Science Today: BaBar Data Acquisition
In the BaBar interaction region at PEP-II, bunches of electrons and positrons cross each other 250 million times per second. The "luminosity" of PEP-II is a measure of how often this produces electron–positron collisions. A fraction of these collisions leads to interesting physics, such as the production of B mesons, charmed particles, or tau leptons—in recent running, about 50 times per second. However, other types of interactions of the beams with each other or with the accelerator occur at a much higher rate, and many of these "background events" leave a detectable trace in the experiment.
PEP-II is now able to operate at four times its design luminosity, thanks to very dedicated efforts by many people at SLAC, and more improvements are expected. It is thus essential that BaBar be able to analyze the maximum possible fraction of the interesting interactions produced.
When the particles produced in a beam interaction enter the BaBar detector, its systems record their passage electronically, producing data that can be stored for further analysis. The detector hardware makes a decision about eight million times per second of whether an interaction has occurred that is interesting enough to merit further study. In recent BaBar running, the decision is "yes" 3,000 to 5,000 times per second. Each time, it is the responsibility of the detector's data acquisition system to take an electronic "snapshot" of the traces of the particles, amounting to about 35 kilobytes of data.
If the resulting flow of data—currently 100 to 150 megabytes per second, or over 100 times the data rate of a DVD movie—were to exceed the capacity of the system, interesting physics could be lost; accordingly, the BaBar team has had to follow the improvements to PEP-II, making sure that the capacity always stays ahead of the requirement.
A farm of 50 "online" computers immediately analyzes the acquired data in more detail, selecting about 1 in 10 events for permanent storage at SCCS and collecting data quality information to be monitored by the detector shift crew.
The online system is responsible for controlling and monitoring the detector itself and the power supplies and gas systems on which it depends, and handles configuring, starting, and stopping data acquisition.
BaBar and PEP-II run 24 hours a day, seven days a week, for eight or nine months every year. It is crucial to keep the data acquisition and online systems running efficiently all the time, and to coordinate them with the PEP-II control system, so that data are recorded whenever useful luminosity is available.
BaBar's online and data acquisition systems have met this goal as well as or better than any previous large experiment, averaging well over 95% efficiency over the lifetime of the experiment. Achieving this remarkable performance has depended on a constant and uncompromising commitment to this goal by the systems' developers and the BaBar operations and shift teams.
—Gregory Dubois-Felsmann and Steffen Luitz, May 10, 2007
Members of the BaBar online and data acquisition team. From left to right, they are: Victor Serbo (SCCS), Leonid Sapozhnikov (PPA Elec & SW Eng), Andy Salnikov (SCCS), Ray Rodriguez (PPA Elec & SW Eng), Viola Sordini (LAL, Orsay), Rainer Bartoldus (EC/ATLAS), Walt Innes (EE), Seffen Luitz (SCCS), Gunther Haller (PPA Elec & SW Eng), Chris O'Grady (PPA Elec & SW Eng), Matthias Wittgen (SCCS), Gregory Dubois-Felsmann (BaBar), Sherry Chu (SCCS), and Jim Hamilton (SCCS). (Click image for larger version. Photo courtesy Diana Rogers.)