Imagine two schools of sardines swimming toward each other. The water is murky, so the fish don't realize they are about to collide. As the schools converge, some fish hit one another, but others slip by unscathed. Obviously, there will be more collisions between oncoming fish if the schools are packed together tightly since there is less space to slip through. But how does one characterize the density of these schools, and therefore the likelihood of collisions? Particle physicists use the word "luminosity" to describe the potential for collisions in this kind of situation. Instead of sardines, they deal with particles.
"In the Tevatron, higher luminosity causes more collisions between protons and antiprotons," said Pushpa Bhat the project manager for Run II upgrades. "Collisions give researchers a chance to discover exotic forms of matter. So high luminosity is what we strive for."
To produce collisions, Fermilab scientists send billions of protons and antiprotons in opposite directions through the 4-mile Tevatron ring. "The protons and antiprotons circulate through the accelerator in hair-width, sausage-shaped bunches," explained Fermilab's Associate Director for Accelerators, Steve Holmes. Thirty-six of these sausage-shaped bunches orbit in each direction for several hours during what physicists call a "store." Each time the bunches in the store converge, a large number of protons and antiprotons slip past each other, but, like the sardines, a few meet head-on.
There are two ways to express the likelihood of collisions, or luminosity, during any particular store: peak luminosity, which describes the initial (and highest) luminosity in a store, and integrated luminosity, which describes how many total collisions are produced over the lifetime of that store. Since an increase in total collisions provides more chances to produce new particles, high integrated luminosity is the key to discovery (see graphic).
The Accelerator Division, which is responsible for providing protons and antiprotons for the Tevatron and getting them up to speed, has recently improved both kinds of luminosity. "Electron cooling has resulted in a significant improvement in the peak luminosity," explained Roger Dixon, who currently manages the Accelerator Division. Cooling the antiprotons before injecting them into the Tevatron packs them close together so that they are more likely to collide with oncoming protons. "We also improved the optics at the collision points," said Dixon. Much as light is focused by a lens, magnets squeeze the protons and antiprotons together on either side of the interaction regions of the two collider detectors, CDF and DZero. This concentrates the beam just before the bunches collide. In addition to raising peak luminosity, the upgrades have also helped maintain high collisions rates for each store. "We've increased the number of antiprotons in each store, and we have made many upgrades that increase reliability," said Bhat. The combination of a higher peak luminosity, and being able to maintain it, have led to better overall levels of integrated luminosity.
Due to these and other upgrades (which will be detailed in upcoming Fermilab Today stories) the Tevatron's integrated luminosity has improved by more than a factor of six since Run I. "Due to Run II upgrades, we are breaking records and making new ones," said Bhat, who came to the Accelerator Division in 2003 to help lead the Run II Upgrade Project. "It required lots of hard work from all across the lab," added Jeff Spalding, who managed the upgrades with Bhat. "But in the last three months we've seen a higher integrated luminosity than all of Run I combined. And that's pretty darn impressive."