Can't see them, can't feel them, but know they exist

Neutrino - the ghost particle

Neutrinos are a leptons, an elementary particle. They do not have electric charge, and they interact with matter via the weak nuclear force. The weakness of the weak force gives neutrinos the property that matter is almost transparent to them. For the over a half a century scientists believed that neutrinos are massless, and that they travel at the speed of light


By MARIJA MITROVIĆ
from Belgrade, SERBIA


Look at your thumbnail! Did you know that in this moment about 100 billion solar neutrinos pass through your thumbnail! You didn't noticed that?!Maybe if you look again. Nothing? That is strange, or it's not.

Neutrinos are a leptons, an elementary particle. They do not have electric charge, and they interact with matter via the weak nuclear force. The weakness of the weak force gives neutrinos the property that matter is almost transparent to them. For the over a half a century scientists believed that neutrinos are massless, and that they travel at the speed of light. This believes were in line with The Standard Model, a theory which describes three of the four known fundamental interactions between the elementary particles that make up all matter. Today we know that neutrino has mass, but we can only estimate its value, and we know that it is very small.

The neutrino was first postulated in December, 1930 by Wolfgang Pauli to explain conservation of energy in beta decay. When neutron decays into a proton and electron, we would expect that energies of electrons emitted by beta decay had a discrete spectrum. But experiments were in disagreement with theory. In 1911 Lise Meitner and Otto Han performed an experiment that showed that the energies of electrons had a continuous spectrum. To resolve this inconsistency, Pauli suggested that in addition to electrons and protons atoms also contained an extremely light neutral particle which he called the neutron. He suggested that this "neutron" was also emitted during beta decay and had simply not yet been observed. In 1931 Enrico Fermi renamed Pauli's "neutron" to neutrino. Actually, that third particle is antiparticle of neutrino, called anti-neutrino.

Pauli believed that scientists will never detect neutrinos, because of their "ghostly" properties. But, in 1956 Clyde Cowan, Frederic Reines, and their colleagues preformed an neutrino experiment. In this experiment, neutrinos created in a nuclear reactor by beta decay were shot into protons producing neutrons and positrons (antiparticle of electron) both of which could be detected. This way they indirectly proved that neutrino exists. Today we know that this way they proved existing of anti-neutrinos.

Still there was a problem of detecting neutrino. Physicist Bruno Pontecorvo, suggested that one might be able to capture neutrinos from a reactor using chlorine. The neutrinos from the reactor would convert some of the chlorine atoms to argon atoms, which are radioactive and could be counted in small quantities in small counters. One could be sensitive to even something as weakly interacting as a neutrino if one had a huge vat of chlorine. If we can detect neutrinos we can use them to look deep into the Sun. How? Well, light, as we all know, doesn't penetrate anything. If you put your hand in front of your face, others can not see your faces; the light won't go through your hand. It doesn't penetrate any appreciable amount of material. Neutrinos can go through unimaginable amounts of material without being affected. There is less than a percent chance that anything would ever happen to them as they passed through the sun, certainly through the Earth.

Raymond Davis Jr. was the first one who successfully detected solar neutrinos. Davis developed an experiment based on this idea by placing a 400 000 liters tank of dry-cleaning chemical which is a good source of chlorine, one and a half kilometer underground in the Homestake Gold Mine in South Dakota and developing techniques for quantitatively extracting a few atoms of argon from the tank. The chlorine target was located deep underground to protect it from cosmic rays. This experiment confirmed that the sun produces neutrinos, but only about one-third of the number of neutrinos predicted by theory of John Bahcall could be detected. This problem was known as a "solar neutrino puzzle" and it is resolved a few years ago, in 2001. John Bahcall predicted the number of solar neutrinos, using all our knowledge about nuclear reaction which are happening on Sun. So everything we knew about Sun would be brought into the question if Davis's experiment was right. Bouth, Davis and Bahcall insisted that they were right, and they were. Strange? Well, actually it is not so strange. There are three known types ( flavors ) of neutrinos: electron neutrino, muon neutrino and tau neutrino, named after their partner leptons in the Standard Model. The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicist's intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics. During their trip, from the Sun to the Earth, neutrinos are changing their flavor. This is called neutrinos oscillation. Only one third of electron neutrinos, produced in the Sun is reaching Earth as electron neutrinos, which we can detect. This explanation has one problem. If neutrinos do not have a mass, they can not interact with each other, so they can not change flavor.

The "solar neutrino puzzle" gave birth to different experiments by scientists around the world, all working to confirm the solar neutrino deficit and to prove that they have a mass. First came Kamiokande in Japan, then SAGE in the former Soviet Union, GALLEX in Italy, and then Super Kamiokande. Finally, in 2001-2002, scientists working at SNO, the Sudbury Neutrino Observatory in Ontario, Canada, found strong evidence that the neutrino has the ability to oscillate.

They are here, we can not see them, we can not feel them, but we know that they exist.