Meson

Key Stage 5

Meaning

Mesons are a type of hadron consisting of a quark-antiquark pair.

About Mesons

Mesons have a very short half life, decaying via the weak nuclear interaction or though the quark-antiquark pair annihilating one another.

Mesons consisting of the quarks; up-quark, down-quark and strange-quark are divided into two main categories; π-mesons and k-mesons.

K-mesons have a strangeness of +1 or -1. However, π-mesons do not carry strangeness.

Mesons are affected by the:

• Strong Nuclear Interaction
• Weak Nuclear Interaction
• Electromagnetic Interaction
• Gravitational Interaction

Determining Meson Composition

NB: You do not need to memorise the exact composition of a given meson. However, you should be able to determine a possible quark composition given the charge and strangeness of the meson and by remembering the properties of the up-quark, down-quark and strange-quark.

Mesons can be shown in the following diagram:

The composition of these mesons can be determined given their charge and strangeness.

Examples

This table is for referencing the properties of the quarks.

Quark	Charge	Strangeness
Up-quark	\(q=+\frac{2}{3}\)	\(S=0\)
Antiup-quark	\(q=-\frac{2}{3}\)	\(S=0\)
Down-quark	\(q=-\frac{1}{3}\)	\(S=0\)
Antidown-quark	\(q=+\frac{1}{3}\)	\(S=0\)
Strange-quark	\(q=-\frac{1}{3}\)	\(S=-1\)
Antistrange-quark	\(q=+\frac{1}{3}\)	\(S=+1\)

Determine the composition of the π+	Determine the composition of the K-
Strangeness π+ has a strangeness of 0, therefore it contains no strange-quark or antistrange-quark. Charge π+ has a positive charge. This can be achieved with one charge of \(+\frac{2}{3}\) (an up-quark) and \(+\frac{1}{3}\) (an antidown-quark). Since \(+1 = +\frac{2}{3} + \frac{1}{3}\) Therefore \(π^+ = u + \bar{d}\) An up-antidown pair.	Strangeness K- has a strangeness of -1, therefore it contains a strange-quark. Charge \(-1 = -\frac{1}{3} + x\) K- has a negative charge. This can be achieved with one its strange-quark with a charge of \(-\frac{1}{3}\) and another charge of \(-\frac{2}{3}\) (an antiup-quark). Since \(-1 = -\frac{1}{3} + -\frac{2}{3}\) Therefore \(K^- = S + \bar{u}\) A strange-antiup pair.