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The Level-2 Trigger Pilot Project of ATLAS, one of the two general purpose LHC experiments, is part of the on-going program to develop the ATLAS high-level triggers (HLT). The Level-2 Trigger will receive events at up to 100 kHz, which has to be reduced to a rate suitable for full event-building of the order of 1 kHz. To reduce the data collection bandwidth and processing power required for the challenging Level-2 task it is planned to use Region of Interest guidance (from Level-l) and sequential processing. The Pilot Project included the construction and use of testbeds of up to 48 processing nodes, development of optimized components and computer simulations of a full system. It has shown how the required performance can be achieved, using largely commodity components and operating systems, and validated an architecture for the Level-2 system. This paper describes the principal achievements and conclusions of this project. Index Terms--ATLAS, computer network performance, distributed computing, LHC, message passing, object oriented methods, parallel processing, protocols, technology assessment, triggering.

Hera-B is a fixed target experiment using a halo target inside the HERA proton ring to generate B mesons in 920 GeV proton-nucleus interactions. The first-level trigger (FLT) (see Table I for a list of the acronyms used in this paper) of the experiment has to reduce the primary 10-MHz input rate by a factor 200 in less than 10 [micro]s to make it acceptable to the second level trigger (SLT). The trigger strategy is based on the tracking of charged particles and on the reconstruction of their kinematic parameters. The combination of track pairs can also be used for the final decision. A parallel and pipelined set of approximately 60 dedicated boards was designed and built to perform this job. In this paper, the working principle of the system and some results obtained by analyzing the data collected during the run in the year 2000 are described. Index Terms--First-level trigger (FLT), Hera-B, latency, pretrigger, rate, TDU, TFU, TPU, tracking, trigger.

The HERA-B detector aims- to measure CP violation in the B meson system. The B mesons under study are produced in interactions of 820 GeV/c protons with the fixed target. Together with the B mesons a 6 orders of magnitude larger background of minimum bias interactions is produced. A highly selective and efficient trigger system is required and has been designed to acquire a sufficient amount of signal decays. It is able to find lepton and hadron tracks and reconstruct their kinematics on the first level of the trigger chain. The final first level trigger decision is based on the properties of the reconstructed pair of tracks. This is mainly targeting at J/[Psi] [right arrow] [l.sup.+][l.sup.-] and [B.sup.0] [right arrow] [[Pi].sup.+][[Pi].sup.-] signatures. A parallel and pipelined system of approximately 100 processors is designed to perform this job. It reduces a 10 MHz input rate by a factor of 200 with a latency of 5 to 10 [[micro]seconds]. A special detailed simulation package was designed to study the system performance and prove the algorithms used. Being a constituent part of the general HERA-B software, it provides not only the possibility of trigger studies but also selects FLT passing events to be considered as input stream for the studies of the following levels of the HERA-B trigger system.

A parallel Hough transform processor has been developed that identifies circular particle tracks in a 2D projection of the OPAL jet chamber. The high-speed requirements imposed by the 8 bunch crossing mode of LEP could be fulfilled by computing the starting angle and the radius of curvature for each well defined track in less than 4 ||micro~second~. The system consists of a Hough transform processor that determines well defined tracks, and a Euler processor that counts their number by applying the Euler relation to the thresholded result of the Hough transform. A prototype of a systolic processor has been built that handles one sector of the jet chamber. It consists of 35x32 processing elements that were loaded into 21 programmable gate arrays (XILINX). This processor runs at a clock rate of 40 MHz. It has been tested offline with about 1000 original OPAL events. No deviations from the off-line simulation have been found. A trigger efficiency of 93% has been obtained. The prototype together with the associated drift time measurement unit has been installed at the OPAL detector at LEP and 100k events have been sampled to evaluate the system under detector conditions.

In the context of Research and Development (R & D) activities for future hadron colliders, competitive implementations of real-time algorithms for feature extraction have been made on various forms of commercial pipelined and parallel architectures. The algorithms used for benchmarking serve for decision making and are of relative complexity; they are required to run with a repetition rate of 100 kHz on data sets of kilobyte size. Results are reported and discussed in detail. Among the commercially available architectures, pipelined image processing systems can compete with custom-designed architectures. General-purpose processors with systolic mesh connectivity can also be used. Massively parallel systems of the SIMD type (many processors executing the same program on different data) are less suitable in the presently marketed form.

The Enable Machine is a systolic 2nd level trigger processor for the transition radiation detector (TRD) of ATLAS/LHC. It is developed within the EAST/RD-11 collaboration at CERN. The task of the processor is to find electron tracks and to reject pion tracks according to the EAST benchmark algorithm in less than 10 [[micro]second]. Track are identified by template matching in a ([Phi], z) region of interest (RoI) selected by a 1st level trigger. In the ([Phi], z) plane tracks of constant curvature are straight lines. The relevant lines form mask templates. Track identification is done by histogramming the coincidences of the templates and the RoI data for each possible track. The Enable Machine is an array processor that handles tracks of the same slope in parallel, and tracks of different slope in a pipeline. It is composed of two units, the Enable histogrammer unit and the Enable z/[Phi]-board. The interface daughter board is equipped with a HIPPI-interface developed at JINR/-Dubna, and Xilinx 'corner turning' data converter chips. Enable uses programmable gate arrays (XILINX) for histogramming and synchronous SRAMs for pattern storage. With a clock rate of 40 MHz the trigger decision time is 6.5 [[micro]second] and the latency 7.0 [[micro]second]. The Enable machine is scalable in the RoI size as well as in the number of tracks processed. It can be adapted to different recognition tasks and detector setups. The prototype of the Enable Machine has been tested in a beam time of the RD6 collaboration at CERN in October 1993.

Recent studies of the level-two (LVL2) trigger of the ATLAS detector [1] show that it will be possible to run the trigger algorithms at high luminosity with a reasonable number of general-purpose processors, using a sequential selection scheme and guidance from the Region-of-Interest (RoI) provided by the LVL1 trigger. The computing power requirements for B-physics, which is studied at low luminosity, are much greater than those at high-luminosity as there is no LVL1-guidance for the track finding algorithms. Instead, track finding is performed for the entire Inner Detector volume. Currently, 2500 commodity CPUs would be required to supply the necessary computing power for the B-physics trigger. We describe a system of only 200 computing nodes which would be capable of performing the B-physics triggering(2). Each of these nodes is made up of a commodity PC and a FPGA co-processor board. Each node processes an entire event. The different tasks are allocated to the appropriate hardware device (CPU or FPGA). Track reconstruction requires a variety of different steps, some of which are suited to parallel processing, whereas others require sequential execution. For some tasks, floating-point arithmetic is needed. The flexibility of the PC/FPGA combination meets these varied requirements well.

The aim of this study was to investigate the association of gastro-oesophageal reflux disease (GORD) with radiographic pulmonary changes. One hundred and thirty-seven patients aged over 60 years referred for endoscopy for abdominal symptoms were included in the study. In patients with any suspicion of GORD (n = 95), 24-hour oesophageal pH monitoring was carried out. All the patients were interviewed before the examinations. Chest radiography was performed in all patients. Odds ratio of respiratory symptoms was 8.7 (95% confidence interval 3.4-22.4) in patients with total reflux time of more than 10% compared with those who had total reflux time of less than 10%. In chest radiography, bilateral pleural adhesions and thickenings and bilateral parenchymal scars were significantly more common in patients with, than in those without, regurgitation or large hiatal openings. The occurrence of bilateral scars and pleural thickenings increased with the extent of gastro-oesophageal reflux in pH monitoring. Odds ratios of bilateral pleural thickening and pulmonary scars were 3.1 (95% confidence interval 0.8-11.5) and 5.8 (1.1-29.6) in patients with total reflux time over 10%, compared with patients who had normal total reflux time. The findings indicate that respiratory involvement is a characteristic feature of GORD in elderly patients. [ABSTRACT FROM AUTHOR]

Concentrated potassium chloride produces ulceration of the gastrointestinal mucosa. Dilute solutions are frequently not acceptable to the patient. A tablet containing potassium chloride crystals suspended in a waxy matrix has been developed to avoid these drawbacks. We measured fecal blood loss by a radioactive chromium technique on 20 subjects during five days while they were given potassium chloride, 40mEq/day; ten subjects took a 10% solution; the others the waxy-matrix tablet. Fecal blood loss did not differ significantly between these groups. Mean fecal blood losses in both groups were slightly increased during the potassium chloride administration but never exceeded acceptable normal limits.