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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 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

𝜈𝑒

0

𝜈̅𝑒

Muon

𝜇

-1

𝜇+

Muon-neutrino

𝜈𝜇

0

𝜈̅𝜇

Tau

𝜏

-1

𝜏+

Tau-neutrino

𝜈𝜏

0

𝜈̅𝜏

 

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

̅Σ̅̅̅

Xi 

Ξ0

0

̅Ξ̅0̅

 Ξ

-1

 ̅Ξ̅̅̅

Omega 

Ω

-1

̅Ω̅̅̅

 

 

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


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