Mesons (AQA AS Physics) : Revision Note

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Katie M

Last updated

24 April 2025

Pions & The Strong Nuclear Force

Pions

Pions (π–mesons) can be positive $(π^{+})$ , negative $(π^{-})$ , or neutral $(π^{0})$
They contain up and down quarks only, so they have zero strangeness
The antiparticle of the positive pion is the negative pion (and vice versa)
- The neutral pion is its own antiparticle
Pions are the lightest mesons, making them more stable than other types of mesons
- They were originally discovered in cosmic rays and can be observed in a cloud chamber

Table of pions and their properties

Particle	Quark composition	Relative charge	Strangeness
$π^{+}$	$u \bar{d}$	$\frac{2}{3} + \frac{1}{3} = + 1$	$S = 0$
$π^{-}$	$d \bar{u}$	$- \frac{1}{3} - \frac{2}{3} = - 1$	$S = 0$
$π^{0}$	$u \bar{u}$	$\frac{2}{3} - \frac{2}{3} = 0$	$S = 0$
$π^{0}$	$d \bar{d}$	$\frac{1}{3} - \frac{1}{3} = 0$	$S = 0$

Pions as exchange particles

The strong nuclear force, one of the four fundamental interactions, keeps the protons and neutrons bound together in a nucleus
Fundamental interactions are mediated by exchange particles
The pion is the exchange particle of the strong nuclear force between baryons
In a nucleus, protons and neutrons exhibit the strong force by exchanging pions
- Pions are said to mediate (bring about) the strong nuclear force
The pion created is a temporary violation of energy and mass conservation, but since it is a virtual particle, it is not directly observed

Examiner Tips and Tricks

When reading about the strong force, you may also come across a particle called the gluon. This is also an exchange particle of the strong force, but you don't need to know about it for your exam.

For clarity or your own interest, the difference between pions and gluons as mediators of the strong force are:

Gluons are responsible for binding quarks together. This is referred to as the strong interaction
Pions are responsible for binding nucleons together. This is referred to as the strong nuclear force

Collectively, these are referred to as the strong force

Kaon Decay

Kaons

Kaons (K-mesons) can also be positive $(K^{+})$ , negative $(K^{-})$ , or neutral $(K^{0})$
Kaons have a property known as strangeness
- This is because they contain a strange quark, which can be paired with either an up or down quark
The antiparticle of the positive kaon is the negative kaon (and vice versa)
- Unlike the neutral pion, the neutral kaon is not its own antiparticle
Kaons are heavy and unstable and normally decay into pions
They are known to have unusually long lifetimes (compared to other mesons)
- This is characteristic of particles containing strange quarks

Table of kaons and their properties

Particle	Quark composition	Relative charge	Strangeness
$K^{+}$	$u \bar{s}$	$\frac{2}{3} + \frac{1}{3} = + 1$	$S = + 1$
$K^{-}$	$s \bar{u}$	$- \frac{1}{3} - \frac{2}{3} = - 1$	$S = - 1$
$K^{0}$	$d \bar{s}$	$- \frac{1}{3} + \frac{1}{3} = 0$	$S = + 1$
${\bar{K}}^{0}$	$s \bar{d}$	$- \frac{1}{3} + \frac{1}{3} = 0$	$S = - 1$

Kaon production

Kaons can be produced in pairs (pair production) via the strong interaction, where strangeness is always conserved
An example of kaon pair production $(K^{+} / K^{-})$ is through a high-energy proton-proton collision:

$p + p \overset{}{\to} p + p + K^{+} + K^{-}$

The strangeness on both sides is zero, so strangeness is conserved

Kaon decay

Kaons decay through the weak interaction
An example of a kaon decay would be a neutral kaon decaying into a positive pion and a negative pion:

$K^{0} \to π^{+} + π^{-}$

Pions do not contain strange quarks, therefore, strangeness is not conserved in weak interactions

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