Specifically, why is the neutron heavier than the proton?

[says]

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?

 

[mitchell porter]

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.

 

[Vanadium 50]

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.

 

[mathman]

Anthropic principle. Otherwise there would be no universe.

 

[Vanadium 50]

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.