03 April 2014
A particle accelerator that generated collisions between
beams of protons and antiprotons at the highest energies
The people behind the invention:
Robert Rathbun Wilson (1914- ), an American physicist and
director of Fermilab from 1967 to 1978
John Peoples (1933- ), an American physicist and deputy
director of Fermilab from 1987
Putting Supermagnets to Use
The Tevatron is a particle accelerator, a large electromagnetic device
used by high-energy physicists to generate subatomic particles
at sufficiently high energies to explore the basic structure of matter.
The Tevatron is a circular, tubelike track 6.4 kilometers in circumference
that employs a series of superconducting magnets to accelerate
beams of protons, which carry a positive charge in the atom, and
antiprotons, the proton’s negatively charged equivalent, at energies
up to 1 trillion electron volts (equal to 1 teraelectronvolt, or 1 TeV;
hence the name Tevatron). An electronvolt is the unit of energy that
an electron gains through an electrical potential of 1 volt.
The Tevatron is located at the Fermi National Accelerator Laboratory,
which is also known as Fermilab. The laboratory was one of
several built in the United States during the 1960’s.
The heart of the original Fermilab was the 6.4-kilometer main accelerator
ring. This main ring was capable of accelerating protons to
energies approaching 500 billion electron volts, or 0.5 teraelectronvolt.
The idea to build the Tevatron grew out of a concern for the
millions of dollars spent annually on electricity to power the main
ring, the need for higher energies to explore the inner depths of the
atom and the consequences of new theories of both matter and energy,
and the growth of superconductor technology. Planning for a
second accelerator ring, the Tevatron, to be installed beneath the
main ring began in 1972.
Robert Rathbun Wilson, the director of Fermilab at that time, realized
that the only way the laboratory could achieve the higher energies
needed for future experiments without incurring intolerable
electricity costs was to design a second accelerator ring that employed
magnets made of superconducting material. Extremely powerful
magnets are the heart of any particle accelerator; charged particles
such as protons are given a “push” as they pass through an electromagnetic
field. Each successive push along the path of the circular
accelerator track gives the particle more and more energy. The enormous
magnetic fields required to accelerate massive particles such
as protons to energies approaching 1 trillion electronvolts would require
electricity expenditures far beyond Fermilab’s operating budget.
Wilson estimated that using superconducting materials, however,
which have virtually no resistance to electrical current, would
make it possible for the Tevatron to achieve double the main ring’s
magnetic field strength, doubling energy output without significantly
increasing energy costs.
Tevatron to the Rescue
The Tevatron was conceived in three phases. Most important,
however, were Tevatron I and Tevatron II, where the highest energies
were to be generated and where it was hoped new experimental findings
would emerge. Tevatron II experiments were designed to be
very similar to other proton beam experiments, except that in this
case, the protons would be accelerated to an energy of 1 trillion
electron volts. More important still are the proton-anti proton colliding
beam experiments of Tevatron I. In this phase, beams of protons
and antiprotons rotating in opposite directions are caused to collide
in the Tevatron, producing a combined, or center-of-mass, energy
approaching 2 trillion electron volts, nearly three times the energy
achievable at the largest accelerator at Centre Européen de Recherche
Nucléaire (the European Center for Nuclear Research, or CERN).
John Peoples was faced with the problem of generating a beam of
antiprotons of sufficient intensity to collide efficiently with a beam
of protons. Knowing that he had the use of a large proton accelerator—
the old main ring—Peoples employed the two-ring mode in
which 120 billion electron volt protons from the main ring are aimed
at a fixed tungsten target, generating antiprotons, which scatter
from the target. These particles were extracted and accumulated in a
smaller storage ring. These particles could be accelerated to relatively
low energies. After sufficient numbers of antiprotons were
collected, they were injected into the Tevatron, along with a beam of
protons for the colliding beam experiments. On October 13, 1985,
Fermilab scientists reported a proton-antiproton collision with a
center-of-mass energy measured at 1.6 trillion electron volts, the
highest energy ever recorded.
The Tevatron’s success at generating high-energy proton antiproton
collisions affected future plans for accelerator development
in the United States and offered the potential for important
discoveries in high-energy physics at energy levels that no other accelerator
Physics recognized four forces in nature: the electromagnetic
force, the gravitational force, the strong nuclear force, and the weak
nuclear force. A major goal of the physics community is to formulate
a theory that will explain all these forces: the so-called grand
unification theory. In 1967, one of the first of the so-called gauge theories
was developed that unified the weak nuclear force and the
electromagnetic force. One consequence of this theory was that the
weak force was carried by massive particles known as “bosons.”
The search for three of these particles—the intermediate vector bosons
W+, W-, and Z0—led to the rush to conduct colliding beam experiments
to the early 1970’s. Because the Tevatron was in the planning
phase at this time, these particles were discovered by a team of
international scientists based in Europe. In 1989, Tevatron physicists
reported the most accurate measure to date of the Z0 mass.
The Tevatron is thought to be the only particle accelerator in the
world with sufficient power to conduct further searches for the elusive
Higgs boson, a particle attributed to weak interactions by University
of Edinburgh physicist Peter Higgs in order to account for
the large masses of the intermediate vector bosons. In addition, the
Tevatron has the ability to search for the so-called top quark. Quarks
are believed to be the constituent particles of protons and neutrons.
Evidence has been gathered of five of the six quarks believed to exist.
Physicists have yet to detect evidence of the most massive quark,
the top quark.
Atomic bomb; Cyclotron; Electron microscope; Field ion
microscope; Geiger counter; Hydrogen bomb; Mass spectrograph;
Neutrino detector; Scanning tunneling microscope; Synchrocyclotron.