RADIATION DAMAGE STUDIES IN THE LHCB VELO DETECTOR AND MEASUREMENT OF THE FLAVOUR-SPECIFIC ASYMMETRY IN SEMILEPTONIC B-DECAYS

Abstract

This thesis presents several studies of data collected at the LHCb detector duringits first two years of operation. A detector upgrade study is first presented, usingsimulated events at an increased luminosity. A second study involves radiationdamage to the silicon sensors of the LHCb Vertex Locator. During 2010 and 2011the silicon sensors were exposed to a range of fluences, with sensors in the mostactive regions exposed to fluences of up to approximately 45x10^12 1MeV neutronequivalent (1MeV neq). The first observation of n+-on-n sensor type inversion at theLarge Hadron Collider is reported, occurring at a fluence of around (10-15)x10^12of 1MeV neq. The effective depletion voltages of the only n+-on-p sensors in use atthe Large Hadron Collider have also been studied, with decreases of around 25Vobserved after initial irradiation. Following this, the effective depletion voltage inn+-on-p type sensors is observed to increase at a comparable rate to type invertedn+-on-n type sensors. A reduction in the charge collection efficiency due to anunexpected effect involving the sensor readout lines is also observed.A third study relates to CP violation in neutral B-meson mixing, by the measurementof the flavour-specific asymmetry. In the Standard Model, CP violationfrom this source is expected to be of order 10^-4. Any measured enhancement ofthis would be a strong indication of new physics. The DØ collaboration has measuredthe flavour specific asymmetry from B0 and B0s mixing, and found it to beinconsistent with the Standard Model at a confidence level of 3.9 standard deviations,thus motivating an independent measurement from the LHCb experiment.Using the full 2011 LHCb dataset, corresponding to 1.0 fb^-1 of recorded luminosity,the B0s-meson component of the flavour-specific asymmetry is measured to beafs_s = (-0.12 +/- 0.48 +/- 0.34)%, where the first uncertainty is statistical and the secondis systematic. This is the single most accurate measurement of afs_s, and is consistentwith both the DØ measurement and the Standard Model prediction.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George Lafferty (Supervisor)