ATLAS Muon Spectrometer Research Articles

Commissioning of the ATLAS Muon Spectrometer with cosmic rays

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 track reconstruction efficiency and the momentum measurement. The results show that the detector is close to the design performance and that the Muon Spectrometer is ready to detect muons produced in high energy proton–proton collisions.

Development of fast high-resolution muon drift-tube detectors for high counting rates

Pressurized drift-tube chambers are efficient detectors for high-precision tracking over large areas. The Monitored Drift-Tube (MDT) chambers of the muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) reach a spatial resolution of 35 μ m and almost 100% tracking efficiency with 6 layers of 30 mm diameter drift tubes operated with an Ar:CO 2 (93:7) gas mixture at 3 bar and a gas gain of 20 000. The ATLAS MDT chambers are designed to cope with background counting rates due to neutrons and γ rays of up to about 300 kHz per tube which will be exceeded for LHC luminosities larger than the design value of 10 34 cm −1 s −1. Decreasing the drift-tube diameter to 15 mm while keeping the other parameters, including the gas gain, unchanged reduces the maximum drift time from about 700 to 200 ns and the drift-tube occupancy by a factor of 7. New drift-tube chambers for the endcap regions of the ATLAS muon spectrometer have been designed. A prototype chamber consisting of 12 times 8 layers of 15 mm diameter drift tubes of 1 m length has been constructed with a sense wire positioning accuracy of 20 μ m . The 15 mm diameter drift-tubes have been tested with cosmic rays in the Gamma Irradiation Facility at CERN at γ counting rates of up to 1.85 MHz.

Muon identification procedure for the ATLAS detector at the LHC using Muonboy reconstruction package and tests of its performance using cosmic rays and single beam data

ATLAS is one of the four experiments at the Large Hadron Collider (LHC) at CERN. This experiment has been designed to study a large range of physics including searches for previously unobserved phenomena such as the Higgs Boson and super-symmetry. The ATLAS Muon Spectrometer (MS) is optimized to measure final state muons in a large momentum range, from a few GeV up to TeV. Its momentum resolution varies from (2-3%) at 10-100 GeV/c to 11% at 1 TeV, taking into account the high level background environment, the inhomogeneous magnetic field, and the large size of the apparatus (24 m diameter by 44 m length). A robust muon identification and high momentum measurement accuracy is crucial to fully exploit the physics potential of the LHC.The basic principles of the muon reconstruction packages Muonboy, STACO, MuTag are discussed in this paper. Details of the modifications done in order to adapt the pattern recognition to the cosmic-ray configuration as well as its performance with the recent cosmic-rays and single beam data are presented.

The ATLAS MDT remote calibration centers

The precision chambers of the ATLAS Muon Spectrometer are built with Monitored Drift Tubes (MDT). The requirement of high accuracy and low systematic error, to achieve a transverse momentum resolution of 10% at 1 TeV, can only be accomplished if the calibrations are known with an accuracy of 20 μm. The relation between the drift path and the measured time (the socalled r-t relation) depends on many parameters (temperature T, hit rate, gas composition, thresholds,...) subject to time variations. The r-t relation has to be measured from the data without the use of an external detector, using the autocalibration technique. This method relies on an iterative procedure applied to the same data sample, starting from a preliminary set of constants. The required precision can be achieved using a large (few thousand) number of non-parallel tracks crossing a region, called calibration region, i.e. the region of the MDT chamber sharing the same r-t relation.

The offline Data Quality Monitoring system of the ATLAS Muon Spectrometer

The ATLAS detector has been designed to exploit the full discovery potential of the LHC proton-proton collider at CERN. Its Muon Spectrometer (MS) has been optimized to measure final state muons from those interactions with good momentum resolution in a wide momentum range. In order to ensure that the hardware, DAQ and reconstruction software of the ATLAS MS is functioning properly, Data Quality Monitoring (DQM) tools have been developed both for the online and the offline environment. The offline DQM is performed on histograms of interesting quantities, which are filled during data processing with the ATLAS software framework at the CERN Tier0 facility. Then those histograms can be displayed and browsed by shifters and experts. They are also given as input to the Data Quality Monitoring Framework (DQMF) application, which performs the actual data quality assessment and sets status flags. The offline muon DQM structure and content, as well as the corresponding tools developed, are presented, with examples from the cosmic ray data collected for the MS Barrel during the commissioning phase.

First results from the ATLAS muon spectrometer optical alignment system

The muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) at CERN consists of 1182 muon chambers for precision track measurements, arranged in three concentric cylinders of up to 25m length in the central (barrel) region, and in four wheels of up to 25m diameter in each of the two endcaps. They are located between 5 and 22m from the interaction point, and each muon track is detected in three equally spaced chambers. The muon chambers are equipped with a complex high-precision optical alignment system to monitor their positions and deformations during ATLAS data-taking to an accuracy of 30–40μm, which is ultimately required for reconstructing high-momentum final-state muons with the desired momentum resolution of 10% at 1TeV. The alignment system, the sensors, and the readout and reconstruction software are described. The installation and commissioning of the more than 12000 sensors in the ATLAS cavern were completed in 2008, and first data from the full system are presented.

ATLAS monitored drift tube chambers for super-LHC

After the high-luminosity upgrade of the Large Hadron Collider (LHC), the ATLAS muon spectrometer is expected to work at 10-fold background rates of gammas and neutrons. As the momentum resolution of the spectrometer is expected to be 10% a single tube resolution of the muon drift tubes of 80μm is required. At background rates around 1000Hz/cm 2 space charge effects will lead in the slow and non-linear AR:CO2=93:7 gas mixture degrade the drift-tube spatial resolution. While this was studied for gammas and low energetic neutrons, almost no information exists for fast neutrons.This paper shows that:•11MeV neutrons at flux densities of 4 and 16kHz/cm2 degrade the position resolution by only 10μm at a detection efficiency of 4×10−4.•two modified inert gas mixtures selected by fastness and linearity of their position-drifttime ( r–t) relation show similar resolution as the standard gas in agreement with Garfield simulations.•the resolution of newly developed 15mm ∅ drift tubes are mostly unaffected by space charge effects due to gamma irradiation up to 1400Hz/cm2 according to Garfield simulations.

Status and performance of the ATLAS MUON spectrometer

The ATLAS experiment at the Large Hadron Collider at CERN is currently waiting to record first data from proton–proton collisions in 2009. During the last two years the MUON spectrometer was commissioned with cosmic rays while being assembled in the underground experimental cavern. In summer 2008, during the start-up phase of the LHC, the first events from single beams were recorded with the fully functional ATLAS detector. Subsequently the whole ATLAS detector was operated and commissioned with cosmic particles. The acquired data samples (of more than 100 million events, both with and without magnetic field) allowed a detailed study of the detector performance. In addition they were used to fine tune the spectrometer for an optimal operation with the first proton–proton collisions. Here the observed performance of the ATLAS MUON detection system is presented. Both the precision and the trigger chambers are studied with cosmic rays, as well as the level-1 muon trigger. First results from combined studies with other subdetectors are also presented.

Study of a micromegas chamber in a neutron beam

The micromegas detectors are proposed as one of the options to upgrade the ATLAS muon spectrometer in the very forward/backward region. One of the problems of all detectors in the LHC and especially the ones near the beam pipe is the high neutron background. A study of a tracking micromegas detector with high flux neutrons, focusing on the sparking properties, is performed for the first time. The neutrons are supplied by the Tandem accelerator, at the N.C.S.R. ``Demokritos'' in Athens, which can provide quasi monochromatic neutrons with energies up to 25.7 MeV. The first results of this study are presented.

The ATLAS muon Micromegas R&D project: towards large-size chambers for the s-LHC

Detectors based on the bulk-Micromegas technology exhibit position resolution better than 100 μm at counting rates of up to several tens of kHz/cm2, along with trigger capabilities. These characteristics, combined with the detector's mechanical robustness and the possibility for cost-effective industrial production, makes them a promising candidate for the ATLAS Muon Spectrometer upgrade in a future luminosity enhancement of the LHC. The R&D project status will be presented together with the obtained results in the effort to define the baseline system specifications.

Finding heavy long-lived charged particles with the ATLAS detector

Long-lived charged particles are predicted by many models of physics beyond the Standard Model (SM). At the LHC, the common signature is a heavy long-lived charged particle with velocity smaller than the speed of light, β < 1 . This paper presents methods we developed for identifying slow particles and measuring their mass using the ATLAS muon spectrometer. The efficacy of these methods is demonstrated using two different models.

Development of large size Micromegas detector for the upgrade of the ATLAS Muon system

With the luminosity upgrade of the LHC machine (SLHC, Super-LHC), the Muon system of the ATLAS experiment at CERN will also need a detector upgrade in the highest rapidity region. MAMMA, Muon ATLAS Micromegas Activity, is an ongoing R&D activity with the aim to develop large detectors based on the bulk-Micromegas technology for use in the ATLAS Muon Spectrometer. Micromegas is a good potential candidate for the construction of large muon chambers that combine trigger and tracking capability and can sustain high particle rates expected at the SLHC. A medium size Micromegas prototype, in scale 1:10 of the final chambers, has been built and evaluated in the laboratory and in beam tests at CERN. Results from the analysis of test-beam data are presented. The results indicate that large size Micromegas is a viable candidate for ATLAS Muon upgrade

Bringing the ATLAS Muon Spectrometer to life with cosmic rays

The Muon Spectrometer of the ATLAS experiment at the Large Hadron Collider is the largest device ever built to track high energy particles. It has been designed to provide muon identification and measurement in the hard environment of proton–proton collisions at high energy and high luminosity at the LHC. An impressive number of elements, spread over the large volume of the spectrometer, were commissioned for many months with cosmic rays and were ready to take data when the first beam was circulated in the LHC. A systematic study of the detector's performance was done in the following months. More than 200 million cosmic ray triggers were taken in different conditions. We present the status of the muon detectors and the main results from the reconstruction of this event sample, showing that the ATLAS Muon Spectrometer is well advanced towards physics data taking.

Alignment of the ATLAS muon spectrometer with tracks

The muon spectrometer of the ATLAS experiment at the Large Hadron Collider is designed to measure the muon momenta of up to 1 TeV/ c with a resolution of better than 10% in a toroidal magnetic field of superconducting air–core magnets. The muon track sagitta is measured in three layers of pressurized drift-tube chambers. The precision muon chambers have to be aligned with an accuracy of better than 30 μ m in the track bending plane. An optical alignment system monitors movements of the muon chambers with a precision of few microns. In order to determine the chamber positions in the spectrometer, the initial chamber positions have to be measured with 30 μ m accuracy using straight muon tracks from proton–proton collisions in a dedicated run of the ATLAS detector with the toroid magnets turned off. A Least Square algorithm has been developed which determines the misalignment parameters of a complete azimuthal sector of the barrel part of the muon spectrometer. It has been tested with straight cosmic muon tracks during the commissioning of the ATLAS experiment. Simulations show that the required alignment accuracy is reached with 100,000 muons per sector originating from the interaction point with transverse momentum greater than 10 GeV. About half of the barrel chambers have already been aligned with 30 μ m accuracy using cosmic muons.

A multi Gbit/s backplane for the RPC DAQ and L1 Trigger of the ATLAS Muon Spectrometer

The Read Out Driver (ROD) of the RPC detectors in the ATLAS Muon Spectrometer is a VME64x board designed to perform sub-event building for one of the 32 sectors in which the apparatus is divided. Each ROD receives data fragments from two companion boards placed in the adjacent VME slots. These two boards also send trigger data to the nearest slot, respectively at their right and left hand side. Due to the large amount of data to be transferred with a known latency, an auxiliary bus named RODbus has been designed and deployed in order to link the five boards.In this paper we present the logical and physical architecture of the RODbus, which implements multiple point-to-point LVDS links for high-speed data transfer and parallel TTL busses for commands, flags and flow-control signals. A prototype of the bus embedded into a VME64x 21-slot backplane has been recently developed together with HARTING, the firm which designed the connectors specified by the VME64x standard. We also discuss the test results on signal integrity, crosstalk and impedance profile made with Time Domain Reflectometry.

Study of the ATLAS MDT spectrometer using high energy CERN combined test beam data

In 2004, a combined system test was performed in the H8 beam line at the CERN SPS with a setup reproducing the geometry of sectors of the ATLAS Muon Spectrometer, formed by three stations of Monitored Drift Tubes (MDT). The full ATLAS analysis chain was used to obtain the results presented in this paper. The basic design performances of the Muon Spectrometer were verified. The stability of MDT calibration constants, the alignment system using optical devices and high energy tracks, as well as the intrinsic sagitta resolution of the Muon Spectrometer were studied and found to agree with expectations. The reconstruction of muon tracks using the combined information from both the Inner Detector and the Muon Spectrometer are also presented.

ATLAS Muon Drift Tube Electronics

This paper describes the electronics used for the ATLAS monitored drift tube (MDT) chambers. These chambers are the main component of the precision tracking system in the ATLAS muon spectrometer. The MDT detector system consists of 1,150 chambers containing a total of 354,000 drift tubes. It is capable of measuring the sagitta of muon tracks to an accuracy of 60 μm, which corresponds to a momentum accuracy of about 10% at pT= 1 TeV. The design and performance of the MDT readout electronics as well as the electronics for controlling, monitoring and powering the detector will be discussed. These electronics have been extensively tested under simulated running conditions and have undergone radiation testing certifying them for more than 10 years of LHC operation. They are now installed on the ATLAS detector and are operating during cosmic ray commissioning runs.

Pre-commissioning and test results of MDT chambers produced in IHEP (Protvino) for ATLAS

Trapezoidal-shape Monitored Drift Tube (MDT) chambers are used in end caps of ATLAS muon spectrometer. IHEP-Protvino has produced 192 EO (End Cap Outer) and 62 EE (Extra End Cap) MDT chambers for the ATLAS muon spectrometer. After transportation from Protvino to CERN the chambers have been finally integrated and tested. Some of the test results of EO MDT chambers are presented.

Implementation of chamber misalignments and deformations in the ATLAS muon spectrometer simulation

The implementation of run-time dependent corrections for alignment and distortions in the detector description of the ATLAS Muon Spectrometer is discussed, along with the strategies for studying such effects in dedicated simulations.

ATLAS muon spectrometer simulation and its validation algorithms

The ATLAS detector, currently being installed at CERN, is designed to make precise measurements of 14 TeV proton-proton collisions at the LHC, starting in 2007. Arguably the clearest signatures for new physics, including the Higgs boson and supersymmetry, will involve the production of isolated final-stated muons. The identification and precise reconstruction of muons are performed using a combination of detector components, including the Inner Detector, comprising a silicon tracker, pixel detector, and transition radiation tracker, housed in a uniform solenoidal field, and the precision standalone Muon Spectrometer, comprising monitored drift tubes and cathode strip chambers, triggered by resistive plate chambers and thin-gap chambers, and housed in a toroidal field. In order to manage the complexity and to understand the performance of the ATLAS Muon Spectrometer, a detailed full detector simulation is required and it should be kept under control by means of automatic validation procedures.We describe the implementation and the functionalities of the recently developed MuonValidation package, which has been developed as a dedicated tool to monitor and validate the performance of the Full simulation and digitization of the Muon System. Its flexible design allows comparisons between different Muon Spectrometer geometrical layouts and different software releases. Validation results based on fully simulated Geant4 events, using the complete detailed geometrical description of the detector, are shown.