KAHIBARO
Table of Contents
Identity and Basic Properties
The up quark is one of the fundamental particles of matter. It is called a fundamental particle because, as far as we know, it is not made of anything smaller. It belongs to the quark family and is one of the two lightest quarks.
The up quark is especially important because ordinary matter around us contains many of them. Protons and neutrons, which make up atomic nuclei, are built from up and down quarks. The proton contains two up quarks and one down quark.
A compact summary of the up quark is shown below.
| Property | Up quark |
|---|---|
| Symbol | $u$ |
| Type | Quark |
| Electric charge | $+\frac{2}{3}e$ |
| Generation | First generation |
| Spin | $\frac{1}{2}$ |
| Color charge | Yes |
| Approximate mass | A few $\text{MeV}/c^2$ |
| Antiparticle | Anti-up quark, $\bar{u}$ |
The up quark has electric charge $+\frac{2}{3}e$, where $e$ is the magnitude of the elementary charge.
Place in Ordinary Matter
The up quark is one of the main building blocks of visible matter. Since protons and neutrons form atomic nuclei, and nuclei form atoms, the up quark is present in nearly everything we can touch.
The proton has quark content
$$
uud
$$
This means two up quarks and one down quark. Adding their charges gives the proton charge:
$$
+\frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +1e
$$
The neutron has quark content
$$
udd
$$
Its total charge is
$$
+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0
$$
A proton is made of $uud$, and a neutron is made of $udd$.
Electric Charge
One of the most distinctive features of the up quark is its positive fractional electric charge. Unlike familiar objects, which usually carry whole-number multiples of $e$, quarks carry fractional charge.
For the up quark,
$$
q_u = +\frac{2}{3}e
$$
This value helps explain the charges of hadrons made from up quarks. When combined with the charges of other quarks, it produces the observed charges of particles such as protons, pions, and many others.
First-Generation Quark
The up quark belongs to the first generation of matter particles. First-generation particles are the lightest and most stable ones in ordinary matter. Because the up quark is very light, it appears naturally in stable hadrons.
Its low mass is one reason why protons and neutrons are built from up and down quarks rather than heavier quarks such as strange, charm, bottom, or top quarks.
Spin and Fermion Nature
The up quark has spin
$$
s = \frac{1}{2}
$$
This means it is a fermion. As a fermion, it follows the quantum rules that apply to matter particles. In hadrons, the spins of constituent quarks combine to give the total spin of the particle.
Antiparticle
Every quark has an antiparticle. The antiparticle of the up quark is the anti-up quark, written as $\bar{u}$.
The anti-up quark has the same mass and spin as the up quark, but opposite electric charge:
$$
q_{\bar{u}} = -\frac{2}{3}e
$$
It also carries the corresponding anticolor rather than color charge.
The anti-up quark has charge $-\frac{2}{3}e$.
Role in Simple Hadron Examples
The up quark appears in many common hadrons. A few simple examples are listed below.
| Particle | Quark content | Total charge |
|---|---|---|
| Proton | $uud$ | $+e$ |
| Neutron | $udd$ | $0$ |
| Positive pion | $u\bar{d}$ | $+e$ |
| Neutral pion | mixture including $u\bar{u}$ and $d\bar{d}$ | $0$ |
These examples show that the up quark is not only part of baryons like the proton and neutron, but also part of mesons.
Simple Charge Check
A useful skill is checking particle charge from quark content. For example, for the positive pion,
$$
u\bar{d}
$$
the charge is
$$
+\frac{2}{3}e + \frac{1}{3}e = +e
$$
because the anti-down quark has charge $+\frac{1}{3}e$.
To find the charge of a hadron, add the charges of its quarks and antiquarks.
Visual Picture
The diagram below gives a simple picture of an up quark inside a proton.
Why the Up Quark Matters
The up quark matters because it is one of the particles that make stable matter possible. Without up quarks, there would be no protons in their familiar form, and the structure of atoms would be completely different.
Together with the down quark, the up quark forms the basic quark content of the nuclei of atoms. For this reason, it is one of the most important elementary particles in all of physics.