Quark charges and antiquarks

8 minAS Level11.2.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can state the charge of any quark or antiquark in units of $e$ , and add quark charges to find the total charge of a combination.

Definition

Quark charges

Charges are quoted in units of the elementary charge $e = 1.60 \times 10^{-19}\ \text{C}$ . Quark charges are fractions of $e$ :

flavour	symbol	charge
up	$\text{u}$	$+\tfrac{2}{3}$
charm	$\text{c}$	$+\tfrac{2}{3}$
top	$\text{t}$	$+\tfrac{2}{3}$
down	$\text{d}$	$-\tfrac{1}{3}$
strange	$\text{s}$	$-\tfrac{1}{3}$
bottom	$\text{b}$	$-\tfrac{1}{3}$

Notice that you do not have to remember six separate numbers. The pattern is by type:

up-type quarks (up, charm, top) carry $+\tfrac{2}{3}$ ;
down-type quarks (down, strange, bottom) carry $-\tfrac{1}{3}$ .

Why fractions are allowed

Every particle you can isolate carries a whole number of elementary charges: an electron is $-1e$ , a proton is $+1e$ .

A quark is the one exception, and you can never catch one on its own. Quarks always come in combinations whose fractional charges add to an integer, so the charge you can ever measure is a whole number. The fractions are hidden inside the composite particle.

Antiquarks flip the sign

Every quark has an antiquark with the same mass and the opposite charge. Its symbol gets a bar: $\bar{\text{u}}$ is the up antiquark.

antiquark	charge
$\bar{\text{u}}$ , $\bar{\text{c}}$ , $\bar{\text{t}}$	$-\tfrac{2}{3}$
$\bar{\text{d}}$ , $\bar{\text{s}}$ , $\bar{\text{b}}$	$+\tfrac{1}{3}$

So the antiparticle of a $+\tfrac{2}{3}$ quark is a $-\tfrac{2}{3}$ antiquark, and the antiparticle of a $-\tfrac{1}{3}$ quark is a $+\tfrac{1}{3}$ antiquark. Nothing else changes: only the sign.

Worked example: Example 1: adding three quark charges

Find the total charge, in units of $e$ , of the combination $\text{u} + \text{u} + \text{d}$ .

Have a go first?

Check 1 — the charge table

Multiple choiceQuestion 1 · Recall

Answer the 3 questions above to continue.