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Leptons and Quarks

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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-
vτ

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
Q/e

Spin

Mean Lifetime
(s)

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

- or n π0

uds

\(\sum\)+

1189

+1

½

0 or n π-

uus

\(\sum\)0

1193

0

½

Ù0Ƴ

uds

\(\sum\)-

1197

-1

½

-

dds

\(\Xi\)0

1315

0

½

Ù0 π0

uss

\(\Xi\)-

1321

-1

½

Ù0 π-

dss

\(\Delta\)++

1232

+2

½

+

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.

  1. Mesons consist of one quark and one anti-quark.
  2. Baryons consist of three quarks.
  3. 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.



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.

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