[EvaH (OP)]

Jeff wants to know:

I would expect protons to have greater mass than neutrons, because protons have a positive charge, which should involve mass to confer the charge, and neutrons have none.  Why do neutrons then have greater mass?


What do you think?

[alancalverd]

A free neutron is not stable. It decays into a proton and a pion. Pion mass is very close to that of an electron and it bears a negative charge, so it is closer to the truth to imagine a neutron as a proton plus an electron, rather than the other way around.

This is intellectually satisfying as the simplest atom we know, hydrogen, consists of one proton and one electron. It also suggests that if there is no upper limit to gravitational force, you could collapse a star into a block of neutrons - I'm sure astronomers on this forum will put me right on this!

[Bored chemist]

A simplistic answer is that a neutron also has an electron "added".

Proton
1.674929 x 10-27 kg
Neutron
1.672623 x 10-27 kg

Difference
0.002306 x 10-27 kg

Electron
9.109390 x 10-31 kg
i.e.
0.0009109390 x10-27 Kg

The numbers don't square up because of the mass energy relationship and the binding  energy of the electron.

[Bored chemist]

A free neutron is not stable. It decays into a proton and a pion.

Sort of...
https://en.wikipedia.org/wiki/Free_neutron_decay

[alancalverd]

....at least it used to when I was a student!  Antineutrinos indeed. What will they think of next?

[Bored chemist]

Antineutrinos indeed. What will they think of next?

All bets are off.
But, even in "your day", they knew about beta  emission

[evan_au]

- The positron (charge +) is the antiparticle of the electron (charge -)
- Light (charge 0) is it's own antiparticle
- Even today, it is not clear if antineutrinos (charge 0) are different from neutrinos (charge 0).

It's very hard to get neutrinos to interact with anything (including antineutrinos), so at this point, we really can't tell experimentally.
See: https://en.wikipedia.org/wiki/Majorana_fermion

[chris]

Brilliant debate / explanations; I learned a lot from that; thanks guys!

[alancalverd]

Antineutrinos indeed. What will they think of next?

All bets are off.
But, even in "your day", they knew about beta  emission

And we still do. Betas (and gammas) are emitted from nuclei, whose halflives vary from attoseconds to megayears, but negative pions come from decaying free neutrons with a halflife of about 14 minutes. It's a bit more than a historical oddity that we name what appear to be identical particles (beta, electron, pion: gamma, x-ray) according to their source rather than their properties.

[Janus]

Antineutrinos indeed. What will they think of next?

All bets are off.
But, even in "your day", they knew about beta  emission

. It's a bit more than a historical oddity that we name what appear to be identical particles (beta, electron, pion: gamma, x-ray) according to their source rather than their properties.

??
Beta and electron can be synonymous, ( though there is such a thing as a beta+, which is a positron that is emitted during the decay of some isotopes which ends up converting a proton to a neutron in the nucleus)
x-ray and gamma are both electromagnetic radiation of different frequency.
But a pion (or pi-meson) is not even a fundamental particle, but the union of two quarks, and has a mass 270 times that of the electron. It not only comes in a positive and negative version, but also a neutral version.
The pion is one of the particles that participate in the force binding nucleons together. ( When this particle was first theorized and the search for it started, the muon was found.   When it was discovered that this was not the particle they were looking for, but an entirely different one, it prompted I.I. Rabi to quip "Who ordered that?")

[alancalverd]

Mea culpa. Must be getting old.