Why are neutrons heavier than protons?

1 Answer
Dec 28, 2015

Up quarks and down quarks are slightly different in mass.


This question strays into the realms of particle physics, but fortunately the answer is not too in-depth.

Image courtesy of the trapped electron

Nucleons is the group term used to refer to both protons and neutrons. The image above shows the quark composition of these two sub-atomic particles. But what are quarks?

Quarks are fundamental particles, i.e. they are, to the best of our knowledge, indivisible. There are six types of quark, but I shall discuss only two of those types here. These two quarks are the 'up' quark ( u ) and the 'down' quark ( d ). You will notice that a nucleon contains at least 1 up quark, and at least 1 down quark: the identity of the final quark, therefore, is what determines the identity of the nucleon itself.

It follows, then, that a nucleon's charge is determined by the identity of its constituent quarks, and this is correct, because the up and down quarks have different charges themselves. An up quark has a charge of #+2/3#, whilst a down quark has a charge of #-1/3# . When you total the charges of the two different combinations, you will observe the following:

#Q_p = Q_u + Q_u + Q_d = +2/3 +2/3 -1/3 = +1#

#Q_n = Q_u + Q_d + Q_d = +2/3 -1/3 -1/3 = 0#

This explains why the charge of a proton is +1 whilst a neutron is neutral.

It is not well understood why an up quark has a different mass to a down quark, but this would be the only explanation for why a neutron is heavier than a proton. The following article provides come contextual information on quarks, which the may be used to explain these mass differences in the future: