Everything about Neutrino Oscillation totally explained
Neutrino oscillation is a
quantum mechanical phenomenon predicted by
Bruno Pontecorvo whereby a
neutrino created with a specific
lepton flavor (
electron,
muon or
tau) can later be
measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates. Neutrino oscillation is of
theoretical and
experimental interest since observation of the phenomenon implies that the neutrino has a non-zero mass, which isn't part of the original
Standard Model of
particle physics.
Observations
A great deal of evidence for neutrino oscillations has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies.
Solar neutrino oscillation
The first experiment to detect the effects of neutrino oscillations was
Ray Davis's Homestake Experiment, in which he observed a deficit in the flux of
solar neutrinos using a
chlorine-based detector. This gave rise to the
Solar neutrino problem. Many subsequent radiochemical and water
Cerenkov detectors confirmed the deficit, but neutrino oscillations weren't conclusively identified as the source of the deficit until the
Sudbury Neutrino Observatory provided clear evidence of neutrino flavor change.
Solar neutrinos have energies below 20
MeV and travel an
astronomical unit between the source and detector. At energies above 5 MeV, solar neutrino oscillation actually takes place in the Sun through a resonance known as the
MSW effect, a different process from the vacuum oscillations described later in this article.
Atmospheric neutrino oscillation
Large detectors such as
IMB,
MACRO, and
Kamiokande II observed a deficit in the ratio of the flux of muon to electron flavor atmospheric neutrinos (see
muon decay). The
Super Kamiokande experiment provided a very high precision measurement of neutrino oscillations in an energy range of hundreds of MeV to a few TeV, and with a baseline of the radius of the
Earth.
Reactor neutrino oscillations
Many experiments have searched for oscillations of electron
anti-neutrinos produced at
nuclear reactors. A high precision observation of reactor neutrino oscillation has been made by the
KamLAND experiment. Neutrinos produced in nuclear reactors have energies similar to solar neutrinos, a few MeV. The baselines of these experiments have ranged from tens of meters to over 100km.
Beam neutrino oscillations
Neutrinos beams produced at a
particle accelerator offer the greatest control over the neutrinos being studied. Many experiments have taken place which study the same neutrino oscillations which take place in atmospheric neutrino oscillation, using neutrinos with a few GeV of energy and several hundred km baselines. The
MINOS experiment recently announced that it observes consistency with the results of the K2K and Super-K experiments. The MINOS result hasn't yet been published in a peer reviewed journal but it's expected that their results will be published soon.
The controversial observation of beam neutrino oscillation at the
LSND experiment was tested by
MiniBooNE. Results from MiniBooNE appeared in Spring 2007, and appeared to contradict the predictions of the
LSND experiment.
The upcoming
T2K experiment will direct a neutrino beam through 295 km of earth, and will measure the parameter
), oscillations become visible for neutrinos travelling several hundred km, which means neutrinos that reach the detector from below the horizon.
From atmospheric and
solar neutrino oscillation experiments, it's known that two mixing angles of the MNS matrix are large and the third is smaller. This is in sharp contrast to the CKM matrix in which all three angles are small and hierarchically decreasing. Nothing is known about the CP-violating phase of the MNS matrix.
If the neutrino mass proves to be of
Majorana type (making the neutrino its own antiparticle), it's possible that the MNS matrix has more than one phase.
Origins of neutrino mass
The question of how neutrino masses arise hasn't been answered conclusively. In the Standard Model of particle physics,
fermions only have mass because of interactions with the Higgs field (see
Higgs boson). These interactions involve both left- and right-handed versions of the fermion (see
chirality). However, only left-handed neutrinos have been observed so far.
Neutrinos may have another source of mass through the
Majorana mass term. This type of mass applies for electrically-neutral particles since otherwise it would allow particles to turn into anti-particles, which would violate conservation of electric charge.
The smallest modification to the Standard Model, which only has left-handed neutrinos, is to allow these left-handed neutrinos to have Majorana masses. The problem with this is that the neutrino masses are implausibly smaller than the rest of the known particles (at least 500,000 times smaller than the mass of an electron), which, while it doesn't invalidate the theory, isn't very satisfactory.
The next simplest addition would be to add right-handed neutrinos into the Standard Model, which interact with the left-handed neutrinos and the Higgs field in an analogous way to the rest of the fermions. These new neutrinos would interact with the other fermions solely in this way, so are not phenomenologically excluded. The problem of the disparity of the mass scales remains.
Seesaw mechanism
The most popular solution currently is the
seesaw mechanism, where right-handed neutrinos with very large Majorana masses are added. If the right-handed neutrinos are very heavy, they induce a very small mass for the left-handed neutrinos, which is proportional to the inverse of the heavy mass.
If it's assumed that the neutrinos interact with the Higgs field with approximately the same strengths as the charged fermions do, the heavy mass should be close to the
GUT scale. Note that, in the Standard Model there's just one fundamental mass scale (which can be taken as the scale of
breaking) and all masses (such as the electron or the mass of the Z boson) have to originate from this one.
There are other varieties of seesaw and currently it isn't clear which, if any, nature has chosen.
The apparently innocent addition of right-handed neutrinos has the effect of adding new mass scales, completely unrelated to the mass scale of the Standard Model. Thus, heavy right-handed neutrinos look to be the first real glimpse of physics beyond the Standard Model. It is interesting to note that right-handed neutrinos can help to explain the origin of matter through a mechanism known as
leptogenesis.
Other sources
There are alternative ways to modify the standard model that are similar to the addition of heavy right-handed neutrinos (for example, the addition of new scalars or fermions in triplet states) and other modifications that are less similar (for example, neutrino masses from loop effects and/or from suppressed couplings). One example of the last type of models is provided by certain versions supersymmetric extensions of the standard model of fundamental interactions, where
R parity isn't a symmetry. There, the exchange of supersymmetric particles such as
squarks and
sleptons can break the lepton number and lead to neutrino masses. These interactions are normally excluded from theories as they come from a class of interactions that lead to unacceptably rapid proton decay if they're all included. These models have little predictive power and are not able to provide a cold dark matter candidate but they're considered interesting since they'd be compatible with new observable signals in particle colliders.
Further Information
Get more info on 'Neutrino Oscillation'.
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