1. HCP: Particle Physics Module, Lecture 3
Plan for today:
Review where we are on the Standard Model Chart
1. Discuss the strong force between quarks: gluons and QCD
2. Introduce the weak force
3. Discuss the unification of electromagnetic and weak forces
predictions and observations
unobserved (so-far) predictions

3. Fermions (half-integer spin particles) satisfy the
Pauli Exclusion Principle : no more than
one identical particle can occupy the same quantum state

4. For the -, all the quarks are
identical (sss) and in the same
quantum state: isn’t this a
problem (ie. conflict with Pauli
Exclusion Principle)?

5. Quarks and “color”
States like ++ = (u u u) or - = (s s s) with three identical fermions in the
same quantum state violate the Pauli principle UNLESS
the three quarks inside the baryon are somehow different
This problem was dealt with in 1964 by introducing the concept of
Quark color: each type of quark comes in three colors
Red Blue Green
Observed hadrons are “color neutral”
Baryons: (q R ) (q B ) (q G ) R + B + G = “white” (color neutral)
Mesons: (q R ) (q R ) or
B B
G G
When it was introduced (1964), color seemed “ad hoc”; doesn’t it have
some kind of real physical meaning?

6. Quantum Chromodynamics (QCD)
Theory of how the strong interaction between quarks works
(analogous to QED – quantum electrodynamics – of the electromagnetic
interaction)
The strong force between quarks is carried by exchange particles
called gluons (analogous to the photons in QED)
The gluons are attracted to the color charge (just like the photons in
QED are attracted to the electric charge)
There are 8 types of gluons (they come in different color – anticolor
combinations):
R B , R G , B G , etc.
Example of two colored quarks interacting via gluon exchange: u R u B RB R B u u
Gluons (like photons) are massless BUT gluons carry color charge
(compare to photons which have zero electric charge)

7. Strength of the Interaction Between Quarks
Confinement: At larger (say ~ 1 F) distances the force between quarks
becomes VERY LARGE. The quarks are permanently confined inside
the hadrons.
confinement
strength of
the force
strength of
the force
between two
quarks
asymptotic freedom
1. Asymptotic freedom: At short (< 0.3 F) distances, the force between
quarks is very weak. They essentially move about freely.
distance between particles
distance between quarks
Electromagnetism: (interaction between
proton and electron in atom, say)
Quarks and strong interaction (QCD):
(interaction between two quarks)
Two special properties of Quantum Chromodynamics (QCD)

8. Discovery of Asymptotic Freedom in QCD – awarded
Nobel Prize in Physics 2004

9. Why Don’t We Observe Free Quarks? Confinement
Electromagnetism (QED):
F  1/R2
weaker at larger separations
Strong (QCD):
F  R
stronger at larger separations
Confinement: We can never observe a free quark.
attempt to pull two quarks apart
so much energy is stored that a new
q q pair is created

10. Trying to pull two quarks apart
results in more quarks being formed
 JETS of hadrons
What evidence do we have that
gluons exist?
Processes like below where a gluon
is “radiated” lead to 3 JET events
3 Jet Event
2 Jet Event

11. Think about the proton’s mass  something doesn’t add up

12. “Picture” of the Proton in QCD
Recall that the proton is made up of three quarks:
proton: u u d
electric charge: /3e +2/3e -1/3e = +e d u
“bag”
radius ~ 1 F
Picture: The quarks are trapped inside a spherical “bag” with a radius
of ~ 1 F (CONFINEMENT) inside of which they move freely at nearly the
speed of light (ASYMPTOTIC FREEDOM).
Most of the mass of the proton is in the kinetic energy of the quarks
(not in the rest mass of the quarks). Recall the Einstein equation E=mc2.

13. Return to Discussing Forces
Note: any number of
identical bosons can
exist in the same
quantum state

14. LIGO: Laser Interferometic Gravitational
Wave Observatory:
The search for gravity waves

15. LISA: Laser Interferometer Space Antenna

16. Return to Discussing Forces
Note: any number of
identical bosons can
exist in the same
quantum state

17. How Do We Classify the “Strength” of Interactions?
Consider decays of particles:
Decay Force Lifetime of particle
++  + + p strong x seconds
o   +  electromagnetic x seconds
-  e- + e +  weak x seconds
Force Typical particle decay lifetime
strong seconds
electromagnetic seconds
weak minutes seconds
Basic point: particle lifetime  (1 / strength of interaction)
So stronger forces  shorter lifetimes
and vice versa

18. Weak Force
Examples:
Neutron beta decay: n  p + e- + e lifetime = 900 seconds d u
d (-1/3)
neutron
u (+2/3)
proton ? -1
time
e- (-1)
e (0)
 decay:   p +  lifetime = 2.6 x seconds d u
s (-1/3) 
u (+2/3)
proton ? -1
time
d (-1/3)
u (-2/3) -
We need a new force carrier to describe the weak interaction. Note
that it has to have electrical charge in order for the above processes
to work.

19. Force Carriers of the Weak Interaction
Steven Weinberg (1967) proposed three force carriers for the weak
interaction:
particle electric charge mass
W e GeV/c2
W e GeV/c2
Z GeV/c2
The W+ and W- were necessary to explain already known charged current
processes like: d u
d (-1/3)
neutron
u (+2/3)
proton W- -1
time
e- (-1)
e (0)
neutron beta decay:
n  p + e- + e
The prediction of the Z0 implied the existence of new, previously
unobserved neutral current processes like:  
 + e-   + e- Z0 e- e-

20. neutral current processes like:
The prediction of the Z0 implied the existence of previously unobserved
neutral current processes like: Z0  e-
 + e-   + e-
These processes were first discovered in 1973:

21. Experimental Confirmation of Electroweak Theory
Many predictions of the electroweak theory (like neutral currents) were
experimentally confirmed in the 1970’s BUT
The masses of the W+- and Z0 bosons were predicted, but no accelerator
was energetic enough to create them at that time.
Discovery of W and Z (1983) by Carlo Rubbia and many others
p p  W “stuff” 
“stuff”
p p  Z “stuff”
Done at CERN in Geneva, Switzerland
W and Z observed with predicted masses
Triumph of electroweak theory!

22. Observation of the Z0 particle at CERN
The energies of the observed e- and e+ decay pair added up to the rest
mass of the Z0

23. Desire for Simplicity in Particle Physics
The ultimate goal of particle physics is to describe Nature in terms of
the smallest number of particles interacting through
the smallest number of forces.
Particles (2005): quarks
6 leptons
Forces (2005): Gravity
Weak, Electromagnetic (unified as Electroweak)
Strong
 Goal is to eventually unify all four forces into one force.

24. Electroweak theory (unification of electromagnetism and
the weak force)
Weinberg and Salam (1967) proposed that the electromagnetic and weak
forces were really two aspects of the same underlying force.
How can this be? Consider the relation between the range of a force
carrier and its mass:
At very high energies (>> 100 GeV) we probe very short distances
(<< m). At such short distances even the weak force appears
“long-range” like the electromagnetic interaction.

25. How Does the Unification Work?
At high energies (>> 100 GeV) and short distances (<< m) 
4 massless exchange particles W+
W B0 W-
“triplet” “singlet”
Symmetric 
all massless,
all interacting with the same strength
Spontaneous
symmetry
breaking
Low energy world we live in 
3 massive particles massless particle
(weak force) (electromagnetism) W+
Z  W-
 Symmetry broken. The fact that the electromagnetic and weak forces
are the same is hidden from us due to the low energy world we live in.

26. Example of spontaneous symmetry breaking – the bar magnet
Iron bar magnet at high temperature
Spins randomly oriented
Symmetric  no special direction
Reflects the underlying symmetries of the laws of electromagnetism
Iron bar magnet at low (room) temperature
Spins all oriented along the same direction
Special direction picked out  not symmetric
Broken symmetry  the symmetries of the underlying laws of
electromagnetism are hidden

27. How does the spontaneous symmetry breaking occur in
electroweak theory?
Higgs mechanism
There are postulated to exist 4 Higgs particles
3 of them are absorbed (“eaten”) by the W+ , W-, and Z0 to give them
their mass
Since the photon has no mass, there should be one remaining, “uneaten”
particle
the Higgs boson
 Not yet observed, but expected to be more massive than the W+- , Z0
The primary goal of the world’s newest, biggest accelerator will be to
detect the Higgs boson:
LHC: Large Hadron Collider
7 x 1012 eV = 7 TeV proton – proton collider
 will start in to take data at CERN in Geneva, Switzerland