Quarks and antiquarks

In a nutshell

Quarks are fundamental particles and are the building blocks of hadrons like the proton and neutron. There are six types of quark, as well as their antiquark equivalents. This lesson will discuss the names of each type of quark as well as their properties and it will list the quark combinations that make up some of the more common particles. In addition, it will show how to construct balanced nuclear equations involving quarks.

Definitions

Keyword	definition
Quark	A fundamental particle with six types that combine to form hadrons.
Baryon number	A quantum number that has a value of $1$ for all baryons, $-1$ for antibaryons and $0$ for other particles.
Lepton number	A quantum number that has a value of $1$ for all leptons, $-1$ for antileptons and $0$ for other particles.
Strangeness	A quantum number that has a value of $1$ for particles with strange quarks, $-1$ for particles with strange antiquarks and $0$ for other particles.

Types of quarks

There are six types of quark, each with an antiquark equivalent. Each type has different properties of charge, baryon number and strangeness.

Quark types and properties

Quark	symbol	Charge	Baryon number	Strangeness
$up\ quark$	$u$	$+\dfrac{2}{3}e$	$+\dfrac{1}{3}$	$0$
$down\ quark$	$d$	$-\dfrac{1}{3}e$	$+\dfrac{1}{3}$	$0$
$charm\ quark$	$c$	$+\dfrac{2}{3}e$	$+\dfrac{1}{3}$	$0$
$strange\ quark$	$s$	$-\dfrac{1}{3}e$	$+\dfrac{1}{3}$	$-1$
$top\ quark$	$t$	$+\dfrac{2}{3}e$	$+\dfrac{1}{3}$	$0$
$bottom\ quark$	$b$	$-\dfrac{1}{3}e$	$+\dfrac{1}{3}$	$0$

Physics; Fundamental particles; KS5 Year 12; Quarks and antiquarks

Note: $e$ represents the charge of an electron, equal to $1.6 \times 10^{-19}C$ .

Antiquark types and properties

antiquark	symbol	charge	baryon number	strangeness
$up\ antiquark$	$\overline{u}$	$-\dfrac{2}{3}e$	$-\dfrac{1}{3}$	$0$
$down\ antiquark$	$\overline{d}$	$+\dfrac{1}{3}e$	$-\dfrac{1}{3}$	$0$
$charm\ antiquark$	$\overline{c}$	$-\dfrac{2}{3}e$	$-\dfrac{1}{3}$	$0$
$strange\ antiquark$	$\overline{s}$	$+\dfrac{1}{3}e$	$-\dfrac{1}{3}$	$+1$
$top\ antiquark$	$\overline{t}$	$-\dfrac{2}{3}e$	$-\dfrac{1}{3}$	$0$
$bottom\ antiquark$	$\overline{b}$	$+\dfrac{1}{3}e$	$-\dfrac{1}{3}$	$0$

Quark combinations

Different combinations of the twelve quark types will result in different hadrons. A baryon is produced with three quarks and a meson is produced with a quark-antiquark pair.

Quark makeup of common hadrons

hadron	symbol	quark combination
$proton$	$p$	$uud$
$neutron$	$n$	$udd$
$antiproton$	$\overline{p}$	$\overline{uud}$
$antineutron$	$\overline{n}$	$\overline{udd}$
$pion$	$\pi^+$	$u\overline{d}$
	$\pi^0$	$u\overline{u}\ or\ d\overline{d}$
	$\pi^-$	$d\overline{u}$
$kaon$	$K^+$	$u\overline{s}$
	$K^-$	$s\overline{u}$
	$K^0$	$d\overline{s}$
	$\overline{K}^0$	$s\overline{d}$

Nuclear equations

Nuclear equations are a way of writing out interactions between various particles. In kinematics, it is important for an equation of motion to conserve both energy and momentum. In particle physics, these are still important, but it is equally vital to remember that charge, baryon number, lepton number and strangeness are all to be conserved if an interaction is valid.

Example

Determine if the following interaction is valid.

$n\to p + e^- + \overline{\nu}_e$

The four quantities to consider in an interaction are charge, baryon number, lepton number and strangeness.

First, check to see if charge is conserved. The neutron has a charge of $0$ , so check to see if the right side sums to $0$ . The proton has a charge of $+1$ , the electron has a charge of $-1$ and the antineutrino has a charge of $0$ .

$1-1+0 = 0$

Therefore, charge is conserved.

Next, check to see if baryon number is conserved. The neutron has a baryon number of $+1$ , so the right side should sum to $+1$ . The proton has a baryon number of $+1$ and both the electron and antineutrino have baryon numbers of $0$

$1 + 0 + 0 = 1$

Therefore, baryon number is conserved.

Next, check to see if lepton number is conserved. The neutron has a lepton number of $0$ , so the right side should sum to $0$ . The proton has a lepton number of $0$ , the electron has a lepton number of $+1$ and the antineutrino has a lepton number of $-1$ .

$0 + 1 - 1 = 0$

Therefore, lepton number is conserved.

Finally, check to see if strangeness is conserved. None of the particles have a non-zero value for strangeness, so it is also conserved.

This equation is valid.