Tevatron luminosity makes an uphill climb
Collider Run II at Fermilab's Tevatron officially began on March 1, 2001. Since Tevatron operations resumed in November, 2001, after a two-month shutdown for accelerator and detector upgrades, luminosity has increased more slowly than hoped for. Fermilab has in place a plan to raise the luminosity to the desired levels by the end of 2002. Intense efforts now underway to address a range of technical issues in the accelerator complex are beginning to show results. Great physics awaits increased Tevatron luminosity, and so do the approximately 1,200 experimenters at Fermilab's CDF and DZero detectors.
The luminosity of an accelerator is a measure of the number of particle collisions that occur each second. Since the energy of Tevatron collisions cannot be significantly raised, increasing the number and rate of collisions is the only path to discovery. Because the types of collisions that yield discoveries are extremely rare, increasing the total number of collisions raises the probability of finding the ones that lead to discoveries.
To increase the number of collisions, accelerator experts try to maximize the "peak" luminosity, measured in inverse centimeters squared per second, of each "shot" of protons and antiprotons to the Tevatron. A shot produces about 10 to 20 hours of collisions. Raising the peak luminosity for each shot maximizes the "integrated luminosity," the cumulative total of particle collisions over the entire collider run. The higher the integrated luminosity, measured in inverse femtobarns, the greater the possibility of discovery.
Fermilab has completed a number of improve-ments to the accelerator complex—including the addition of a new $260-million injection accelerator, the Main Injector—to increase the Tevatron peak luminosity above the Run I level by a factor of about five in Run IIa, with the complex as now configured. Fermilab has established a Run IIa goal of 8 x 1031 cm-2sec-1 by the end of calendar year 2002. With the insertion of the Antiproton Recycler into the accelerator complex in mid-2003, Run IIa luminosity is expected to increase by an extra factor of two or three to about 20 x 1031.
"There is no fundamental problem with the Tevatron accelerator complex to prevent the achievement of Run IIa's luminosity goals," said Fermilab Director Michael Witherell.
The integrated luminosity goal of Run IIa is to achieve two inverse femtobarns by the end of 2004. Further upgrades will then raise the luminosity by an additional factor of two or more for Run IIb, beginning in about 2005, for an integrated luminosity of four or more inverse femtobarns per year. Run IIb's luminosity goal is 15 inverse femtobarns by about 2008.
As the Tevatron complex began stable Run II operations in late summer of 2001, it became clear that achieving Run II goals would not be as straightforward as anticipated, said Associate Director Steve Holmes.
"We encountered a number of surprises," Holmes said. "We saw antiproton emittances in the Accumulator that were 50 to 100 percent larger than in Run I. Our antiproton transfer efficiencies from the Accumulator to Tevatron collisions were less than 30 percent when they should be about 80 percent. Issues in the Tevatron appeared related to operations with 36 proton bunches, compared to the six bunches of Run I. The effect of the intense proton beam on the antiproton beam, the 'long-range beam-beam effect,' at the Tevatron injection energy of 150 GeV has a greater impact than we anticipated. However, at this point, we believe we understand all the issues, and we have defined a systematic approach to resolving them."
Fermilab is devoting all possible resources to improving Tevatron performance, laboratory officials said.
"There is no higher priority for our laboratory than producing more high-energy collisions in the Tevatron," Witherell said. He described efforts to mobilize help from across the laboratory.
"We are working to match needs with volunteers," Witherell said. "We have asked physicist Hugh Montgomery to act as matchmaker on some of these efforts. We are receiving help from individuals in the Particle Physics Division on making Tevatron profile monitors work. We're about to get help from Computing Division personnel on the shot data acquisition system; and the Technical Division will take on some longer-term Run II jobs in order to free Beams Division personnel to work on more immediate problems."
Assistance may also come from beyond Fermilab's borders.
"We have received offers of help from the university community and from SLAC, the Stanford Linear Accelerator Center," Associate Director Holmes said. "In general, help works best on projects needed on time scales of months rather than days or weeks. A number of instrumentation and data analysis needs fit well with skills of particle physics experimenters."
On January 1, 2002, Fermilab put a 12-month plan in place, with well-defined luminosity goals. It is beginning to show results. However, Holmes said, Fermilab did not meet the first goal on February 15.
"The February 15 luminosity goal was 1.8 x 1031," Holmes said. "We have achieved 1.2 x 1031. The shortfalls come from the number of antiprotons per bunch, at 82 percent of goal; the number of protons per bunch at 93 percent; and beam sizes at 120 percent. It adds up to achieving 64 percent of the goal. In practice, we have had difficulty increasing the proton and antiproton intensities simultaneously. As we raise proton intensity, our efficiency for transferring antiprotons from the Antiproton Accumulator to the Tevatron suffers, presumably because of beam-beam effects."
To address this problem, Holmes said, the Beams Division is pursuing its broad-based plan.
"The Beams Division is making systematic progress on many of the underlying issues that need to be attacked in parallel," he said. "The plan shows us reaching Run IIa luminosity goals by the end of 2002. This journey will not be completed overnight. The goals we have established are there to measure our progress. Falling short on the February goal does not mean that our strategy is incorrect or that we will abandon its pursuit."
Indeed, a number of significant indicators have recently showed progress.
On February 3, the Tevatron set a Run II peak luminosity record of 1.18 x 1031 cm-2sec-1. Peak luminosities for recent stores have been running routinely near this level.
A troublesome problem of magnet quenching (quenching occurs when a superconducting magnet "goes normal" with the release of significant amounts of energy) at the end of each store has been stopped.
The efficiency of antiproton transfer has risen from about 25 percent to 35 to 40 percent. Work is underway to raise it towards the goal of 80 percent.
In January, the new Anitproton Recycler Ring achieved a successful store of 40 hours, an important step toward a beam lifetime well over 100 hours.
During the week of February 11, the Antiproton Accumulator set a record for the average antiproton stacking rate of 7.58 milliamps per hour for the week.
Reliability in the Tevatron has been exceptional. Nearly 90 percent of stores have ended intentionally rather than by failure.
Witherell said that the regular goals for the 12-month plan give Fermilab a clear picture of progress.
"To achieve the long-range goal of 15 inverse femtobarns in Run II will require further planned upgrades to the accelerators," Witherell said. "Intense efforts over the next six months will determine the luminosity achievable in Run IIa."
To those who hold that the current luminosity problems are part of normal Tevatron start-up patterns, Holmes has a clear response.
"I disagree with that one hundred percent," he said. "We have set ambitious goals and we are pushing the accelerator complex very hard. The Beams Division is working hard on these issues, and they are starting to effectively integrate help from the outside. I expect we will see the payoff from the effort they are investing over the next few months. I have confidence in these folks."
Meanwhile, out at the experiments...
"Being on shift this week, it is clear to me that we really are finally taking data for physics analyses. Even some of our most critical colleagues will now agree with this assessment. Yes there are still a few warts on the detector, and the Tevatron losses are a worry (we have worked around this for now). But for many analyses the data we are taking could appear in future publications. It is wonderful to see people on shift excited about our data taking, and working across language, gender and age boundaries to keep the detector running efficiently."
Al Goshaw, CDF Cospokesman
"DZero is also making important progress. Charged particle trajectories are now clearly reconstructed with data from the fiber tracker. The readout system underwent a big upgrade last week, and data can now be written to tape at rates up to 50 Hz. Starting at midnight on February 10, DZero wrote over a million events before lunchtime, a new record for the experiment."
John Womersley, DZero Cospokesman