Quarks and anti-quarks

In a nutshell

Quarks are fundamental particles that make up all hadrons. The standard model has six quarks and six anti-quarks and their charges are fractions of the elementary charge $e$ . Baryons are particles with three quarks and mesons are particles with two quarks.

Quarks and charges

Quarks are particles that make up all hadrons and just like leptons they are considered to be fundamental particles as they cannot be split further. There are six quarks and six anti-quarks in the standard model, each with its own symbol and a charge that is a fraction of the elementary charge $e$ as shown in the table below:

Quark	symbol	charge ( $e$ )	anti-quark	symbol	charge ( $e$ )
up	$u$	$+\dfrac{2}{3}$	anti-up	$\overline{u}$	$-\dfrac{2}{3}$
down	$d$	$-\dfrac{1}{3}$	anti-down	$\overline{d}$	$+\dfrac{1}{3}$
charm	$c$	$+\dfrac{2}{3}$	anti-charm	$\overline{c}$	$-\dfrac{2}{3}$
strange	$s$	$-\dfrac{1}{3}$	anti-strange	$\overline{s}$	$+\dfrac{1}{3}$
top	$t$	$+\dfrac{2}{3}$	anti-top	$\overline{t}$	$-\dfrac{2}{3}$
bottom	$b$	$-\dfrac{1}{3}$	anti-bottom	$\overline{b}$	$+\dfrac{1}{3}$

Note: at this level you only need to remember the charges of up, down, strange and their antiparticles.

Making hadrons

Hadrons are divided into baryons and mesons. Baryons are particles composed of three quarks while mesons are particles composed of a quark anti-quark pair. The charge of the hadron depends on the quark combination that makes it up.

Physics; Particle physics; KS5 Year 12; Quarks and anti-quarks

Protons and neutrons

Protons and neutrons are both baryons and are found in the nucleus of atoms.

Example

A proton has a quark combination of $uud$ .

Show that the charge of a proton is $+ 1 \ e$ .

Firstly, write down the quarks and their charges:

$\begin{aligned}u = + \dfrac {2}{3} \ e \\ d = - \dfrac {1}{3}\ e \end{aligned}$

Next, add them in the combination $uud$ and calculate the charge of the particle:

$u + u + d$

$Q = (+ \dfrac{2}{3}) e + (+ \dfrac{2}{3}) e (- \dfrac{1}{3}) e = +1 \ e$

The charge on a proton is $\underline{+ 1 \ e}$ .

Example

A particle has a quark combination of $udd$ .

What is the charge of the particle? Suggest what this particle could be.

Firstly, write down the quarks and their charges:

$\begin{aligned}u&=+\dfrac{2}{3} \ e\\d&=-\dfrac{1}{3} \ e \end{aligned}$ 

Next, add them in the combination $udd$ and calculate the charge of the particle:

 $u + d + d$ 

$Q = (+ \dfrac{2}{3}) e + (- \dfrac{1}{3}) e + (- \dfrac{1}{3}) e = 0 \ e$ 

The charge of the particle is $\underline {0 \ e}$ . This is a neutron particle as it has a neutral charge and a quark combination of $udd$ .