Transcript Chapter 30: Particle Physics

My
Chapter 30
Lecture
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Chapter 30: Particle Physics
•Fundamental Particles (quarks and leptons)
•Fundamental Interactions
•Unification
•Particle Accelerators
•21st Century Particle Physics
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§30.1 Fundamental Particles
Protons and neutrons are not fundamental particles. They
are composed of three quarks each.
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Each quark has an antiquark that has the same mass, but
opposite charge.
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An isolated quark has not been seen, but combinations of
quarks make up the particle groups known as mesons and
baryons. Both groups fall under the name of hadron.
A meson is a bound quark/antiquark pair.
A baryon is composed of three bound quarks.
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The proton and neutron are baryons. A proton is made of
two up quarks and one down quark (uud) and a neutron is
two down quarks and one up quark (udd).
A free neutron decays with a half-life of 10.2 minutes, but
a neutron in a nucleus can be stable. The proton appears
to be stable with a half-life of at least 1029 years.
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The electron belongs to a group of particles called leptons.
No internal structure of an electron has been observed yet.
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The leptons and quarks are grouped into three generations.
Ordinary matter is made up of particles from the first
generation.
The muon and tau leptons are not stable, but the electron
is stable.
The three “flavors” of neutrinos are able to change from
one flavor to another (a neutrino oscillation).
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§30.2 Fundamental Interactions
Each fundamental force in nature occurs by the exchange
of a mediator or an exchange particle.
The exchange particle can transfer momentum and energy
between particles.
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Electromagnetic interactions are mediated by the photon.
Weak interactions are mediated by the W+, W-, and Z0.
Strong interactions are mediated by gluons.
Gravity is mediated by the graviton.
Photons, gluons, and gravitons have no charge and are
massless.
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The strong interaction holds quarks together to form
hadrons.
Quarks have a property called color charge that determine
their strong interactions. Leptons have no color charge and
so do not “feel” the strong force.
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There are three types of color charges (red, green, and
blue). They form colorless combinations. One each of red,
green, and blue will form a colorless baryon. For example,
one red and one antired quark can form a meson.
It is the need to have colorless quark combinations which
prevents them from being removed from a colorless group.
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A gluon mediates a strong interaction. Quarks emit and
absorb gluons, which carry a color charge. Absorbing or
emitting a gluon changes the color of a quark.
The strong interaction keeps both quark systems and
atomic nuclei bound together.
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The weak interaction proceeds by the exchange of the
W+, W-, and Z0. These particles have nonzero masses.
Quarks and leptons have weak charge and so feel the
weak force.
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The weak interaction allows one flavor of quark to change
into any other flavor of quark. In beta-minus decay, a
neutron changes into a proton. This occurs when a down
quark changes into an up quark by emitting a W-, which
then decays into an electron and an electron antineutrino.
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The quantum mechanical description of the strong, weak,
and electromagnetic forces along with the three generations
of quarks and leptons is called the standard model. The
standard model is not complete.
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§30.3 Unification
Just after the Big Bang, it is believed that all four
fundamental forces were unified together as a single force.
As the universe expanded and cooled, the force of gravity
split off, followed by the strong force, which was followed by
the splitting of the weak and electromagnetic forces.
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Experiments have been done to show that under certain
conditions the electromagnetic and weak forces are
unified into the electroweak force.
So far a quantum theory of gravity has not been developed.
General relativity works on large size scales, but fails on
the size scale of atoms.
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Supersymmetry is an attempt at unifying the strong and
electroweak interactions.
It has been found that including extra dimensions is a
way to unify gravity with the other forces.
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String theories treat the fundamental particles as vibrating
loops of energy. These “strings” vibrate in 10 or 11
dimensions. The extra dimensions are very compact and
cannot be observed directly.
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§30.4 Particle Accelerators
A particle accelerator is used to give beams of charged
particles high kinetic energy before colliding them with
each other or with a stationary target.
The results of the collision are recorded by various
detectors for later study.
Two types of particle accelerators are the synchrotron
and the linear accelerator.
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§30.5 Twenty-First Century Particle
Physics
Some open questions physicists hope to answer:
•Is there a pattern to the masses of quarks and leptons?
•Are there only three generations of quarks and leptons?
•Will the Higgs particle be found?
•Are quarks & leptons fundamental particles?
•Is the proton stable?
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•Why is the force of gravity so weak in comparison to
the other three fundamental forces?
•What is dark matter?
•What is dark energy?
•What happened to all of the antimatter formed in the
early universe?
•Can gravity be unified with the other three fundamental
forces?
•Does our universe only have four dimensions? If so,
why?
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Example (text problem 30.12): A proton in Fermilab’s
Tevatron is accelerated through a potential difference of
2.5 MV during each revolution around the ring of radius
1.0 km. In order to reach an energy of 1 TeV, how many
revolutions must the proton make? How far has it
traveled?
During each revolution, the proton is given kinetic
energy of qV = 2.5 MeV.
totalenergy
number of revolutions 
energy gain per revolution
1 T eV

 4 105
2.5 MeV
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Example continued:
The distance traveled is
d  circumference of ring* number of revolutions
 2.5 106 km.
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Summary
•Fundamental Particles (quarks & leptons)
•Fundamental Interactions (strong, weak, EM, gravity)
•Exchange Particles
•Unification
•Particle Accelerators
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