Getting Ready for the Fixed-Target Run
Two experiments explore CP violation
by Sharon Butler
Asked what the scientists who rely on Fermilabs accelerators want most from the Laboratory, Patricia McBride, a scientist herself, once said, "Beam, beam and more beam."
When the Tevatron cranks up again, two fixed-target experiments will get under way and the scientists involved should be delighted.
The Tevatron will be running at 40 percent of its 1997 intensityabout all the intensity the two experiments can handle. But theyll be getting 1.5 times more beam, enabling experimenters to collect more data with better statistics for deeper insights into the way matter is put together and falls apart.
Staff in the Beams Division are now busily preparing for the upcoming run: refurbishing the septa in the switchyard, looking for shorts in the power supplies, testing the magnets, making sure all the utilities (the water lines, safety systems, and instrumentation) are in order.
To get more useful beam to experimenters, the Fermilab staff are configuring the "new" beam to have a longer "flat top."
"In the first run, we had a cycle of 60 seconds total for each pulse of beamramping up to 800 GeV for 20 seconds, then ramping down," explained Craig Moore, head of the External Beams Department. "In this run, the cycle will be 80 seconds long, with the beam at 800 GeV [the "flat top"] for 40 seconds of that time."
"Before we were taking data one-third of the time; now well be taking data one-half of the time," said Craig Dukes, cospokesperson for the HyperCP experiment.
Besides studying the kaons and rare decays that emerge along its beamline, HyperCP, experiment number 871, is looking for CP violation in hyperons, particles that have at least one strange quark and are heavier than protons.
Scientists have long known that CP violation exists. In 1964, scientists studying neutral kaons in experiments at Brookhaven Laboratory discovered an asymmetry in the behavior of the neutral kaon and its antiparticle, subverting the long-held belief that the combination of charge conjugation (C) and parity inversion (P) was an inviolate fundamental symmetry of nature. But while theory suggests that CP violation exists in other particles as well, including B mesons and hyperons, no firm evidence for the phenomenon has yet surfaced.
According to Dukes, 30-some physicists associated with HyperCP are still analyzing data from Run I. They accumulated the largest number of events ever recorded on tape by a particle physics experiment, Dukes said: 75 billion events in all, which take up "only" about 33 terabytes of information (since the detector is simple). From these data, they have been able to amass the profiles of 1.3 billion hyperons and 0.3 billion antihyperons. This was a tremendous experimental feat, since, as HyperCP physicist Catherine James pointed out, "you dont get hyperons and antihyperons at the same time or at the same rate."
By comparing the spatial distributions of decay products of hyperons and antihyperons, the experimenters hope to discover an asymmetry in their behavior. If CP violation exists, the experimenters expect the phenomenon to be as large as several parts in 10,000 (one of the largest manifestations of direct CP violation outside of decays involving b quarks).
In Run II, Dukes said, the HyperCP collaboration expects to collect four times as much data. With more data to shore up their analyses, the scientists will be able to reach firmer conclusions than they would with less data. The quality of the data should be better, as numerous minor but significant upgrades have been made to the apparatus.
To collect more data, the collaboration has been focusing on upgrading its data acquisition system. In particular, the scientists have been working to increase the size of the memory so that the data acquisition system can cope with the barrage of data from the new beam. As James described it, the memory in the system serves as a buffer, a kind of shock absorber. Data stream in from the ramped-up 800-GeV beam and are stored in this buffer memory until the slower readout system can sort through the masses of digital signals and write the relevant ones to tape.
KTeV, or Kaons at the Tevatron, is also studying CP violation, but in kaons.
"Were not only going to collect more data in Run II, but we expect the quality of the data to be better," said Ed Blucher, a physicist from the University of Chicago who is cospokesperson for the KTeV collaboration, along with Fermilab physicist Bob Tschirhart.
Several changes to the detector will make the experiment run "more smoothly and efficiently," according to Blucher, enabling the collaboration to collect more data. For example: In the last run, because of a fabrication problem in the custom integrated circuits that read out the detectors cesium iodide calorimeter, the collaboration lost considerable data-collecting time. Each time a chip failedas often as twice a day, Blucher saidexperimenters had to halt operations and replace the faulty components.
"The failures in these chips caused more than half of the downtime in the experiment due to detector problems," Blucher said. New chips were tested last fall, and proved to be far more reliable; there was only one failure during the four-week test. The new circuitry has now been installed.
To improve the quality of their data, the experimenters are also modifying certain features in their drift chambers, where crisscrossing, hair-thin wires help reconstruct the paths that particles take under the influence of a magnetic field. Delineating those paths enables the scientists to calculate the particles momenta.
Ideally, the "spark" that occurs when a particle passes by one of the fine wires is amplified 100,000-fold, giving experimenters precise information on the particles whereabouts. But in the last run, the electronics sometimes left the signal too weak, making it difficult to pinpoint a particles location. The problem arose, said Tschirhart, "largely because external noise sources, like the data acquisition system and even radio station transmissions, prevented the electronics from operating at the original design amplification." Improvements to the grounding and shielding of the electronics should ensure optimal amplification.
Both Tschirhart and Blucher noted that the collaboration was in an ideal position. Because of the size of the collaboration (about 90 scientists), and the hard work of postdocs and graduate students, much of the data set from the last run has been analyzed already. Consequently, the scientists have been able to study methodically the operation of their detector and decide where to make improvements that would give them more and still better data.
Said Tschirhart, "The KTeV experiment ended a year and a half ago. Its not a lot of time but in that time weve finished many of our analyses, and weve gotten to a point where we clearly understand what changes in the detector would be beneficial."
The next run, Blucher said, is a chance to work with a detector whose bugs have been shaken out. "Weve learned where we made small mistakes and where there were weaknesses. All of these issues have been addressed for the upcoming run."
Physics results, said Tschirhart, are dependent not just on more data but on higher-quality datajust as advances in astronomy, to use Tschirharts analogy, depend not just on larger lenses to collect more light, but on finer optics of the lenses to better resolve the images of the galaxies.
|last modified 4/30/1999 email Fermilab|