Performance characterization of the Micromegas detector for the New Small Wheel upgrade and Development and improvement of the Muon Spectrometer Detector Control System in the ATLAS experiment

The ATLAS, an abbreviation for A Toroidal LHC ApparatuS, detector is one of the two general purpose high luminosity experiments (along with CMS) that have been built for probing p-p and Pb-Pb or p-Pb collisions in the LHC. The muon spectrometer encircles the rest of the ATLAS detector subsystems defining the ATLAS overall dimensions. Its principle of operation is based on the magnetic deflection of muon tracks by a system of superconducting air-core toroid magnets providing high resolution muon momentum measurement. The upgrade of the ATLAS muon spectrometer is primarily motivated by the high background radiation expected during Run-3 (2021) and ultimately at $\mathcal{L}=7\times 10^{34}\,\mathrm{cm^{-2}s^{-1}}$ in HL-LHC (2026). Owing to this the detectors that occupy the innermost muon station called Small Wheel (SW), MDT, CSC \& TGC, will go beyond their design luminosity limit. In addition, the muon trigger rate will exceed the available bandwidth because of the fake endcap muon triggers ($90\%$ is coming from low energy particles, generated in the material located between the Small Wheel and the outer endcap muon station). The collaboration has decided to replace the SW with a NSW system combining sTGC \& resistive MM detectors in order to maintain the excellent performance of the muon system beyond Run-2. Both detector technologies should provide tracking and triggering information for redundancy. The NSW will contribute to the suppression of fake triggers by reconstructing high quality $\left(\sigma_\theta\sim 1\,\mathrm{mrad}\right)$ IP pointing segments online. Moreover, efficient \& precise offline tracking ($\sigma_r\leq100\,\mu\mathrm{m}$) even for the maximum expected rate of $15\,\mathrm{kHz/cm^2}$ is required. The NSW requires fully efficient high-rate capable MM chambers with a single plane spatial resolution better than $100\,\mu\mathrm{m}$ independent of the track incidence angle and the magnetic field ($B \leq 0.3\,\mathrm{T}$). Along with the precise tracking the MM should be able to provide a trigger signal, complementary to the sTGC, thus a decent timing resolution is required. In order to demonstrate and prove that the MM satisfy the NSW requirements several tests have been performed on small ($10\times10\,\mathrm{cm^2}$) resistive-strip MM chambers using medium ($10\,\mathrm{GeV/c}$) and high ($150\,\mathrm{GeV/c}$) momentum hadron beams at CERN and electron beam at DESY. The studies presented focus on the efficiency as well as hit and time reconstruction accuracy measured during these tests demonstrating the excellent characteristics of the MM. Exploiting the ability of the MM to work as a Time Projection Chamber a novel method, called the $\mu$TPC, has been developed for the case of inclined tracks, allowing for a precise segment reconstruction using a single detection plane. A detailed description of the method along with thorough studies towards refining the method's performance are shown. Moreover, owing to the moderate magnetic field expected in the NSW region ($0.3\,\mathrm{T}$), dedicated studies have been devoted in understanding the effect of the Lorentz force on the MM performance when operated inside a magnetic field. The effect of the field has been well understood using test beams, with chambers operated inside magnets, and in simulation. Owing to the quantitative agreement between simulation and test beam measurements the exact knowledge of the magnetic field value has become a prerequisite for a precise track reconstruction in the NSW Micromegas in order to correct for the field effect. Complementary to this, novel methods, for measuring the magnetic field using MM data, have been developed. Complementary to studying and optimising the MM performance, the use of readout geometries with different characteristics had been studied. The adoption of these schemes for the MM readout minimise the number of detection layers and electronic channels, decreasing the complexity, and make the application of Micromegas in large scale experiments feasible. The use of a stereo readout scheme, as foreseen for the NSW MM, allows for a two-dimensional hit position reconstruction with a decent accuracy. The validation of the stereo principle has been performed studying a medium size quadruplet prototype MM, following the NSW stereo readout scheme. Moreover, the possibility of multiplexing MM strips into a smaller number of electronic channels has been studied. The results demonstrate the feasibility of the multiplexing concept, proving that the excellent intrinsic characteristics of the detector remain unaffected by the multiplexing that significantly reduces the number of electronics channels.In parallel to the R&D on the MM detector 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 realised concern the implementation of a calibration tool for the current offset of MDT high voltage channels using ATLAS luminosity data and a complete and independent DCS for the monitoring of the muon end-cap alignment system. Both developments are integrated into the ATLAS DCS and are used on the regular operation of the ATLAS detector. The initial idea of the current calibration tool, that uses the luminosity data, has been expanded towards developing a technique that can calculate the ATLAS luminosity using the MDT high voltage current data.