# Why are neutrons heavier than protons?

Dec 28, 2015

Up quarks and down quarks are slightly different in mass.

#### Explanation:

This question strays into the realms of particle physics, but fortunately the answer is not too in-depth. 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 $+ \frac{2}{3}$, whilst a down quark has a charge of $- \frac{1}{3}$ . When you total the charges of the two different combinations, you will observe the following:

${Q}_{p} = {Q}_{u} + {Q}_{u} + {Q}_{d} = + \frac{2}{3} + \frac{2}{3} - \frac{1}{3} = + 1$

${Q}_{n} = {Q}_{u} + {Q}_{d} + {Q}_{d} = + \frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$

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:

https://www.quora.com/Why-is-a-down-quark-heavier-than-an-up-quark