Commissioning of the ATLAS Pixel Detector with Cosmics Ray Data

The ATLAS Pixel Detector is the innermost detector of the ATLAS experiment at the Large Hadron Collider at CERN. It consists of silicon sensors equipped with approximately 80 M electronic channels and will allow to detect particle tracks and secondary vertices with very high precision. After connection of cooling and services and verification of their operation the ATLAS Pixel Detector is now in the final stage of its commissioning phase. Prior to the first beams expected in Autumn 2009, a full characterization of the detector was performed. Calibrations of optical connections, verification of the analog performance and special DAQ runs for noise studies were done. Combined operation with other sub-detectors in ATLAS allowed to qualify the detector with physics data from cosmic muons. This paper will show all aspects of detector operation, including the monitoring and safety system, the DAQ system and calibration procedures. The summary of calibration tests on the whole detector as well as analysis of physics runs with cosmics data will be presented

Electron/photon Identification and Standard Model Higgs Process Studies at the High Level Trigger for the ATLAS experiment

The ATLAS experiment, one of the four detectors at the Large Hadron Collider currently under construction at CERN with startâup date in 2007, will search for Standard Model Higgs Bosons and for new physics beyond the Standard Model up to the TeV scale. The LHC will collide proton beams at a centerâofâmass energy of 14 TeV with an interaction rate of 10^9 Hz. The ATLAS online selection will face the challenge of efficiently selecting interesting candidate events whilst rejecting the enormous number of minimum bias and QCDâjet background events reducing the bunch crossing rate of 40 MHz to around 200 Hz for permanent storage. The ATLAS trigger is performed in a three level selection: Level1, Level2 and the Event Filter; the last two are viewed as the High Level Trigger. In this thesis the High Level Trigger strategy and physics performance for the electron and photon selection and the potential to trigger on a Standard Model Higgs with electron and photon decays in the low mass region mH < 2mZ has been evaluated. In addition, to validate the electron trigger selection the electron/pion separation capability with the High Level Trigger has been studied with real data from the ATLAS Combined Test Beam 2004

Physics with photons at the ATLAS experiment

ATLAS is a general-purpose detector due to start operation next year at the Large Hadron Collider (LHC). The LHC will collide pairs of protons at a centre-of-mass energy of 14 TeV, with a bunch-crossing frequency of 40 MHz, and luminosities up to L = 10^34 cm^-2s^-1. The identification of photons is crucial for the study of a number of physics channels, including the search for a Higgs boson decaying to photon pairs, and measurements of direct production of single photons and photon pairs. Events containing true high-p_T photons must be selected with high efficiency, while rejecting the bulk of high-p_T jet events produced with enormously larger rate through QCD processes. The photon--photon and photon--jet channels are interesting in their own right, allowing the study of QCD at high energy. It is also essential to understand these proceses as the dominant background in the search for certain new physics processes, notably the production and decay of Higgs bosons to photon pairs. There are large uncertaintines in model predictions at LHC energies, and experimental determination of the differential cross-sections is mandatory. The photon-identification strategy (including the trigger and the offline event selection) in ATLAS will be presented along with estimates of signal efficiencies and background rates. Emphasis will be placed on the perfomance for identifying photons from direct-photon production in the early running (with instantaneous luminosities starting around L = 10^31 cm^-2s^-1), and on the search for Higgs to two-photon decays at higher luminosities (L = 10^33 cm^-2s^-1 and above). Techniques for determining identification efficiencies from the data, minimizing model dependence on the cross-section determination, will also be addressed

ATLAS Trigger System, performance and strategy for Physics

ATLAS Trigger System, performance and strategy for Physic

The ATLAS High Level Trigger Steering

The High Level Trigger (HLT) of the ATLAS experiment at the Large Hadron Collider receives events which pass the LVL1 trigger at ~75 kHz and has to reduce the rate to ~200 Hz while retaining the most interesting physics. It is a software trigger and performs the reduction in two stages: the LVL2 trigger and the Event Filter (EF). At the heart of the HLT is the Steering software. To minimise processing time and data transfers it implements the novel event selection strategies of seeded, step-wise reconstruction and early rejection. The HLT is seeded by regions of interest identified at LVL1. These and the static configuration determine which algorithms are run to reconstruct event data and test the validity of trigger signatures. The decision to reject the event or continue is based on the valid signatures, taking into account pre-scale and pass-through. After the EF, event classification tags are assigned for streaming purposes. Several powerful new features for commissioning and operation have been added: comprehensive monitoring is now built in to the framework; for validation and debugging, reconstructed data can be written out; the steering is integrated with the new configuration (presented separately), and topological and global triggers have been added. This paper will present details of the final design and its implementation, the principles behind it, and the requirements and constraints it is subject to. The experience gained from technical runs with realistic trigger menus will be described

Implementation and Performance of the High-Level Trigger electron and photon selection for the ATLAS experiment at the LHC

The ATLAS three-tier trigger system faces the challenge to reduce the incoming rate of 40 MHz to â¼ 200 Hz. It consists of hardware based Level-1, and a software based High-Level Trigger (HLT). In this paper an overview of the selection algorithms for electrons and photons will be given as well as the expected performance. The electron and photon trigger menu and the strategy for the initial phase of LHC exploitation

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

International audienceThe 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 Level1 trigger will reduce this rate to around /spl Oscr/(100kHz). Subsequently, the high level trigger (HLT), which is comprised of the Second Level trigger and the Event Filter, will need to reduce this rate further by a factor of /spl Oscr/(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 preparation 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 H8 SPS extraction line; from these tests a validation of the selection architecture chosen in a real on-line environment is expected

The ATLAS Data Acquisition and High-Level Trigger: Concept, Design and Status

The Trigger and Data Acquisition system (TDAQ) of the ATLAS experiment at the CERN Large Hadron Collider is based on a multi-level selection process and a hierarchical acquisition tree. The system, consisting of a combination of custom electronics and commercial products from the computing and telecommunication industry, is required to provide an online selection power of 105 and a total throughput in the range of Terabit/sec. This paper introduces the basic system requirements and concepts, describes the architecture of the system, discusses the basic measurements supporting the validity of the design and reports on the actual status of construction and installation

The ATLAS Trigger/DAQ Authorlist, version 1.0

This is a reference document giving the ATLAS Trigger/DAQ author list, version 1.0 of 20 Nov 2008

The ATLAS trigger - high-level trigger commissioning and operation during early data taking

The ATLAS experiment is one of the two general-purpose experiments due to start operation soon at the Large Hadron Collider (LHC). The LHC will collide protons at a centre of mass energy of 14~TeV, with a bunch-crossing rate of 40~MHz. The ATLAS three-level trigger will reduce this input rate to match the foreseen offline storage capability of 100-200~Hz. This paper gives an overview of the ATLAS High Level Trigger focusing on the system design and its innovative features. We then present the ATLAS trigger strategy for the initial phase of LHC exploitation. Finally, we report on the valuable experience acquired through in-situ commissioning of the system where simulated events were used to exercise the trigger chain. In particular we show critical quantities such as event processing times, measured in a large-scale HLT farm using a complex trigger menu