Abstract

The ATLAS Level-1 Calorimeter Trigger is one of the main elements of the first stage of event selection for the ATLAS experiment at the LHC. The input stage consists of a mixed analogue/digital component taking trigger sums from the ATLAS calorimeters. The trigger logic is performed in a digital, pipelined system with several stages of processing, largely based on FPGAs, which perform programmable algorithms in parallel with a fixed latency to process about 300 Gbyte/s of input data. The real-time output consists of counts of different types of physics objects, and energy sums. The final system consists of over 300 custom-built VME modules, of several different types. The installation at ATLAS of these modules, and the necessary infrastructure, was completed at the end of 2007. The system has since undergone intensive testing, both in standalone mode, and in conjunction with the whole of the ATLAS detector in combined running. The final steps of commissioning, and experience with running the full-scale system are presented. Results of integration tests performed with the upstream calorimeters, and downstream trigger and data-flow systems, are shown, along with an analysis of the performance of the calorimeter trigger in full ATLAS data-taking. This includes trigger operation during the cosmic muon runs from before LHC start-up, and a first look at LHC proton beam data.

Highlights

  • The CERN Large Hadron Collider presents huge challenges for the trigger systems of its experiments

  • ATLAS will deal with this using a three-level selection scheme in order to select about 200 bunch-crossings per second for permanent recording

  • The PPr consists of 124 PreProcessor Modules (PPMs), which provide the digital input data used by both the Cluster Processor and the Jet/Energy-sum Processor

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Summary

INTRODUCTION

The CERN Large Hadron Collider presents huge challenges for the trigger systems of its experiments. The first level of trigger selection, Level-1, uses reducedgranularity information from the calorimeters and muon information from dedicated fast triggering detectors to reduce the 40 MHz bunch-crossing rate down to 75 kHz (and potentially 100 kHz) within a fixed latency maximum of 2.5 μs. Level-2 can access the full-granularity detector readout data from all components of ATLAS, and further reduces the rate to about 3.5 kHz within about 40 ms To do this, it uses so-called “region-of-interest” (RoI) information from Level-1 to access potentially interesting regions in the readout data. The Event Filter works on fully built events, and can use software selections similar to the offline analysis This cuts the rate to the desired 200 Hz within about 4 s. Most steps are carried out using a 40 MHz clock, but in a few places 80 and 160 MHz multiples are used

General design features
Input signals
The PreProcessor
Analogue and digital links
Pedestals and noise
Coarse and fine timing
FIRST COSMIC AND PROTON-BEAM RESULTS
Findings
SUMMARY AND OUTLOOK

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