# Proton + Electron = Neutron?

## Main Question or Discussion Point

Dear PF Forum,
I have a question to ask.
Supernovae produce neutron star (or Black Hole).

This is what I summarize from wikipedia.
Proton
Composition: 2 up quarks, 1 down quark
Mass: 1.672621777(74)E-27 kg

Neutron
Composition: 1 up quark, 2 down quarks
Mass: 1.674927351(74)E-27 kg

Electron
Composition: Electron
Mass: 9.10938291(40)×10E-31 kg
1. Is P + e = N? Is it that simple?
Judging by its mass, altough slightly off.
2. Is Up Quark + e = Down Quark?

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jtbell
Mentor
1. Is P + e = N?
No. As you note, p = uud and n = udd, in terms of quarks.

2. Is Up Quark + e = Down Quark?
No. Up and down quarks are both fundamental particles, not composed of other particles, as far as we know.

No. As you note, p = uud and n = udd, in terms of quarks.
No. Up and down quarks are both fundamental particles, not composed of other particles, as far as we know.
So, what makes neutron star?
Matter is we know it, consist atom. Proton, Neutron, Electron and I read somewhere, there are abundant neutrinos, too. Altough their combined mass are still much less than combined baryon mass in the universe.
So, if there are Proton, Neutron and Electron, and the result is Neutron star, what makes it?

Thanks.

Orodruin
Staff Emeritus
Homework Helper
Gold Member
So, what makes neutron star?
Mostly neutrons ...

That particles are fundamental does not mean that they cannot be converted into each other. An up quark and an electron may be converted into a down quark and a neutrino by exchanging a W boson. This is what happens in beta decay and when a neutron star is formed (many many many times of course).

Mostly neutrons ...

That particles are fundamental does not mean that they cannot be converted into each other. An up quark and an electron may be converted into a down quark and a neutrino by exchanging a W boson. This is what happens in beta decay and when a neutron star is formed (many many many times of course).
As I know it. The process of producing neutron star is supernova, which happens in 1 seconds. How long is "beta decay"?

Thanks.

ChrisVer
Gold Member
As I know it. The process of producing neutron star is supernova, which happens in 1 seconds. How long is "beta decay"?
The exact name of the interaction would be "electron capture" or one of the inverse beta decays. But still time of interaction won't tell you much. An energy approach can help. In particular the interaction is:
$p + e^- \rightarrow n + \nu_e -0.8~MeV$
This becomes energetically favorable around at the energies/momenta of the electrons when the white dwarf becomes gravitationally unstable to collapse. And by the end of the collapse, the protons in the core have been converted to neutrons. Neutrons then can't decay back because there is an electron/proton atmosphere in a degenerate state and so the resulting electrons/protons from a neutron beta decay can't find any state to exist in (Pauli-block due to Pauli exclusion principle)

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