Class 3 - Forces, Interactions, and
Antimatter
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Forces and Interactions
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Force Carriers
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Feynman Diagrams
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Antimatter
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Pions
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Muons
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Taus
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Kinds of Neutrinos
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Particle Classifications
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Homework
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Forces and Interactions
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Newton’s laws give us an operational definition of a
force as something that changes the momentum of an
object
In the subatomic world, when two particles exert a
force on each other they exchange a particle called a
force carrier
When two particles exchange a force carrier it is called
an interaction
Four forces are presently considered to be fundamental
Gravitational force
– Electromagnetic force
– Strong force
– Weak force
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Gravitational Force
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Acts between all particles that have mass
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Attractive
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Proportional to the product of the masses
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Gets weaker as the distance between the masses increases
Binding force of the solar system and galaxies
Not important in the subatomic world because the
masses are so very small
The force carrier is called the graviton (mass=0, charge=0,
spin=2, range=infinite, never been observed)
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Electromagnetic Force
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Acts between all objects that have electric charge
Attractive for oppositely charged objects and repulsive for objects with the same charge
Gets weaker as the distance between the objects increases
Holds the atom together
The force carrier is the photon (mass=0, charge=0,
spin=1, range=infinite)
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Strong Force
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Acts between nucleons (protons and neutrons)
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Attractive
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Holds the nucleus together
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We will see later that the strong force actually acts between quarks and it is what we call the residual strong
force that holds two nucleons together
The force carrier is called the gluon (mass=0, charge=0,
spin=1, range=10−15 m, and we have experimental
evidence that it exists)
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Weak Force
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Called weak force because it’s weak compared to the
strong force
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Responsible for beta decay
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Neutrinos interact only through the weak force
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Whenever a neutrino is involved in a reaction, the
weak force must be responsible
The force carriers are the W+, W−, and Z0 (mass=82(W),
91(Z), charge=+1/-1(W), 0(Z), spin=1, range=10−19
m, have been detected)
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Feynman Diagrams
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Feynman diagrams are used as an aid in performing
complex calculations of particle interactions
Rules for drawing Feynman diagrams
Incoming and outgoing particles are drawn as straight
lines
– Force carriers are drawn as wavy lines connecting
the particles
– Arrows indicate the direction of motion in time
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e
e + p
e + p
e
γ
p
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p
Feynman Diagram for Beta Decay
p + e−
n + ν
p
n
W
−
e−
ν
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Antimatter
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In his famous work to combine special relativity with
quantum mechanics, Paul Dirac predicted the existence of a particle with the same mass and spin as the
electron but opposite electric charge
This new particle called the positron (e+) was discovered in 1932 and is now known as the antielectron
Antiparticles were then proposed for the proton and
neutron and the antiproton (p-bar, p) and antineutron
(n-bar, n) were discovered with the use of accelerators in 1955
Antiprotons have the same mass and spin as protons
but with opposite charge
Antineutrons have the same mass, spin, and charge
as the neutron (we’ll see that they really are different particles when we talk about the substructure of
nucleons)
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Properties of Antimatter
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When a property has an opposite value, the antiparticle will have the opposite value of that property than
the particle (e.g. electric charge)
When a property has no opposite value, the antiparticle will have the same value of that property as the
particle (e.g. mass, spin)
When a particle meets its antiparticle in a reaction
they can annihilate into energy, for example
p + p → energy (γ)
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Energy equivalent to at least twice the electron mass
can create an electron-positron pair
energy (γ) → e+ + e−
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Pions
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In the 1930s Yukawa predicted the mass of the force
carrier of the residual strong force between nucleons
from the range of the interaction to be 1/7 the mass of
the proton
He was also able to predict that it came in three charge
states: positive, negative, and neutral
The particle was eventually named the pion
The charged pion was discovered in cosmic rays in
1947 and the neutral pion was discovered in accelerator experiments in 1950
Pions are unstable - they live for a short period of time
and then spontaneously decay into other particles
Pion Charge Mass Lifetime Most Common
(s)
Decay Mode
π+
+1
1/7
10−8
π + → µ+ + ν
π−
-1
1/7
10−8
π − → µ− + ν
π0
0
1/7
10−16
π0 → γ + γ
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Discovery of the Pion
The picture below shows the particle tracks left in a photographic emulsion (exposed at high altitude) during the
decay of a pion.
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Muons
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In the search for pions, physicists discovered a new
particle called the muon (µ) with 1/9 the mass of a
proton
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It was discovered in 1937 in a cloud chamber
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Pions can decay into muons
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The muon is related to the electron, but is 200 times
as massive
Muon Charge Mass Lifetime Most Common
(s)
Decay Mode
µ+
+1
1/9
10−6 µ+ → e+ + ν + ν
µ−
-1
1/9
10−6 µ− → e− + ν + ν
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Taus
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There’s an even heavier cousin of the electron and
muon, called the tau (τ ), that was discovered in 1975
The tau has the same charge and spin as the electron
and muon, but has a mass of about twice the proton
Of course it has an antiparticle τ +
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Three Kinds of Neutrinos
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Actually, there are three kinds of neutrinos
One associated with the electron (νe)
– One associated with the muon (νµ )
– One associated with the tau (ντ )
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Of course, each kind of neutrino has an antineutrino
The kind of neutrino involved in a reaction depends
on whether the electron, the muon, or the tau is involved
The positive pion can decay like this π + → µ+ +νµ
or π + → e+ + νe, but it cannot decay like this
π + → µ + + νe
– When the electron neutrino interacts with a neutron, this will occur: n + νe → e− + p, but this will
not: n + νe → µ− + p
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Particle Classifications
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Leptons - Spin
strong force
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1
2
particles that do not experience the
e−, µ−, τ −, νe, νµ, ντ (and their antiparticles)
Hadrons - particles that experience the strong force
Baryons - Particles with half-integer spin (e.g. p,
n)
+
−
– Mesons - Particles with integer spin (e.g. π , π ,
π 0)
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Homework
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Read Chapters 3 & 4 of TSZ
Read about Cosmic Rays at:
http://www2.slac.stanford.edu/vvc/cosmic_rays.html
Do Homework Activity 1
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