Lecture 29 Elementary Particles and Quarks

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Lecture 29Elementary Particles and Quarks
Chapter 30.4 ? 30.12
Outline

• Fundamental Forces
• Elementary particles
• Quarks

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Fine Structure of Matter
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.
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Elementary Particles
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.
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Elementary particles
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.
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Fundamental Interactions
Elementary particles interact with each other in
4 ways.

• The strong interaction
• holds protons and neutrons together
• acts over sizes of 10?15 cm
• does not affect electrons

• The electromagnetic interaction
• gives rise to electric and magnetic forces
• responsible for the structure of matter
• 100 times weaker than the strong interaction
• is unlimited in range and acts on electrons

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Fundamental Interactions

• The weak interaction
• affects all particles and helps to determine the
  compositions of atomic nuclei (beta-decay)
• acts over a shorter range than the strong
  interaction and 10 trillion times weaker

• The gravitational interaction
• responsible for the attractive force one mass
  exerts on another
• dominates on a large scale
• the weakest on the smallest scales

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Leptons and Hadrons
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 (10?15 cm), and
have structure (proton and neutron).
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Quarks
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.
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Quarks
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Summary
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).