Präzisionsmessungen an Myondriftkammern für den CMS-Detektor und die Bedeutung des Myonsystems für die Higgs-Suche am LHC

This thesis deals with preparations for the experiment CMS (Compact Muon Solenoid) of the LHC (Large Hadron Collider) planned at CERN in Geneva (Switzerland). The construction of the muon system, especially of the drift chambers in the barrel detector constructed with participation of the Aachen group, is explained. In the first part of the thesis the results from a Monte-Carlo-study for a decay channel interesting for the search for the Higgs boson at CMS is presented. In this channel the Higgs finally decays into four muons. The second part describes the running production of the drift chambers as well as results of tests developed and implemented for purposes of quality control. The LHC will accelerate protons to a center of mass energy of 14 TeV, aspiring to a luminosity of L = 10^34 cm^-2s^-1, allowing for the production of a sufficient number of such processes as rare as the production of Higgs bosons, which were predicted, but not yet proven. The Higgs bosons, created mainly by gluon gluon fusion in proton proton collisions at the LHC, favor moving into the forward direction of the detector, because the center of mass system of the gluons moves into this direction. By means of a fully simulated detector study based on Monte-Carlo calculations of the channel H -> ZZ -> 4µ the consequences for the final state muons are analyzed. Furthermore, a more detailed analysis of the angular dependent efficiencies of detecting the muons shows that both parts of the muon system, forward and barrel detectors, are of equally great importance. On the one hand, at least one of the four muons moves into the barrel region of the detector for 95% of the events. On the other hand 84% of the events show at least one muon in the endcaps (In case, the muons are equally distributed over the entire angular range one would expect 99% of the events with at least one muon in the barrel detector and 81% with at least one muon in the end caps.). Furthermore, the mass resolution within reach for the Higgs is discussed. Drift chambers are used to detect muons in the barrel region. In collaboration with three other institutes 250 drift chambers are constructed, where 70 of them are made and tested in Aachen. Each drift chamber is 2,5 m wide, up to 4 m long and 0,3 m high and consists of 12 layers being composed of single drift cells. A crossing muon creates electron-ion-pairs by ionization in the gas volume of a drift cell. Consequently, the electrons accelerated by high voltage drift towards the anode wire at the center of the cell. Due to an approximately constant drift velocity, it is possible to achieve a precise reconstruction of the muon track from the measurement of the drift time and the knowledge of the wire position. The complete process of producing the muon drift chambers is described in the fifth chapter, whereby a focal point lies on the crimping machine developed in Aachen and modified during this study. In order to detect the muons with certainty and sufficient accuracy, a necessary central detector parameter is the spatial resolution to be reached in the measurement of muon tracks. A spatial resolution of 100 µm of the muon drift chambers constitutes an adequate value. Consequently, with a preset number of measuring points on a track segment a spatial resolution of 250 µm arises for the single drift cell, thereby constituting a great challenge for the drift tube production. All single components of a drift cell have to be positioned and constructed with an accuracy of a few hundredth millimeters. The precision of the 70 drift chambers being produced in Aachen is to be guaranteed by inspection with comprehensive measurements. For the spatial resolution of a drift cell, the anode wire situated in the middle of the cell is the decisive component. Its exact spatial position in each single cell constitutes the foundation for ensuing accuracy in the measurement of muon tracks. By applying a tension of 3 N to the anode wires of a given length with almost maximal force at the edge of the elastic range, and by measuring the wire tension after the installation into the drift cells, it is possible to ensure a minimal sag of the wire. A new type of wire tension meter has been developed to the state of serial production in cooperation with the University of Purdue, whose special characteristic is a contact-less and simultaneous measurement of up to 64 wires in a process of about five minutes duration. Measurements and further developments concerning the functional ability and accuracy (0,6%) of the wire tension meter have been carried out within the scope of this thesis. Today, the device is used daily for purposes of quality control at all institutes involved in the drift chamber production with success. Every drift chamber produced in Aachen so far conforms to the demands for accuracy of the mechanical wire tension of ± 10%. The position of the anode wire in the drift cell is primarily defined through the position of the wire ends. In order to determine the latter with an accuracy of a few hundredth of a millimeter, a special measuring system based upon computer-aided, optical measurements has been developed in Aachen. The further development of this system into a functioning instrument for the position measurement in the CMS reference system is part of this thesis. Thanks to a comprehensive calibration process for the entire system it was possible to reach a sufficient accuracy of about 50 µm for both axes of the system. This is sufficient to control the desired position accuracy of the wires of ±100 µm. Within the measuring accuracy no deviation with respect to precise wire positions worth mentioning was found. Production mistakes could be found and consistently removed by means of the reached measuring accuracy in the daily wire position measurements thereby significantly improving the total mechanical positioning and gluing procedure. Thanks to the engagement of all persons concerned, the desired production speed of two muon chambers per month could be reached. Till the end of last year 36 chambers have really been glued, so today more than half of all muon chambers were already constructed and tested. Consequently, they are being constructed according to schedule. Due to the impressive achieved progress, the CMS-project is expected to become a successful experiment.