RPCs as trigger detector for the ATLAS experiment: performances, simulation and application to the level-1 di-muon trigger

The Large Hadron Collider is the proton-proton collider in construction at CERN. It will provide the highest ever realized energy in the center of mass, reaching the value of √s = 14TeV, thus giving the possibility to investigate a wide range of physics up to masses of ∼ 1TeV. The most prominent issue is the search for the origin of the spontaneous symmetry-breaking mechanism in the electroweak sector of the Standard Model. ATLAS will be one of the four experiments installed at the LHC. It has been designed to be a general purpose experiment, and among its characteristics it has a large stand-alone muon spectrometer, which allows high precision measurements of the muon momentum. A general overview of the ATLAS experiment and of its muon spectrometer is given in chapters 1 and 2. In the muon spectrometer different detectors are used to provide trigger functionality and precision momentum measurements. In the pseudorapidity range |η| ≤ 1 the first level muon trigger is based on Resistive Plate Chambers, gas ionization detectors which are characterized by a fast response and an excellent time resolution σt ≤ 1.5ns. The working principles of the Resistive Plate Chambers will be illustrated in chapter 3. Given the long time of operation expected for the ATLAS experiment (∼10 years), ageing phenomena have been carefully studied, in order to ensure stable long-term operation of all the subdetectors. Concerning Resistive Plate Chambers, a very extensive ageing test has been performed at CERN’s Gamma Irradiation Facility on three production chambers. The results of this test are presented in chapter 4. One of the most commonly used gases in RPCs operation is C2H2F4 which, during the gas discharge can produce fluorine ions. Being F one of the most aggressive elements in nature, the presence of F− ions on the plate surface is dangerous for the integrity of the surface itself. For this reason a significant effort has been put in the last years to understand the mechanisms of F− production in RPCs operated with C2H2F4-based gas mixtures. The results of the measurements performed in the INFN-Roma2 ATLAS laboratories, in collaboration with the Dept. of Science and Chemical Technology of the Univeristy of Rome "Tor Vergata", are presented in chapter 5. The old Geant3 software toolkit, which has been the de facto standard for high energy physics simulation in the last 15 years, is being progressively replaced by the completely re-written toolkit Geant4. The migration from Geant3 to Geant4 has required, in the case of the ATLAS experiment, a re-writing from scratch of most of the simulation software. In chapter 6 the work done on RPC Geant4 simulation will be described. Many interesting physics processes to be observed in ATLAS will be characterized by the presence of pairs of muons in the final state. For this reason, the ATLAS first level muon trigger has been designed to allow to select di-muon events. While foreseen in the design of the trigger system, this possibility was never intensively tested with the final detector layout. In chapter 7 the first results of such a test will be summarized.