Information For the Public

The Standard Model of Elementary Particles and Forces

A new experiment at Fermilab will explore the question whether the mass of the subatomic particles called neutrinos is zero, as the Standard Model says, or if, instead, neutrinos have mass.

Physics experiments of the past hundred years have revealed the atom's structure, with its nucleus and orbiting electrons. Today's experiments use powerful particle accelerators to explore the deepest substructure of matter, the particles and forces inside the proton and neutron of the atom's nucleus. Decades of research have now given us a remarkably simple theoretical model of the elementary particles and forces of matter. View Illustration.

The Standard Model

The discovery of the top quark at Fermilab in March, 1995, provided strong evidence for the Standard Model, the prevailing theory that describes the elementary particles and forces. These particles are the matter particles called leptons and quarks; and the force-carrying particles called bosons.

The six leptons include the electron, the muon, and the tau; and three neutral particles postulated to be massless — the electron neutrino, the muon neutrino, and the tau neutrino.
The six quarks include the up and down quarks that make up the proton and neutron, as well as the strange, charm, bottom, and top quarks that were present at the birth of the universe and that we now produce in particle collisions.
The gauge bosons include the gluon that transmits the strong force that holds quarks together in the nucleus; the W and Z bosons that transmit the weak nuclear force responsible for radioactive decay; and the photon that transmits electromagnetic force.

Beyond the Standard Model

Experiment after experiment has tried to find flaws in the Standard Model's predictions, but so far all the the experimental evidence supports it. Nevertheless, scientists do not believe that the Standard Model provides complete answers to all our questions about matter.

For example, data from astrophysics, cosmology, and nuclear and particle physics experiments suggest that the neutrinos might, in fact, have mass, despite the Standard Model's description of these leptons as massless particles. Although the Standard Model now assigns zero mass to neutrinos, it does not rule out the possibility that they might have mass. New experiments at Fermilab are designed to test this possibility, to "weigh" the neutrino. Physicists will design the experiments to look for evidence of neutrino mass in the mass ranges suggested by results of earlier experiments. If experimenters do find evidence for neutrino mass, such a discovery would not only have profound implications for our understanding of cosmology but might also provide a clue to physics beyond the Standard Model.

The discoveries of the past few years have brought us to a deeper understanding of matter and energy, but difficult questions remain: Why does matter have mass? What accounts for the enormous preponderance—crucial to the existence of the universe—of matter over antimatter? What is the invisible matter that accounts for so much of the universe that we cannot see? Are there forces and particles as yet undiscovered? The experiments of the future will explore these questions, in the long, continuing search to understand the nature of matter and energy, space and time.