1. Highlights of Contemporary Physics: Particle Physics Module
Mark Pitt, 309 Robeson,
Particle Physics: physics of Nature's most fundamental particles and the
forces that act between them
Goals: At the end of this module you will hopefully know the answer to some of the following
questions:
1. Why do physicists study particle physics?
2. What is the "standard model of particle physics" and what are some examples of its
successes and remarkable predictions?
3. Why is there a connection between particle physics (small scales) and cosmology (large
scales)?
4. Why is such large-scale equipment required to study particle physics?
5. What is still unexplained about particle physics and what sorts of new results can we look
forward to over the next ~ 10 years?
6. What are some examples of pure physics research with applications useful to society
as a whole?

2. Highlights of Contemporary Physics: Particle Physics Module
Mark Pitt, 309 Robeson,
Administrative information:
Writing Assignment:
Write a story (or poem) with at least two elementary particles as characters. The
behavior of the characters should reflect the properties of the particles.
Read a Scientific American article I give you on particle physics and answer a few
questions about it.
Due on Tuesday December 7, 2010 in class (during last class)
Test:
Dec. 14, 2010, Tuesday, 10:05 AM – 12:05 PM in Robeson 210
Preparation materials for test (webpage should be ready by this Thursday Nov. 18)
4-5 page writeups for each lecture on the web
Complete powerpoint version of lectures on web (but the 4-5 page writeup should
contain everything you need to know
Worksheet with list of concepts to study and practice problems (we will go over
the practice problems at the beginning of the fifth lecture on Tuesday Dec. 7)

3. Composition of the Universe

4. Some Length Scales in Particle Physics
(1 m = 1 meter)
Size of typical complex molecule
(collection of atoms)
~ 10-9 m
Radius of the orbit of the electron
in a hydrogen atom
~ m
Radius of a Pb (lead) nucleus:
(contains 82 protons and 126 neutrons)
~ m = 10 F
(note: 1 F = 1 Fermi = m)
Radius of proton
~ m = 1 F
See more about length scales in
the "Powers of 10" film
"Radius" of quark or electron
< m = F
(ie. "pointlike" as best we can tell currently)

5. The Full Range of Observed Length Scales
Observed length scales range from size of observable universe ~ 1026 m
down to smallest sizes studied at high energy accelerators ~ m
 a 44 order of magnitude range of sizes!
Observable by
(aided) eye
with visible light
Only observable
with radiation
of shorter wavelength
(higher energy)
than light

7. Properties of Particles
1. Mass
Recall: a) Rest energy E = m c2
b) Units of energy are eV = 1 electron-volt
= energy gained by an electron when
it is accelerated through a 1 volt
potential difference
Since m = E / c2 the units for mass are:
(unit of energy) / c2 = eV/c2
Note: 1 KeV = 103 eV = Kilo-eV
1 MeV = 106 eV = Mega-eV
1 GeV = 109 eV = Giga-eV 
Some common masses:
electron: 9.1 x kg = GeV/c2
proton: x kg = .938 GeV/c2
Note: electron is ~2000 times lighter than the proton

8. Properties of Particles, continued
2. Electric charge:
Electric force causes  opposite charges to attract
like charges to repel
electron: e Q = -e
proton: p Q = +e
neutron: n Q = 0
3. Spin:
Particles are "spinning" on their own internal axis
BUT spin is quantized (only certain values allowed by quantum mechanics)
 1/2, 3/2, 5/2, ...  fermions (half-integral spin particles)
 0, 1, 2, 3,  bosons (integral spin particles)
4. Magnetic moment:
= electric charge + spin
Example: The proton is like a miniature bar magnet; when you put it in a
strong magnetic field it orients itself along the magnetic field direction.
 magnetic resonance imaging (MRI)

9. Important Constant to Know for Calculations

10. We will now (briefly) follow a historical path to see how we got to this
set of fundamental particles and interactions

11. A Brief Overview of the History of Particle Physics
Pre-1897: People knew about atoms, periodic table, atomic masses, etc.
1. Thomson (1897): Discovery of electron
First discovery in particle physics
observed with a cathode ray discharge tube similar to your television
picture screen (see demonstration of this in next lecture)
2. Rutherford (1911): Discovery of atomic nucleus
Rutherford experiment lead to the conclusion that there was a
small positively-charged nucleus at the center of the atom
Before that, people thought the atom was described by the
"plum-pudding" model:
positive "pudding"
negative electrons ("raisins")
10-10 m

12. Rutherford Experiment
 source
Au (gold) foil
detector
= alpha particle = 4He Au = gold nucleus
(2 protons, 2 neutrons) (79 protons, 118 neutrons)
Calculated: typical deflection angle for  particle interacting with ONE
Au plum pudding atom is  ~ 0.01o
In typical thin gold foil there are lots of atoms, so the  particle passes
within the atomic radius (~10-10 m) of many Au atoms
In fact, expect ~ = 104 scatterings
Statistical theory says: average total scattering angle ~

13. Rutherford Experiment, continued
For large (  > 90o) scattering ALL of the individual scatterings must be in
the same direction
like flipping “heads” times in a row!
Probability of  > 90o : probability of flipping heads times in a row:
= (1/2)10000 ~
Even with 1012  particles/second incident on target it would take
~ years to observe 1 event!
For comparison, universe ~ 1010 years old
BUT Rutherford, Geiger, and Marsden (1909) found
Probability ( > 90o) ~ 10-4
Conclusion: The positive charge of the atom is concentrated in a small
nucleus at the center.
In Rutherford’s own words:
The proton was explicity “discovered” in 1919 by Rutherford:
 + 14N  p + 17O

14. A Brief Overview of the History of Particle Physics, cont.
3. Antiparticles (1928): Dirac Equation
Relativistic version of the quantum mechanical Schroedinger equation
Dirac equation predicts the existence of antiparticles
Antiparticle: same mass, but opposite charge and magnetic moment
Example: e- : electron (Q = -e) and e+ : positron (Q = +e) are antiparticles
Positron discovered in 1933 by Anderson
matter + antimatter  energy
e e   + 
p p   +   = photon
4. Neutron (1932):
Neutron: electric charge = 0, mass ~ mass of proton
discovered by Chadwick in 1932
At this point (1933), all constituents of everyday matter (e-, p, n) were
known.
Why did people continue on doing this research after that?

15. A Brief Overview of the History of Particle Physics, cont.
5. Create new particles (~1920's 's):
Use cosmic rays from outer space
Example: Iron (Fe) nucleus with kinetic energy 5000 GeV bombarding
atoms in the upper atmosphere
 This high energy particle interacts with nuclei in the upper
atmosphere to create new, short-lived (10-6  seconds)
Examples of new particles observed during this period:
Muons: + - mass ~ .105 GeV/c2
Pions: + - mass ~ .140 GeV/c2
both ~ 1/10 of the mass of the proton

16. Example of a Cosmic Ray "Shower" of Particles
5000 GeV Iron nucleus
~ 200 new particles
created (mostly
pions = + - )
nuclear photographic emulsion

17. A Brief Overview of the History of Particle Physics, cont.
’s: Advent of particle accelerators (Berkeley, Brookhaven)
Accelerate particles (mostly electrons or protons) to high energies
Collide with targets (like protons in hydrogen atoms) use particle
detectors to observe NEW particles
 The new particles created are short-lived (10-6  seconds)
 Hundreds of new particles observed (see Particle Data Book)
At this point, some organization was needed to this
“particle zoo.”
It was believed that nature couldn’t be so complicated at its
most “fundamental level.”

18. A Brief Overview of the History of Particle Physics, cont.
7. Organization arrives in 1964:
Quark model (Murray Gell- Mann)
up u : q = charge= +2/3 e
down d : /3 e
strange s : /3 e
 Prediction of fractionally charged particles which compose
all hadrons (of which the proton and neutron are the simplest examples)
 Gell-Mann’s theory could explain all known hadrons at that time
Hadrons
Baryons Mesons
3 quark objects quark objects
Proton (u u d) 2/3 + 2/3 – 1/3 = + (u d ): 2/3 + 1/3 = +1
Neutron (d d u) -1/3 – 1/3 + 2/3 = 0

19. A Brief Overview of the History of Particle Physics, cont.
8. Further quark discoveries:
1974: charm (c) quark mass = 1.5 GeV/c2
1977: bottom (b) quark mass = 4.7 GeV/c2
1996: top (t) quark mass = 175 GeV/c2
Current picture of fundamental “matter constituents”:
3 families of leptons + 3 families of quarks

20. How do these fundamental particles interact with each other?
Current picture of fundamental “matter constituents”:
3 families of leptons + 3 families of quarks
How do these fundamental particles interact with each other?
 Through the four fundamental forces

21. Classical Description of Forces
Particles feel a force “field” emanating from other particles
Examples: M1 M2 R Q1 Q2 R

22. Modern Description of Forces
“Quantum Field Theory”
 Forces between particles are caused by the exchange of discrete
particles (called exchange particles or exchange bosons)
The type of exchange particle depends on the force
examples: electromagnetic force: photon () m = 0
weak force: Z boson: mZ = 90 GeV/c2
BUT doesn’t this violate energy conservation?