Upgrade of the ATLAS Muon Trigger for the SLHC

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 system (Level-1 trigger). When the luminosity of the LHC will be upgraded to 4-5 times the present nominal value (SLHC) in about a decade from now, an improvement of the selectivity of the ATLAS Level-1 triggering system will be mandatory in order to cope with the maximum allowed trigger rate of 100 kHz. For the Level-1 trigger of the ATLAS muon spectrometer this means an increase of the p$_T$ threshold for single muons. Due to the limited spatial resolution of the trigger chambers, however, the selectivity for tracks above ~20 GeV/c is insufficient for an effective reduction of the Level-1 rate. We describe how the track coordinates measured in the MDT precision chambers can be used to decisively improve the selectivity for high momentum tracks. The resulting increase in latency will also be discussed.Comment: These are the proceedings of a presentation given at the Topical Workshop of Electronics for Particle Physics 2010 in Aachen, Germany (sept., 20-24, 2010

Performance of the ATLAS Muon Drift-Tube Chambers at High Background Rates and in Magnetic Fields

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

Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider

The muon detectors of the experiments at the Large Hadron Collider (LHC) have to cope with unprecedentedly high neutron and gamma ray background rates. In the forward regions of the muon spectrometer of the ATLAS detector, for instance, counting rates of 1.7 kHz/square cm are reached at the LHC design luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher background rates are expected which require replacement of the muon chambers in the critical detector regions. Tests at the CERN Gamma Irradiation Facility showed that drift-tube detectors with 15 mm diameter aluminum tubes operated with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns provide efficient and high-resolution muon tracking up to the highest expected rates. For 15 mm tube diameter, space charge effects deteriorating the spatial resolution at high rates are strongly suppressed. The sense wires have to be positioned in the chamber with an accuracy of better than 50 ?micons in order to achieve the desired spatial resolution of a chamber of 50 ?microns up to the highest rates. We report about the design, construction and test of prototype detectors which fulfill these requirements

Resolution and Efficiency of the ATLAS Muon Drift-Tube Chambers at High Background Rates

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 to 82 microns without irradiation and from 128 microns to 108 microns at the maximum expected rate. The measured drift-tube efficiency agrees with the expectation from the dead time of the read-out electronics up to the maximum expected rate

Performance of the ATLAS Precision Muon Chambers under LHC Operating Conditions

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. The resolution and efficiency of a chamber from the serial production for ATLAS has been investigated in a 100 GeV muon beam at photon irradiation rates as expected during LHC operation. A silicon strip detector telescope was used as external reference in the beam. The spatial resolution of a chamber is degraded by 4 ?m at the highest background rate. The detection efficiency of the drift tubes is unchanged under irradiation. A tracking efficiency of 98% at the highest rates has been demonstrated

A Cosmic Ray Measurement Facility for ATLAS Muon Chambers

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 calibration procedures. Until the start of muon chamber installation in ATLAS, 88 chambers built at the Max Planck Institute for Physics in Munich have to be commissioned and calibrated. With a data taking period of one day individual wire positions can be measured with an accuracy of 8.3 micrometers in the chamber plane and 27 micrometers in the direction perpendicular to that plane.Comment: 14+1 pages, 11 figures, contributed paper to the EPS2003 conference, Aache

Rate effects in high-resolution drift chambers

The impact of high counting rates on the spatial resolution of cylindrical drift tubes is investigated in detail and the results are compared with simulations. Electronics effects and space-charge effects are quantitatively analysed. A spatial resolution of $\sigma < 80\,\mu\mathrm{m}$ can be achieved even at rates as high as 1500\,Hz/cm wire length (300\,kHz per wire)

Resolution limits of drift tubes

Measurements of the drift-tube response to charged particle tracks are compared with a complete simulation. The measured resolution of typically 80\,$\mu$m agrees well with the simulation and allows the individual factors limiting the resolution such as diffusion, charge deposit fluctuations, gas gain fluctuations and signal processing to be studied. The results with respect to the dependence of the drift chamber resolution on gas gain, gas pressure and electronics parameters are reported

Front-end electronics for drift tubes in a high-rate environment

A front-end electronics readout for drift tubes in a high-rate environment is presented. This system allows us to encode several pieces of information (leading edge time, trailing edge time, signal charge and piled-up hits from multiple tracks) into a single readout channel that is presented to the TDC. The advantage of active baseline restoration compared to bipolar signal shaping is discussed