Classification of Particles

Classification of Particles

Key Words

Hadron, Meson, Baryon, Lepton

In the exam you are expected to know about:

Hadrons: baryons (proton, neutron) and antibaryons (antiproton and antineutron) and mesons (pion, kaon).
Hadrons are subject to the strong nuclear force.
The proton being the only stable baryon into which other baryons eventually decay; in particular, the decay of the neutron should be known.
Leptons: electron, muon, neutrino (electron and muon types).
Leptons being subject to the weak interaction.
Baryon numbers for the hadrons. Lepton numbers for the leptons will be given in the data booklet.

Leptons

There are six particle-antiparticle pairs known. Leptons (Greek – “light thing” or “small coins”) are the smallest of the fundamental particles. They have the following properties:

fundamental particles without structure
they interact by the weak interaction. If they are charged, they interact by the electromagnetic interaction, but NOT the strong interaction.
charge and lepton number are conserved in all allowed lepton processes.

There are three categories of lepton number, L_e, L_m, and L_t. Each lepton has a lepton number, 0 or 1, in each category, and each antilepton has a number 0 or -1 in each category. You need to know the lepton numbers.

The names of the leptons are:

*Lepton*	*Symbol*	*Charge*	*Lepton Number*
electron	e-	-1e	L_e= 1, L_m, & L_t = 0
electron neutrino	n_e	0	L_e= 1, L_m, & L_t = 0
muon	m-	-1e	L_m= 1, L_e, & L_t = 0
neutrino	n_m	0	L_m= 1, L_e, & L_t = 0
tau	t-	-1e	L_t= 1, L_m, & L_e = 0
tau neutrino	n_t	0	L_t= 1, L_m, & L_e = 0

Each particle has an antiparticle; for the electron, it is the positron, the muon the antimuon, and the tau, the antitau. We show the anti-particle either by an opposite charge (e+) or by putting a bar across the symbol.

Hadrons

There are a very large number of particles that are classified as hadrons, which are subdivided into two further classifications, the mesons, and the baryons.

Hadrons interact by the strong, weak, and electromagnetic force.
They are not fundamental particles but have a structure.
They have non-zero rest masses, about 1 GeV/c²
They have an associated value of charge, Q,and baryon number B.

Hadrons with zero baryon number are called mesons; those with baryon number of 1 are called baryons.

Mesons

These particles have a smaller rest mass than the baryons (and a lower rest mass than the tau lepton). They have:

Zero baryon number.
Short lifetimes.
Antiparticles

Here are a few mesons:

*Name*	*Symbol*	*Q B*	*Lifetime (s)*	*Antiparticle*
Pion	p⁰	0 0	0.8 x 10^-16	Itself
	p+	1 0	2.6 x 10^-8	p-
Kaon	K+	1 0	1.2 x 10^-8	K-
	K⁰	0 0	8.9 x 10^-11 5.2 x 10^-8

Notice how short the lifetimes are of these mesons.

Question 1

Why does the neutral pion seem to have a particularly short lifetime?

ANSWER

We should note the following:

Mesons have TWO quantum numbers that must be conserved in interactions. The charge is denoted by Q, the baryon number by B. Mesons have a baryon number of 0.
Mesons have a lepton number of 0. This must be conserved in any interactions with leptons.

Here is a typical decay:

Notice the conservation of charge and baryon number.

Here are some more:

Question 2

Show that this interaction can proceed:

p+ ---> m+ + n_m

ANSWER

Baryons

These are the heavyweights of particle physics, and include the familiar proton and neutron.

They are made up of three quarks
They have quantum numbers such as charge and baryon number, which must be conserved in interactions.

Let us look at the properties of the baryons:

*Name*	*Symbol*	*Q B*	*Lifetime (s)*	*Antiparticle*
Proton	p	1 1	stable	p
Neutron	n	0 1	898	n
Lambda	L⁰	0 1	2.6 x 10^-10	L⁰
Sigma	S+	1 1	0.8 x 10^-10	S+
	S⁰	0 1	7.4 x 10^-20	S⁰
	S-	-1 1	1.5 x 10^-10	S-
Omega	W-	-1 1	0.8 x 10^-10	W+

Typical Decay

The proton is the only stable baryon. All the others spontaneously decay, although the neutron within a nucleus is stable, apart from beta decay. The decay times are incredibly short, except the isolated neutron which takes about 8 to 10 minutes. Baryons decay to protons, either directly (S+ --> p + p⁰) or indirectly (W- --> L⁰ + K, then L⁰ --> p + p-). Mesons decay to photons or leptons.

Question 3

Show that this decay is possible:

L⁰ -----> p+ + p-

ANSWER

As in radioactivity, the decay of particles is random. The values quoted are the mean lifetimes, not half-lives.

It is believed that gluons are shuttled backward and forward between the quarks like rugby footballs.

Mesons bind the baryons together with the strong force.

The proton is the only baryon that is stable in isolation. The neutron on its own decays to a proton by beta minus decay after about 14 minutes. The decay is as a result of the weak interaction that occurs within nucleons.

Symbol

Charge