Neutrinos

A neutrino is an elementary particle, which was first postulated by Wolfgang Pauli (1930) I order to explain the missing energy in radioactive decays. It was only later in 1965, that neutrino was first detected by Clyde Cowan and Fredrick Reines. 

A neutrino is an obscure particle, with zero electric charge and rarely interacts with matter, and according to recent studies has a small mass (which was earlier believed to be massless according to standard model of particle physics). About 100 billion neutrinos from sun, pass through your thumbnail every second, but you don’t feel them because they interact so rarely and weakly with matter. Because they interact so rarely, neutrinos can escape from solar interior and bring direct information about the solar fusion reactions to us on earth. The neutrinos from the fusion process in the sun can pass through several light years of solid lead before being absorbed by matter. 

There are 3 types of neutrinos: electron neutrino, muon neutrino and tau neutrino. Nuclear fusion in the sun produces neutrino related with electrons called electron-neutrino. Muon-neutrino and tau-neutrino are produced in lab accelerators and exploding stars along with heavier versions of the electron, the muon and tau. 

The neutrinos can be divided into two categories. The low energy neutrinos and the high energy neutrinos. The low energy neutrinos (about 10th MeV) are produced in nuclear processes such as fusion reactions of the sun or in the center of an exploding supernova. High energy neutrinos (more than 10th GeV) are produced due to high energy particle collisions which produce short-lived mesons decaying into neutrinos and other particles. 

Only low energy neutrinos have been detected so far: the solar neutrinos and the supernova neutrinos. Raymond Davis Jr., 2002 Nobel Laureate in Physics, managed to catch an average of half an electron neutrino (solar neutrino) interaction per day in his detector during his 20 years of research. While the Supernova Neutrinos were observed during 10 seconds in 1987 when a star in the Large Magellanic Cloud exploded as a supernova. The neutrinos from the inner part of the collapse reached the earth after a journey of 170,000 years, a few hours before the arrival of light from the supernova. The neutrinos were able to travel more or less directly from the central collapse in the inner part of the star. About 25 neutrino interactions were observed by the detectors at Kamiokande (Japan), Baksan (Sovjet union) and IMB (USA) during those 10 seconds. This observation of neutrinos from the sun and the supernova represented a new kind of astronomy since the neutrinos give us information from processes deep inside objects.

The unknown sources of high-energy cosmic rays produce high energy neutrinos called the GZK neutrinos when the accelerated high-energy protons (possible sources are gamman ray bursts, super heavy stars collapsing to black holes or two neutron stars falling into each other) collide with the photon gas around the sources, or with the microwave background (remnant of the Big Bang). These collisions will produce short-lived mesons, which decay to neutrinos and other particles. These neutrinos will travel, unaffected by the magnetic field in space and if detected on earth, will point back to the sources of the cosmic rays, which would help us identify the source, unlike cosmic rays which are electrically charged and get deflected due to the magnetic field in space. 

High energy neutrinos, might be produced in connection with another strange observation called "dark matter" in the universe. Galaxies and groups of galaxies rotate as if they contain more matter than what we can observe with our standard astronomical instruments. With only the observed visible matter, the galaxies should eject stars and matter out in the empty space due to the fast rotation. But this is not happening, which indicates that there is more matter than what we can observe. It is only the gravitational force, which feels the unknown hidden matter. This matter is called the "dark matter." One popular explanation of the dark matter is that the large part of this different kind of matter consists of weakly interacting massive particles (WIMPs) that were created in the Big Bang at the same time as our ordinary matter. Today, these WIMPs surround us, but can’t be felt as they weakly interact with matter. But when WIMPs pass through strong gravitational fields, they would tend to get gravitationally trapped within the core of the strong gravitational fields. When WIMPs interact they annihilate each other and produce high-energy neutrinos and other particles. Detection of these high energy neutrinos from the core would prove the existence of WIMPs and stand as an indirect proof of dark matter. 

The neutrino saga that started as a theoretical postulation to recent detection and experimental proof in the 20th century has much in store. It forces us to revise the standard model of particle physics that states neutrino as massless. It provides us an eye into the beginning of universe: the Big Bang and explore into the unsolved mysteries of the universe about dark matter. From particle physics to astrophysics, the neutrino holds the key to unravel the unexplained and unseen.