Quarks and antiquarks (AQA A-Level Physics): Revision Notes

2.1.6 Quarks and Antiquarks

Introduction to Quarks and Antiquarks

Quarks are fundamental particles that combine to form larger particles, such as hadrons. Hadrons, in turn, include baryons and mesons, which are bound by the strong nuclear force. Quarks come in different "flavours," each with distinct properties like charge, baryon number, and strangeness.

Type of Quark	Charge	Baryon Number	Strangeness
Up ($u$)	$+\frac{2}{3}e$	$+\frac{1}{3}$	$0$
Down ($d$)	$-\frac{1}{3}e$	$+\frac{1}{3}$	$0$
Strange ($s$)	$-\frac{1}{3}e$	$+\frac{1}{3}$	$-1$

Note: The antiquarks of these quarks have opposite signs for their charge, baryon number, and strangeness.

Quark Combinations in Baryons and Mesons

1. Baryons:

• Baryons are formed from three quarks.

• Protons and neutrons are examples of baryons. Examples:

• Proton $(uud )$: The quark combination $uud$ gives the proton an overall charge of $+1e$ and a baryon number of $+1$ .

• Neutron $(udd )$: The quark combination $udd$ results in a neutral charge but still has a baryon number of $+1$.

2. Antibaryons:

• Antibaryons are made up of three antiquarks.
• To find the quark combination of an antibaryon, take the corresponding baryon's quarks and replace them with their antiquarks.

3. Mesons:

• Mesons are composed of a quark and an antiquark.
• Examples of mesons include pions and kaons, which have various charges and strangeness values depending on their quark composition.

Example Explanation:

• The $K^+$ meson has a quark composition of $u \bar{s}$, which gives it a charge of $+1$ and a strangeness of $+1$.

Decay of Baryons

All baryons (except protons) are unstable and will eventually decay into protons. For example, neutrons decay into protons through beta-minus decay:

$$n \rightarrow p + e^- + \bar{\nu}_e$$

In this decay process, a neutron converts into a proton, an electron, and an electron antineutrino. This reaction is an example of how particle transformations adhere to conservation laws, including baryon and lepton number conservation.