Research and Development of the Micromegas Detector for the New Small Wheel upgrade in the ATLAS Experiment

The Large Hadron Collider (LHC) is the world's largest and most powerful particle accelerator ever made. LHC commenced on 10 September 2008, consisting of a 27 km ring of superconducting magnets, operating at temperatures (-271.3 "^{o}"C) colder than the outer space, guiding the particles through their journey to the collision point. The magnet system includes 1232 dipole magnets 15 meters in length bend the beams, and 392 quadrupole magnets, each 5-7 meters long, focus the beam. A number of accelerating structures, called radio-frequency cavities, boost the particles close to the speed of light preparing them for the upcoming collisions. The collisions in the LHC occur at four positions around the ring, that corresponds to the locations of the four particle detectors, namely ATLAS, CMS, ALICE and LHCb. In order to surpass the scientific expectations and the challenges that will be presented during the High Luminosity LHC (HL-LHC) era, the four experiments need to be upgraded. This dissertation aims to present a comprehensive overview of one of the two general multi-purpose high luminosity experiments, the ATLAS detector and mainly the Muon Spectrometer (MS) and the upgrade of its first forward muon station, the so-called, New Small Wheel (NSW). ATLAS, an abbreviation for A Toroidal LHC ApparatuS, has been built in order to study a broad spectrum of physics, evaluating the nature of the fundamental cornerstones of our universe by either confirming the Standard Model or revealing possible clues for new ones. The Muon Spectrometer lies outside the hadronic calorimeter, completing in such way the detector's entity. The spectrometer is devoted to the identification and the measurement of the momenta of muons based on the magnetic deflection of the muon tracks by a system of superconducting air-core toroid magnets. The motivation of this upgrade project stems from the current conditions of the ATLAS muon spectrometer which will exceed its design capabilities in the high background radiation expected during Run-3 (2021), and ultimately in HL-LHC (2026). Additionally, the muon trigger rate will exceed the available bandwidth due to the fake endcap muon triggers, where more than 90% emerges from low energy charged particles emitted by the radiation shielding and the materials of the endcap toroid. In order to cope with the foreseen increasing rate, the collaboration has decided to replace the SW with a New SW (NSW) system, that combines sTGC and resistive MM detectors maintaining in such way the excellent performance of the muon system beyond Run-2. The NSW requires that both technologies will provide level-1 trigger and pattern recognition performance of the reconstruction of track segments, consisting of a fully redundant multi-layered system. The tracking performance should achieve a position resolution of about 50 μm, which results in a position resolution better than 100 μm per plane for the planned 4-layer detector. The online reconstructed track segments for triggering should achieve an angular resolution better than 1mrad (RMS) at high efficiency (> 95%) in the full pseudo-rapidity coverage of the detector (1.3 < |η| < 2.7). These can also confirm that the muon tracks originate from the IP, reduce the fake EM trigger rate to an acceptable level (< 20kHz). In order to demonstrate and prove that the MM satisfies the NSW requirements several tests have been performed on small (10 x 10 cm2) resistive-strip MM chambers using medium (10 GeV/c) and high (150 GeV/c) momentum hadron beams at CERN. For the NSW of the ATLAS experiment a gas distribution system has to be appropriately designed in order to ensure the required gas renewals rate among the Micromegas Multiplets and to achieve a uniform distribution of the gas flow through the different gas supply branches. The gas mixture that will be used for the operation of the Micromegas is the well-known Ar+7%CO2 at atmospheric pressure. The gas distribution scheme is optimized for the minimization of the gas pressure and its variation due to the gravity by simulating the gas flow along the pipes of the inlet and outlet channels. Two consecutive wedges of the same type are supplied by one gas channel, with a total of 16 input and 16 output channels per wheel. Each gas channel is equipped with an impedance (Z), in order to keep the chamber's pressure below 1 mbar, and with the use of a custom-made manifold distributes the gas from both sides of the outer module to the inner module via interconnections. From the output of the inner module, the gas is circulated back to the main lines. Complementary to the R&D of the MM detector, an intuitive control system was of vital importance. The Micromegas for the Small Wheel (MMSW) DCS has been developed, following closely the existing look, feel and command architecture of the other Muon sub-systems, in view of being a basis for future developments for the final NSW DCS integration. The principal task of the DCS is to enable the coherent and safe operation of the detector by continuously monitoring its operational parameters and its overall state. Additionally, several contributions have been made in expanding and improving the efficiency and functionality of the ATLAS Muon Detector Control System (DCS). The two major developments that were accomplished concern the implementation of a tool to study the current behavior of the chambers during the High luminosity runs in ATLAS, while the second one was the integration of the small-diameter Muon Drift Tube (sMDT) chambers, which serve the increase of the acceptance for the precision muon momentum measurement, improving the rate capability of the muon chambers in the high-background regions. Both developments are integrated into the ATLAS DCS and are used on the regular operation of the ATLAS detector. As in all discovery journeys, and so in this one, there may be pitfalls that will make you deviate from your initial track. Obstacles that are in fact an opportunity for new knowledge. Everything in life is a matter of perspectives; it doesn't matter how many times you will lose your track, what actually counts is to continue the journey until you reach your final destination!