Hadrons, Baryons and Mesons

In a nutshell

Hadrons are particles that are made from quarks and are influenced by the strong nuclear force. They are separated into two categories, named baryons and mesons. This lesson will discuss the difference between baryons and mesons, understand what the baryon quantum number is and how it is always conserved and understand the interaction of the strong nuclear force.

Definitions

keyword	definition
Hadron	Particles made from quarks that are influenced by the strong nuclear force.
Quark	A fundamental particle with six types that make up hadrons.
Strong nuclear force	The fundamental force that defines interactions between any particles containing quarks.
Baryon	Hadrons that are made up of three quarks or antiquarks.
Meson	Hadrons that are made up of a quark and an antiquark.
Standard Model	The theory classifying all elementary particles and forces within the universe.

Baryons

Baryons are hadrons that are made up of three quarks and since each quark can be of a different type, there are many different types of baryons. Similarly, a hadron made of three antiquarks is called an antibaryon. The two most common types of baryons are the proton and the neutron, known for making up most of the Universe.

Beyond these, other baryons are not stable and can only be observed in specific high-energy conditions, such as high in the atmosphere as a result of cosmic rays.

Within the Standard Model, the proton is known to be the most stable baryon. A free proton has never been observed to decay and if it did, the proposed lifetime of a proton is many orders of magnitude greater than the age of the Universe.

All other baryons will decay, eventually decaying into protons. Even the neutron, when it is isolated, will only last for around $15\ minutes$ before decaying. Typical baryons have lifespans between $10^{-10}\,s$ and $10^{-24}\,s$ .

Baryon quantum number

All baryons are given a baryon quantum number of $1$ , while all antibaryons have a baryon quantum number of $-1$ . Any other particle has a baryon quantum number of $0$ . These baryon quantum numbers allow an interaction between baryons to be correctly tracked, as baryon quantum number is always conserved.

Example

Show that baryon quantum number is conserved during electron capture.

Electron capture is given by the following interaction:

$p + e \to n+ \nu_e$

where $p$ denotes a proton, $e$ denotes an electron, $n$ denotes a neutron and $\nu_e$ denotes an electron neutrino. To validate if the baryon number is conserved, check both sides and see if the resulting baryon number totals are the same.

$1+0 \to 1 + 0$

The proton and neutron are the only particles with a baryon number of $1$ , thus the both sides have a total baryon number of $1$ . This means that the baryon number is conserved.

Mesons

Contrary to baryons, mesons are particles that are made up of a quark and antiquark pair and have a baryon quantum number of zero. Mesons are typically seen in high-energy conditions, such as within a particle detector, but some do appear as a result of high-energy cosmic rays hitting the upper atmosphere.

The pion (given the symbol $\pi$ ) is one example of a meson produced in this way. The pion is a particle that is exchanged between protons and neutrons within atomic nuclei. Due to the pion also being made of quarks, they are susceptible to the strong nuclear force much like protons and neutrons.

The strong interaction between protons and neutrons is caused by particle exchange. As the pion is the particle exchanged in this interaction, it is called a 'force carrier' of the strong nuclear force. This means that the strong force is passed between nucleons only by the exchange of the pion.

The kaon (given the symbol $K$ ) is another example of a meson produced in cosmic ray interactions with the atmosphere. Kaons are unstable, with lifetimes of only about $10^{-8}\,s$ to $10^{-10}\,s$ and are known to decay through the weak interaction back into pions.

Physics; Fundamental particles; KS5 Year 12; Hadrons, Baryons and Mesons