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A great and very useful lecture on Modern Physics. These lecture slides include: Elementary Particles, Building Blocks of Matter, Positron, Antiparticles, Feynman Diagram, Fundamental Interactions, Graviton, Classification of Elementary Particles, Higgs Boson, Mesons, Particles and Lifetimes, Additional Conservation Laws, Strangeness, Hypercharge, Quarks
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If I could remember the names of all these particles, I’d be a botanist.
We began our study of subatomic physics in Chapter 12. We investigated the nucleus in Chapters 12 and 13. We now delve deeper, because finding answers to some of the basic questions about nature is a foremost goal of science: What are the basic building blocks of matter? What is inside the nucleus? What are the forces that hold matter together? How did the universe begin? Will the universe end, and if so, how and when?
In 1930 the known elementary particles were the proton, the electron, and the photon. Thomson identified the electron in 1897, and Einstein’s work on the photoelectric effect can be said to have defined the photon (originally called a quantum ) in 1905. The proton is the nucleus of the hydrogen atom. Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron, and Carl Anderson identified the positron in cosmic rays.
Dirac in 1928 introduced the relativistic theory of the electron when he combined quantum mechanics with relativity. He found that his wave equation had negative, as well as positive, energy solutions. His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies. If enough energy is transferred to the “sea”, an electron can be ejected with positive energy leaving behind a hole that is the positron , denoted by e + .
The Japanese physicist Hideki Yukawa had the idea of developing a quantum field theory that would describe the force between nucleons analogous to the electromagnetic force. To do this, he had to determine the carrier or mediator of the nuclear strong force analogous to the photon in the electromagnetic force which he called a meson (derived from the Greek word meso meaning “middle” due to its mass being between the electron and proton masses).
Yukawa’s meson , called a pion (or pi-meson or π- meson), was identified in 1947 by C. F. Powell (1903–
Figure 14.3: A Feynman diagram indicating the exchange of a pion (Yukawa’s meson) between a neutron and a proton.
We have learned that the fundamental forces act through the exchange or mediation of particles according to the quantum theory of fields. The exchanged particle in the electromagnetic interaction is the photon. All particles having either electric charge or a magnetic moment (and also the photon) interact with the electromagnetic interaction. The electromagnetic interaction has very long range.
It has been suggested that the particle responsible for the gravitational interaction be called a graviton. The graviton is the mediator of gravity in quantum field theory and has been postulated because of the success of the photon in quantum electrodynamics theory. It must be massless, travel at the speed of light, have spin 2, and interact with all particles that have mass- energy. The graviton has never been observed because of its extremely weak interaction with objects.
14.3: Classification of Elementary Particles
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