- Slides: 14
Standard Model of Particles What is the standard model and why do we have one? The standard model is a theory that scientists have been looking for since people have asked ”what’s that made of? ” It consists of three simple principles 1. All matter is composed of a relatively few types of fundamental particles. 2. These particles interact with each other in a few specific ways. 3. Everything that takes place in the universe results from interactions between fundamental particles
A brief history of finding the fundamental The first person to create a standard model was Caveman Thag. • Model contained two fundamental things • Food • Not food (this will be on the test) The first recorded standard model in the western world was Greek. There were several ideas but we will discuss atomism.
Leucippus and Democritus (late 5 th century BCE) First Greeks to ponder if matter could be cut so fine that it could not be cut into smaller bits. • All objects are made from arrangements of • Atoms • Void • Properties of substances depend upon only a few properties of atoms • Solids are made of hard pointy atoms that hook together • Water is made from round soft atoms that roll past each other.
Socrates and Plato (mostly Plato) Plato was more interested in ideas than perceived reality but had a theory of how things exist in the world. • Theory of the forms • Four element idea (earth, water, wind, fire) • Each element was associated with one of the perfect solids
Aristotle- know it all- had his own ideas on everything. Did not believe Democritus’ philosophy. Could not get his mind around the idea of void Believed everything was a continuous mixture of the four elements. Always something there. A note (some people blame Aristotle for setting science back for 2000 years. It was not Aristotle but everyone who believed him) Problem with all these theories: NOT TESTED
The idea of atoms as being fundamental returned during the renaissance with the rise of the alchemist. Unfortunately most alchemists were charlatans. The philosophers Thomas Hobbs, Francis Bacon, René Descartes as well as Galileo, and Robert Boyles had an atomistic idea of matter.
Atomic Theory with a capital T Late in the 18 th century an English school teacher name John Dalton noticed that oxygen and hydrogen always combine in the same ratio. From these observations he developed his atomic theory part of which stated that atoms were indivisible and included the law of conservation of mass. The rest is another lecture.
Ideas need to be organized Dmitri Ivanovich Mendeleev organized then discovered elements into table form with blank spaces for undiscovered but predicted elements. He even predicted masses, densities and other properties and was correct. A few fundamental elements (92) each made of unique indivisible atoms are now organized into a table.
Someone had to go and mess it all up by discovering something smaller that the indivisible atom. J. J. Thompson discovered the electron in 1897 Ernest Rutherford announced the discovery of the proton in 1911. James Chadwick found the neutron in 1932 OK now everything was made from these three fundamental particles- much simpler than 92.
Paul Dirac messed everything up in 1928 by predicting the existence of antimatter particularly the positron. Carl D. Anderson is credited with discovering and naming positrons in 1932. (Chung-Yao Chao, a grad student found them in 1930 but didn’t know what he had found. ) He then went and discovered the muon in 1936. Wolfgang Pauli postulated the existence of the neutrino in 1930. The detection of the neutrino was announced in 1956 by this group of people Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. Mc. Guire
Particle zoo Hideki Yukawa in 1935 hypothesized the existence of a particle responsible for the strong interactions between protons and neutrons in the nucleus. He called it a pion. It was found in 1947. In the fifties scientists were finding hundreds of hadrons (thick particles) in cosmic rays and the simple fundamental models fell apart…again.
In 1961 Murray Gell-Mann proposed that all these hadrons being discovered were actually composed of three sub particle he called quarks /kworks/. They were named up, down, and strange. Predictions of the omega particle consisting of three strange quarks were vindicated in 1964 when it was discovered. James Bjorken had an idea of how to observe quarks and in the early 1970 s at the Stanford Linear Accelerator Center (SLAC) the were found.
More quarks. In the 1970 s more quarks were needed to explain things. Sheldon Glashow, John Iliopoulos and Luciano Maiani proposed the charm quark in 1970 and it was observed in 1974. Makoto Kobayashi and Toshihide Maskawa predicted a third generation of quarks to preserve symmetry. The bottom (beauty) quark was discovered in 1977 and the top (truth) quark was finally found in 1996.
With the discovery of the bottom quark physicists had an idea of what the fundamental particles were and in the 1980 s the standard model as we know it was