This is the story of the neutrino (Greek letter nu ; ν), a little piece of spinning nothing (i.e. a mass;less particle, but with angular momentum) whose existence was theoretically required by the need to balance certain equations in nuclear physics. How a mass;less thing could possibly have momentum of any sort however, was a paradox which was left unaddressed for the time being, and even today, though we mostly agree that it must have some mass, it is so miniscule (even in particle physics, where things are notoriously tiny) that we have no accurate idea of what that mass might be.
It was Wolfgang Pauli who, in 1930, in order to explain how beta;decay could work while conserving mass, momentum, charge and angular momentum, postulated that there must be a new;to;nuclear;physics particle involved in the reaction. Pauli tentatively called this theoretically required particle a ‘neutron.’ However, James Chadwick discovered and named the ‘real’ neutron (i. e. the particle we now know as the neutron) in 1932. Chadwick’s neutron was a particle with mass, and for a while there were two particles called neutrons, hence, in order to remove the confusion, a new name had to be found for the ‘other neutron’. It was finally baptised, sight still unseen, by Enrico Fermi in 1933. who came up with the term ‘neutrino’ (little neutral one) in order to distinguish between the two neutral particles known at the time, one having ameasurable mass, and the other apparently having none at all.In fact, actual physical evidence of the neutrino’s existence was not discovered until 1956.
The fact that it has no charge and essentially no mass is what makes it an extremely difficult thing to detect: it does not interact much with anything at all. There are massive numbers of them being emitted from the Sun as a consequence of the nuclear fusion reactions going on there for example, but until very recently, we didn’t even have a clue that they were around despite the fact that around 65 thousand million of them they pass right through every square centimetre of our bodies (as well as the entire Earth) both day and night (if you are wondering about this, because they pass right through the Earth, almost entirely without interacting with it, the neutrino flux density on the day;side of the planet is essentially the same as the neutrino flux density on the night;side of the planet). It’s been calculated that a neutrino’s mean free path through water (the distance it would have to travel through a given substance – in this case water – before there was a reasonable chance of the particle interacting with anything) is about 10 times the Earth;Sun distance. In other words, one would need a tank of water stretching from us to well beyond the orbit of Saturn – a distance of more than a thousand million kilometres – if we were to stand any chance of trying to detect a neutrino by its interaction with water!
In 1962, Leon Lederman, Jack Steinberg & Melvin Schwartz demonstrated that more than one type of neutrino existed. This work eventually led to the discovery of first the muon neutrino (νμ) by Lederman et(al, and eventually to that of the tau (ντ) neutrino by Martin Lewis Perl and his colleagues.In addition to the first neutrino discovered, the electron neutrino (νe), this brought the total up to three neutrinos. These discoveries were stimulated by the discovery of two new leptons, the muon, and the tau lepton. Leptons are a class of elementary particle of which the most well known is the electron (e;), and the other two, less well known leptons, are the muon (μ), and the tau (τ). The table below summarises the various names which the 6 types of leptons we now know of go by.Le
|Electron neutrino||Electron antineutrino|
There is a plethora of names because of historical factors associated with their discoveries. To find out more about them, please go to: http://en.wikipedia.org/wiki/Lepton, http://en.wikipedia.org/wiki/Neutrino, http://lappweb.in2p3.fr/neutrinos/anhistory.html or http://hyperphysics.phy; astr.gsu.edu/hbase/particles/neutrino.html, where they are explained in more detail. Happy reading. Pages and pages have been written about them, and Nobel Pries have been won for making discoveries about them. Not a bad story for a little bit of spinning nothing.