KAHIBARO
Table of Contents
Composition of the Neutron
The neutron is a baryon, which means it is a particle made of three quarks. Its quark content is
$$n = udd$$
This means the neutron contains one up quark, $u$, and two down quarks, $d$.
The electric charges of these quarks are fractional. The up quark has charge $+\frac{2}{3}e$, and each down quark has charge $-\frac{1}{3}e$. Adding them gives the total neutron charge:
$$+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0$$
So the neutron is electrically neutral, even though it is built from charged parts.
The neutron quark structure is
$$n = udd$$
and its total charge is
$$Q_n = +\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0$$
Charge Balance
A useful way to see the neutron structure is to compare the contribution of each quark to the total charge.
| Quark | Number in neutron | Charge of each | Total contribution |
|---|---|---|---|
| Up, $u$ | 1 | $+\frac{2}{3}e$ | $+\frac{2}{3}e$ |
| Down, $d$ | 2 | $-\frac{1}{3}e$ | $-\frac{2}{3}e$ |
| Total | 3 | $0$ |
Although the total charge is zero, the charge is not absent inside the neutron. Instead, positive and negative charge are distributed among its constituent quarks.
Internal Structure
The neutron is not a simple rigid object with three tiny balls fixed in place. The quarks move inside it and are held together by the strong interaction. Because of this motion and interaction, the neutron has an internal structure that affects measurable properties such as magnetic moment and scattering behavior.
In a simple quark model, the neutron is described by its three valence quarks, one up and two down. These are the quarks that determine its main identity as a neutron.
Valence quarks determine the basic type of hadron. For the neutron, the valence quarks are
$$udd$$
Comparison with the Proton
The neutron is closely related to the proton. They differ only by swapping one up quark and one down quark.
| Particle | Quark structure | Total charge |
|---|---|---|
| Proton | $uud$ | $+e$ |
| Neutron | $udd$ | $0$ |
This small change in quark content leads to an important difference in electric charge, while many other properties remain similar because both are baryons made from light quarks.
A Simple Picture
A basic sketch of the neutron can be drawn as three quarks bound together.
This drawing is only a simplified model. In reality, the quarks are quantum objects, and the neutron is a dynamic bound state rather than a static arrangement.
Why the Structure Matters
The neutron quark structure explains why a neutral particle can still participate in complex interactions. Since it contains charged quarks, the neutron has internal electromagnetic structure even though its net charge is zero. This is one reason the neutron can have a magnetic moment and finite size.
Experiments that scatter electrons or other particles from neutrons reveal that the neutron is not elementary. Its internal quark structure can be probed through how momentum and charge are distributed inside it.
A neutron is neutral overall, but it is not structureless. Its charged quarks give it internal charge distribution and other measurable properties.
Summary
The neutron is a baryon made of three valence quarks:
$$n = udd$$
One up quark contributes $+\frac{2}{3}e$, and two down quarks contribute $- \frac{2}{3}e$, so the total charge is zero. This simple quark structure is the key to understanding the neutron as a composite particle rather than an elementary one.