Tadashi Maeno

The ATLAS level-1 central trigger system

The central part of the ATLAS level-1 trigger system consists of the central trigger processor (CTP), the local trigger processors (LTPs), the timing, trigger and control (TTC) system, and the read-out driver busy (ROD/spl I.bar/BUSY) modules. The CTP combines information from calorimeter and muon trigger processors, as well as from other sources and makes the final level-1 accept decision (L1A) on the basis of lists of selection criteria, implemented as a trigger menu. Timing and trigger signals are fanned out to about 40 LTPs which inject them into the sub-detector TTC partitions. The LTPs also support stand-alone running and can generate all necessary signals from memory. The TTC partitions fan out the timing and trigger signals to the sub-detector front-end electronics. The ROD-BUSY modules receive busy signals from the front-end electronics and send them to the CTP (via an LTP) to throttle the generation of L1As. An overview of the ATLAS level-1 central trigger system will be presented, with emphasis on the design and tests of the CTP modules.

The ATLAS Level-1 central trigger processor

ATLAS is a multi-purpose particle physics detector at CERNs Large Hadron Collider where two pulsed beams of protons are brought to collision at very high energy. There are collisions every 25 ns, corresponding to a rate of 40 MHz. A three-level trigger system reduces this rate to about 200 Hz while keeping bunch crossings which potentially contain interesting processes. The Level-1 trigger, implemented in electronics and firmware, makes an initial selection in under 2.5 mus with an output rate of less than 100 kHz. A key element of this is the central trigger processor (CTP) which combines trigger information from the calorimeter and muon trigger processors to make the final Level-1 accept decision in under 100 ns on the basis of lists of selection criteria, implemented as a trigger menu. Timing and trigger signals are fanned out to all sub-detectors, while busy signals from all sub-detector read-out systems are collected and fed into the CTP in order to throttle the generation of Level-1 triggers

The ATLAS local trigger processor (LTP)

The local trigger processor (LTP) receives timing and trigger signals from the central trigger processor (CTP) and injects them into the timing, trigger and control (TTC) system of a sub-detector front-end TTC partition. The LTP allows stand-alone running by using local timing and trigger signals or by generating them from memory. In addition, several LTPs of the same sub-detector can be daisy-chained. The LTP can thus be regarded as a switching element for timing and trigger signals with input from the CTP or the daisy-chain, from local input, or from the internal data generator, and with output to the daisy-chain, to the TTC partition, or to local output. Finally, in combined mode several LTPs can be connected together using their local outputs and local inputs to allow stand-alone running of combinations of different sub-detectors.

Design, deployment and functional tests of the online event filter for the ATLAS experiment at LHC

The Event Filter (EF) selection stage is a fundamental component of the ATLAS Trigger and Data Acquisition architecture. Its primary function is the reduction of data flow and rate to values acceptable by the mass storage operations and by the subsequent offline data reconstruction and analysis steps. The computing instrument of the EF is organized as a set of independent subfarms, each connected to one output of the Event Builder (EB) switch fabric. Each subfarm comprises a number of processors analyzing several complete events in parallel. This paper describes the design of the ATLAS EF system, its deployment in the 2004 ATLAS combined test beam together with some examples of integrating selection and monitoring algorithms. Since the processing algorithms are not explicitly designed for EF but are adapted from the offline ones, special emphasis is reserved to system reliability and data security, in particular for the case of failures in the processing algorithms. Other key design elements have been system modularity and scalability. The EF shall be able to follow technology evolution and should allow for using additional processing resources possibly remotely located

The ATLAS Level-1 Central Trigger Processor (CTP)

The ATLAS Level-1 Central Trigger Processor (CTP) com- bines information from the Level-1 calorimeter and muon trig- ger processors, as well as from other sources such as calibration triggers, and makes the final Level-1 Accept deci- sion. The CTP synchronises the trigger inputs from different sources to the internal clock and aligns them with respect to the same bunch crossing. The algorithm used by the CTP to com- bine the different inputs allows events to be selected on the basis of trigger menus. The CTP provides trigger summary information to the data acquisition and to the Level-2 trigger system, and allows one to monitor various counters of bunch- by-bunch as well as accumulated information on the trigger inputs. The design of the CTP with its six different module types and two dedicated back-planes will be presented.

Implementation and performance of the high level trigger electron and photon selection for the ATLAS experiment at the LHC

The ATLAS experiment at the Large Hadron Collider (LHC) will face the challenge of efficiently selecting interesting candidate events in pp collisions at 14 TeV center of mass energy, while rejecting the enormous number of background events, stemming from an interaction rate of up to 10/sup 9/ Hz. The First Level trigger will reduce this rate to around O(100 kHz). Subsequently, the High Level Trigger (HLT), which is comprised of the Second Level trigger and the Event Filter, will need to further reduce this rate by a factor of O(10/sup 3/). The HLT selection is software based and will be implemented on commercial CPUs, using a common framework built on the standard ATLAS object oriented software architecture. In this paper an overview of the current implementation of the selection for electrons and photons in the HLT is given. The performance of this implementation has been evaluated using Monte Carlo simulations in terms of the efficiency for the signal channels, rate expected for the selection, data access and manipulation times, and algorithm execution times. Besides the efficiency and rate estimates, some physics examples will be discussed, showing that the triggers are well adapted for the physics programme envisaged at LHC. The electron and photon trigger software is also being exercised at the ATLAS 2004 Combined Test Beam, where components from all ATLAS subdetectors are taking data together along the the H8 SPS extraction line at CERN; from these tests a validation of the selection architecture chosen in a real on-line environment is expected.

ATLAS Level-1 Trigger Timing-In Strategies

The ATLAS detector at CERN’s LHC will be exposed to proton-proton collisions at a bunch-crossing rate of 40 MHz. In order to reduce the data rate, a three-level trigger system selects potentially interesting events. Its first level is implemented in electronics and firmware, and aims at reducing the output rate to under 100 kHz. The Central Trigger Processor (CTP) combines information from the calorimeter and muon trigger processors, and makes the final Level-1 Accept (L1A) decision, which is transferred to all sub-detector front-ends. The functioning of the Level-1 Trigger is based on the correct timing of the signals. In this paper we present various strategies for sub-detector timing-in, in particular how to arrive at a decent initial timing setup using test-pulses in standalone mode, and in global mode with the CTP. In addition we describe how the beam pick-up detectors are a powerful tool to further refine the timing with bunches in the LHC machine. In this context we describe new developments on a proposal for precision read-out of the ATLAS beam pick-up detectors with commercial oscilloscopes in order to monitor the phase of the clock with respect to the LHC bunches. I. I NTRODUCTION

Implementation and Performance of the Seeded Reconstruction for the ATLAS Event Filter Selection Software

ATLAS is one of the four LHC experiments that will start data taking in 2007, designed to cover a wide range of physics topics. The ATLAS trigger system has to cope with a rate of 40 MHz and 23 interactions per bunch crossing. It is divided in three different levels. The first one (hardware based) provides a signature that is confirmed by the following trigger levels (software based) by running a sequence of algorithms and validating the signal step by step, looking only to the region of the space indicated by the first trigger level (seeding). In this presentation, the performance of one of these sequences that run at the event filter level (third level) and is composed of clustering at the calorimeter, track reconstruction and matching, were presented

Implementation and performance of the event filter muon selection for the ATLAS experiment at LHC

The ATLAS trigger system is composed of three levels: an initial hardware trigger level (LVL1) followed by two software-based stages (LVL2 trigger and event filter) included in the high level trigger (HLT) and implemented on processor farms. The LVL2 trigger starts from LVL1 information concerning pointers to restricted so-called regions of interest (ROI) and performs event selection by means of optimized algorithms. If the LVL2 is passed, the full event is built and sent to the event filter (EF) algorithms for further selection and classification. After that, events are finally collected and put into mass storage for subsequent physics analysis. Even if many differences arise in the requirements and in the interfaces between the two HLT stages, they have a coherent approach to event selection. Therefore, the design of a common core software framework has been implemented in order to allow the HLT architecture to be flexible to changes (background conditions, luminosity, description of the detector, etc.). Algorithms working in the event filter are designed to work not only in a general purpose or exclusive mode, but they have been implemented in such a way to process given trigger hypotheses produced at a previous stage in the HLT dataflow (seeding concept). This is done by acting in separate steps, so that decisions to go further in the process are taken at every new step. An overview of the HLT processing steps is given and the working principles of the EF offline algorithms for muon reconstruction and identification (MOORE and MuId) are discussed in deeper detail. The reconstruction performances of these algorithms in terms of efficiency, momentum resolution, rejection power and execution times on several samples of simulated single muon events are presented, also taking into account the high background environment that is expected for ATLAS.

The ATLAS level-1 central trigger processor core module (CTP/spl I.bar/CORE)

ATLAS is a detector at CERNs Large Hadron Collider where bunches of protons in counter-rotating beams will cross every 25 ns producing, on average, about 25 collisions for a total interaction rate of 1 GHz. A three-level trigger system selects bunch crossings potentially containing interesting processes. The Level-1 trigger, implemented in electronics and firmware, makes an initial selection in under 2.5 mus with an output rate of less than 100 kHz. A key element of this is the core module of the Central Trigger Processor which combines trigger information from the calorimeter and muon trigger processors to make the final Level-1 accept decision in under 75 ns. The event-selection algorithm used by the core module is based on lists of selection criteria, i.e., trigger menus, and is implemented in fully programmable look-up tables and content-addressable memories. In addition to the event selection, the core module generates dead-time in order to limit the frequency of Level-1 accepts to a rate that the sub-detector front-end electronics can support. The core module further provides trigger-summary information to the Level-2 trigger and to the data acquisition system. The design of the core module is presented, and results from recent laboratory tests and from tests with the calorimeter and muon trigger processors connected to detectors in a particle beam are shown