Section 9.2 Particle Types
It appears that on the fundamental level, all matter consists of particles. But what are these particles? How can they be distinguished and identified? Are there a few basic types from which all matter is constructed? These are some of the questions to be addressed in this section.
Now to the question of identifying particles. Basically, a particle is characterized by its mass and charge, but there are many other properties that contribute to identification. In the relativistic momentum and energy lab from Physics 211, you calculated the mass and charge of an invisible particle by invoking the conservation laws of energy and momentum. You studied the reaction
and determined that particle \(X\) had a mass about as big as a proton, but was charge neutral. This was sufficient information to identify \(X\) as a neutron.
In the 1930's, the only particles known were protons, neutrons, electrons, and photons. As better detectors became available, new particles were identified from reactions initiated by cosmic rays and as products of radioactive decay. For instance, when cosmic rays interact with gas in the upper atmosphere muons are created. These are negatively-charged particles that decay quickly into electrons, releasing large amounts of energy by the reaction \(\mu^- \to e^- + \mbox{energy}\text{.}\) Muons are not a part of normal matter and their existence was a puzzle for physicists.
Another milestone in particle physics was the discovery of the positron, also known as an antielectron. In 1932, Carl Anderson succeeded in identifying a positive particle in cosmic rays with the same mass as an electron. Using a cloud chamber in a magnetic field, he photographed an electron-like track being deflected in the opposite direction that one would expect from an electron. This was the first direct evidence of an antiparticle and served to confirm Dirac's theoretical prediction that all particles have antiparticles. Since then antiparticles have been found for essentially all known particles. Antiparticles have the same mass and spin as their corresponding particles, but take the opposite sign for additive properties like charge.
Useful as these studies were, the experimental physicists' dependence on naturally occurring energetic particles was very restrictive. In the 1940's and 50's, particle accelerators were developed that enabled physicists to extend vastly the range of energies and particles available for study. Particles like pions (\(\pi\)) and neutrinos (\(\nu\)) were found in reactions such as
and
Most of the particle physics of the 1950's and 1960's consisted of banging particles together with more and more energy and seeing what came out. With higher energy accelerators, more and more kinetic energy of the incident particle could be converted to rest energy of new particles. An explosion of new particles, with names like lambda (\(\Lambda\)), sigma (\(\Sigma\)), kaon (\(K\)), tau (\(\tau\)) and omega-minus (\(\Omega^-\)), appeared on the scene (see Tables 9.1 through Table 9.3). From the three or four particles known in the first third of the 20\(^\text{ th }\) century, the list of “elementary” particles had grown to several hundred. Fermi is said to have remarked that if he had known that there were so many particles whose properties he was expected to memorize, he would have taken up botany! 1
A list of particles known today would extend to many hundreds of entries. The problem of devising separate symbols to name the particles long ago exhausted the resources of the Greek and Latin alphabets and numbering particles with their masses is now commonplace. A database maintained by the Particle Data Group (see http://pdg.lbl.gov/) contains entries like \(\Delta\)(1910) or \(f'\)(1523), with a listing of the particle's properties (mass, charge, spin, lifetime, etc.). But to make any sense of this huge list of particles and properties, we must look at the classification schemes available.