1. CHAPTER 14 Elementary Particles
Homework due Monday December 7th
Chapter 14: 1, 3, 5, 6, 8, 10, 11, 15, 22, 25
No class this Friday, Dec. 4th!
Steven Weinberg ( )

2. Quark Properties The spin of all quarks (and anti-quarks) is 1/2.
The spin of all quarks (and anti-quarks) is 1/2.

3. Quark Description of Particles
Baryons normally consist of three quarks or anti-quarks.
A meson consists of a quark-anti-quark pair, yielding the required baryon number of 0.

4. The W− and Quantum Chromodynamics (QCD)
This isn’t possible unless some other quantum number distinguishes each of these quarks in one particle.
A new quantum number called color circumvents this problem and its properties establish Chromodynamics (QCD).
The bubble chamber picture of the first omega-minus. An incoming K- meson interacts with a proton in the liquid hydrogen of the bubble chamber and produces an omega-minus, a K° and a K+ meson which all decay into other particles. Neutral particles which produce no tracks in the chamber are shown by dashed lines. The presence and properties of the neutral particles are established by analysis of the tracks of their charged decay products and application of the laws of conservation of mass and energy.
Discovery of the W-

5. Gluons
The particle that mediates the very strong interaction between quarks is called a gluon (for the “glue” that holds the quarks together); it’s massless and has spin 1, just like the photon.
Like the photon, the gluon has two transverse polarization states.
There are eight independent types of gluon (depending on the colors of the quarks involved).
Computed image of quarks and gluons in a nucleon

6. Quark-Antiquark Creation
J/y
No one’s ever measured a free quark.
Physicists now believe that free quarks cannot be observed; they can only exist within hadrons. This is called confinement.
This occurs because the force between the quarks increases rapidly with distance, and the energy supplied to separate them creates new quark-anti- quark pairs.

7. Fundamental and Composite Particles
We call certain particles fundamental; this means that they aren’t composed of other, smaller particles. We believe leptons, quarks, and gauge bosons are fundamental particles.
Other particles are composites, made from the fundamental particles.
Some of these fundamental particles (W, Z, m, t) have short lifetimes and decay, but this is okay.

8. The Families of Matter
The three generations (or families) of matter. Note that both quarks and leptons exist in three distinct sets. One of each charge type of quark and lepton make up a generation.
All visible matter in the universe is made from the first generation.
Second- and third-generation particles are unstable and decay into first-generation particles. Second-generation particles occur in astrophysical objects and cosmic rays. Third-generation particles were probably important in the early universe. All are produced in accelerators.

9. The Four Fundamental Interactions

10. The Four Fundamental Interactions
Photons and gravitons are massless. W and Z bosons are heavy. Gluons are also massless and appear to violate our calculation of the inverse relation between the range and mass of such particles, but, because quarks are confined within hadrons, this effectively limits the range of the strong interaction to 10−15 meters, roughly the size of an atomic nucleus.
But why are there four fundamental interactions?

11. The Standard Model
Over the latter half of the 20th century, numerous physicists combined efforts to model the electromagnetic, weak, and strong interactions, which has resulted in The Standard Model.
It is currently widely accepted.
Image from
It is a relatively simple, comprehensive theory that explains hundreds of particles and complex interactions with six quarks, six leptons, and four force-mediating particles.
It’s based on three independent interactions, symmetries and coupling constants.

12. The Higgs Boson What about all the particle masses?
The Standard Model of particle physics proposes that there’s a field called the Higgs field that permeates all of space.
By interacting with this field, particles acquire mass. Particles that interact strongly with the Higgs field have heavy mass; particles that interact weakly have small mass.
The Higgs field requires another boson.
It’s called the Higgs particle (or Higgs boson) after Peter Higgs. .
Wikipedia

13. The Higgs boson was recently detected!
That little bump? That's where CERN has seen a significant number of unusual events at about 125 GeV, which means that something new is going on.
Are we sure it’s the Higgs? All evidence to date suggests yes.
Higgs!

14. James Clerk Maxwell (1831-1879)
In the mid-19th century, Maxwell unified electricity and magnetism into a single force with his now famous equations.
In free space:
Ampère’s Law
The Scottish physicist James Clerk Maxwell, b. Nov. 13, 1831, d. Nov. 5, 1879, did revolutionary work in electromagnetism and the kinetic theory of gases. After graduating (1854) with a degree in mathematics from Trinity College, Cambridge, he held professorships at Marischal College in Aberdeen (1856) and King's College in London (1860) and became the first Cavendish Professor of Physics at Cambridge in Maxwell's first major contribution to science was a study of the planet Saturn's rings, the nature of which was much debated. Maxwell showed that stability could be achieved only if the rings consisted of numerous small solid particles, an explanation still accepted. Maxwell next considered molecules of gases in rapid motion. By treating them statistically he was able to formulate (1866), independently of Ludwig Boltzmann, the Maxwell-Boltzmann kinetic theory of gases. This theory showed that temperatures and heat involved only molecular movement. Philosophically, this theory meant a change from a concept of certainty--heat viewed as flowing from hot to cold--to one of statistics--molecules at high temperature have only a high probability of moving toward those at low temperature. This new approach did not reject the earlier studies of thermodynamics; rather, it used a better theory of the basis of thermodynamics to explain these observations and experiments.
Maxwell's most important achievement was his extension and mathematical formulation of Michael Faraday's theories of electricity and magnetic lines of force. In his research, conducted between 1864 and 1873, Maxwell showed that a few relatively simple mathematical equations could express the behavior of electric and magnetic fields and their interrelated nature; that is, an oscillating electric charge produces an electromagnetic field. These four partial differential equations first appeared in fully developed form in Electricity and Magnetism (1873). Since known as Maxwell's equations they are one of the great achievements of 19th-century physics.
Maxwell also calculated that the speed of propagation of an electromagnetic field is approximately that of the speed of light. He proposed that the phenomenon of light is therefore an electromagnetic phenomenon. Because charges can oscillate with any frequency, Maxwell concluded that visible light forms only a small part of the entire spectrum of possible electromagnetic radiation.
Maxwell used the later-abandoned concept of the ether to explain that electromagnetic radiation did not involve action at a distance. He proposed that electromagnetic-radiation waves were carried by the ether and that magnetic lines of force were disturbances of the ether.
Maxwell’s term (displacement current)
James Clerk Maxwell ( )
where is the electric field, is the magnetic field, and c is the velocity of light.

15. Unifying the Electromagnetic and Weak Forces: the Electroweak Theory
Sheldon Glashow (1932- )
In the 1960s, Sheldon Glashow, Steven Weinberg, and Abdus Salam unified the electromagnetic and weak interactions into what they called the electroweak theory.
Steven Weinberg ( )

16. Grand Unifying Theories
Several grand unified theories now combine the weak, electromagnetic, and strong interactions into one interaction and predict that, at extremely high energies (>1014 GeV), the electromagnetic, weak, and strong forces fuse into a single unified field.
Currently, they can explain why:
Neutrinos have a small, nonzero mass.
The proton and electron charges have the same magnitude.
Massive magnetic monopoles may exist. Just one anywhere in the universe would explain charge quantization.
But current experimental measurements have shown the proton lifetime to be greater than 1035 years. Current theory has it at 1029 to years.
Wikipedia: Physicists feel that three independent gauge coupling constants and a huge number of Yukawa coupling coefficients require far too many free parameters, and that these coupling constants ought to be explained by a theory with fewer free parameters. A gauge theory where the gauge group is a simple group only has one gauge coupling constant, and since the fermions are now grouped together in larger representations, there are fewer Yukawa coupling coefficients as well.

17. Including Gravity: String Theory
For the last two decades there has been a tremendous amount of effort by theorists in string theory, which has had several variations. The addition of super-symmetry resulted in the name theory of super-strings.
In super-string theory, elementary particles don’t exist as points, but rather as tiny, wiggling loops that are only 10−35 m in length.
Presently super-string theory is a promising approach to unify the four fundamental forces, including gravity.
However, because experiments that can confirm or falsify string theory require orders of magnitude more energetic particles than can currently be produced, these theories are very controversial.

18. Super-Symmetry
Super-symmetry is a necessary ingredient in many of the theories trying to unify all four forces of nature.
The symmetry relates fermions and bosons. All fermions have a super-partner: a boson of equal mass, and vice versa.
These particles include sleptons, squarks, axions, winos, photinos, zinos, gluinos, and preons.
The super-partner spins differ by ħ/2.
Presently, none of the known leptons, quarks, or gauge bosons can be identified with a super-partner of any other particle type.

19. M-Theory
Recently theorists have proposed a successor to super-string theory called M-theory.
M-theory has 11 dimensions (ten spatial and one temporal) and predicts that strings coexist with membranes, called “branes” for short.
Only through experiments (which no one currently knows how to do, since they require > 1,000,000 times more energy than current accelerators can produce) will scientists be able to wade through the vast number of unifying theories.
Background:
Membrane:

20. Another Challenge: Matter vs. Antimatter
Galaxy
According to current theory, matter and antimatter should have been created in exactly equal quantities.
And baryon number is conserved!
Anti-galaxy?
But it appears that, in our universe now, matter dominates over antimatter, and the reason for this has puzzled physicists and cosmologists for years.
Events in the early universe may be responsible for this asymmetry. But explanations go far beyond the standard model.
Image from