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What is a neutrino?
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Neutrinos were first proposed by Pauli in order
to explain why electrons coming from the decay of certain radioactive nuclei
behaved as if they were being pushed around by another outgoing particle that
carried energy, but little or no mass. If only an electron was
emitted, then it should carry away all the available energy every time.
Therefore, for a given atom the energy would always be the same. The plot
shown here is a representation of what is
actually seen. The energy of the electron (x-axis) and the number of times
that energy electron comes out (y-axis) is a smooth curve between a minimum
and maximum value - not just a single amount. This is exactly what would
happen if a third, invisible, particle were coming out at the same
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Later, two American scientists, Reines and Cowan (shown here), actually
detected such particles coming from the core of a nuclear reactor!
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After many decades of experimentation, we now know there are three types of
neutrino based on their weight, with the unimaginative names of
ν1, ν2, and ν3.
It is also known that there are three "flavors" of neutrinos based on how they
interact. These are known as νe,
νμ, and ντ. For many years, it
was thought that the weight of all three types of neutrino was exactly zero
and that only the three "flavors" really mattered. Just in the past few
years we have found this simple picture is just not so.
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Where do they come from?
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Why are they interesting?
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How small are they?
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What is a "neutrino oscillation"?
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