Q: The Mystery of Neutrinos from Collapsing Stars: What Kind are They?

[Mk]

The core of the collapsing star is initially composed of iron supported by electron degeneracy pressure, since the nuclear fusion of iron doesn't release energy. When the core collapses, the densities and pressures in the core overcome even the electron degeneracy pressure and the iron atoms' electrons are compressed into their nuclei where they combine with protons to form neutrons.
electron + proton --> neutron + neutrino
But what kind of neutrino is it? Electron?
- Mk

 

[El Hombre Invisible]

I'd say yes - electron lepton number has to be conserved.

 

[jtbell]

Yes, it's an electron neutrino when it's produced. However, when it finally interacts with something (e.g. a neutrino detector here on Earth), it can do so as an electron neutrino, a muon neutrino or a tau neutrino, via the phenomenon of "neutrino oscillations".

A significant fraction of neutrinos produced by the sun do not interact as electron neutrinos here on Earth. Solar neutrino experiments could detect only electron neutrinos, so there was an apparent deficit of solar neutrinos compared to theoretical predictions according to models of how the sun works. This puzzled physicists for many years. But other experiments on neutrino oscillations have convinced most physicists (as far as I know) that neutrino oscillations are the solution to the "solar neutrino problem".

 

[El Hombre Invisible]

Ahhh, is that why a separate conservation law for (non-specific) lepton number exists? I was wondering what the point of that was.

 

[Entropy]

I'd say yes - electron lepton number has to be conserved.

Well, no because tau and muon neutrinos have the same lepton number as electrons and electron neutrinos.

 

[El Hombre Invisible]

Well, no because tau and muon neutrinos have the same lepton number as electrons and electron neutrinos.

They have the same lepton number, but not the same electron lepton number. See the above conversation. Lepton number is conserved. Electron number, muon number and tau number are also conserved in particle decays of the kind outlined by the OP.