Specifically, why is the neutron heavier than the proton?

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Discussion Overview

The discussion centers on the reasons behind the neutron being heavier than the proton, exploring theoretical and experimental aspects of particle physics, particularly focusing on quark composition and interactions. Participants examine various factors contributing to the mass difference, including quark masses, electric charge differences, and theoretical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the proton consists of 2 up quarks and 1 down quark, while the neutron has 2 down quarks and 1 up quark, suggesting that the different masses of these quarks contribute to the mass difference.
  • Another participant references two papers that propose the coupling of up and down quarks to the omega meson field as a significant factor in the mass difference, although they have not worked through the details of these papers.
  • A different participant claims that approximately 60% of the mass difference is due to the quark mass difference, while about 40% arises from the differences in electric charge, which affects internal repulsion and wavefunction characteristics, noting the complexity of these calculations.
  • One participant introduces the anthropic principle as a potential explanation for the mass difference, suggesting that without it, the universe as we know it would not exist.
  • Another participant expresses skepticism towards the anthropic explanation, arguing that it is an argument of last resort and that the universe could have different properties without fundamentally altering the stability of protons or neutrons.

Areas of Agreement / Disagreement

Participants express differing views on the anthropic principle as an explanation for the mass difference, with some supporting it and others rejecting it. Additionally, there is no consensus on the primary factors contributing to the mass difference, as various models and calculations are presented without resolution.

Contextual Notes

Participants acknowledge the complexity of calculations related to quark masses and their interactions, indicating that numerical methods are required to fully understand the contributions to the mass difference. There are also unresolved assumptions regarding the implications of the anthropic principle.

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Proton is made of 2 up, 1 down quarks
Neutron is made of 2 down, 1 up quarks

The up and down quark have different masses, which account for some of the mass difference. I've also read that hadron masses depend upon the interactions/dynamics inside the particle, not just the quarks contained.

What types of interactions/dynamics have an influence on the mass and result in the neutron being heavier than the proton?
 
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These two papers 1 2 seem to say that it's the different coupling of up and down quarks to the omega meson field, that is most responsible for the mass difference. But I have not actually worked through them.
 
About 60% of the mass difference is due to the quark mass difference, and about 40% from the differences in electric charge of the quarks - you can view this as a proton having more internal repulsion than the neutron, which in turn changes the wavefunction slightly. These calculations are complex and need to be done numerically. Also, I believe the two effects are in opposite directions.
 
Anthropic principle. Otherwise there would be no universe.
 
I don't like the anthropic answer.
  1. I feel it is an argument of last resort, and in this case we don't need it.
  2. It's not necessarily true - the universe might have turned out differently and the denizens of that putative universe might be wondering why it had to turn out differently.
  3. I don't think it's true in this case. If the proton were a little lighter than the neutron, the proton would still be stable or observationally stable. BBN would be all screwed up, but I don't think it would make a difference to what we are made out of.
 

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