muon neutrino) and Ft (tau neutrino) are no longer viewed as particles but are axes of a reference frame for a “field”, FN field (Flavor Neutrino field). Every point in this FN field is a vector which has three components (from three axes). Thus, the neutrino oscillation is simply the reference frame transformation from the flavor frame to the rest mass frame. Of course, if the Nature needs no frame transformation, there should not be any oscillation. As the oscillation is a fact, then the Nature demands a frame transformation during its evolution (the passing of time). That is, the Nature demands the existence of two reference frames, and any “point” (either a quark or a neutrino) of a “field” must flip-flap between these two frames. This flip-flap process is the source for giving rise to mass and gravity in “Super Unified Theory” (SUT), and it is described in the article “Higgs Boson, a bad idea, part nine” (http://prebabel.blogspot.com/2011/10/higgs-boson-bad-idea-part-nine.html ). That is, one process (not two) gives raise the rest mass to all particles (including neutrinos).
Furthermore, the recent LHC data has ruled out the fourth generation of quark. Then, a fourth neutrino (sterile or not) will make a very bad design for Nature. In “Prequark Chromodynamics” the “generation” is treated as “color force” (gene-color force) which obeys the color rules, having only three primary colors. The muon decay [μ− → (e−) + ( ν e) + ( ν μ) ] is, in fact, driven by this gene-color force. The detailed description of this muon decay in terms of gene-color is available at http://www.prequark.org/pq11.htm .
Note: The fourth generation of quarks or leptons was ruled out theoretically in the article "Law of Creation" (http://www.prequark.org/Create.htm ) 20 years ago; it states, "In fact, the number of generations of elementary particles can be derived theoretically ...Equation ... gives rise to 64 subspaces. With Eq. (2), only 48 subspaces can manifest into particles; 24 of them are anti-particles. Because that the space is trisected into three seats in the Prequark Chromodynamics, only 8 elementary particles can be generated with three seats for each generation (2 quarks with three colors each [=6] and 2 leptons). Thus, only three generations of particle can ever exist, no more or less, period. This is as simple as 1, 2, or 3 because 24/8 = 3."
With the gene-color of “Prequark Chromodynamics” and the mass giving mechanism of SUT (flip-flap in space-time sheet), both the neutrino mass and the neutrino oscillation become the direct consequences and mystery no more.