Tevatron to Reach Highest Energy Ever

by Kurt Riesselmann

Fermilab scientists are taking yet another step up the energy ladder. After five years of upgrades and tests, scientists are ready to squeeze an extra 10 percent of energy out of the Tevatron accelerator when Run II starts this month.

Technicians have carried out a lot of work to give every proton circulating in the Tevatron an extra 80 GeV (billion electron volts). In everyday life, this extra energy just wouldn't matter. It is orders of magnitude less than the calories contained in a single cornflake. It is negligible compared to the energy a mosquito needs to fly. And it is even less than the energy stored in a single gold atom according to Einstein's famous equation E=mc2.

As a matter of fact, a cosmic particle traveling with 80 GeV energy may well have hit you as you read this story. Cosmic particles constantly arrive here on earth, many traveling at even higher energies. The most powerful particles ever observed had energy of more than 10²⁰ eV, or 100,000,000,000 GeV. That is the energy a high-speed tennis ball transfers when hitting the ground. Confined to a tiny particle, this energy doesn't even cause a dent: Like an x-ray, such a particle traverses thick layers of material, losing its energy along the way.

Compared to the power of the highest-energy cosmic accelerators, which are associated with black holes and violent, star-producing areas in the universe, the new Tevatron beam energy of 980 GeV is about a billion times smaller.

Adding the extra 10 percent should have been an easy task. But not so.

"We pushed the Tevatron to the limit," said Mike Church, the head of the Tevatron Department. He and his group have spent the last three years on maximizing the strength of the Tevatron magnets, which confine protons and antiprotons to the beam pipe while their energy is increased. Eventually, the magnets reached their limit, defining the maximum energy at which protons and antiprotons can circulate in the Tevatron without veering out of the beam pipe.

"You are limited by the weakest link in the chain," Church said. "We weeded out the 20 weakest magnets and swapped other magnets to put the remaining weak ones in cold spots."

Cold spots, according to Church, are areas inside the Tevatron tunnel in which the temperature of the liquid-helium cooling system is less than the usual minus 450 degrees Fahrenheit, or 4.5 kelvins. The more cooling they receive, the more electric current the magnets can conduct and still remain superconducting. Every tenth of a degree is important, but it costs dearly. Fermilab's Cryogenic Department has made significant upgrades to match the increased helium demand by the Tevatron during Run II (FermiNews, Vol. 23, No. 19, Oct. 20, 2000).

This ultimate increase in the Tevatron beam energy could be the critical boost necessary to discover the Higgs boson, the missing keystone in the intricate theoretical construction called the Standard Model of particles and their interactions.

"In a single proton-antiproton collision it is possible to produce the Higgs together with a top and anti-top quark," said Fermilab theorist David Rainwater. "Such an event would leave a distinctive signature in a detector. The increase in Tevatron energy leads to a 65-percent higher chance of producing it." A recent report shows that the Tevatron could produce more than a hundred events with this particular signature in the next years if the Higgs mass is about 115 GeV/c2, a value favored by experimenters at the European research laboratory CERN.

The Tevatron completed its engineering run at the new record beam energy in September of last year and produced the first proton-antiproton collisions. Since then physicists moved two large detectors, CDF and DZero, into the beamline. The Tevatron will resume operation at the beginning of March, and collisions are scheduled to begin at the end of the month.

"It's been a long project," said Church, recalling years of magnet testing and reshuffling. "I'm glad it's over."

For more than 1,000 physicists who will be using the improved Tevatron to conduct their experiments, the fun has just begun.