Faculté des sciences de base SB, Programme doctoral Physique, Institut de physique de l'énergie et des particules IPEP (Laboratoire de physique des hautes énergies 1 LPHE1)

Contribution to the inner tracker design and penguin sensitivity studies for the measurement of sin 2ß in LHCb

Perrin, Aurélie ; Tran, Minh Tâm (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 2008 ; no 4234.

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    LHCb is one of the four large experiments hosted at the Large Hadron Collider (LHC) at CERN in Geneva. It will start taking data in September 2008, and will then operate for several years. It consists of a single-arm forward spectrometer dedicated to precise measurements of CP violation and rare decays in the B sector, with the aim of testing the Standard Model and possibly of discovering the first signatures of New Physics. Building such a large experiment as LHCb is a challenge, and many contributions are needed. The Lausanne lab is responsible for the design and the production of the Silicon Inner Tracker (IT) of LHCb. This detector is made of Silicon sensors which need to be cooled to avoid thermal runaway. We present here a contribution to the design of this sub-detector and a description of the production steps. In particular, a study of the cooling of the Inner Tracker is described. It is shown that the cooling abilities of the IT can avoid thermal runaway. CP violation in B meson decays was first observed in the measurement of the so-called "golden channel", in which a Bd0 meson decays into a J/ψ and a Κs0 . The time-dependent CP asymmetry in Bd0 → J/ψΚs0 allows to measure the angle β of the (d, b) unitary triangle. This parameter is now known with 4% accuracy at B factories. However, this determination of sin 2β is made under the assumption that there is only a single amplitude present in this decay : this means that penguin diagrams which might be present have been neglected. In 1999, Robert Fleischer [29] proposed a theoretical method to access those penguin diagrams in the Bd0 → J/ψΚs0 decay, using the Bs0→ J/ψΚs0 channel. This method relies on U-Spin symmetry and also allows to determine the γ angle of the (d, b) unitary triangle. We have developed a selection method for the Bd0 → J/ψΚs0 channel in order to strongly suppress the background and to allow the separation of the Bs0 and Bd0 peaks. We obtained mass resolution of 8 MeV/c2 and a B/S ratio for the channel Bd0 → J/ψΚs0 estimated to belong to [0, 0.039] at 90% confidence level in a ±2σ mass window around the Bd0 mass, after the first level of trigger (L0). For the channel Bs0 → J/ψΚs0 , the B/S ratio is calculated from the result for Bd0 → J/ψΚs0 assuming known branching fractions. It lies in the interval [0, 3.33] at 90% CL. The annual yield is expected to be around 300 events for an integrated luminosity of 2 fb-1. We have simulated with fast Monte Carlo the Bs0 → J/ψΚs0 signal using the parametrization proposed in [29] and taking as input the results of selection obtained for Bd0 → J/ψΚs0 . The simulation has been repeated several times for different integrated luminosities and B/S ratios. We conclude that after 5 years of normal running, LHCb will be able to determine the penguin contribution in the Bd0 → J/ψΚs0 decay with a sensitivity of (0.172 ± 0.004) using this method based on U-spin symmetry.