A. Di Mattia

Studies for a common selection software environment in ATLAS: from the level-2 trigger to the offline reconstruction

The ATLAS High Level Triggers (HLT) primary function of event selection will be accomplished with a Level-2 trigger farm and an event filter (EF) farm, both running software components developed in the ATLAS offline reconstruction framework. While this approach provides a unified software framework for event selection, it poses strict requirements on offline components critical for the Level-2 trigger. A Level-2 decision in ATLAS must typically be accomplished within 10 ms and with multiple event processing in concurrent threads. To address these constraints, prototypes have been developed that incorporate elements of the ATLAS data flow, high level trigger, and offline framework software. To realize a homogeneous software environment for offline components in the HLT, the Level-2 Steering Controller was developed. With electron/gamma- and muon-selection slices it has been shown that the required performance can be reached, if the offline components used are carefully designed and optimized for the application in the HLT.

Architecture of the ATLAS High Level Trigger Event Selection Software

We present an overview of the strategy for Event Selection at the ATLAS High Level Trigger and describe the architecture and main components of the software developed for this purpose.

Overview of the high-level trigger electron and photon selection for the ATLAS experiment at the LHC

The ATLAS experiment is one of two general purpose experiments to start running at the Large Hadron Collider in 2007. The short bunch crossing period of 25 ns and the large background of soft-scattering events overlapped in each bunch crossing pose serious challenges that the ATLAS trigger must overcome in order to efficiently select interesting events. The ATLAS trigger consists of a hardware-based first-level trigger and of a software-based high-level trigger, which can be further divided into the second-level trigger and the event filter. This paper presents the current state of development of methods to be used in the high-level trigger to select events containing electrons or photons with high transverse momentum. The performance of these methods is presented, resulting from both simulation studies, timing measurements, and test beam studies.

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

Portable Gathering System for Monitoring and Online Calibration at ATLAS.

During the runtime of any experiment, a central monitoring system that detects problems as soon as they appear has an essential role. In a large experiment, like ATLAS, the online data acquisition system is distributed across the nodes of large farms, each of them running several processes that analyse a fraction of the events. In this architecture, it is necessary to have a central process that collects all the monitoring data from the different nodes, produces full statistics histograms and analyses them. In this paper we present the design of such a system, called the gatherer. It allows to collect any monitoring object, such as histograms, from the farm nodes, from any process in the

Commissioning of the ATLAS High Level Trigger with single beam and cosmic rays

ATLAS is one of the two general-purpose detectors at the Large Hadron Collider (LHC). The trigger system is responsible for making the online selection of interesting collision events. At the LHC design luminosity of 1034 cm?2s?1 it will need to achieve a rejection factor of the order of 10?7 against random proton-proton interactions, while selecting with high efficiency events that are needed for physics analyses. After a first processing level using custom electronics based on FPGAs and ASICs, the trigger selection is made by software running on two processor farms, containing a total of around two thousand multi-core machines. This system is known as the High Level Trigger (HLT). To reduce the network data traffic and the processing time to manageable levels, the HLT uses seeded, step-wise reconstruction, aiming at the earliest possible rejection of background events. The recent LHC startup and short single-beam run provided a stress test of the system and some initial calibration data. Following this period, ATLAS continued to collect cosmic-ray events for detector alignment and calibration purposes. After giving an overview of the trigger design and its innovative features, this paper focuses on the experience gained from operating the ATLAS trigger with single LHC beams and cosmic-rays.

The RPC LVL1 trigger system of the muon spectrometer of the ATLAS experiment at LHC

The Atlas Trigger System has been designed to reduce the LHC interaction rate of about 1 GHz to the foreseen storage rate of about 100 Hz. Three trigger levels are applied in order to fulfill such a requirement. A detailed simulation of the ATLAS experiment including the hardware components and the logic of the Level-1 Muon trigger in the barrel of the muon spectrometer has been performed. This simulation has been used not only to evaluate the performances of the system but also to optimize the trigger logic design. In the barrel of the muon spectrometer the trigger will be given by means of resistive plate chambers (RPCs) working in avalanche mode. Before being mounted on the experiment, accurate quality tests with cosmic rays are carried out on each RPC chamber using the test station facility of the INFN and University laboratory of Napoli. All working parameters are measured and the uniformity of the efficiency on the whole RPC surface is required. A summary of the Napoli cosmic rays tests, together with a brief description of the Atlas Trigger, in particular of the Level-1 Muon Trigger in the barrel, and the results of the trigger simulation will be given.

Online muon reconstruction in the ATLAS level-2 trigger system

To cope with the 40 MHz event production rate of LHC, the trigger of the ATLAS experiment selects events in three sequential steps of increasing complexity and accuracy whose final results are close to the offline reconstruction. The Level-1, implemented with custom hardware, identifies physics objects within Regions of Interests and operates with a first reduction of the event rate to 75 kHz. The higher trigger levels, Level-2 and Level-3, provide a software based event selection which further reduces the event rate to about 100 Hz. This paper presents the algorithm (/spl mu/Fast) employed at Level-2 to confirm the muon candidates flagged by the Level-1. /spl mu/Fast identifies hits of muon tracks inside the barrel region of the Muon Spectrometer and provides a precise measurement of the muon momentum at the production vertex. The algorithm must process the Level-1 muon output rate (/spl sim/20 kHz), thus particular care has been taken for its optimization. The result is a very fast track reconstruction algorithm with good physics performance which, in some cases, approaches that of the offline reconstruction: it finds muon tracks with an efficiency of about 95% and computes p/sub T/ of prompt muons with a resolution of 5.5% at 6 GeV and 4.0% at 20 GeV. The algorithm requires an overall execution time of /spl sim/1 ms on a 100 SpecInt95 machine and has been tested in the online environment of the Atlas detector test beam.

The muon spectrometer barrel level-1 trigger of the ATLAS experiment at LHC

The proton-proton beam crossing at the LHC accelerator at CERN will have a rate of 40 MHz at the project luminosity. The ATLAS Trigger System has been designed in three levels in order to select only interesting physics events reducing from that rate of 40 MHz to the foreseen storage rate of about 200 Hz. The First Level reduces the output rate to about 100 kHz. The ATLAS Muon Spectrometer has been designed to perform stand-alone triggering and measurement of Muon transverse momentum up to 1 TeV/c with good resolution (from 3% at 10 GeV/c up to 10% at 1 TeV/c). In the Barrel region of the Muon Spectrometer the Level-1 trigger is given by means of three layers of resistive plate chamber detectors (RPC): a gaseous detector working in avalanche mode composed by two plates of high-resistivity bakelite and two orthogonal planes of read-out strips. The logic of the Level-1 barrel Muon trigger is based on the search of patterns of RPC hits in the three layers consistent with a high transverse momentum Muon track originated from the interaction vertex. The associated trigger electronics is based on dedicated processors, the Coincidence Matrix boards, performing space coincidences and time gates and providing the RPC readout as well. A detailed simulation of the ATLAS Experiment and of both the hardware components and the logic of the Level-1 Muon Trigger in the barrel of the Muon Spectrometer has been performed. This simulation has been used not only to evaluate the performances of the system but also to define the hardware set-up such as the cabling of both the trigger detectors and the trigger electronics modules. A description of both the Level-1 Muon Trigger system in the barrel and the RPC detectors, with their cosmic rays quality tests, will be presented together with the trigger performances and rates calculations evaluated for Muons over a wide range of pT and preliminary studies on the impact of accidental triggers due to low energy background particles in the experimental area

Muon calibration data extraction and distribution for the ATLAS experiment

In the ATLAS experiment, fast calibration of the detector is vital to feed both prompt data reconstruction with fresh calibration constants. At the same time, online data extraction presents several advantages, since the data rate needed to have data sets taken in homogeneous conditions can be achieved without performing special runs that would require special tuning of parameters and architecture of the ATLAS TDAQ system. The best place to get muon tracks suitable for muon detector calibration is the second level trigger, where the pre-selection of data sitting in a limited region by the first level trigger allows to select all (and only) the hits from a single track and to add some useful information to speed up the calibration process. The MDT (monitored drift tube) calibration is shown as a use case, as it can be generalized to the entire muon system. In this case, the extracted fragment size has been evaluated to be about 800 bytes. Since at low luminosity the achievable data throughput is about 12 kHz, the total data throughput is 9.6 MB/s. The data collection model is based on a two level data concentration: pre-selected hits from muon tracks plus some auxiliary information are sent by any second level trigger machine in a rack to a server; a calibration server collects data from the several rack servers and sends them to one or more calibration farm(s). Different options are being explored to ensure the quality of service needed for data distribution