G. Rajasekaran

Dirac and the discovery of anti-electron

Paul Adrian Maurice Dirac discovered the correct quantum mechanical equation for the electron in 1928. This was an important discovery. For, this
Dirac equation succeeded in combining  the two great revolutions of that century, namely relativity and quantum mechanics.

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Through his equation Dirac discovered the truth that there exists positron which is the antiparticle of electron. Both electron and positron have the same mass. Although their electrical charges have equal magnitudes, electron has negative charge while positron's charge is positive. Carl Anderson of CALTECH discovered the positron in a cosmic-ray experiment in 1932. In the same way, all elementary particles have their antiparticles. It is now regarded as an important law of relativistic quantum mechanics.

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When electron and positron collide, at what angles both will emerge? Homi Jahangir Bhabha calculated this correctly. This Bhabha scattering formula is even today used as a tool in high energy physics experiments.

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Left-right asymmetry and matter-antimatter asymmetry

 

In the early 1950's Richard Henry Dalitz discovered that K meson decay into three pi mesons exhibits a different left-right parity from its decay into
two pi mesons. It is this discovery that led to the discovery of left-right asymmetry in weak interactions by TD Lee, CN Yang and CS Wu in 1956. Recognizing the importance of this fundamental discovery the Nobel Committee awarded the Nobel Prize to Lee and Yang almost immediately.

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In the same weak interactions, there is no matter-antimatter symmetry either. But it was thought that if the left to right transformation and matter to antimatter transformation are made at the same time, in this joint transformation, there may be symmetry. If we denote the left to right transformation by the letter P and the matter to antimatter transformation by C, the joint transformation is CP. So it was thought that there may be CP symmetry.

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But in the year 1964, Cronin and Fitch experimentally discovered CP asymmetry. Because of its importance Nobel Prize was awarded for this discovery also.

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How did antimatter disappear from the Universe?

 

The Universe was born in the Big Bang explosion 13.7 billion years ago. In that explosion both particles and antiparticles must have been produced in equal number. But the present-day Universe is made of particles only. How did the antiparticles disappear? This is an important puzzle in Cosmology.

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Sacharov is a famous Russian physicist who is regarded as the father of the hydrogen bomb in Russia. He formulated a set of conditions under which antiparticles may disappear from the universe. One of the conditions is particle-antiparticle asymmetry. 

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We mentioned that Cronin and Fitch had already discovered particle-antiparticle asymmetry in experiment. How to incorporate it into the Standard Model of High Energy Physics?

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Standard Model and three generations

 

As a culmination of hundred years of Fundamental Physics, we have reached Standard Model. This incorporates the three fundamental forces of nature: strong, electromagnetic and weak. (Details of the Standard Model can be obtained from my essay on "Higgs boson and Standard Model"). The fundamental particles are given below:

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The number 1,2,3 denote generations. There are 3 generations of fermions. Kobayashi and Maskawa discovered that Standard Model will have CP asymmetry only if there are at least three generations of fermions. They made this discovery even before most of the particles of the second and third generations were experimentally found. Kobayashi and Maskawa were
awarded the Nobel Prize.

 

Majorana particles

 

A particle and its antiparticle have opposite charges. Electron and positron have opposite charges. But neutrinos do not have charges. So there is a possibility that neutrino and antineutrino are the same particle. If the particle and antiparticle are different, it is called a Dirac particle. If particle and antiparticle are the same, it is called a Majorana particle. (The Italian physicist Ettore Majorana disappeared in his young age. Nobody knows what happened to him. Even after 80 years, this puzzle has not been solved.)

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Is neutrino Dirac particle or Majorana particle?

In recent times, many things have been discovered in neutrino research. In the year 1998 it was discovered that neutrino also has mass. This discovery is expected to lead to important changes in Physics and Astronomy.

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Neutrino research has not found out whether neutrino is a Dirac or Majorana particle. This is the most important question in neutrino research.

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India-based Neutrino Observatory (INO) is planned to be constructed in the Theni District of Tamil Nadu. Research on this question also will be taken up at INO.

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Antimatter bomb

If matter and antimatter collide, they annihilate each other and the whole mass is converted into energy. E = mc^2. The destructive power will be unimaginably larger than that of a hydrogen bomb.

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But nobody has found a way of storing antimatter in large enough quantity. At the CERN laboratory in Switzerland, antimatter is being stored in small quantity for research.

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Using this stored antimatter as an antimatter bomb, Dan Brown has made an interesting novel. In that famous novel called "Angels and Demons", one can find a lot of information
about CERN.