Experiment VI-6

Elementary Particles

finding the building blocks of the universe


The 2000 year old search for the building blocks of the universe started by the Greeks had led to a list of about 90 kinds of atoms build with nuclear cores of protons and neutrons surrounding by electrons.

exchange particleUnderstanding the electric glue that held the electrons to the nucleus provided another challenge.  As early as 1875 James Clerk Maxwell had suggested that atoms must have structure more complicated that rigid bodies.  Many scientists presumed there was a connection between the long known electrical charges on chemical ions, the resulting chemical bonds, and emission of spectra of light when atoms were heated.  Many thought that atomic spectra must originate like radio broadcasts from oscillating electric charges inside of atoms.  In 1905 Albert Einstein explained the photoelectric effect saying that a quanta of light carried energy (E = hν) than when absorbed caused the ejection of an electron.  It was realized by Einstein and others that momentum would also be transferred during emission and absorption of bundles of light resulting in electrical force between atoms.  It seemed that electrical forces could be entirely explained as continuous exchanges of virtual (hidden) quanta of light, finally named photons by G.N.Lewis in 1926.

Paul DiracIn 1924 Louie deBroglie proposed that particles such as electrons and protons should also exhibit wave properties.  By 1926 Werner Heisenberg had invented matrix mechanics, Erwin Schrödinger developed wave mechanics, and Paul Dirac (1902-1984, AIP photo to left) demonstrated their two theories treating electrons as waves are actually equivalent.  By 1928 Dirac had combined Schrödinger's quantum mechanics and Einstein's special relativity formulating mathematical equations describing the electron.  Max Born upon learning of Dirac's equation said Physics as we know it will be over in six months.  But the equations (a bit like the well known quadratic equation) had two solutions: a wave solution and a negative or inverse wave solution.  Dirac first interpreted the second solution to represent protons but by 1931 realized the second solution required the same mass as the electron, but positively charged.  What he called positrons would be anti-matter, that is electron holes in space such that combining an electron wave and the inverse anti-electron would result in cancellation leaving nothing but emitted photons of electromagnetic energy.  The negative electric charge of the electron would cancel the positive charge of the anti-electron.  The following year Carl Anderson recorded photographs of tracks left by cosmic rays in a cloud chamber carried to the upper atmosphere.  The tracks matched the expected properties of anti-electrons; That same year Dirac completed the mathematics for protons and neutrons and predicted the existence of similar anti-protons and anti-neutrons.  Anti-protons were confirmed in tracks from collisions of beam from a particle accelerator in 1954.  Anti-neutrons were confirmed in 1956.  Presumably a whole world made of anti-atoms assembled from anti-particles might be possible.

Hadeki YukawaIt was apparent that the electric force was incapable of holding protons and neutral neutrons together as nuclei.  And gravity was far too weak a force to do so.  Therefore the universe requires a third force.  This was first called the nuclear force and later the strong nuclear force.  It was clear this must be different from the electric force and gravity which diminish by the square of increased distance, 1/d2.  At tiny distances the nuclear force had to be stronger than the electrical repulsions between protons in nuclei, but weaker at larger distances so that neighboring atoms would not collapse to a single giant nucleus.  A new field theory was proposed in 1935 by Hideki Yukawa (1907-1981 photo at right) of Osaka and Kyoto Universities, Japan, to explain the nuclear force.  Similar to electromagnetic forces which are mediated by exchanges of massless photons, the nuclear force would involved exchanges of a similar particle but with mass.  The uncertainty principle of Werner Heisenberg based on the fuzziness of waves allowed a massless particle such as a photon to have diminishing effects out to infinity without violating the restriction known as conservation of energy.  But an exchange particle with non-zero mass could only have effects and be exchanged to a finite distance permitted by Heisenberg's formula Δx ΔP ≥ h/2π.  Using the know size of nuclei, Yukawa predicted the exchange particle needed to account for the nuclear force would need a mass of about 200 times that of the electron (but about 1/10 that of protons and neutrons).

According the theory of matter and anti-matter, if a collision occurred with enough energy specified by Einstein's equation E = mc2, a particle and its anti-particle could be created.  This was observed to happen in collisions of high energy cosmic rays with atoms in the upper atmosphere.  But such collisions are rare and, because of their locations, hard to detect.  So particle accelerators were constructed to speed large numbers of electrons and later protons to energies sufficient for collisions to create and investigate particles such as predicted by Yukawa.  That effort has discovered hundreds of particles.  While the particle predicted by Yukawa, now called the π meson, was discovered in 1947, the large number of unexpected particles meant that the universe is far more complex than a universe made exclusively of atoms moving with energy.  Yukawa was awarded the 1949 Nobel Prize for Physics.


predicting radiation behavior using Monte Carlo simulation

To improve the resolution of the tracks made visible in cloud chambers (described in Experiment VI-6), a half century later the saturated air was replaced with a liquid on the verge of boiling.  In such a bubble chamber, the formation of bubbles would scatter light much like the droplets in a cloud chamber.  If extremely cold liquid Hydrogen was used, occasionally the radiation collided with protons in the Hydrogen, revealing clues about the interactions between the elementary particles which constitute matter.

While cloud chambers of condensing vapor and bubble chambers of boiling liquids have been retired and replaced by faster detectors, archived photographs from such chambers still provide visual insights into the nature of high energy particles and the procedures we still used to learn about the particles' properties and interactions.  Today such trails are electronically detected then presented on computer screens looking much like the older photographs.  So understanding the imaging processes and studying the particle tracks can help us understand the nature of matter.

A single high energy particle may interact with matter in several ways:  Passing close by, a portion of its energy and momentum may be transferred to particles in the target.  Ejected particles leave additional diverging trails.  If energy is great enough, interaction may transform energy into pairs of particles and their anti-particles (E = mc2) which in the presence of perpendicular magnetic fields curl in opposite directions.  Or the radiation might pass through the matter without any interaction.  And if particles without electric charge are formed, they leave no trail at all.  While we cannot predict the exact interaction caused by a single high energy particle, the possibilities and probabilities are thought to be well understood.  (Predictions of the Standard Model described in Experiment VI-9 match experimental results extremely well.)  A computer program called EGS was developed at Stanford University using the probability rules of Monte Carlo games to simulate what a beam of high energy radiation can be expected to do in matter.

In this investigation we will use the EGS computer simulator at Stanford University to calculate in real time possible tracks for high energy collisions of our choosing.  In Experiment VI-7 we will view, compare and interpret real tracks on photographs archived by CERN, the European high energy research institution.


Use this link to proceed to EGS at the Stanford Linear Accelerator Center (SLAC).

  1. You may wish to browse around their site and learn more about EGS.  But be sure to
  2. take the Guided Tour, and
  3. do the Lab Activities.



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created 9/24/2003
revised 12/17/2004
by D Trapp
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