Last update: Apr 1, 2000
Speaker: Andreas GellrichThe HERA-B experiment is currently preparing for its first physics run with the full detector installed, which will start beginning 2000. The main goal is to study CP violation in decays of neutral B mesons into $J/\Psi K^0_s$ and subsequently into two leptons and two pions.
HERA-B's DAQ and trigger system exploits four levels to select a rate of less than 1 Hz of interesting physics events from an initial event rate of 10 MHz with on average four overlaying interactions per event.
To allow for immediate data analysis and to avoid time-consuming re-processing of data, events will be reconstructed online in level-4 of the DAQ and trigger system before being logged to tape.
Since trigger efficiency and background suppression depend heavily on the quality of calibration and alignment of the detector components, online monitoring and updating of the database is foreseen. During event reconstruction, quantities which are needed for calibration and alignment are derived and sent to a central destination. By making use of the data coming from the reconstruction jobs, constants are updated. If necessary, new constants are distributed to the reconstruction processes as well as to the level-2/3 trigger system.
The large computing power of the level-4 farm is used for re-processing during data taking breaks.
Online event reconstruction is performed on the fourth level of the DAQ and trigger system on a PC farm. To process events at a rate of 50 Hz with processing times of typically 4 sec per event, 200 CPUs are needed and will be installed until end 1999. For the farm nodes standard dual-CPU PCs were chosen which are equipped with Intel Pentium III/500MHz processors and hard-disks. Networking is as well exploiting off-the-shelf components. The level-4 farm nodes are grouped into eight so-called mini-farms and are connected via Fast-Ethernet switches. The data link to mass storage media is realized by means of GigaBit-Ethernet.
The usage of Linux as a Unix-like operating system, which has become a standard for software developments and data analysis in HEP, allows to give up the separation line between online and offline software. Reconstruction programs as well as calibration, alignment, and monitoring software, which was developed for offline purposes, can be directly used on the level-4 farm without modifications.
In the contribution to CHEP2000 concept and implementation as well as results on the performance of the online reconstruction farm will be presented and discussed. Main emphasis will be put on HERA-B's first physics running period with the complete detector, starting beginning of 2000.
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