Lecture: April 7, 2021 QED: Quantum Electro Dynamics & QCD: Quantum Chromodynamics
QED and QCD are the ultimate quantum theory that describes Nature:
QED: Describes electrons or in general particles that have charge and interact with electromagnetic forces. In QED, when two charged particles interact, they exchange photons.
QCD describes particles inside the nucleus: Protons and Neutrons, ie Quarks. In general it describes particles that interact with Strong or Nuclear forces. Strong forces are attractive. Nuclear forces are about 100 times ( more precisely 137 times ) stronger than electromagnetic forces.
(These two theories provide complete description of all fundamental particles. Hidden in them are Planck theory, de Broglie theory, Einstein’s theory of light quanta, Bohr model, Schodinger¨ and Dirac equations, Heisenberg quantum theory etc etc...
• Both have charge.
• Both are fermions, ie spin half-particles and hence obey Pauli exclusion principle.
• Two electrons interact with electromagnetic force, exchanging photons. Two protons when far apart ( ie out side the Nucleus) also interact with electromagnetic force- exchange photons.
• However, when two protons are inside the Nucleus, they attract each other. This attractive force is called Strong force or Nuclear force. This interaction is mediated by gluons: that is just like two electrons exchange photons in an electromagnetic interaction, two protons exchange gluons in strong interaction. Just like photons, gluons have zero mass and no charge.
• Note that the Nuclear force between two protons is same as the force between two neutrons and also same as force between a proton and neutron. That is, nuclear force does not depend on charge.
1 • Unlike electrons, protons and neutrons are not elementary particles. They are made up of quarks.
• Quarks and gluons have Color Charge: The ”color charge” of quarks and gluons is completely unrelated to the everyday meaning of color. The term color and the labels red, green, and blue became popular simply because of the loose analogy to the primary colors. Richard Feynman referred to his colleagues as “idiot physicists” for choosing the confusing name.
This color charge differs from electric charge in that electric charge has only one kind of value. However color charge is also similar to electric charge in that color charge also has a negative charge corresponding to each kind of value.
There are three types of quarks and they are labeled as red, green and blue.
A particle with red, green, or blue charge has a corresponding antiparticle in which the color charge must be the anticolor of red, green, and blue, respectively, for the color charge to be conserved in particle-antiparticle creation and annihilation. Particle physicists call these antired, antigreen, and antiblue. All three colors mixed together, or any one of these colors and its complement (or negative), is ”colorless” or ”white” and has a net color charge of zero. Due to a property of the strong interaction called color confinement, free particles must have a color charge of zero:
In quantum chromodynamics (QCD), a quark’s color can take one of three values or charges: red, green, and blue. An antiquark can take one of three anticolors: called antired, antigreen, and antiblue (represented as cyan, magenta, and yellow, respectively). Gluons are mixtures of two colors, such as red and antigreen, which constitutes their color charge. QCD considers eight gluons of the possible nine color–anticolor combinations to be unique
Higgs Field and Higgs Boson
Higgs Boson & WHAT IS MASS ?
In 2012, Higgs bosons were discovered.
2 Higgs bosons explain why particles have mass.
According to QFT, all particles are quanta of some field...
Higgs particles are quanta of some field and imagine that this field permeates the entire universe. This field is special: it does something that NO other field does: rather than attracting or repelling anything, it makes it hard for particles to get going or it slows them down. In other words, the effect of this field is identical to the effect of having inertial mass. The more the field interacts with a particle, the more mass or inertia it has.
It goes one step further and suggests that the inertia generated by a particle interacting with this field is the particle’s mass. Some particles feel this field strongly – meaning that they take lot of force to speed up or slow down: these particles have lot of mass. Other particles hardly feel this field and hence have very little mass. According to Higg’s theory, this is what a mass is.
Theory does not explain why particles have certain masses. IT IS OPEN PUZZLE.
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