What is Elementary Particles ? Details

Elementary particles

The particles which are not made up of other particles are called elementary particles. They are structured less and indivisible. All substances are made up of elementary particles. Each elementary particle is characterized by a well-defined charge, mass, and spin. Every elementary particle has its own antiparticle. If A is a particle then A̅ represents its antiparticle. The antiparticle has the same mass but an opposite charge. Almost all the elementary particles except electrons are unstable.

Pair production and pair annihilation

When an energetic 𝛾-ray photon falls on a heavy substance, it is absorbed by some nucleus of the substance and its energy gives rise to the production of an electron and a positron. Therefore the formation of particle with its antiparticle from the energy is called pair production.

𝛾-photon = electron + positron =₋₁ 𝑒⁰++₁𝑒⁰

According to Einstein’s mass-energy relation

𝐸 = 𝑚₀𝑐²= 9.1 × 10⁻³¹ × (3 × 10⁸ )² = 0.51MeV

Hence for pair production, it is essential that the energy of 𝛾-photon must be at least 2 × 0.51 = 1.02MeV

The converse phenomenon of pair production is also possible which is called annihilation. When a particle comes in contact with its anti-particle, they combine with each other and lose their existence. This process is called annihilation. In this process mass of pair is converted into energy

_-1e⁰ + ₊₁e⁰ = hν + hν = 2𝛾 (1.02MeV)

Classification of elementary particles

Elementary particles can be classified into three groups

1. Leptons

• They are light particles.

• They do not have strong interaction (weak interaction)

• This group contains six particles, electron, electron neutrino, muon, muon neutrino, tau and tau neutrino.

• Each lepton has its anti-particle.

• All are fermions having a spin.

Particle	Symbol	Charge	Anti-particle
Electron	𝑒−	-1	𝑒+
Electron-neutrino	𝜈_𝑒	0	𝜈̅_𝑒
Muon	𝜇−	-1	𝜇+
Muon-neutrino	𝜈_𝜇	0	𝜈̅_𝜇
Tau	𝜏−	-1	𝜏+
Tau-neutrino	𝜈_𝜏	0	𝜈̅_𝜏

2. Quarks

• The elementary particles that make up hadrons are called quarks.

• They are strongly interacting particles.

• There are six quarks.

• Each quark has its anti-quarks.

• All are fermions having spin.

Particle	Symbol	Charge	Anti-particle
Up	𝑢	+ 𝑒	𝑢̅
Down	𝑑	− 𝑒	𝑑̅
Charm	𝑐	+ 𝑒	𝑐 ̅
Strange	𝑠	− 𝑒	𝑠̅
Top	𝑡	+ 𝑒	𝑡̅
Bottom	𝑏	− 𝑒	𝑏̅

3. Mediator particle

• Those particles which mediate interactions among the particles.

• They are boson having integer spin.

Force(interaction)	Particles affected	Range	Relative strength	Elementary particles exchange (mediator) particle
Strong	Quark	≈ 10−15𝑚	1	Gluons
Electromagnetic	Charged particles	∞	≈ 10⁻²	Photons
Weak	Quark and leptons	≈ 10−17𝑚	≈ 10⁻⁵	Intermediate bosons
Gravitational	All	≈ 10−39𝑚		gravitons

Hadrons

Hadrons are heavy particles. They are strongly interacting particles. They can be divided into two groups.

1. Baryons:

Baryons include nucleons and hyperons

i. Nucleons: It includes protons, neutrons, and their anti-particles. All are fermions having spin ½.

ii. Hyperons: The particles of mass greater than that of the nucleon lie on this group. All are fermions having spin half except omega whose spin is 3/2. It includes lambda, sigma, xi, omega and their anti-particles.

particle	symbol	Charge	Anti-particle
Proton	𝑝	1	𝑝̅
Neutron	𝑛	0	𝑛̅
Lambda	Λ0	0	Λ̅0
Sigma	Σ+	1	̅Σ̅̅+̅
	Σ0	0	̅Σ̅0̅
	Σ−	-1	̅Σ̅̅−̅
Xi	Ξ0	0	̅Ξ̅0̅
Xi	Ξ⁻	-1	̅Ξ̅̅−̅
Omega	Ω−	-1	̅Ω̅̅−̅

2. Mesons:

Mass of these particles is greater than that of leptons but less than that of baryons. They all are bosons having spin 0 or 1.

particle	symbol	Charge	Anti-particle
Pion	𝜋⁰	0	Self
	𝜋⁺	1	𝜋−
	𝜋⁻	-1	𝜋+
Kaon	𝑘0	0	Self
	𝑘⁺	1	𝑘−
	𝑘−	-1	𝑘+
Eta	𝜂0	0	Self

Quark combination of hadrons

1. Baryons: A baryon is a three quark system i.e. baryon = 𝑞 𝑞 𝑞 where 𝑞 stands for any quark. Example: proton is made up of two u quarks and a d quark. i.e. 𝑝 = 𝑢𝑢𝑑

Charge= 2/3+2/3-1/3 = 1

Anti-proton 𝑝̅ = 𝑢̅𝑢̅𝑑̅ =-2/3−2/3+1/3 = −1

2. Mesons: A meson is a quark and anti-quark system i.e. meson= 𝑞𝑞̅, where 𝑞 stands for any quark and 𝑞̅ stands for any anti-quark. Example: 𝜋⁺ is made up of a 𝑢 quark and a 𝑑 anti-quark.

i.e. 𝜋⁺= 𝑢𝑑̅ Charge= 2/3+1/3 = 1

Quark combination of baryons and mesons are given below:

Baryons		Quark combination	Charge
	𝑝	𝑢𝑢𝑑	2 2 1 + − = 1 3 3 3
	𝑛	𝑢𝑑𝑑	2 1 1 − − = 0 3 3 3
	Λ0	𝑢𝑑𝑠	2 1 1 − − = 0 3 3 3
	Σ+	𝑢𝑢𝑠	2 2 1 + − = 1 3 3 3
	Σ0	𝑢𝑑𝑠	2 1 1 − − = 0 3 3 3
	Σ−	𝑑𝑑𝑠	1 1 1 − − − = −1 3 3 3
	Ξ0	𝑢𝑠𝑠	2 1 1 − − = 0 3 3 3
	Ξ⁻	𝑑𝑠𝑠	1 1 1 − − − = −1 3 3 3
	Ω−	𝑠𝑠𝑠	1 1 1 − − − = −1 3 3 3

Mesons		Quark combination	Charge
	𝜋⁰	𝑢𝑢̅	2 2 − = 0 3 3
	𝜋⁺	𝑢𝑑̅	2 1 + = 1 3 3
	𝜋⁻	𝑢̅𝑑	2 1 − − = −1 3 3
	𝑘0	𝑑𝑠̅	1 1 − + = 0 3 3
	𝑘⁺	𝑢𝑠̅	2 1 + = 1 3 3
	𝑘−	𝑢̅𝑠	2 1 − − = −1 3 3
	𝜂0	𝑏𝑏̅	1 1 − = 0 3 3