#### LEPTON.CO.UK

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Note that the neutrino masses are known to be non-zero because of neutrino oscillation, but their masses are sufficiently light that they have not been measured directly as of 2008. However there have been measured (indirectly based on the oscillation periods) the differences of the mass squares between the neutrinos, which have been estimated $\Delta m,2_{12} = 80{meV},2$ and $\Delta m,2_{23} \approx \Delta m,2_{13} = 2400{meV},2$. This leads to the following conclusions:

• ?µ and ?t are lighter than 2.2 eV (as ?e is and the mass differences between the neutrinos are of order of millielectronvolts)
• one (or more) of the neutrinos is heavier than 0.040 eV
• two (or three) of the neutrinos are heavier than 0.008 eV

The names "mu" and "tau" seem to have been selected due to their places in the Greek alphabet; µ is seven letters after e, whereas t is seven letters after µ.

## Etymology

According to the Oxford English Dictionary, the name "lepton" (from Greek leptos meaning 'thin') was first used by physicist Léon Rosenfeld in 1948:

Following a suggestion of Prof. C. Møller, I adopt — as a pendant to "nucleon" — the denomination "lepton" (from ?ept??, small, thin, delicate) to denote a particle of small mass.[2]

The name originates from before the discovery in the 1970s of the heavy tau lepton, which is nearly twice the mass of a proton.

## References

1. , a b c Laboratory measurements and limits for neutrino properties.
2. , Rosenfeld, Léon (1948). Nuclear Forces. Interscience Publishers, New York, xvii.

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