Re: neutrino mass

[harvey.picker@mail.cc.trincoll.edu (Harvey Picker)]

> I have a question about a finite mass neutrino. If it is so, then it takes over
> as the smallest non-zero mass particle from the electron. When photons with
> energies above twice the electron rest mass pass through matter, they create
> electron/positron pairs. So if neutrinos of mass 0.07 eV exist why don't
> photons
> of energy > 0.14 eV create neutrino/antineutrino pairs? (And how would optics
> be different if they did?)
>
> Tim Sullivan
> sullivan@kenyon.edu

Dear Tim,

The process you describe--nu-nubar pair production--can occur--and at zero
threshold energy--even if neutrinos have zero rest mass.  But it will occur
incredibly rarely, since photons do not couple directly to neutrinos, but
must do so through virtual electron-positron pairs.  The rate of this
process is therefore suppressed by the weakness of the weak interaction (e+
e- to nu nubar) and the fact that the virtual electron and positron are far
off their mass shells.  So optics is not altered at any detectable level.
(On the other hand, real e+ e- pairs in a hot white dwarf star emit nu
nubar pairs at a rate that contributes significantly to the cooling of the
star.  This is the ORCA process, if my aging memory serves me right.)

By the same token, ordinary H2 molecules are unstable against fusion of the
two protons into deuterium, with the emission of a positron and an electron
neutrino.  While this process is energetically allowed, the combination of
the enormous Coulomb barrier (at the exceedingly low relative energy of the
two protons in the H2 molecule) and the weakness of the weak interaction
causes it to proceed at an amazingly small rate (which I was stupid enough
to actually calculate, once).  The lifetime of H2 against such decay is
many powers of ten times the lifetime of the universe, so it is safe to
regard H2 as stable.

                          Harvey Picker

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