Elementary Particles -------------------------- Joseph F. Alward, PhD Department of Physics University of the Pacific

 Until 1932, the "elementary" particles were the electron,
 proton, and neutron. We now know of hundreds of other
 elementary particles.  

 

   Pion Production

Hidekei Yukawa (1907-1981) In 1935 Yukawa predicted existence of pions, which were finally discovered in 1947.

Pion decays into two gamma photons in 0.8 x 10-16 s.

  Matter from Energy

Other elementary particles can be created out of thin air from pure energy; the positron, for example.

    Antiparticles

Paul Dirac (1902-1984) 1933 Nobel Prize In 1928 Dirac redicted the existence of the positron

The positron was discovered in 1932 by C.D. Anderson. In this false-color cloud chamber photograph a gamma-ray photon  disappears and in its place appear an electron and its antiparticle, the positron, a particle of equal mass but opposite charge.

  Missing Energy in Beta Emission
   (Leads to discovery of another elementary particle)

Electron (beta particle) doesn't have the same kinetic energy each time.  Why?

  The Neutrino

Wolfgang Pauli, Austrian physicist (1900-1958) In 1931 predicted existence of a particle which Enrico Fermi called "neutrino". 1945 Nobel Prize

The neutrino was found in 1956.  It is believed to have zero charge, and practically zero mass.

  The Anti-Neutrino

The antineutrino carries away the "missing" energy.

  Neutrinos from Supernova

Supernova 1987A 170,000 light years away emitted a blast of neutrinos traveling near the speed of light.

Two hours later the supernova began emitting a blaze of light a million times brighter than the Sun.

Today, every square centimeter on Earth is struck by a neutrino each second.

   Positron-Electron Annihilation

The positron is the anti-particle to the electron, and vice-versa.    When they come together, they annihilate one another; pure  energy appears in their place in the form of gamma photons  moving in opposite directions.

  Positron Emission Tomography 

"tomos":  slice

8O15 -------> 7N15 + 1e0   then  1e0  +  -1e0 -------> g + g

  PET Scan Image of Brain

Healthy brain        Brain with Alzheimer's                                               disease

 
  Material Particles

Leptons Examples:       muon (-)    electron        neutrino ----------------- No internal structure

Hadrons Examples:    neutron      proton    pion (+) ---------------- Believed to contain quarks.

                                                                                (The pion is also called a pi-meson.)

 Quarks

Hadrons Examples:    neutron      proton    pion (+) ---------------- Believed to contain quarks.

Murray Gell-Mann took the name quark from "Three quarks for muster Mark", in James Joyce's book Finnegan's Wake. (1963)

Whimsical names-- called "flavors"--for the quarks.

  Molecules, Atoms, and Nuclei

   Nuclei, Nucleon, and Quarks

   Quark Charge

The neutron contains three quarks.  Which three quarks could be used to make a neutron? The proton contains three quarks.  Which three quarks could be used to make a proton? The pion has a charge of +1 and contains two quarks.   Which two quarks--if any--could be used to make a pion?

  Quarks and Anti-Quarks

The Pion  (Two quarks; charge:  + 1) Which two quarks could be used to make a pion? The Neutron     (Three quarks; charge:  0) Could a neutron have one or more antiquarks?

 Quark Structure of the Pion, Proton, and Neutron

  Quark Color Causes Strong Force (Nuclear Force)

Moo-Young Han, Duke Univ

In 1965 Moo-Young Han and Yoichiro Nambu suggested quarks possess color.  The "color" attribute is not traditional color; the name is somewhat arbitrary, and almost as whimsical as the names of the quarks.

Color is also called color charge. Like colors repel. Unlike attract. Color-AntiColor attraction is stronger. Blue-AntiBlue stronger than Blue-Red, for example

  Quark-Containing Particles are White

Protons and neutrons contain  three quarks:  Protons and neutrons each  contain a  red, blue, and  green quark.

Pi-mesons contain only two quarks. For example:  Anti-red is white light minus red,  or, blue-green.

  Quarks Exert Force by Exchanging Color

Material particles: Leptons  Examples:     muon (+)  electron      neutrino -------------   Leptons   contain   no quarks  Hadrons  Examples:  neutron  proton  pion (+) ----------- Hadrons contain quarks

Color charge is the cause of the color force, also known as the     strong force.

 Quarks and Gluons

The strong force is caused by the emission and absorption of gluons.

Rule:  Sum of colors conserved

Red-antigreen gluon is emitted by a red quark, which is transformed to a green quark. A green quark, not shown, absorbs the red-antigreen gluon and becomes a red quark.