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#### The Standard Model

The theory that describes the properties and structure of the fundamental particles as well on their interactions is called, “The standard Model”. This theory assumes that the matter is made up of the two kinds of particles leptons and quarks to which we add bosons that carry the interactions between particles.

According to the theory there are six leptons and six quarks all with spin ½ and therefore all are fermions obeying Pauli’s exclusion principle. The six leptons are electron (e), muon (µ), tau (τ) all with charge –e and the three corresponding neutrinos: electron neutrino (ue), muon neutrino (uµ), tau neutrino (uτ). Thus, we have three groups of leptons:
\begin{matrix} \text{Charge}\:\:\:\:\:\: 0 && \text {v}_e && \text{v}_{\mu} &&\text {v}_{\tau} \\\text {Charge} \: -e &&\text {e} &&\mu &&\tau\end{matrix}

Similarly theory assumes six quarks up(u), down(d), charm(c), strange (s), bottom (b) and top (t). thus we have three group of quarks

\begin{matrix} \text{Charge}\:\:\:\: 2e/3 && \text{u}& &\text {c} &&\text {t}\\ \text {Charge} \: -d/3& &\text {d} &&\text {s}& &\text {b}\end{matrix}

On the basis odd standard model fundamental particles are classified as follows:

##### Explanation of some important phenomena

Residual interactions

Two colourless hadrons feel practically no color force when they are far apart. But if two hadrons come close to each other, the colored quarks in each may feel the color force from the quarks in the other. This force is called residual interactions. Nuclear force between protons and neutrons is due to residual effect.

Magnetic Moment of Neutron

Neutrons have zero electric charges but have a magnetic moment that results from the combination of the magnetic moments of the three quarks.

Gluons and Quantum Chromodynamics

The colour force or strong interaction between quarks is mediated by bosons called gluons which suppose to be massless, spin 1 and carry a colour charge but no electric charge. The theory dealing with colour carrying particles is called quantum chromodynamics.

Interaction of Leptons and Quarks

Leptons which possess no colour charge interacts among themselves and with quarks only through electromagnetic and weak interactions. Both interactions can be combined together which is called the electroweak theory. Accordingly electron-positron annihilation is not limited to the production of two photons by e+ + e $$\longrightarrow$$\ $$\gamma$$ + $$\gamma$$, which is purely electromagnetic process.

If the

Β-decay

In this process due to strong interaction quark can change the flavour but not colour through the action of weak bosons. Example: $$\beta \: \text {decay}: d \rightarrow \:u + \beta$$

Confinement:

Once quarks or antiquarks combine to form hadron, they cannot be separated, a property called confinement. The nature of gluon is such that it produces a force between quarks that increases very rapidly with distance. This explains why there are no free quarks in the universe. They were all confined within hadrons shortly after the big bang.

#### Universe

All matter, energy, and space that exists, is called the universe. The branch of physics which deals with the study of the universe is called astronomy. The branch of astronomy, which deals with physical processes connected with the celestial bodies and the intervening region of space, is called astrophysics. The study of the origin, evolution, and nature of the universe is called cosmology.

##### The Solar System

The solar system, in which we live, is part of the universe which includes the planets, asteroids, and comets revolving round the sun, in an elliptical orbit.

##### The Sun

Sun is a star which mainly contains extremely hot hydrogen gas and radiates energy in all directions. The temperature of the outer region is photosphere which is about 6000 K and has a diameter of 1.4 ´ 109 m. the diameter of the invisible part called chromospheres, is much greater than this. The mean distance between the sun and earth is 1.496 ´ 1011 m. and is called one astronomical unit (A.U.). The mass of the sun is 2 ´ 1030 kg which is called one solar mass.

##### The planets

There are eight planets revolving round the sun in their elliptical orbits. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn and Neptune. Mercury is the nearest to the sun while Neptune is the farthest one.

##### Comets

Comets are astronomical bodies, moving round highly elongated elliptical orbits. It consists of frozen gases, like ammonia, methane, water and nuclei of solid particles. When moving near the sun, a comet has a head and a tail. Substances like water in the comet get vaporised and the radiation pressure forces the vapour away in the shape of the tail.

##### Asteroids

Minor planets revolving in elliptical orbit around the sun, mostly between the orbits of mars and Jupiter, in the same plane as that of the earth are called asteroids. The diameter of the asteroids varies from 1.6 km to 1000 km. Among the ten thousand asteroids Ceres is the largest with the size of 1000 km, Pallas about 600 km and Vesta is about 540 km.

##### Meteor, Meteoroids, and Meteorites

Meteor is an object orbiting the sun which when enters the earth’s atmosphere is vigorously accelerated, due to gravity and becomes incandescent. Meteors are collectively called as meteoroids. When these objects come close to the earth’s surface they are heated to high temperatures due to friction and look like bright lines of fire and are called shooting stars. Some of the meteoroids survive while passing through the earth surface and they hit the surface. They are called meteorites.

##### The stars

A star is a self-luminous celestial body which converts nuclear energy into heat and light through nuclear fusion. There are billions of stars in the universe which are not uniformly distributed but collected together in the groups, called galaxies.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, .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 theory that describes the properties and structure of the fundamental particles as well on their interactions is called, “The standard Model”.

The nuclear force between protons and neutrons is due to residual effect.

he theory dealing with color carrying particles is called quantum chromodynamics.

In beta decay process due to strong interaction quark can change the flavor but not color through the action of weak bosons.

Once quarks or antiquarks combine to form hadron, they cannot be separated, a property called confinement.

The branch of astronomy, which deals with physical processes connected with the celestial bodies and the intervening region of space, is called astrophysics.

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