Advanced General Physics 323/324

Modern Physics Unit EP Elementary Particles

Overview: This unit gives an introduction to the physics of elementary particles. It discusses the different classes of particles, conservation laws which are obeyed during interactions.

Prerequisite: QM5

Read:

R. Eisberg and R. Resnick Quantum Mechanics of Atoms, Solids, Nuclei and Particles (2nd Ed.) , Chapt. 17 - Introduction to Elementary Particles; Chapt. 18 - More Elementary Particles, Sec. 3,4.

COMMENT:

The material on elementary particles in Eisberg and Resnick is somewhat dated, and has little on quarks, and nothing on the standard model and quantum chromodynamics, not to speak of string theory and supersymmetry. More up-to-date material in this field can be found in a website called the particle adventure . It is strongly suggested that you go through the material in this website and take a few notes.

The chart below shows the elementary fermions of the standard model. These are divided into 3 "generations", each of which contains two quarks and two leptons. Note that the upper and lower quarks have charges 2/3 and -1/3, respectively, while in each generation one of the leptons has a negative charge while the other (the neutrino) is neutral. Each of these fermions has an anti-particle with opposite quantum numbers, including "flavor" and charge.

The charts below show the elementary bosons of the standard model, the exchange of which is responsible for the four different "forces" or interactions. The electromagnetic and weak forces are closely related in the standard model, with the "weakness" of the latter at low energy due to the massive nature of the W and Z bosons. The gluon carries color charge and therefore interacts directly with itself, unlike the neutral photon

The chart below displays the two different types of fermions and bosons involved in the standard model. These are the "elementary" ones described in the charts above, and the composite "everyday" baryons and mesons such as protons, neutrons and pions which are constructed from quarks and anti-quarks. Because of color confinement free quarks anf gluons do not exit in nature: only the color-neutral baryons and mesons can be found in isolation, along with the non-colored leptons and electroweak bosons.

After completing "the particle adventure" you should understand::

  1. How many fermions are there, and into which two categories do they fall?
  2. Which of the fermions interact via strong, electromagnetic, and weak forces?
  3. What quantum numbers are conserved by the various interactions (forces)?
  4. The composite strongly-interacting particles known as hadrons fall into two classes: baryons and mesons. What are the quark contents and possible spins of these particles?
  5. The meaning of confinement and asymptotic freedom.
  6. The meaning and role of the "color" degree of freedom in quantum chromodynamics.

Problems:

Chapt. 17: Questions 1,4,5,6,12,18,21,22,26 and Problems 17,18;

Indicate which of the reactions below violates one or more conservation laws and name the law(s) in each case.

  1. p → π+ + e+ + e- .
  2. p + pbar → γ + γ .
  3. n → p + π- .
  4. μ- → e- + ν + νbar .
  5. e+ + e- → γ .
  6. ν + p → n + e+ .

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