Information AboutNeutrino |
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Neutrinos are Elementary Particle s denoted by the Greek letter ν . Travelling close to the Speed Of Light , lacking Electric Charge and able to pass through ordinary matter almost undisturbed, they are extremely difficult to detect. Neutrinos have a minuscule, but non-zero, Mass that is too small to be measured as of 2007 . Neutrinos are created as a result of certain types of partner, called an Antineutrino . Electron neutrinos or antineutrinos are generated whenever Neutron s change into Proton s or vice versa, the two forms of Beta Decay . Most neutrinos which pass through the Earth emanate from the sun, and more than 50 trillion solar electron neutrinos pass through the human body every second. HISTORY . The collision occurred at the point where three tracks emanate on the right of the photograph.]] The neutrino was first postulated in December 1930 by '', a result that was rewarded with the 1995 Nobel Prize . In this experiment, now known as the Neutrino Experiment , neutrinos created in a nuclear reactor by beta decay were shot into protons producing Neutron s and Positron s both of which could be detected. We now know that both the proposed and the observed particles were antineutrinos. The name ''neutrino'' was formed by Enrico Fermi , who developed the first theory describing neutrino interactions, as a Pun on ''neutrone'', the Italian name of the Neutron : ''neutrone'' seems to use the ''-one'' suffix (even if it is a complete word, not a compound), which in Italian indicates a large object, whereas ''-ino'' indicates a small one. In 1962 Leon M. Lederman , Melvin Schwartz and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the Muon neutrino, which earned them the 1988 Nobel Prize . When a third type of Lepton , the Tau , was discovered in 1975 at the Stanford Linear Accelerator , it too was expected to have an associated neutrino. First evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay that had led to the discovery of the neutrino in the first place. The first detection of actual tau neutrino interactions was announced in summer of 2000 by the DONUT collaboration at Fermilab , making it the latest particle of the Standard Model to have been directly observed. Starting in the late 1960s, several experiments found that the number of electron neutrinos arriving from the sun was between one third and one half the number predicted by the Standard Solar Model , a discrepancy that became known as the Solar Neutrino Problem which remained unresolved for some thirty years, until low energy neutrinos were detected in the Sudbury Neutrino Detector from 1999 to 2001. The Standard Model of particle physics in use at the time postulated massless neutrinos; for theoretical reasons, massless neutrinos cannot change flavors. However, non-zero neutrino mass and accompanying flavor oscillation remained a possibility. A practical method for investigating neutrino masses (that is, flavour oscillation) was first suggested by Bruno Pontecorvo in 1957 using an analogy with the neutral Kaon system; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev and Alexei Smirnov (expanding on 1978 work by Lincoln Wolfenstein ) noted that flavour oscillations can be modified when neutrinos propagate through matter. This so-called MSW Effect is important to understand neutrinos emitted by the Sun, which pass through its dense atmosphere on their way to detectors on Earth. Starting in 1998, experiments began to show that neutrinos indeed have mass and can change flavors (see Super-Kamiokande , Sudbury Neutrino Observatory , KamLAND and MINOS ). This resolved the solar neutrino problem: the electron neutrinos produced in the sun had partly changed into other flavors which the experiments could not detect. Raymond Davis Jr. and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize In Physics for their work on solar neutrinos. Detection of solar neutrinos, and detection of neutrinos of the SN 1987A Supernova in 1987 marked the beginning of Neutrino Astronomy . PROPERTIES The neutrino has half-integer Spin () and is therefore a Fermion . Because it is an electrically neutral Lepton , the neutrino interacts neither by way of the Strong nor the Electromagnetic force, but only through the Weak Force and Gravity . Because the Cross Section in weak nuclear interactions is very small, neutrinos can pass through matter almost unhindered. For typical neutrinos produced in the sun (with energies of a few MeV ), it would take approximately one Light Year (~1016m) of Lead to block half of them. Detection of neutrinos is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams. All neutrinos observed to date have left-handed Chirality . Types of neutrinos There are three known types (''s in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicsts' 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. |
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