Lecture 29 Elementary Particles and Quarks. Chapter 30.4 30.12. Outline. Fundamental Forces Elementary particles Quarks. Fine Structure of Matter. Ancient Greeks suggested that matter consists of tiny particles called atoms .
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Chapter 30.4 30.12
Ancient Greeks suggested that matter consists of tiny particles called atoms.
Only in the 20-th century building blocks of atoms were discovered.
The main blocks (protons, electrons, and neutrons) are stable at least within the atomic nucleus.
Free neutrons decay within minutes.
Protons have half-life of 1031 years.
It was found by 1930s that kinetic energies of electrons in decay reactions are lower than predicted.
Pauli (1930) and Fermi proposed a new, electrically neutral, particle to account for the missing energy.
It was discovered in 1956 and called neutrino.
In 1960s, many other subatomic particles were discovered experimentally.
Most of them turned out to be short-lived.
Almost each elementary particle has an antiparticle, that has the same mass, but the electric charge of an opposite sign (e and e+,electron and positron).
When a particle and its antiparticle come together, they destroy each other (annihilation).
The lost mass reappears as energy in the form of rays.
Elementary particles interact with each other in 4 ways.
All elementary particles fall into 2 broad categories with respect to their response to the strong interaction.
Leptons (light) are not affected and seem to be point particles with no internal structure (electron).
Hadrons (heavy) are affected by the strong interaction, have definite sizes (~1015 cm), and have structure (proton and neutron).
Quarks are particles which make up hadrons.
Only 6 kinds of quarks are needed to account for all hadrons.
The proton, neutron, and heavier hadrons consist of 3 quarks.
Quarks have fractional electric charges 1/3e and 2/3e.
They do not seem to exist outside hadrons.
All the evidence for quarks is indirect, but the theory correctly predicts new hadrons.
Matter has finer structure than atoms and the three main subatomic particles.
There are only 4 fundamentally different forces in nature (strong, electromagnetic, weak, and gravity).