Contribution to the Inner Tracker design and penguin sensitivity studies for the measurement of sin 2 $\beta$ in LHCb

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 Bd meson decays into a J/psi and a Ks. The time-dependent CP asymmetry in Bd -> J/psi Ks allows to measure the angle beta of the (d,b) unitary triangle. This parameter is now known with 4% accuracy at $B$ factories. However, this determination of sin(2 beta) is made under the assumption that there is only a single amplitude present in this decay : this m eans that penguin diagrams which might be present have been neglected. In 1999, Robert Fleischer proposed a theoretical method to access those penguin diagrams in the Bd -> J/psi Ks decay, using the Bs -> J/psi Ks channel. This method relies on U-Spin symmetry and also allows to determine the gamma angle of the (d,b) unitary triangle. We have developed a selection method for the Bd -> J/psi Ks channel in order to strongly suppress the background and to allow the separation of the Bs and Bd peaks. We obtained mass resolution of 8MeV/c^2 and a B/S ratio for the channel Bd -> J/psi Ks estimated to belong to [0, 0.039] at 90% confidence level in a +- 2 sigma mass window around the Bd mass, after the first level of trigger (L0). For the channel Bs -> J/psi Ks, the B/S ratio is calculated from the result for Bd -> J/psi Ks 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 2fb^-1. We have simulated with fast Monte Carlo the Bs -> J/psi Ks signal using the parametrization proposed in and taking as input the results of selection obtained for Bd -> J/psi Ks. 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 Bd -> J/psi Ks decay with a sensitivity of (0.172 +- 0.0 04) using this method based on U-spin symmetry. Sensitivities to the physics parameters, which are related to the penguin contributions of the Bd -> J/psi Ks decay in the sin(2 beta) determination, have been determined in each case. With the expected 100fb^-1, the penguin contributions in the Bd -> J/psi Ks can be measured with a sensitivity between 0.7% and 1.2% using this method.