Inner detector alignment and top-quark mass measurement with the ATLAS detector

Abstract

This thesis is divided in two parts: one related with the alignment of the ATLAS InnerDetector tracking system and other with the measurement of the top-quarkmass. Both topics are connected by the Globalχ2 fitting method. In order to measure the properties of the particles with high accuracy, the Inner Detector (ID) is composed by devices with high intrinsic resolution. If by any chance the position of the modules in the detector is known with worse precision than their intrinsic resolution this may introduce a distortion in the reconstructed trajectory of the particles or at least degrade the tracking resolution. The alignment is the responsible of determining the location of each module with high precision and avoiding therefore any bias in the physics results. My contribution in the ID alignment has been mainly related with the developing and commissioning of the Globalχ2 algorithm. During the commissioning of the detector, different alignment exercises were performed for preparing the Globalχ2 algorithm: the CSC exercise allowed to work under realistic detector conditions, whilst the FDR exercises were used for integrating and running the ID alignment software within the ATLAS data taking chain. In addition, special studies were continuously done for maintaining the weak modes under control. At the same time, the ATLAS detector was collecting million of cosmic rays which were used to align the modules with real data. The alignment with cosmic rays provided a large residual improvement for the barrel region producing therefore a good detector description for the first LHC collisions. Subsequently, the data collected during the pilot runs was used for performing the first ID alignment with real collisions. Here, not only the residuals but also physics observable distributions were used to monitor the detector geometry and therefore obtain a more accurate ID alignment (specially in the end-cap region). The Inner Detector alignment achieved with the work presented in this thesis was crucial for fixing the basis of the ID alignment, getting a good initial ID performance and leading to the first ATLAS physic paper . The physics analysis part of this thesis is focused on measuring the top-quark mass with the Globalχ2 method. This measurement is important since the top quark is the heaviest fundamental constituent of the SM and may be a handle to discover new physics phenomena BSM. The analysis used the 4.7 fb−1 of data collected by ATLAS during the 7 TeV LHC run of 2011 in order to obtain a mtop measurement with real data. This measurement has been performed in the tt → ℓ + jets channel with two b -tagged jets in the event. This topology contains a W boson decaying hadronically which is used to determine the global jet energy scale factor for this kind of events. This factor helps to reduce the impact of the Jet Energy Scale uncertainty in the final measurement. For each event the mtop is evaluated from a Globalχ2 fit which exploits the full kinematics in the global rest frame of each top. Finally, the mtop distribution has been extracted using a template method and the obtained mtop value is: mtop = 173. 22 +- 0. 32 (stat.) +- 0. 42 (JSF) +- 1. 67 (syst.) GeV The total uncertainty is dominated by the systematic contribution. The result of this analysis is compatible with the recent ATLAS and CMS combination.