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3. Fudamental Interactions and associated particles

4. Classes of elementary particles

5. Leptons

6. Hadrons

7. Quarks

8. The Standard Model

9. The Higgs boson

10. Dark Matter


12. Future of particle Physics Research


One of the prominent research areas, which have gained a lot of attention of the scientific community as well as that of the general public is the study of the basic structure of matter. ‘Elementary particle physics’ as it is called is a balancing act of theoretical predictions and experimental confirmations.The widespread attention and media coverage on the latest discoveries which are being announced frequently by CERN, the best particle physics study centre in the world, is undoubtedly due to the inherent interest and curiosity of human beings to understand the framework of the world they are in. This book provides an overview of the things that are going on in the micro world, to an audience who may not be specialised in science, but are interested in understanding the study of ‘matter’ with a little effort. Special emphazis is given to ‘ particle accelarators’ section, which is the practical side of particle physics research.


Protons and neutrons are the macro constituents of the atomic nucleus.Nuclei of the same element have the same number of protons, but can have different number of neutrons.The varieties of an element that differ in the numbers of neutrons their nuclei contain are called isotopes. The protons and neutrons are held together in the nuclei by a short range attractive force called strong force.This force is powerful enough to overcome the electric repulsion of the positive charged protons, provided the neutrons are also present to help. The atomic number (Z) of an element is the number of protons its nucleus pocess, and this will be the same as the number of electrons in that atom, since the atom is electrically neutral.The atomic mass refers to the mass of neutral atoms, not of the bare nuclei. Thus the atomic mass always includes the masses of its Z electrons. A mass number (A) is associated with each element, which is the sum of the number of protons and the number of neutrons in the atom of the element. Atomic masses are expressed in a unit called a.m.u (atomic mass unit) .It is found that the volume of a nucleus is directly proportional to the number of nucleons it contains, which is its mass number.When ever the number of nuterons (indicated by N) is very different from number of protons (Z) in a nucleus, it makes the nucleus unstable.

Nuclear Decay

Nuclear decay occurs in an unstable nuclei, till a final stable one is formed.Nuclear decay occurs in three modes viz. alpha decay, beta decay (positive and negative), and electron capture.Nuclear deacy is not at all a straight forward approach.This involves a lot of elementary particles which are therefore part of nuclear structure. Hundreds of elementary particles have been discovered so far.

3. Fudamental Interactions and associated particles

The Fudamental physical forces in the universe are strong force, electromagnetic force weak force and gravitational force. Force results due to interactions among particles.It is postulated to be occuring by the exchange of another particles. The following table gives a rough picture of the fundamental forces, the details will be dealt with later.

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4. Classes of elementary particles

Elementary particles fall into two classes - leptons and hadrons, depending on whether they respond to the strong nuclear force between nucleons.hadrons do respond while leptons do not.Leptons are truly elementary with no hint of internal structures.The electron is a typical example.Hadrons (eg. protons, nuetrons) occupy space and are composed of either two or three quarks, which like leptons are structureless and elementary as present measurements can establish.The strong force that acts between hadrons is the external manifestation of the more basic interactions among the quarks they contain.Hadrons that consist of three quarks, such as proton and neutron are called baryons, and those contain two quarks such as pion are called mesons.There are convincing evidence that quarks exist, but it is not possible to isolate them from hadrons.

5. Leptons

The lepton called electron was the first elementary particle for which a satisfactory theory was developed.It was paul dirac (1928) who obtained a relativistically correct wave equation for a charged particle in an electromagnetic field.An unexpected result of Dirac’s theory was its requirement that an electron can have negative as well as positive energies.But the very existence of stable atoms disprove negative energy of electrons. Dirac rescued his theory by postulating that all negative energy states are normally filled. So by a famous principle called Pauli’s exclusion principle, it prevents any other electron from dropping into this negative energy ‘sea’. But if any electron is given enough energy (by lighting, or when interacted with photons), then this electron can jump out of this negative energy sea, and becomes a positive energy electron. This leaves behind a hole in the negative energy sea, an entity which behaves as a positive charge.The entire process is that a gamma ray (photon) materialises into an electron and positive charged particle. This positive charged particle was later discovered and has been called positron, the anti particle of electron. The other leptons are the muon and tau, and the neutrinos associated with them.

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Spin’ property of elementary particles

In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles.

All elementary particles of a given kind have the same spin quantum number, an important part of the quantum state of a particle., The spin of electrons results in the Pauli’s exclusion principle Wolfgang Pauli was the first to propose the concept of spin, but he did not name it. In 1925, Ralph Kronig, George Uhlenbeck, and Samuel Goudsmit suggested a physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927. When Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it.

There are two types of angular momentum in quantum mechanics: Orbital angular momentum, which is a generalization of angular momentum in classical mechanics (L = r × p), and spin, which has no analogue in classical mechanics. Since spin is a type of angular momentum, it has the same dimensions: Js in Si units In practice, however, SI units are never used to describe spin: instead, it is written as a multiple of the reduced Planck constant ħ. In natural units, the ħ is omitted, so spin is written as a unit less number. The spin quantum numbers are always unit less numbers by definition.

Neutrino and anti neutrino

A neutrino is one of the elementary particles, a lepton with zero charge, spin ½ and extremely small mass. Neutrinos come in three varieties, each associated with an electron-like lepton: the electron neutrino, the muon neutrino and the tau neutrino.

The need for neutrinos (sticky electron neutrinos) was first pointed out by Wolfgang Pauli, in 1930, to explain the missing energy in beta decay (Also to conserve the linear momentum) . Later in 1956 neutrino were proved to exist, and in 1980’s cosmologists started to explore the possibility that neutrinos may make up some of the dark matter in the universe.Lacking charge and mass, and not electromagnetic in nature as the photo is, the neutrino can pass unimpeded through vast amounts of matter.A neutrino would have to pass through over 100 light years of solid iron on the average before interacting.

Distinction between neutrino and anti neutrino

The spin of the neutrino is anticlockwise, when viewed from behind, while the spin of antineutrino is clockwise.Experiments on them showed that, neutrinos and anti neutrinos are distinguishable, having right handed and left handed spins respectively- on all other things they are the same.

6. Hadrons

Unlike leptons, hadrons are subjected to the strong interaction and they have known internal structures.They are of two types mesons and baryons as already mentioned.Mesons consists of a quark and an anti quark, about 140 types are known.Baryons consists of three quarks, about 120 types are known.The lightest baryon is the proton, which is also the only apparantly stable hadron in free space.All baryons other than nucleons decay with mean lives of less than 10-[9] seconds, in a variety of ways, but the end result is always a proton or neutron.

Resonance particles.

Most hadrons have a measurable time period existence.But some experiments prove that there are also hadrons, which are very short lived, that it is difficult to measure their life time directly.They are called resonance particles.. The lifetime of these particles is on the order of 10-[23] seconds. Traveling at the speed of light, these particles could only travel about 10-[15] meters, or about the diameter of a proton, before decaying. Distances of this magnitude cannot be measured in bubble chambers or any other device for detecting subatomic particles. How can we know anything about particles we cannot detect? To understand how we deduce properties of resonance particles, it is first necessary to examine another, more complicated, explanation of their existence. This explanation involves the ‘scattering cross-section ‘of a particle. When two particles move towards each other and collide, it is possible to say that the collision was caused by the cross-section of the particles. The greater the cross-section of the particles is, the more likely it is that there will be a collision. So, if we have two beams of particles, the amount of scattering that occurs is related to the cross-section of the particles that make up the beams (this is a simplification, but it helps to understand resonances) . We can measure the cross-section of a particle by knowing how much scattering occurs when two beams of particles collide.




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overview particle physics




Titel: An Overview of Particle Physics