- Stacking--the number of antiprotons produced;
- Number of consecutive stores without failure--the entire process of injecting bunches of protons and antiprotons and making them collide;
- Peak, or initial, luminosity--the number of collisions per second at the beginning of a Tevatron store;
- And integrated, or delivered, luminosity--the number of collisions produced in the course of a store.
When Mary Convery moved to the Accelerator Division about one and a half years ago, people told her that with major upgrades finished there wasn't much to do but to keep the nearly three-decades-old Tevatron particle accelerator running smoothly.
"'There are no more home runs to get,' I was told," says Convery, Fermi National Accelerator Laboratory deputy run coordinator. "Yet, we keep continuing to make improvements."
In the past five years at the Tevatron, the luminosity--the number of collisions per second--has increased sixfold. In the last six weeks alone, overall luminosity has improved 10 percent, generating more than a dozen luminosity records, sometimes multiple records in one week. Since October, the Tevatron has had nine of the top ten stores in its history.
The Tevatron consistently tops past integrated luminosity records even without increasing the number of protons and antiprotons injected into the accelerator ring.
The higher the luminosity, the better the chance for discovery--including sighting the Higgs boson, theorized to impart mass to other particles. Higher luminosity increases the probability of collisions producing a discovery of new physics, that discovery coming sooner and the precision at which scientists can study the Standard Model.
The Tevatron is on track to beat the FY 2009 estimated maximum weekly integrated luminosity by 20 to 25 percent. Earlier this month, the machine showed its stamina with a run of 15 uninterrupted stores. Achieving a run of more than six stores without interruption is considered good.
"So we are getting more bang for the buck," says Cons Gattuso, run coordinator.
The Tevatron's FY2009 luminosity consistently exceeds that of past years.
The Tevatron stores counter-rotating bunches of protons and antiprotons comparable to handfuls of roughly 100 billion marbles, which circle the ring, increasing their energy until they collide. The more collisions per bunch, the more data.
Quantifying exactly how well the Tevatron is running is difficult because it operates more than 300 times better than it was initially designed to do. Each time physicists think they know the top performance of the machine, they break through it, hitting the equivalent of a home run.
Last Saturday, the Tevatron set a new top record for peak, or initial, luminosity--the number of particle collisions per second when the beam is first brought to collisions. Saturday's store had a luminosity of 3.55 x 1032 cm-2 sec-1 up from a record of 3.45 x 1032 cm-2 sec-1 set Nov. 30.
Although a strong peak luminosity indicates a powerful, well-run machine, weekly integrated luminosity numbers give a better picture of a machine's reliability and overall strength. Fermilab gets top grades there as well.
The fiscal year 2009 goal for the Accelerator Division is to acheive and maintain at least 41.5 inverse picobarns of weekly integrated luminosit--the sum of all the collisions in all the stores during a week's time. The division has been averaging 55 inverse picobarns.
So far, the weekly integrated luminosity record is 63.5 inverse picobarns from the week of Nov. 24 to Dec. 1, up from a record of 57.4 inverse picobarns on July 7. That amounts to a six-fold increase from the weekly integrated luminosity record of 10.65 inverse picobarns set in July 2003 in the beginning of Run II.
The purple dots show the Tevatron's performance in FY2009 excceds the goal, shown by the top line. The lower line shows the minimum acceptable luminosity range for FY2009. The weeks without lines coincide with scheduled maintenance shutdowns.
Big bang for the buck
The AD's knack for fine-tuning the Tevatron to squeeze out the most data while prolonging the life of its parts is a powerful argument for funding to extend the Tevatron's lifespan by a year through 2010.
"It is like a little Fiat we have, but it is running like a Ferrari," Gattuso says. "We are doing extraordinary things with the number of people we have in the division and the laboratory. We have lost a lot of talent through retirements, budget cuts, and transfers to other avenues of employment."
Yet nothing seems to throw the steady progress off course for long.
Seemingly unstoppable
A power outage in early November shut down the Tevatron for several days. Still, crews were able to set another luminosity record a day after restarting.
During the October shutdown, the AD staff fixed a kink in the beam pipe near CDF that had limited the room to send stores of protons and antiprotons. Fixing that kink, which could have been caused by a beampipe shift over time, is the main factor in the recent improvements.
"In a sense, we have opened a door," says Gattuso, who points out that the improvement resulted from cooperation by every division and department.
How much more productive can the Tevatron get?
"As far as the upper limits, we are not sure," Gattuso says.
Fine tuning and efforts to pre-empt breakdowns to prolong the life of the equipment have pulled more out of the Tevatron than ever thought possible.
Still, the AD faces the challenge of finding a balance between delivering beam with as little down time and as many tightly packed protons and antiprotons as possible while experimenters maximize the capacity of the detectors. The AD strives to slowly increase the intensity of the beam to give the detector crews time to adapt to recording increased amounts of data. Working together, the AD and detector groups hope to close in on the Higgs boson.
"We are going to keep pushing," Convery says. "We are trying to squeeze as much luminosity out of this machine as we can."
