The performance of the ATLAS muon trigger system is evaluated with proton-proton ($pp$) and heavy-ion (HI) collision data collected in Run 2 during 2015-2018 at the Large Hadron Collider. It is primarily evaluated using events containing a pair of muons from the decay of $Z$ bosons to cover the intermediate momentum range between 25 GeV and 100 GeV. Overall, the efficiency of the single-muon triggers is about 68 % in the barrel region and 85% in the endcap region. The $p_{\text{T}}$ range for efficiency determination is extended by using muons from decays of $J/ψ$ mesons, $W$ bosons, and top quarks. The performance in HI collision data is measured and shows good agreement with the results obtained in $pp$ collisions. The muon trigger shows uniform and stable performance in good agreement with the prediction of a detailed simulation. Dedicated multi-muon triggers with kinematic selections provide the backbone to beauty, quarkonia, and low-mass physics studies. The design, evolution and performance of these triggers are discussed in detail.
The ATLAS detector has been designed to have good muon momentum resolution up to momenta in the TeV range. The muon momentum resolution of the ATLAS spectrometer has been measured with p-p collision data recorded in 2011. The measurement combines the di-muon mass resolution in J/Ψ->μμand Z->μμdecays with measurements of the alignment accuracy of the detector based on straight muon tracks, which are acquired with special runs without magnetic field in the ATLAS detector.
The muon spectrometer of the ATLAS experiment is one of the largest detectors ever built. At the LHC, new physics signs could appear through high momentun muons (1 TeV). Identification and precise momentum measurement of such muons are two of the main challenges of the ATLAS muon spectrometer. In order to get a good resolution for high energy muons (i.e. 10% at 1 TeV), the accuracy on the alignment of precision chambers must be of the order of 50 microns. Several procedures have been developed to reach such a precision. This document describes complementary techniques used to align the muon su…
The ATLAS muon spectrometer is designed to measure muon momenta with a resolution of 4% @ 100 GeV/c rising to 10% @ 1 TeV/c track momentum. The spectrometer consists of precision tracking and trigger chambers embedded in a 2T magnetic field generated by three large air-core superconducting toroids. The precision detectors provide 50 micron tracking resolution to a pseudo-rapidity of 2.7. The system also includes an optical monitoring system which measures detector positions with 40 micron precision. This paper reports on the calibration and performance of the ATLAS muon spectrometer.
The performance of the three-level ATLAS muon trigger as evaluated by using LHC data is presented. Events have been selected by using only the hardware-based Level-1 trigger in order to commission and to subsequently enable the (software-based) selections of the High Level Trigger. Studies aiming at selecting prompt muons from J/ψ and at reducing non prompt muon contamination have been performed. A brief overview on how the muon triggers evolve with increasing luminosity is given.
The goals of the ongoing and planned ATLAS muon detector upgrades are to increase the acceptance for precision muon momentum measurement and triggering and to improve the rate capability of the muon chambers in the high-background regions corresponding to the increasing LHC luminosity. Smalldiameter Muon Drift Tube (sMDT) chambers have been developed for these purposes. With half the drift-tube diameter of the current ATLAS Muon Drift Tube (MDT) chambers with 30 mm drift tube diameter and otherwise unchanged operating parameters, the sMDT chambers share all the advantages of the MDTs, but have…
Large-scale computing in ATLAS is based on a grid-linked system of tiered computing centers. The ATLAS Great Lakes Tier-2 came online in September 2006 and now is commissioning with full capacity to provide significant computing power and services to the USATLAS community. Our Tier-2 Center also host the Michigan Muon Calibration Center which is responsible for daily calibrations of the ATLAS Monitored Drift Tubes for ATLAS endcap muon system. During the first LHC beam period in 2008 and following ATLAS global cosmic ray data taking period, the Calibration Center received a large data stream f…
In this talk I discuss the use of calorimeter timing both for detector commissioning and in searches for new physics. In particular I present real and simulated cosmic ray muons data results for the ATLAS Tile Calorimeter system. The analysis shows that several detector errors such as imperfect calibrations can be uncovered. I also demonstrate the use of ATLAS Tile Calorimeters excellent timing resolution in suppressing cosmic ray fake missing transverse energy in searches for supersymmetry.
The ATLAS muon spectrometer uses drift-tube chambers for precision tracking. The performance of these chambers in the presence of magnetic field and high radiation fluxes is studied in this article using test-beam data recorded in the Gamma Irradiation Facility at CERN. The measurements are compared to detailed predictions provided by the Garfield drift-chamber simulation programme.
Calibration of the discriminator threshold of the Amplifier-Shaper-Discriminator Chip for the Upgrade of the ATLAS Muon Drift-Tube Chambers for the High-Luminosity LHC
The Monitored Drift Tubes, as a part of the ATLAS muon spectrometer, are precision drift chambers designed to provide excellent spatial resolution and high tracking efficiency independent of the track angle. Through the life of the LHC and ATLAS experiment, this detector has already demonstrated that they provide precise tracking over large areas. The aim of the ATLAS muon spectrometer upgrade is to increase the muon trigger efficiency, precise muon momentum measurement and to improve the rate capability of the muon system in the high-background regions during the High-Luminosity LHC runs. To …
The ATLAS muon spectrometer consists of several major components: Monitored Drift Tubes (MDTs) for precision measurements in the bending plane of the muons, supplemented by Cathode Strip Chambers (CSC) in the high eta region; Resistive Plate Chambers (RPCs) and Thin Gap Chambers (TGCs) for trigger and second coordinate measurement in the barrel and endcap regions, respectively; an optical alignment system to track the relative positions of all chambers; and, finally, the world's largest air-core magnetic toroid system. We will describe the status and commissioning of the muon system with cosmi…
The ATLAS detector is intended to verify the standard model and to search for new physics at the Large Hadron Collider (LHC, CERN). In addition to this primary goal, it also allows detection of muons of cosmic rays. On the other hand, unusual bursts of the muon intensity, which correlate with powerful solar flares were recorded and investigated earlier at the Baksan Underground Scintillation Telescope (BUST, INR, Russia) in period from 1981 to 2006 (~2.5 solar cycles). The nature of the muon bursts and their probable relation to the solar cosmic rays is still not quite clear. ATLAS has an exce…
The ATLAS detector at the Large Hadron Collider has collected several hundred million cosmic ray events during 2008 and 2009. These data were used to commission the Muon Spectrometer and to study the performance of the trigger and tracking chambers, their alignment, the detector control system, the data acquisition and the analysis programs. We present the performance in the relevant parameters that determine the quality of the muon measurement. We discuss the single element efficiency, resolution and noise rates, the calibration method of the detector response and of the alignment system, the…
A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bb final states, and pp collision data at sqrt(s) = 7 TeV collected with the ATLAS detector at the LHC during 2011.
The High-Luminosity LHC (HL-LHC) will provide the unique opportunity to explore the nature of physics beyond the Standard Model of strong and electroweak interactions. Highly selective first-level triggers are essential for the physics programme of the ATLAS experiment at HL-LHC, where the instantaneous luminosity will exceed the instantaneous LHC Run 1 luminosity by about an order of magnitude. The ATLAS first-level muon trigger rate is dominated by low momentum muons, which are accepted because of the moderate momentum resolution of the RPC and TGC trigger chambers. This limitation can be ov…
Small-diameter Muon Drift Tube (sMDT) chambers have been developed for the ATLAS muon detector upgrade. They possess an improved rate capability and a more compact design with respect to the existing chambers, which allows to equip detector regions uninstrument at present. The chamber assembly methods have been optimized for mass production, while the sense wire positioning accuracy is improved to below ten microns. The chambers will be ready for installation in the winter shutdown 2016/17 of the Large Hadron Collider. The design and construction of the new sMDT chambers for ATLAS will be disc…
Precision drift tube chambers with a sense wire positioning accuracy of better than 20 microns are under construction for the ATLAS muon spectrometer. 70% of the 88 large chambers for the outermost layer of the central part of the spectrometer have been assembled. Measurements during chamber construction of the positions of the sense wires and of the sensors for the optical alignment monitoring system demonstrate that the requirements for the mechanical precision of the chambers are fulfilled.
The ATLAS experiment at the Large Hadron Collider (LHC) at CERN is currently waiting to record the first collision data in spring 2009. Its muon spectrometer is designed to achieve a momentum resolution of 10% pT(mu) = 1 TeV/c. The spectrometer consists of a system of three superconducting air-core toroid magnets and is instrumented with three layers of Monitored Drift Tube chambers (Cathode Strip Chambers in the extreme forward region) as precision detectors. Resistive Plate Chambers in the barrel and Thin Gap Chambers in the endcap regions provide a fast trigger system. The spectrometer pass…
With half the drift-tube diameter of the Monitored Drift Tube (MDT) chambers of the ATLAS muon spectrometer and otherwise unchanged operating parameters, small-diameter Muon Drift Tube (sMDT) chambers provide an order of magnitude higher rate capability and can be installed in detector regions where MDT chambers do not fit. The chamber assembly time has been reduced by a factor of seven to one working day and the sense wire positioning accuracy improved by a factor of two to better than ten microns. Two sMDT chambers have been installed in ATLAS in 2014 to improve the momentum resolution in th…
The innermost station of the present ATLAS forward muon detector needs to be upgraded for the super-LHC. We present a proposal to replace it with a sandwiched detector composed of several layers of small-radius Monitored Drift Tube chambers (sMDT) for precision tracking measurement and two stations of multi-gap Resistive Plate Chambers (mRPC) for triggering purpose. We describe the layout of the upgraded detector and the trigger strategy. Several modifications to the RPCs used in the ATLAS barrel region are needed to satisfy the super-LHC requirements. Various studies with the proposed mRPC ti…
This paper reports on the development of a resource-efficient FPGA-based neural network regression model for potential applications in the future hardware muon trigger system of the ATLAS experiment at the Large Hadron Collider (LHC). Effective real-time selection of muon candidates is the cornerstone of the ATLAS physics programme. With the planned ATLAS upgrades for the High Luminosity LHC, an entirely new FPGA-based hardware muon trigger system will be installed that will process full muon detector data within a 10 $μs$ latency window. The large FPGA devices planned for this upgrade should …
We propose a new fixed latency scheme for Xilinx gigabit transceivers that will be used in the upgrade of the ATLAS forward muon spectrometer at the Large Hadron Collider. The fixed latency scheme is implemented in a 4.8 Gbps link between a frontend data serializer ASIC and a packet router. To achieve fixed latency, we use IO delay and dedicated carry in resources in a Xilinx FPGA, while minimally relying on the embedded features of the FPGA transceivers. The scheme is protocol independent and can be adapted to FPGA from other vendors with similar resources. This paper presents a detailed impl…
The ATLAS Muon Spectrometer is designed to measure the momentum of muons with a resolution of dp/p = 3% and 10% at 100 GeV and 1 TeV momentum respectively. For this task, the spectrometer employs 355,000 Monitored Drift Tubes (MDTs) arrayed in 1200 Chambers. Calibration (RT) functions convert drift time measurements into tube-centered impact parameters for track segment reconstruction. RT functions depend on MDT environmental parameters and so must be appropriately calibrated for local chamber conditions. We report on the creation and application of a gas monitor system based calibration progr…
For the upgrade of the ATLAS muon spectrometer in March 2014 new muon tracking chambers (sMDT) with drift-tubes of 15 mm diameter, half of the value of the standard ATLAS Monitored Drift-Tubes (MDT) chambers, and 10~$μ$m positioning accuracy of the sense wires have been constructed. The new chambers are designed to be fully compatible with the present ATLAS services but, with respect to the previously installed ATLAS MDT chambers, they are assembled in a more compact geometry and they deploy two additional tube layers that provide redundant rack information. The chambers are composed of 8 laye…
The ATLAS trigger has been used very successfully to collect collision data during 2009 and 2010 LHC running at centre of mass energies of 900 GeV, 2.36 TeV, and 7 TeV. This paper presents the ongoing work to commission the ATLAS trigger with proton collisions, including an overview of the performance of the trigger based on extensive online running. We describe how the trigger has evolved with increasing LHC luminosity and give a brief overview of plans for forthcoming LHC running.
The outer shell of the ATLAS experiment at the LHC consists of a system of toroidal air-core magnets in order to allow for the precise measurement of the transverse momentum p$_T$ of muons, which in many physics channels are a signature of interesting physics processes. For the precise determination of the muon momentum Monitored Drift Tube chambers (MDT) with high position accuracy are used, while for the fast identification of muon tracks chambers with high time resolution are used, able to select muons above a predefined p$_T$ threshold for use in the first Level of the ATLAS triggering sys…
Monitored Drift Tube (MDT) chambers will constitute the large majority of precision detectors in the Muon Spectrometer of the ATLAS experiment at the Large Hadron Collider at CERN. For commissioning and calibration of MDT chambers, a Cosmic Ray Measurement Facility is in operation at Munich University. The objectives of this facility are to test the chambers and on-chamber electronics, to map the positions of the anode wires within the chambers with the precision needed for standalone muon momentum measurement in ATLAS, and to gain experience in the operation of the chambers and on-line calibr…
For Run 3 (from 2021), the LHC will undergo a significant increase in instantaneous luminosity to 1.5 times its current value which will lead to larger collected statistics and an enhanced sensitivity to new physics. The Phase-1 upgrade of the ATLAS Level-1 endcap muon trigger is essential to keep the physics acceptance at Run 3. A new trigger logic to take coincidence with detectors inside and outside the magnetic field is described and is shown to reduce the trigger rate to lower than the required level in Run 3. A new trigger board, NSL, to integrate all the information from various detecto…
The High-Luminosity LHC will provide the unique opportunity to explore the nature of physics beyond the Standard Model of strong and electroweak interactions. Highly selective first level triggers are essential for the physics programme of the ATLAS experiment at the HL-LHC where the instantaneous luminosity will exceed the LHC Run 1 instantaneous luminosity by almost an order of magnitude. The ATLAS first level muon trigger rate is dominated by low momentum muons, selected due to the moderate momentum resolution of the resistive plate and thin gap trigger chambers. This limitation can be over…
The ATLAS experiment at the Large Hadron Collider (LHC) employs a trigger system consisting of a first-level hardware trigger (L1) and a software-based high-level trigger. The L1 muon trigger system selects muon candidates, assigns them to the correct LHC bunch crossing and classifies them into one of six transverse-momentum threshold classes. The L1 muon trigger system uses resistive-plate chambers (RPCs) to generate the muon-induced trigger signals in the central (barrel) region of the ATLAS detector. The ATLAS RPCs are arranged in six concentric layers and operate in a toroidal magnetic fie…
The Monitored Drift Tube (MDT) chambers for the muon spectrometer of the AT- LAS detector at the Large Hadron Collider (LHC) consist of 3-4 layers of pressurized drift tubes on either side of a space frame carrying an optical monitoring system to correct for deformations. The full-scale prototype of a large MDT chamber has been constructed with methods suitable for large-scale production. X-ray measurements at CERN showed a positioning accuracy of the sense wires in the chamber of better than the required 20 ?microns (rms). The performance of the chamber was studied in a muon beam at CERN. Cha…
ATLAS muon detector upgrades aim for increased acceptance for muon triggering and precision tracking and for improved rate capability of the muon chambers in the high-background regions of the detector with increasing LHC luminosity. The small-diameter Muon Drift Tube (sMDT) chambers have been developed for these purposes. With half of the drift-tube diameter of the MDT chambers and otherwise unchanged operating parameters, sMDT chambers share the advantages of the MDTs, but have an order of magnitude higher rate capability and can be installed in detector regions where MDT chambers do not fit…
We report on the design of a serializer ASIC to be used in the ATLAS forward muon detector for trigger data transmission. We discuss the performance of a prototype chip covering power dissipation, latency and stable operating line rate. Tests show that the serializer is capable of running at least at 5.76 Gbps with a bit error ratio below 1x10^{-15}, and a power consumption of 200 mW running at 4.8 Gbps. The latency between the start of loading 30 bits into the serializer to the transmission of the first bit from the serializer is measured to be about 6 ns.
Highly selective triggers are essential for the physics programme of the ATLAS experiment at HL-LHC where the instantaneous luminosity will be about an order of magnitude larger than the LHC instantaneous luminosity in Run 1. The first level muon trigger rate is dominated by low momentum muons below the nominal trigger threshold due to the moderate momentum resolution of the Resistive Plate and Thin Gap trigger chambers. The resulting high trigger rates at HL-LHC can be sufficiently reduced by using the data of the precision Muon Drift Tube chambers for the trigger decision. This requires the …
The Monitored Drift Tube (MDT) chambers for the muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) consist of 3-4 layers of pressurised drift tubes on either side of a space frame carrying an optical deformation monitoring system. The chambers have to provide a track position resolution of 40 microns with a single-tube resolution of at least 80 microns and a sense wire positioning accu- racy of 20 ?microns (rms). The feasibility was demonstrated with the full-scale prototype of one of the largest MDT chambers with 432 drift tubes of 3.8 m length. For the ATLAS muon spec…
The resolution and efficiency of a precision drift-tube chamber for the ATLAS muon spectrometer with final read-out electronics was tested at the Gamma Irradiation Facility at CERN in a 100 GeV muon beam and at photon irradiation rates of up to 990 Hz/square cm which corresponds to twice the highest background rate expected in ATLAS. A silicon strip detector telescope was used as external reference in the beam. The pulse-height measurement of the read-out electronics was used to perform time-slewing corrections which lead to an improvement of the average drift-tube resolution from 104 microns …
The instantaneous luminosity of the Large Hadron Collider at CERN will be increased by up to a factor of five to seven with respect to the design value. To maintain an excellent detection and background rejection capability in the forward region of the ATLAS detector, part of the muon detection system will be upgraded during LHC shutdown periods with the replacement of part of the present first station in the forward regions with the so-called New Small Wheels (NSWs). The NSWs will have a diameter of approximately 10 m and will be made of two detector technologies: Micromegas and small-strip T…
Many models for physics Beyond the Standard Model predict an increased production rate of tau leptons. Therefore hadronically decaying tau leptons play an important role in searches for new physics at the LHC. By triggering on them, the discovery power of ATLAS can be greatly enhanced for many Beyond the Standard Model searches. In this contribution we present a brief description of the ATLAS tau trigger system and demonstrate its robustness and reliability during 2011.
The ATLAS experiment at the Large Hadron Collider employs a two-level trigger system to record data at an average rate of 1 kHz from physics collisions, starting from an initial bunch crossing rate of 40 MHz. During the LHC Run 2 (2015-2018), the ATLAS trigger system operated successfully with excellent performance and flexibility by adapting to the various run conditions encountered and has been vital for the ATLAS Run-2 physics programme. For proton-proton running, approximately 1500 individual event selections were included in a trigger menu which specified the physics signatures and select…
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 centre-of-mass energy, whilst rejecting the enormous number of background events. Therefore it is equipped with a three level trigger system. The first level is is hardware based and uses coarse granularity calorimeter information and fast readout muon chambers. The second and third level triggers, which are software based, will need to reduce the first level trigger output rate of ~ 75 kHz to ~ 200 Hz written out to mass storage. Th…
The ionization signals in the liquid argon of the ATLAS electromagnetic calorimeter are studied in detail using cosmic muons. In particular, the drift time of the ionization electrons is measured and used to assess the intrinsic uniformity of the calorimeter gaps and estimate its impact on the constant term of the energy resolution. The drift times of electrons in the cells of the second layer of the calorimeter are uniform at the level of 1.3 % in the barrel and 2.8 % in the endcaps. This leads to an estimated contribution to the constant term of (0.29 +0.05 -0.04) % in the barrel and (0.54 +…
This paper reports on the design and construction of infrastructure and test stations for small-diameter monitored drift tube (sMDT) assembly and testing at the University of Michigan (UM) to prepare for the ATLAS Muon Spectrometer upgrade for the high-luminosity program of the Large Hadron Collider. Procedures of the tube assembly and quality assurance and control (QA/QC) tests are described in detail. More than 99% of the tubes meet the tube QA/QC specifications based on 2100 tubes built at UM. The UM test stations are also used for QA/QC testing on the tubes constructed at Michigan State Un…
The upgrade to the ATLAS trigger for LHC Run 2 is presented including a description of the design and performance of the newly reimplemented tracking algorithms. The profiling infrastructure, constructed to provide prompt feedback from the optimisation procedure is described including the methods used to monitor the relative performance improvements as the code evolves. The performance of the trigger on the first data collected in the LHC Run 2 is presented.
The ATLAS detector at CERN's Large Hadron Collider will be exposed to proton-proton collisions from beams crossing at 40 MHz. A three-level trigger system was designed to select potentially interesting events and reduce the incoming rate to 100-200 Hz. The first trigger level (LVL1) is implemented in custom-built electronics, the second and third trigger levels are realized in software. Based on calorimeter information and hits in dedicated muon-trigger detectors, the LVL1 decision is made by the central-trigger processor yielding an output rate of less than 100 kHz. The allowed latency for th…
A new small-diameter Monitored Drift Tube (sMDT) chamber has been developed for the muon spectrometer of the ATLAS experiment to handle the higher collision rates expected at the CERN High Luminosity Large Hadron Collider (HL-LHC). This paper presents measurements of the tracking resolution and hit efficiency of two prototype sMDT chambers constructed at the University of Michigan. Using cosmic-ray muons the sMDT tracking resolution of 103.7$\pm8.1$ \textmu m was measured for one chamber and 101.8$\pm$7.8 \textmu m for the other, compared with a design resolution of 106 \textmu m. A further tr…
We present a multi-layer single-event upset mitigation strategy implemented in a low-cost Xilinx Artix-7 FPGA. The implementation is targeted for a trigger data router for the ATLAS muon spectrometer upgrade. The mitigation strategy employs three layers of protection to improve overall FPGA design robustness: use of triple-modular redundancy for FPGA fabric logic and embedded soft-error mitigation in the first layer; further enhancement with multi-boot FPGA reconfiguration across multiple copies of configuration memory in the second layer; and FPGA power cycling and configuration memory re-ini…
For the muon spectrometer of the ATLAS detector at the large hadron collider (LHC), large drift chambers consisting of 6 to 8 layers of pressurized drift tubes are used for precision tracking covering an active area of 5000 m2 in the toroidal field of superconducting air core magnets. The chambers have to provide a spatial resolution of 41 microns with Ar:CO2 (93:7) gas mixture at an absolute pressure of 3 bar and gas gain of 2?104. The environment in which the chambers will be operated is characterized by high neutron and background with counting rates of up to 100 per square cm and second. T…
Highly selective first-level triggers are essential to exploit the full physics potential of the ATLAS experiment at High-Luminosity LHC (HL-LHC). The concept for a new muon trigger stage using the precision monitored drift tube (MDT) chambers to significantly improve the selectivity of the first-level muon trigger is presented. It is based on fast track reconstruction in all three layers of the existing MDT chambers, made possible by an extension of the first-level trigger latency to six microseconds and a new MDT read-out electronics required for the higher overall trigger rates at the HL-LH…
Resistive Plate Chambers are used in the ATLAS experiment for the muon trigger and two coordinate measurements in the central region. In preparation for the coming years of LHC running at higher luminosity, besides the New Small Wheel project which is expected to complement the ATLAS muon spectrometer in the endcap regions, a smaller size project, known as BIS78, is being developed with a foreseen installation during the LHC Long Shutdown 2. The BIS78 project will reinforce the fake muon rejection and the selectivity of the muon trigger in the transition region between the ATLAS barrel and end…
This paper describes the small-diameter monitored drift-tube detector construction at the University of Michigan as a contribution to the ATLAS Muon Spectrometer upgrade for the high-luminosity Large Hadron Collider at CERN. Measurements of the first 30 chambers built at Michigan show that the drift tube wire position accuracy meets the specification of 20 microns. The positions of the platforms for alignment and magnetic field sensors are all installed well within the required precision. The cosmic ray test measurements show single wire tracking resolution of 100 +- 7 microns with an average …
Single muon triggers are crucial for the physics programmes at hadron collider experiments. To be sensitive to electroweak processes, single muon triggers with transverse momentum thresholds down to 20 GeV and dimuon triggers with even lower thresholds are required. In order to keep the rates of these triggers at an acceptable level these triggers have to be highly selective, i.e. they must have small accidental trigger rates and sharp trigger turn-on curves. The muon systems of the LHC experiments and experiments at future colliders like FCC-hh will use two muon chamber systems for the muon t…
Electron and photon triggers covering transverse energies from 5 GeV to several TeV are essential for signal selection in a wide variety of ATLAS physics analyses to study Standard Model processes and to search for new phenomena. Final states including leptons and photons had, for example, an important role in the discovery and measurement of the Higgs particle. Dedicated triggers are also used to collect data for calibration, efficiency and fake rate measurements. The ATLAS trigger system is divided in a hardware-based (Level 1) and a software based High-Level Trigger (HLT), both of which wer…
Large area Micromegas detectors will be employed for the first time in high-energy physics experiments. A total surface of about $\mathbf{150~m^2}$ of the forward regions of the Muon Spectrometer of the ATLAS detector at LHC will be equipped with 8-layer Micromegas modules. Each layer covers more than $\mathbf{2~m^2}$ for a total active area of $\mathbf{1200~m^2}$. Together with the small strip Thin Gap Chambers they will compose the two New Small Wheels, which will replace the innermost stations of the ATLAS endcap muon tracking system in the 2018/19 shutdown. In order to achieve a 15$\mathbf…
The LHC upgrade scheduled for 2018 is expected to significantly increase the accelerator's luminosity, and as a result the radiation background rates in the ATLAS Muon Spectrometer will increase too. Some of its components will have to be replaced in order to cope with these high rates. Newly designed small-strip Thin Gap chambers (sTGC) will replace them at the small wheel region. One of the differences between the sTGC and the currently used TGC is the alignment of the wires along the azimuthal direction. As a result, the outermost wires approach the detector's edge with a small angle. Such …
Muons from cosmic-ray interactions in the atmosphere provide a high-statistics source of particles that can be used to study the performance and calibration of the ATLAS detector. Cosmic-ray muons can penetrate to the cavern and deposit energy in all detector subsystems. Such events have played an important role in the commissioning of the detector since the start of the installation phase in 2005 and were particularly important for understanding the detector performance in the time prior to the arrival of the first LHC beams. Global cosmic-ray runs were undertaken in both 2008 and 2009 and th…
The ATLAS liquid argon calorimeter has been operating continuously since August 2006. At this time, only part of the calorimeter was readout, but since the beginning of 2008, all calorimeter cells have been connected to the ATLAS readout system in preparation for LHC collisions. This paper gives an overview of the liquid argon calorimeter performance measured in situ with random triggers, calibration data, cosmic muons, and LHC beam splash events. Results on the detector operation, timing performance, electronics noise, and gain stability are presented. High energy deposits from radiative cosm…
Small-diameter muon drift tube (sMDT) detectors have been developed for upgrades of the ATLAS muon spectrometer. With a tube diameter of 15 mm, they provide an about an order of magnitude higher rate capability than the present ATLAS muon tracking detectors, the MDT chambers with 30 mm tube diameter. The drift-tube design and the construction methods have been optimised for mass production and allow for complex shapes required for maximising the acceptance. A record sense wire positioning accuracy of 0.005 mm has been achieved with the new design. 14 new sMDT chambers are already operational i…
