Download Particle Physics

Essential Physics II
E
英語で物理学の
エッセンス II
Lecture 14: 18-01-16
Last lecture of EP2!
Congratulations!
This was a hard course.
Be proud!
Next week’s exam
Next Monday!
All lecture slides on course website:
http://astro3.sci.hokudai.ac.jp/~tasker/teaching/ep2
Remember your calculator!
Dictionary is OK!
(Phone is not)
SHOW ALL WORKING!
Next week’s exam
10 multiple choice questions
(A) .....
(B) .....
(C) .....
(D) .....
3 thermodynamics
5 electromagnetism
2 modern physics
Homework
Attendance / clickers
Exam
Total
40 %
20 %
40 %
100 %
Pass > 60 %
Next week’s exam
This is a question.
(A) .....
(B) .....
(C) .....
(D) .....
F
F 1
B=
=
Il sin ✓
l I sin ✓
0.31N/m
=
= 49mT
(15A) sin(25 )
If this is wrong....
You will get marks!
SHOW YOUR WORKING!
But parts of this are right...
Modern Physics
Particle Physics
What are we made from?
In 1924, Physicists thought the Universe was made from:
4 elementary particles
Cannot be divided.
Proton
Not made from smaller particles.
Neutron
Electron
Neutrino
charge (q) = 0
Positron
Electron antiparticle
Same mass
Opposite charge
Every elementary particle
has an antiparticle
What are we made from?
In 1924, Physicists thought the Universe was made from:
Electromagnetic
radiation
4 Forces
Gravity
Photon
Electromagnetic
N
P
Nuclear
N
P
Weak
e
radioactive decay
What are we made from?
In 1924, Physicists thought the Universe was made from:
2 questions:
How is it connected?
Photon
N
N
P
e
P
Forces
Particles
Is this everything?
Forces as particles
Lecture 12: Interaction between EM field and matter:
EM field
Matter
Quantum
theory
EM field made from photon particles.
But photons can also explain the electromagnetic force.
Quantum electrodynamics
Forces as particles
How do photons create a force?
A ball is thrown between two boats.
Forces as particles
How do photons create a force?
A ball is thrown between two boats.
Conservation of momentum:
mb vb
mB v B
mB v B
mb vb
As green man throws the ball, he moves to the right
As blue man catches the ball, he moves to the left
The boats feel a repulsive force
Men fight for the ball
Boats feel an attractive force
Forces as particles
How do photons create a force?
2 electrons are repelled (Coulomb force) by exchanging a photon
Forces as particles
How do photons create a force?
2 electrons are repelled (Coulomb force) by exchanging a photon
The electron and positron are attracted (Coulomb force) by exchanging
a photon
Quantum description of Coulomb force.
Quantum electrodynamics (QED)
Forces as particles
How about other forces?
Gravity
N
P
Nuclear
N
P
Weak
e
radioactive decay
Are all forces transmitted by particles?
Forces as particles
How about other forces?
The nuclear force holds protons and neutrons together in an
atom’s nucleus
Without the nuclear force, the protons would be repelled by
the Coulomb force.
In 1935, Physicist Hideki Yukawa (日本人)
predicted the particle for the nuclear force.
he called it a ‘meson’
Greek word for ‘intermediate’
me < mmeson < mp
But does this particle exist?
because he predicted the mass to be
between the proton and electron:
Forces as particles
Hunting particles: How do you look for a particle?
Cosmic radiation:
High energy particles from space.
1930s: only source of high energy particles
(Now, we can use particle accelerators: 加速器)
American Physicist Carl Anderson
found particle with mass 207me
?
1me
207me
2000me
This particle is named the muon.
2 types of muon were found:
µ
µ
+
(antiparticle)
Could this muon = the meson?
Forces as particles
µ
The muon’s mass ' predicted meson mass
meson
But the muon only interacted weakly with the nucleons
(protons and neutrons)
µ
Cannot transmit nuclear force.
The muon is not the meson.
In 1947,Yukawa’s meson was discovered (also in cosmic radiation).
This meson was named a pion
1me
270me
2000me
+
⇡
3 types of pions:
positive
⇡
negative
⇡0
neutral
The name ‘meson’ now means a particle with particular properties.
A pion is a type of meson.
Can we put all particles into groups?
Forces as particles
Put these particles in order of mass (lightest to heaviest):
(A) electron, pion, muon, proton
(B) pion, electron, muon, proton
(C) electron, muon, proton, pion
(D) electron, muon, pion, proton
(E) pion, electron, proton, muon
Quiz
Classifying Particles
By 1980, over 100 particles had been discovered.
Could they be sorted into different particle types?
Type 1: Leptons
whatever
lepton
e
µ
⌧
electron
muon
tau
⌫e
electron
neutrino
⌫µ
muon
neutrino
⌫⌧
tau
neutrino
Spin:
anti-particles
Leptons don’t feel the nuclear force
Wait! What is spin?
1
2
Spin
Every particle has intrinsic properties:
e.g. mass
(m)
charge (q)
Always the same
Does not change
spin
(s)
Quantum mechanical property
(not used in classical mechanics)
1
Particles can have spin:
2
1
Fermions
Bosons
leptons are fermions
Classifying Particles
Type I1: Hadrons
Mesons
spin: 1
Force!
Bosons
includes...
Force!
Hadrons DO feel the nuclear force
2 types of hadrons
⇡+ ⇡
pions
⇡0
⌘
⇢
K
eta
rho
kaon
.... and others
Baryons
1
spin:
2
includes...
proton
neutron
Fermions
(much higher mass than leptons)
⇤
⌃
lambda
sigma
.... and others
Classifying Particles
Type I1I: Gauge Bosons: Force carriers
N
N
P
P
bosons
Electromagnetic (QED)
Photon (γ)
Spin 1
Nuclear (strong) force
Gluon (g)
Spin 1
Weak force
W and Z bosons
Spin 1
e
Gravity
WAIT!
?????
W+ W
Z
Spin 1
+
⇡
Isn’t the carrier for a nuclear force a pion?!
Classifying Particles
Quiz
Which is TRUE?
(A) Mesons are bosons, baryons are fermions
(B) Mesons are fermions, baryons are bosons
(C) Mesons are fermions, baryons are fermions
(D) Mesons are bosons, baryons are bosons
1
Fermions spin
2
Bosons
spin 1
Classifying Particles
Leptons
Hadrons
Gauge bosons
Mesons
electron
electron
neutrino
muon
muon
neutrino
tau
tau
neutrino
anti-particles
pions
⇡+ ⇡
⇡0
⌘
⇢
K
eta
rho
kaon
photon
.... and others
W+ W
Baryons
proton neutron lambda
⌃ .... and others
sigma
Are these really elementary?
Z
W bosons Z boson
⇤
Many many particles!
gluon
Quarks
Are hadrons really elementary? No!
They are made from quarks
neutron
proton
Quarks
In 1964, physicists Murray Gell-Mann & George Zweig predicted quarks.
They thought hadrons were made from 3 quarks:
up quark
u
down quark
d
s
strange quark
u
anti-particles
d
s
Quarks carry fractional charge!
u
2
+ e
3
spin 1
2
d
1
e
3
spin 1
2
s
1
e
3
spin 1
2
Baryons
1
3 quarks : spin (1) 2
q=0
neutron
(u, d, d)
Mesons
quark + anti-quark: spin 1
q=1
proton
(u,u,d)
pion
(u, anti-down)
Quarks
The real nuclear force
Pions transmit the nuclear force,
but they are made from quarks
The real force is the strong force:
Holds quarks together.
transmitted by the gluon
3 more quarks were later found:
1974, the charm quark was discovered
c
1977, the bottom quark was discovered
b
1995, the top quark was discovered
t
Quarks
Which contains quarks?
(A) Baryons
(B) Leptons
(C) Gauge Bosons
Quiz
e.g.
proton
neutron
⇤
⌃
lambda
sigma
e
µ
⌧
electron
muon
tau
⌫e
electron
neutrino
⌫µ
muon
neutrino
⌫⌧
tau
neutrino
Photon
Gluon
W/Z bosons
W+ W
Z
The Standard Model
The discovery of quarks makes sorting particles much simpler!
Gauge
bosons
u
Quarks
Leptons
c
t
up
charm
top
photon
d
s
b
Z
down strange bottom
Z boson
⌫e
⌫µ
⌫⌧
W
electron muon
tau
neutrino neutrino neutrino
W boson
e
µ
⌧
g
electron
muon
tau
gluon
H
(+ anti-particles)
These particles are called
‘The Standard Model’
Higgs
boson
Gives particles mass
Discovered July 2012
Are we done?
In 2012, the Higgs boson was discovered.
This was the LAST particle predicted by the Standard
Model.
H
Higgs
boson
So.... does this explain all Particle Physics?
Professor Naoyuki Haba (Hokkaido University)
“No!”
The Standard Model leaves questions....
Are we done?
Problem 1:
3 groups of particles: “3 generations”
mass increase
u
Quarks
up
d
c
charm
s
t
top
b
down strange bottom
Leptons
⌫e
⌫µ
⌫⌧
e
µ
⌧
electron
muon
tau
electron muon
tau
neutrino neutrino neutrino
Difference between generations is mass
Why 3?
Why not 4?
or 2?
The Standard Model cannot explain this.
Are we done?
Problem 2:
Gauge
bosons
photon
Electromagnetic force
Einstein’s theory of General Relativity
Z
Z boson
W
Amazingly accurate for gravity
Weak force
W boson
g
gluon
But uses bending of space-time
Not quantum!
Strong force
G?
What about gravity?
Graviton?
Is there a ‘graviton’?
???
Are we done?
Problem 3:
What is dark matter?
Only ~4% of the Universe is made from atoms
~ 75% is energy
~ 21% is ...... something else??
The Standard Model has no possible particle for dark matter!
The Standard Model
What do you need to build a “simple” Universe?
(A) protons, neutrons, electrons
(B) protons, electrons, muons
(C) electrons, pions, Higgs
(D) pions, mesons, muons
Quiz
The Standard Model
Quiz
At the start of the 20th Century (~1930s), where did physicists find
new particles?
(A) particle accelerators
(B) from the sun (northern lights)
(C) cosmic rays
(D) deep underground
The Standard Model
Quiz
What properties did physicist use to classify (sort) the particle ‘zoo’?
(A) mass, size, colour, charge
(B) charge, shape, spin, density
(C) lifetime, charge, colour, density
(D) mass, charge, spin, lifetime
The Standard Model
Quiz
How was the particle ‘zoo’ simplified (less elementary particles)?
(A) Electrons actually not particles
(B) Many particles were found to be the same
(C) Quarks predicted
(D) Many particles actually molecules
The Standard Model
What is the force carrier for the electromagnetic force?
(A) meson
(B) photon
(C) gluon
(D) W and Z bosons
Quiz
The Standard Model
What is the elementary force carrier for the strong force?
(A) meson
(B) pion
(C) gluon
(D) W and Z bosons
Quiz
The Standard Model
What was the “Holy Grail” of particle physics?
(A) Dark matter
(B) The graviton
(C) The gluon
(D) The Higgs boson
Quiz
Good luck!
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