Download Introduction to Particle Physics

From Quarks to the Cosmos!
 Prof. Richard E. Hughes
 3046 Physics Research Building, 614-688-5690
 Email: [email protected]
 Course Web Address:
 http://www.physics.ohio-state.edu/~hughes/freshman_seminar/
 Course goals:
 Particle physics and astronomy have seen incredible gains over the
past twenty years. And yet, though
 particle physics concerns the very small
 astronomy concerns the extremely large,
 it is clear that these two disciplines are very closely related.
 This course will introduce the non-expert to these most exciting
sciences, and describe the major research aims of each.
 We will focus on important questions at the intersection of physics
and astronomy that have some hope of being answered over the next
decade.
Richard E. Hughes
Lecture 1; p.1
Course Structure
 Class meets once per week
 Each class will focus on one major research area in particle
physics/particle astrophysics
 Many of these but not all have some participation by OSU physicists
 Each class will be organized like a “Press Conference”
 Except this one!
 YOU are the press:
 After each class, writeup a ~two paragraph summary of the press
conference
 Like you might expect to see in your local paper
 This should be easy: expect it should take you about 30 minutes outside
of class
 Prior to / after class: explore topics on web
 Today’s class: brief introduction to particle physics and the
important questions physicists are trying to answer
Richard E. Hughes
Lecture 1; p.2
What is particle physics?
 Particle physics addresses some of the most
fundamental questions that people have been
pondering for centuries:
 What are the building blocks of matter?
 Why are these blocks what they are? Can we
explain their properties, such as mass?
 How do they interact?
 In a way, particle physics is complementary
to cosmology:
 cosmology studies the largest possible objects
(such as galaxies, with hundreds of billions of
stars!), and particle physics studies the smallest
possible objects imaginable.
Richard E. Hughes
Lecture 1; p.3
Building Blocks of Man
Build this…
Richard E. Hughes
…or build this!
Lecture 1; p.4
Distance Scales
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Football Field 109m
Person: ~1.7m
Hand: ~15cm
Mosquito: ~2cm
Ant 5mm
Human hair: 100microns
Human red blood cell, bacterium:10microns
HIV virus: 100nm
Diameter of DNA: 2nm
Width of Protein: 0.5nm
Radius of Hydrogen: 25pm
Size of the atomic nucleus: 10fm
Size of proton: 1fm
Size of quarks: <10^-18m
Planck Length: 10^-35m distances below this make no sense!
Richard E. Hughes
Lecture 1; p.5
What is a building block?
 What is the most elementary building block of matter?
First, we need to define elementary:
 Let us define an elementary particle as something that
 has no discernable internal structure;
 appears “pointlike”. (At least in current
experiments…)
• First, people thought that the atom was elementary:
The atom, as it was envisioned around 1900 -a ball with electrical charges inside, bouncing around!
Richard E. Hughes
Lecture 1; p.6
Rutherford Scattering Experiment
Rutherford
Experiment
gold
foil
a
Hard central core!
Like firing a cannon ball at a paper towel
and having the ball bounce back
Most of the atom is
empty space.
The alpha particle is probing the structure of the gold in the foil. This
basic idea has been repeated many times over the last hundred years to
further probe the structure of matter.
Richard E. Hughes
Lecture 1; p.7
The atom has a rich structure!
 Eventually, it was realized that
the atom is not elementary:
 it consists of a positively charged
nucleus and negatively charged
electrons.
electron
nucleus
 The properties of outermost
electrons in atoms give rise to
chemistry and biochemistry,
with all of its complexity!
 The electron, as far as we
know, is elementary!
Richard E. Hughes
If the nucleus were as big as a
baseball, then the entire atom's
diameter would be greater than
the length of thirty football fields!
Lecture 1; p.8
Is the nucleus elementary, too?
 Unlike the electron, the nucleus
is not structureless! It consists
of protons and neutrons.
 But protons and neutrons are not
elementary, either!
 They consist of quarks, which to
the best of our knowledge are
elementary.
nucleus
Experiment in 1960’s
High Energy
Electrons
Richard E. Hughes
neutron
proton
Lecture 1; p.9
Break down H20
O
H
H


p  e
p  e
8 p  8n  8e 
u
d

Richard E. Hughes
8
p
p
28u

p
n
26d

10e

Lecture 1; p.10
Break down Pb
Pb

82 
125
p

Richard E. Hughes
289u

332d

n
82 

e
82e

Lecture 1; p.11
H20 vs Pb
H 2O
Pb


28u

289u

26d

332d

10e
82e


The sizes of the piles are different, but ratio of u/d is not all that
different and e/u ratio is not all that different. Looking at H2O and Pb
this way…they don’t look all that different.
Richard E. Hughes
Lecture 1; p.12
The Standard Model
 The most comprehensive theory developed so far that explains what the
matter is made out of and what holds it together is called the Standard
Model.
 In the Standard Model, the elementary particles are:
 6 quarks (which come in three sets)
 6 leptons (which also come in three sets)
 Why do quarks and leptons come in sets (which are called generations)?
Why are there three of them? We don't know.
 Note that the Standard Model is still a model because it's really only a theory
with predictions that need to be tested by experiment!
 Going to very high energies the theory begins to breakdown. (Effective Theory)
Richard E. Hughes
Lecture 1; p.13
How many quarks?
 Quarks: They are fundamental particles…make up protons
and neutrons…but other exotic forms of matter as well.
First proposed in 1960’s.
There are 6 quarks, and they come in pairs:
up
charm
top/truth
1995
1974
1978
down
Richard E. Hughes
strange
beauty/bottom
Lecture 1; p.14
What about the electron?
 We said earlier that apart from the six quarks, the electron was also
elementary.
 It turns out that the electron is not alone -- it belongs to a group of six
particles called leptons! Just like quarks, leptons come in pairs:
Electron neutrino
Muon neutrino
Tau neutrino
ne
nm
nt
e
m
t
electron
Richard E. Hughes
(mass = 205 x mass of e)
muon
(mass = 3503 x mass of e)
tau
Lecture 1; p.15
What are neutrinos?
 W. Pauli postulated their existence in order to save the energy
conservation principle in certain types of radioactive decays, known
as beta-decays:
n  p  e n
neutron decays into proton plus electron plus neutrino
 E. Fermi called them "neutrinos" -- "little neutrons" in Italian.
 Neutrinos hardly interact with anything at all. In fact, the earth
receives more than 40 billion neutrinos per second per cm2. Most
of them just pass through the earth, as if it's not even there!
Richard E. Hughes
Lecture 1; p.16
What particles are important?
Everything you can look at
contains the simple protons
neutrons, and electrons.
So the natural expectation is that protons, neutrons, and electrons are
the most common particles in the universe. But you would be very wrong!
There are about:
 0.5 protons per cubic meter of universe
 330 million neutrinos per cubic meter
 One billion photons per cubic meter
Richard E. Hughes
Lecture 1; p.17
Antimatter!
 The quarks and leptons discussed so far make up “ordinary”
matter.
 For every one of these there is an antimatter counterpart.
 Antiup quark, Antidown quark, etc.
 antielectron (positron), antielectron neutrino, etc.
 Antihydrogen:
e

Matter
Antimatter
u
d
p
Never shake hands with your antiself!
An oddity: as far as we can tell, all of the luminous material we see in the
universe is MATTER not ANTI-MATTER!
The predominance of matter over antimatter in the Universe is one of the
biggest mysteries of modern high energy physics and cosmology!
Richard E. Hughes
Lecture 1; p.18
What holds everything together?
 Things are not falling apart because fundamental particles
interact with each other.
 An interaction is an exchange of something.
A rough analogy of an interaction:
the two tennis players exchange a ball
? But what is it that particles exchange? There is no choice - it has to be some other special type of particles! They
are called force particles (Intermediate Vector Bosons).
Richard E. Hughes
Lecture 1; p.19
Four fundamental interactions
 There are four fundamental interactions between particles:
Interaction
Strong
Gluon (g)
Who feels this force
Quarks and gluons
Photon (g)
Everything electrically charged
Weak
W and Z
Quarks, leptons, photons, W, Z
Gravity
Graviton (?)
Electromagnetic
Richard E. Hughes
Mediating particle
Everything!
Lecture 1; p.20
The strong interaction
 The strong force holds together
quarks in neutrons and protons.
 It's so strong, it's as if the quarks
are super-glued to each other! So
the mediating particles are called
gluons.
 This force is unusual in that it
becomes stronger as you try to
pull quarks apart.
 Eventually, new quark pairs are
produced, but no single quarks. That's
called quark confinement.
Richard E. Hughes
Lecture 1; p.21
The electromagnetic interaction
 The residual electromagnetic interaction is
what's holding atoms together in molecules.
 The mediating particle of the electromagnetic
interaction is the photon.
 Visible light, x-rays, radio waves are all
examples of photon fields of different energies.
Richard E. Hughes
Lecture 1; p.22
The weak interaction
 Weak interactions are indeed
weak:
 Neutrinos can only interact
with matter via weak
interactions -- and so they
can go through a light year
of lead without experiencing
one interaction!
nm
 Weak interactions are also
responsible for the decay of
the heavier quarks and leptons.
 So the Universe appears to be
made out of the lightest quarks
(u and d), the least-massive
charged lepton (electron), and
neutrinos.
Richard E. Hughes
Lecture 1; p.23
Gravity
 The Standard Model does not include
gravity because no one knows how to do
it.
 That's ok because the effects of
gravity are tiny comparing to those
from strong, electomagnetic, and weak
interactions.
 People have speculated that the
mediating particle of gravitational
interactions is the graviton -- but it has
not yet been observed.
Richard E. Hughes
Lecture 1; p.24
Seething Underworld
 Lots of gluons, photons, even strange and charm quarks
inside protons and neutrons.
g
u
e
c
c
g
p
d
u
d
g
g

g
e

g
d
Richard E. Hughes
e

g u
n
Lecture 1; p.25
The Big Questions
How was matter formed at the beginning of the universe?
How does it stay together?
What are the fundamental building blocks of nature?
What are the basic laws upon which the universe operates?
Astrophysicists have found that less than 5 percent of the
mass of the entire universe consists of the kind of
"luminous" matter that we can see. What is the dark matter
that makes up the rest of the universe?
 Why is our universe is made of matter, while antimatter has
all but disappeared?
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Richard E. Hughes
Lecture 1; p.26
Fermi National Accelerator Laboratory
Proton-antiproton collider:
Question: What are the fundamental building
blocks of nature?
Only place in the world where
top quarks can be made
Richard E. Hughes
Lecture 1; p.27
Gamma-ray Large Area Space Telescope
Gamma Ray Bursts: Power at
maximum up to
1,000,000,000,000,000,000
(quintillion) times the Sun's
power
Matter that radiates across the
entire electromagnetic spectrum is
only 10% of the total mass of the
universe: 90% of the mass of the
universe does not emit light at any
wavelength. Can detect this socalled dark matter by its
gravitational effects on luminous
matter
Compton Observatory all sky gamma-ray image of the unidentified
sources (active galactic nuclei, pulsars, supernova remnants, dense
molecular clouds, and stellar-mass black holes within our Galaxy?)
Richard E. Hughes
Lecture 1; p.28
ATLAS
Proton-proton collider
increase energy by factor
of 7 over Fermi Tevatron!
Main purpose: Search for
a special particle –
- the Higgs – that gives all
other particles MASS!
Richard E. Hughes
Lecture 1; p.29
NUMI/MINOS
Idea: make neutrinoes, shoot them underground approximately 450 miles to
Minnesota;
study neutrino mass
Richard E. Hughes
Lecture 1; p.30
Supernova / Acceleration Probe
Studying the Dark Energy of the Universe
A star's distance can be estimated from its
brightness as seen on Earth, if its total emitted
light is known — the farther away it is, the
dimmer it appears. Accurate estimates of total
emitted light are possible for only a few kinds of
astronomical objects such as type Ia supernovae
most distant supernovae are dimmer than they
would be if the universe were slowing under the
influence of gravity; they must be located farther
away than would be expected – the conclusion is:
the Universe is expanding!
some form of dark energy does indeed appear to
dominate the total mass-energy content of the
Universe, and its weird repulsive gravity is pulling
the Universe apart
Richard E. Hughes
Lecture 1; p.31