Neutrinos

Neutrinos :
There are two types of leptons, the electron and the neutrino. The neutrino is a strange particle, not discovered directly,
but by inference from the decay of other particles by Wolfgang Pauli in 1930. It has no charge and a very small mass. It
interacts with other particles only through the weak force (i.e. it is immune to the strong and electromagnetic forces). The
weak force is so weak, that a neutrino can pass through several Earth's of lead with only a 50/50 chance of interacting
with an atom, i.e. they are effectively transparent to matter.
The weakly interacting nature of neutrinos makes them very difficult to detect, and therefore measure, in experiments.
Plus, the only sources of large amounts of neutrinos are high energy events such as supernova, relics from the early
Universe and nuclear power plants. However, they are extremely important to our understanding of nuclear reactions
since pratically every fusion reaction produces a neutrino. In fact, a majority of the energy produced by stars and
supernova are carried away in the form of neutrinos (the Sun produces 100 trillion trillion trillion neutrinos every
second).
Detecting neutrinos from the Sun was an obvious first experiment to measure neutrinos. The pioneering experiment was
Ray Davis's 600 tonne chlorine tank (actually dry cleaning fluid) in the Homestake mine, South Dakota. His experiment,
begun in 1967, found evidence for only one third of the expected number of neutrino events. A light water Cherenkov
experiment at Kamioka, Japan, upgraded to detect solar netrinos in 1986, finds one half of the expected events for the
part of the solar neutrino spectrum for which they are sensitive. Two recent gallium detectors (SAGE and GALLEX),
which have lower energy thresholds, find about 60-70% of the expected rate.

The clear trend is that the measured flux is found to be dramatically less than is possible for our present understanding of
the reaction processes in stars. There are two possible answers to this problem: 1) The structure and constitution of stars,
and hence the reaction mechanisms are not correctly understood (this would be a real blow for models that have
otherwise been very successful), or 2) something happens to the neutrinos in transit to earth; in particular, they might
change into another type of neutrino, call oscillation (this idea is not as crazy as it sounds, as a similar phenomenon is
well known to occur with the meson particles). An important consequence to oscillation is that the neutrino must have
mass (unlike the photon which has zero mass).

By the late 1990s, the oscillation hypothesis is shown to be correct. In addition, analysis of the neutrino events from the
supernova 1987A indicates that the neutrinos traveled at slightly less than the speed of light. This is an important result
since the neutrino is so light that it was unclear if its mass was very small or exact zero. Zero mass particles (like the
photon) must travel exactly the speed of light (no faster, no slower). But objects with mass must travel at less than the
speed of light as stated by special relativity.

Since neutrino's interact very weakly, they are the first particles to decouple from other particles, at about 1 sec after the
Big Bang. The early Universe is so dense that even neutrinos are trapped in their interactions. But as the Universe
expands, its density drops to the point where the neutrinos are free to travel. This happens when the rate at which
neutrinos are absorbed and emitted (the weak interaction rate) becomes slower than the expansion rate of the Universe.
At this point the Universe expands faster than the neutrinos are absorbed and they take off into space (the expanding
space).
Now that neutrinos have been found to have mass, they also are important to our cosmology as a component of the
cosmic density parameter. Even though each individual neutrino is much less massive than an electron, trillions of them
are produced for every electron in the early Universe. Thus, neutrinos must make up some fraction of the non-baryonic
matter in the Universe (although not alot of it, see lecture on the large scale structure of the Universe).