All spin and no substance – the story of the neutrino, the little neutral one.

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.

Source: http://www.scientificlib.comIt 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

Lepton nomenclature

Particle name

Antiparticle name

Electron Antielectron
Electron neutrino Electron antineutrino
Mu lepton
Antimu lepton
Muon neutrino
Muonic neutrino
Mu neutrino
Muon antineutrino
Muonic antineutrino
Mu antineutrino
Tau lepton
Antitau lepton
Tau neutrino
Tauonic neutrino
Tau neutrino
Tau antineutrino
Tauonic antineutrino
Tau antineutrino

There is a plethora of names because of historical factors associated with their discoveries. To find out more about them, please go to:,, or http://hyperphysics.phy;, 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.


Jothi's a guy who's inordinately curious about all manner of things, and has been trying to understand the way the world, in all its myriad facets, works, as well as to convey his understanding of this to all and sundry for most of his life. He has tertiary-level academic qualifications in a variety of different fields ranging from the hard sciences, through the social and behavioural sciences, to the humanities. He's willing to try almost anything at least once, and many things more than once.

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    In the December seminar ATLAS peerentsd the full amount of data for the diphoton and four lepton channels, plus 2.01 fb-1 of data for the H →WW(∗) channel.In the accompanying note ATLAS-CONF-2011-163 one can read that the combined local significance of the detected excess at 126 GeV is 3.4 sigma, taking into account the two high resolution channels, further increased to 3.6 sigma by the inclusion of the third channel.In the slide 26 of the Moriond ATLAS talk the same combination is updated graphically. The change of the situation is dramatic: the maximum of the joint local significance of the two high mass resolution channels (red line) now drops to 2.7 sigma (blu line) upon inclusion of the H →WW(∗) channel (4.7 fb-1 of data), and then further reduces to 2.5 sigma by adding the H → tau tau and H→bbar channels .Thus, even ignoring the two new H → tau tau and H→bbar channels , which anyhow contribute marginally, the simple inclusion of the full amount of data of the H →WW(∗) channel has reduced substantially the significance of the 126 GeV excess. And this channel is very important, as witnessed by its crucial role in driving most of the exclusion in the low mass region.So, statistically the ATLAS evidence is weaker than before.Unfortunately, low counting statistics is a field perfectly suited to generate surprises.For example, in the golden four lepton channel, on the basis of the predicted background, one could easily conclude a-priori from a probabilistic calculation that it is very unlikely that two independent experiments would observe the same quantitative excess of three events clustered in 1 GeV at two different energies. In practice, instead, this is what happened, three events custered within 1 GeV at 119 GeV for CMS, 3 events clustered at 124 GeV for ATLAS.Another example of a kind of statistical surprise is the premature appearance of the 126 GeV fluctuation in the ATLAS diphoton mass distribution, which was somehow already present in the distribution shown at the Grenoble EPS-HEP conference in July and based only on 1 fb-1 of data.

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