DPF Public Information

The Division of Particles and Fields is part of the American Physical Society, the professional organization of phsyicists in the U.S. This web page is designed to provide resources for those interested in learning more about particle physics. It is intended, in particular, to be of use to journalists and science writers looking for background on new results.

What is Particle Physics?

What are things made of? Our curiosity as children compels us to ask such questions. For particle physicists, finding the answer is a lifetime quest. The surprising result of the research of the past fifty years is that the subatomic world is astonishingly simple. There are altogether very few components, all of which are very similar to each other. Similarly, there are very few forces and these again resemble each other to a remarkable degree.

An atom has two basic components: a nucleus and cloud of electrons surrounding it. Inside the nucleus, protons and neutrons are bound together by the "strong force." Particles that feel the strong force are called "hadrons" and are made of quarks. Particles, like the electron, that do not feel the strong force are called "leptons." All of this, and much more are explained in the informative and entertaining Particle Adventure.

Research in particle physics is an international enterprise. There are major accelerator laboratories in China, Japan, Germany, Switzerland, France, and the U.S. The World Wide Web is a very important tool for particle physics research and was, in fact, invented at the European Laboratory for Particle Physics, CERN. A single experiment may have scientists from twenty or thirty nations. There are perhaps 10,000 particle physicists worldwide, with about 3,000 in the U.S.

In the U.S., particle physics is supported by the Department of Energy and by the National Science Foundation. DOE funds work at

  • Stanford Linear Accelerator Center (SLAC),
  • Fermi National Accelerator Center (Fermilab),
  • Brookhaven National Laboratory ,
  • Lawrence Berkeley National Laboratory (LBNL) and Argonne National Laboratory. It also funds many research groups at universities. NSF funds the Cornell Electron Storage Ring (CESR) and many university research teams.

    What's Happening Today

    The last of the six quarks, the

  • t or top was discovered in 1995 at Fermilab. This was the result of a search begun in 1977, with the discovery of the fifth quark, the b or bottom. No one anticipated that the top quark would turn out to be 40 times heavier than its predecessor. It will be a continuing topic of research as we try to understand its anomalously large mass. It will be a particular target of research Fermilab's improved proton-antiproton collider.

  • The b quark has a special fascination because particles containing it are expected to display CP violation, a sort of asymmetry between particles and antiparticles. Andrei Sakharov first showed that CP violation is essential to explaining why there is more matter than antimatter in the Universe. Many laboratories are rushing to explore this phenomena. Among the experiments aiming to measure CP violation in b particles are

  • Neutrinos are the neutral partners of the electron and similar particles. They are known to be very light compared to particles like the electron, but it is not known whether they are, in fact, massless. If they have mass, the different kinds of neutrinos actually transform themselves back and forth between one species and another. It would then be possible to observe these "oscillations," which would demonstrate that neutrinos are massive. One source of neutrinos is the Sun. Some experiments are measuring the flux of neutrinos from the Sun in the hope of detecting oscillations. Among the experiments looking for neutrino oscillations are

  • One popular theory, supersymmetry, suggests that there are many more elementary particles yet to be discovered, in fact, one unknown particle for each particle we already know. Evidence for supersymmetry is being sought in the collisions of electrons and positrons at CERN's Large Electron Positron collider (LEP), where there are four experiments: The CDF, and D0 experiments at Fermilab's Tevatron collider will also be looking for supersymmetry when they begin running again in 2000.

  • The accelerators we have today may not provide enough energy to answers the most fundamental questions. The Superconducting Super Collider was designed to address them. Since the cancellation of the SSC in 1993, the hopes of the particle physics community for progress at the highest energies have rested with the Large Hadron Collider project at CERN. There are four experiments planned for this machine, which should begin operation in 2005. Two of these, ATLAS, and CMS are aimed at the very highest energy collisions.


    About one hundred years ago, discoveries by J. J. Thomson, Ernest Rutherford, Marie Curie, Niels Bohr, and their contemporaries demonstrated that all matter is made of atoms. Each atom has a positive nucleus surrounded by negative electrons.

    In the 1920's and 1930's, Erwin Schrodinger, Werner Heisenberg, Wolfgang Pauli, Paul Dirac, and others showed that the atomic and subatomic worlds are governed not by Newton's Laws, but by quantum mechanics, which predicts the probabilities of each possible outcome of any measurement. With the discovery of the neutron by James Chadwick in 1932, it became apparent that the nucleus is made of protons and neutrons.

    Beginning in the 1930's, cosmic rays revealed new particles that do not appear in ordinary matter. These included the positron (anti-electron), the muon (a heavy version of the ordinary electron), and peculiar particles that were dubbed "strange."

    Starting in the 1950's, particle accelerators overtook cosmic rays as the most effective means of revealing new elementary particles. Soon there were dozens of such particles. In the mid-1960's it was realized that the myriad strongly interacting particles (hadrons) could be understood as being built from more fundamental particles, quarks.

    In the 1970's the quark description became more and more persuasive. A simple model was proposed by Sheldon Glashow, Steven Weinberg, and Abdus Salam that could explain the electromagnetic and weak forces as consequences of a single (electroweak) interaction. An analogous theory was proposed that would explain the strong interactions.

    Extensive tests in the 1980's and 1990's verified the "Standard Model," the combination of the electroweak and strong interaction theories. Nevertheless, this raised more questions than it answered. Among these questions are: what determines the masses of elementary particles? what determines the number and types of elementary particles? what explains CP violation? Are the electroweak and strong interactions connected to the gravitational force?

    Other web sites that provide information on particle physics for the public:

  • A Brief Overview of High Energy Physics
  • A Guided Tour of Fermilab Exhibit
  • CERN's Microcosm Exhibit
  • Particle Adventure.
  • Chart of the Standard Model
    Robert N. Cahn
    Last modified: Mon Aug 10 08:02:45 PDT 1998