Leptons and Quarks
Subject: Physics
Overview
The leptons are the lightweight elementary particles which do not have strong interactions.This note provides us an information about leptons and quarks.Leptons and Quarks
Leptons
The leptons are the lightweight elementary particles which do not have strong interactions. There are six types of leptons. They are electron (e-), the muon(µ-), the tau particle (τ-), electron neutrino (ve), muons neutrino (vµ) and tau neutrino (vτ). Each of the six particles has a distinct antiparticle. All leptons have spin and thus are fermions.
The types of leptons are shown in the table
| Particle Name | Symbol | Anti-particle | Mass (MeV/c2) | Lifetime (s) |
| Electron Electron neutrino | e- ve | e- | 0.511< 3× 10-6 | Stable
Stable |
| Muon Muon neutrino | µ- vµ | µ+ | 105.7<0.19 | 2.20 ×10-6 Stable |
| Tau Tau neutrino | τ- | τ+ | 1777<18.2 | 2.9 ×10-13 Stable |
Hadrons
Hadrons are the strongly interacting particles. Each hadron has an antiparticle. There are two subclasses of hadrons: mesons and baryons. Properties of some hadrons are shown in the table below.
Hadrons and their proporties
| Particle | Mass (MeV/c2) | Charge Ratio | Spin | Mean Lifetime | Quark Content |
| Mesons | |||||
| π0 | 135.0 | 0 | 0 | 8.4 × 10-17 | u \(\overline{u}\), d\(\overline{d}\) |
| π+ | 139.6 | +1 | 0 | 2.60 × 10-8 | u\(\overline{d}\) |
| π- | 139.6 | -1 | 0 | 2.60 × 10-8 | d \(\overline{u}\) |
| K+ | 493.7 | +1 | 0 | 1.24 × 10-8 | u \(\overline{s}\) |
| K- | 493.7 | -1 | 0 | 1.24 × 10-8 | s\(\overline{u}\) |
| \(\eta\)0 | 547.3 | 0 | 0 | =10-18 | u\(\overline{u}\), d\(\overline{d}\), u\(\overline{s}\) |
| Baryons | |||||
| P | 938.6 | +1 | ½ | - | uud |
| n | 939.6 | 0 | ½ | pe- | udd |
| \(\Lambda\) | 1116 | 0 | ½ | pπ- or n π0 | uds |
| \(\sum\)+ | 1189 | +1 | ½ | pπ0 or n π- | uus |
| \(\sum\)0 | 1193 | 0 | ½ | Ù0Ƴ | uds |
| \(\sum\)- | 1197 | -1 | ½ | nπ- | dds |
| \(\Xi\)0 | 1315 | 0 | ½ | Ù0 π0 | uss |
| \(\Xi\)- | 1321 | -1 | ½ | Ù0 π- | dss |
| \(\Delta\)++ | 1232 | +2 | ½ | pπ+ | uuu |
| \(\Omega\)- | 1672 | --1 | ½ | Ù0 π- | sss |
| \(\wedge\)+ | 2285 | +1 | ½ | pK- π- | udc |
Mesons
Mesons are intermediate mass particles. They are heavier than leptons but lighter than baryons. Mesons include pions, kaons, eta particles etc. They are all bosons with spin 0 and 1. There are no stable mesons.
Baryons
Baryons include nucleons and hyperons. They are heavy particles and they have half-integer spin and, therefore, all are fermions. The only stable baryon is a proton.
Quarks
Quarks are fundamental constituents of all the hadrons. They are strongly interacting particles. No isolated existence of quark is discovered so far.
Original Quark Model
According to this model all hadrons are a composite system of two or here fundamental constituents called quark. There are three types of quarks up, down and strange. Each quark has anti-quark of opposite charge, baryon number and strangeness.
Main properties of quarks and anti quarks are given in the table:
| Name | Symbol | Spin | Charge | Baryon Number | Strangeness |
| Quarks | |||||
| Up | u | ½ | +2e/3 | 1/3 | 0 |
| Down | d | ½ | -e/3 | 1/3 | 0 |
| Strange | s | ½ | -e/3 | 1/3 | -1 |
| Anti Quarks | |||||
| Anti-up | \(\overline {u}\) | ½ | -2e/3 | -1/3 | 0 |
| Anti-down | \(\overline{d}\) | ½ | +e/3 | -1/3 | 0 |
| Anti-strange | \(\overline{s}\) | ½ | +e/3 | -1/3 | -1 |
The composition of all hadrons could be completely specified by three simple rules.
- Mesons consist of one quark and one anti-quark.
- Baryons consist of three quarks.
- Antibaryons consist of three antiquarks.
Composition of several baryons and mesons are given below
| Baryons | Quarks composition |
| P | uud |
| n | udd |
| \(\lambda\)0 | uds |
| \(\sum\)+ | uus |
| \(\sum\)- | dds |
| \(\sum\)0 | uds |
| \(\Xi\)0 | uss |
| \(\Xi\)- | dss |
| \(\Omega\)- | sss |
| \(\Delta\)- | ddd |
| \(\Delta\)++ | uuu |
| Mesons | Quarks composition |
| π0 | u |
| π+ | u |
| π- | d |
| K+ | u |
| K- | s |
| K0 | u |
The third quark is needed only to construct charged particle with strangeness.
Charm and Other Recent Development
Consequently, fourth quark was proposed by several physicists in 1967. The fourth quark was given the new property or quantum number called charm.
In 1975 researcher at SU reported strong evidence of the τ lepton. This discovery led to the more elaborate quark model and purposed of new quarks called top (t) and bottom (b)
The properties of overall quark with additional quark.
| Name | Symbol | Spin | Charge | Baryon Number | Strangeness | Charm ness | Bottom ness | Top ness |
| Quarks | ||||||||
| Up | u | ½ | +2e/3 | 1/3 | 0 | |||
| Down | d | ½ | -e/3 | 1/3 | 0 | |||
| Strange | s | ½ | -e/3 | 1/3 | 1 | |||
| Charm | c | ½ | +2e/3 | 1/3 | 1 | 0 | 0 | |
| Bottom | b | ½ | -e/3 | 1/3 | 0 | 1 | 0 | |
| Top | t | ½ | +2e/3 | 1/3 | 0 | 0 | 1 | |
| Anti Quarks | ||||||||
| Anti-up | \(\overline{u}\) | ½ | -2e/3 | -1/3 | 0 | |||
| Anti-down | \(\overline{d}\) | ½ | +e/3 | -1/3 | 0 | |||
| Anti-strange | \(\overline {s}\) | ½ | +e/3 | -1/3 | -1 | |||
| Anti-charm | \(\overline{c}\) | ½ | -2e/3 | -1/3 | -1 | 0 | 0 | |
| Anti-bottom | \(\overline {t}\) | ½ | +e/3 | -1/3 | 0 | -1 | 0 | |
| Anti-top | \(\ overline{b}\) | ½ | -2e/3 | -1/3 | 0 | 0 | -1 |
After the discovery of top quark according to the standard model all the matter is composed of six strongly interacting particles the quarks (u, d, s, c, b, t) and six weakly interacting particles the leptons (e, µ, τ, ne, nµ, nτ) together with their antiparticles.
Reference
Manu Kumar Khatry, Manoj Kumar Thapa, et.al.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.
S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.
Things to remember
The leptons are the lightweight elementary particles which do not have strong interactions.
Quarks are fundamental constituents of all the hadrons.
According to original quark model all hadrons are a composite system of two or here fundamental constituents called quark.
After the discovery of top quark according to the standard model all the matter is composed of six strongly interacting particles the quarks (u, d, s, c, b, t) and six weakly interacting particles the leptons (e, µ, τ, ne, nµ, nτ) together with their antiparticles.
- It includes every relationship which established among the people.
- There can be more than one community in a society. Community smaller than society.
- It is a network of social relationships which cannot see or touched.
- common interests and common objectives are not necessary for society.
© 2019-20 Kullabs. All Rights Reserved.