Measurements of B Meson Lifetime Ratios with the LHCb Detector

The LHCb experiment is one of the four main experiments at the Large Hadron Collider (LHC) at CERN. The LHCb detector is a one-arm spectrometer dedicated to precise measurements of the CP-violation and studies or rare b-hadron decays. The centre of mass energy and luminosity (for a hadron collider) have both values never reached before. LHCb started data taking in November 2009. LHCb relies on excellent reconstruction and trigger efficiency, very good mass and proper time resolution, reliable particle identification systems to achieve the wanted precision on CP-violation parameters. These performances cannot be fulfilled without the detector to be calibrated. For instance, the Inner Tracker (IT) is readout by around a thousand chips, and the data are processed by 42 data acquisition cards, called TELL1. The Tracker Turicensis (TT) data are processed by 48 TELL1 boards. Each of these TELl1 boards has more than 18,000 parameters that need to be tuned in order to reduce the fake hits in the tracking systems. A high number of fake hits would result in poorer tracking performances. The first part of this thesis discuss the calibration of the IT and TT TELL1 boards. The first step was to find a way to extract the needed parameter values from data taken without beam. Once the parameters were gathered, a way to store them for future usage had to be found. The chosen solution was to use XML file, since XML files were already used for the detector geometry description and detector conditions. The XML file offered a convenient way to use the TELL1 parameters in the software framework, but those parameters were of course also required for data taking, i.e. they have to be uploaded to the TELL1 boards. A software library had to be developed in order to 'translate' the information contained in the XML file into a format handled by the control software. The second part discusses hit efficiency measurements in the IT. These measurements were shown to allow a monitoring of the detector status, as several hardware problems could be discovered by looking at the obtained efficiencies. Studies of the efficiency as a function of the clustering threshold allowed to improve background rejection in IT and TT by increasing one of the clustering thresholds. Finally, the last part covers the measurement of the B-meson lifetime in three decay channels: B+ → J/ψ(1S) (μ+ μ-) K+, B0 → J/ψ(1S) (μ+ μ-) K*0 (K+ π-), Bs0 → J/ψ(1S) (μ+ μ-) φ (K+ K-). These values are already quite well known for B± and B0, but their measurement represents a milestone in the measurement of the CP violating phase φs in the Bs0 → J/ψ (1S) φ decay. Lifetime ratios are also presented: τB± / τB0, τBs0 / τB0 and τB+ / τB-. The first two allow to probe the Heavy Quark Effective Theory (HQET), whilst the last one is a test of CPT symmetry, foundation of the Quantum Field Theory.

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