1. What are neutrinos ?
Neutrinos (symbolized by v) are fundamental particles, and they are formed in stars. The most common reaction that produces them is the following overall fusion reaction responsible for the sun's output of energy:
4 1H --> 4He + 2 0e+ + 2 ve
Subatomic particles of the low-mass lepton variety, neutrinos are cousins of electrons, muons and taus. (unlike hadrons, which include protons, neutrons and mesons). There are three charged leptons: electrons, muons and tau particles. Only electrons, muons and taus have a charge of -1; but all leptons including neutrinos have a spin quantum number of either (+/-) 1/2. If charged, leptons interact with both the electromagnetic and weak forces. On the other hand, the neutrinos (electron neutrinos, muon neutrinos and tau neutrinos) only interact with the weak force.
At some point after the Big Bang, the universe found itself with neutrons, protons, and among other things, neutrinos, vast numbers of them. When the universe's density was high, some of the neutrinos would react with neutrons to produce protons and electrons in a reversible reaction. But as the universe expanded and its density decreased, the neutrinos stopped reacting.(This all happened in less than three seconds, so when students tell you that they're packing up with three minutes to go before the bell, tell them what great things have been accomplished in a smaller increment of time.)
So this essentially freed neutrinos and their counterparts, anti neutrinos. Now the neutrino is not very massive, maybe only 10-7 as massive as a proton. But there are so many of them that it was hypothesized their combined mass could outnumber the mass of visible matter in the form of galaxies. Then neutrinos would have had gravitational effects on stars that only seemed to be away from the action.
Incidentally, accounting for neutrinos' mass makes the universe's density increase by a factor of 33, from 3 X 10-31 to 10-29
Detecting neutrinos has been an important way of providing evidence for theoretical models of how the sun shines. For about 30 years those theories were in doubt because they predicted a far greater number of neutrinos than were actually detected. Then an experiment in Sudbury Ontario proved that if all three types neutrinos were measured, then the theoretical prediction was essentially correct. Electron neutrinos "mutated" into muon and tau neutrinos on their way to earth. Originally only electron neutrinos had been picked up by early experiments, leading to the erroneous conclusion that the sun was actually producing few electron neutrinos.
See http://www.pbs.org/wgbh/nova/neutrino/missing.html (PBS Nova)
(A short article by Scientific American describing the resolution of the solar neutrino problem.)
http://www.sns.ias.edu/~jnb/Papers/Popular/Nobelmuseum/mystery.html (A summary of how the solar mystery problem was solved. This was written by John Bahcall, the theoretical researcher whose work was vindicated by recent experiments.)
http://cupp.oulu.fi/neutrino/index.html (This Finnish site summarizes neutrino research.)
2. Neutrinos are not influenced by gravity, the strong force or electromagnetism. They only "feel" the weak force . What is the weak force?
Of the four fundamental forces, the weak force is the most obscure. Although the weak force is too weak to bind particles into atoms, it is important because it renders so many of the subatomic particles such as muons, taus and 5 of the 6 quark-types unstable. The weak force governs beta decay, which converts a neutron into a proton and an electron, and it also plays an important role in the sun. In fusion's first step, the sun converts essentially converts a proton into a neutron, a positron and a neutrino. The weak force itself is mediated by either photons or exchange particles known as W+, W- and Z, the latter group being perhaps former photons that mysteriously acquired mass. Exchange particles can keep other particles bonded in the same way that the ball-passing between Malone and Stockton keep them together on the basketball court.
For a more detailed discussion with references, click here.
3. By the way, what is color and what is the strong force?
Quarks, the basic units of neutron, protons and other hadrons, are kept
together by a color force. An exchange process binds
them just as it does with the weak force, but particles known as gluons are involved.
Gluons can only be exchanged over a small distance, acting over distances of 10-15m or less,
about 10000 times smaller than the smallest atom, hydrogen, and thus limiting the range of this force .
When charged atoms of similar charges approach one another, the distance separating them is far greater
than 10-15m. As a result, they repel electromagnetically. But quarks are much closer together, allowing gluons to do their thing.
In 1935, the Japanese physicist, Hideki Yukawa, first proposed the existence of a particle responsible for a strong force. The discovery of the pi meson confirmed his hypothesis and won him the Nobel Prize in 1949. But the existence of quarks was only confirmed about 20 years later by Taylor, Friedmman and Kendall, who fired high energy electrons from a linear accelerator at protons and neutrons. Strangely, electrons were deflected at large angles. Sixty years earlier, Rutherford had obtained similar deflection angles upon firing helium nuclei at gold foil. The trio's results suggested that neither the proton nor the neutron was a solid sphere. Two up quarks and a down quark of charge +2/3 and -1/3, respectively, make up the proton, and two downs and an up make up a neutron.
Protons and neutrons are bound to each other by the strong force. To go into more detail, let's consider an analogy. In an atom, positively charged protons are attracted to negatively charged electrons. Atoms are often electrically neutral because they have an even amount of positive and negative charge. In spite of this, both atoms and molecules can be attracted to each other from an uneven distribution of electrons. Similarly, quarks are not only charged but have a property called "color". When quarks are bound together to make neutrons or protons, the color property is neutralized. But as in the case of atoms and molecules, there can be residual color resulting from an uneven distribution(I'm guessing on the latter two words). This helps constitutes the strong nuclear force that binds protons and neutrons.
(reference: The Particle Garden Kane, Gordon. Helix Books. 1995)