Smashing success: accelerator gets cool upgrade.

A novel scheme for increasing the number of collisions in particle accelerators has boosted the performance of the world's highest-energy collider and promises to rev up others.

This scheme, called high-energy electron cooling, helped the Tevatron collider at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., last October to shatter the 23-year-old world record for particle-collision rates.

For decades, particle physicists have used electron cooling to control the properties of particles in low-energy accelerators, but they were daunted by the difficulty of high-energy cooling, comments beam-cooling specialist Fritz Caspers of the European Organization for Nuclear Research in Geneva. To finally develop and implement such a system is "a really great achievement," he says.

Accelerators must generate vast numbers of collisions to produce even a few of the exceedingly rare elementary particles sought by high-energy-physics researchers. In the Tevatron accelerator, discrete bunches of protons and antiprotons circulate in opposite directions around a 6-kilometer ring. They travel at nearly the speed of light. When these bunches cross paths at two locations in the ring, particles smash into each other and spawn sprays of other elementary particles that are recorded by huge detectors.

However, most protons and antiprotons zoom right past each other, explains physicist Sergei Nagaitsev, leader of electron cooling at Fermilab. That's largely because the antiproton bunches tend to be hot and therefore spread out. To pack antiprotons more tightly in each bunch, Nagaitsev and his colleagues created a separate electron accelerator that serves as the heart of the electron-cooling system. That accelerator ramps up electrons to the same velocity as that of the antiprotons and then injects the electrons into a ring. There, the two types of particles interact before the antiprotons enter the main ring and encounter the protons.

Because each electron weighs only a fraction of what an antiproton weighs, jostling among the particles tends to transfer energy to the electrons. Those energy transfers decrease random vibrations of the antiprotons, in effect cooling them, Nagaitsev explains. That, in turn, makes it possible to have more antiprotons in each bunch, increasing its density and the subsequent collision rate.

"The Fermilab work is particularly significant for us," says Ilan Ben-Zvi of Brookhaven National Laboratory in Upton, N.Y. He and his team plan to build upon it to equip a giant accelerator there with even higher-energy electron cooling.

The Tevatron's surging collision rate increases the chances that the machine will yield important discoveries in coming years, Nagaitsev says. In the debris of future smashups, physicists will search for such long-hunted prizes as the Higgs boson, thought to bestow mass on other particles, and supersymmetric particles, which are hypothetical sister particles to the particles already known (SN: 6/12/04, p. 371).

Many other tweaks to the Tevatron have contributed incrementally to its collision rate. However, electron cooling by itself has so far resulted in a roughly 50 percent increase in the Tevatron's instantaneous collision rate, Nagaitsev says. Another 50 percent boost might be possible with further improvements, he adds.

Nagaitsev and his colleagues describe their system in the Feb. 3 Physical Review Letters.