Thesis BELLE2-MTHESIS-2017-011

Measurement of the time dependent asymmetry in the decay $B^{0} \longrightarrow K_{S}^{0} \pi^{0} \gamma$ using the Silicon Vertex Detector of the Belle II experiment

Alberto Martini ; Francesco Forti ; Stefano Bettarini

2017
Department of Physics Enrico Fermi, University of Pisa, Pisa

Abstract: My thesis work is in the context of the Belle II experiment, a new generation of B factories that is currently in construction in Tsukuba (Japan) at the KEK laboratory. It is placed on the $e^{+} e^{-}$ accelerator machine SuperKEKB, which is planned to achieve the highest instant luminosity ever obtained, $\mathcal{L} = 8\cdot 10^{35} cm^{-2} s^{-1}$, exploiting the new idea of the nano-beams scheme. Belle II expects to collect data for an integrated luminosity of 50 $ab^{-1}$, which is around 40 times more than its predecessor Belle. The accelerator machine operates at a center of mass energy of the $\Upsilon (4S)$, right above the production threshold of a $B\overline{B}$ event.\\ The primary goal of this thesis work is the study of the time dependent asymmetry in the decay $B^{0} \longrightarrow K_{S}^{0} \pi^{0} \gamma$. Since this measurement relies on the new Belle II Silicon Vertex Detector (SVD), a large fraction of my time has also been devoted to issues related to the construction and characterization of this challenging detector. This radiative decay, mediated by the quark transition $b \longrightarrow s \gamma$, is described at the leading order through a loop diagram, since flavour changing neutral currents are forbidden at tree level in the Standard Model, and is therefore potentially sensitive to physics effects not foreseen in the Standard Model. The main contribution given by my analysis consists in the estimation of the resolution of the decay time difference ($\Delta t$) between the $B^{0}$ and the $\overline{B}^{0}$, extracted from the reconstructed vertex positions of the B mesons. Using a simulated data sample of $\Upsilon(4S) \longrightarrow B^{0} \overline{B}^{0}$ with $B^{0} (B^{0}_{sig}) \longrightarrow K_{S}^{0} \pi^{0} \gamma$ and $\overline{B}^{0} (B^{0}_{tag}) \longrightarrow Generic$, the $B^{0}_{sig}$ and all its daughters are reconstructed in order to obtain the information of the vertex position. Normally charged tracks are used to obtain the decay vertex position, but in the decay under study the direction extrapolation towards the interaction point of the $K_{S}^{0}$ (which is forced to decay into $\pi^{+} \pi^{-}$) is the only usable information. For this reason, in the fit procedure an additional beam spot constraint is used, exploiting the very small size of the beam. The $B^{0}_{tag}$ vertex, needed to measure the decay time difference, as well as to determine the B flavour at the time of decay, is reconstructed using the information of the tracks belonging to the rest of event, avoiding a complete exclusive reconstruction that would imply a large drop in efficiency. Using the vertex coordinates along the beam axis z, the time difference $\Delta t$ is obtained through the formula: $\Delta z = \beta \cdot \gamma \cdot c \cdot \Delta t$. The $\Delta t$ resolution is improved requiring that the kaons are reconstructed by the SVD. Another important part of my thesis work concerns the participation in the construction of the Belle II SVD, which is crucial to obtain the physics sensitivity. I worked in the INFN-Pisa Laboratory of High Technology on the assembly of the backward and forward double-sided silicon strip detector modules. The quality of the produced modules is essential to ensure the performance of the detector, and a detailed electrical test is performed before shipment. I actively participated in the module characterization work, in particular developing a detailed electrical simulation of the defects to help understand the module behaviour and improve the quality. In chapter 1, after an introduction to the CP violation mechanism, the physics motivations for the time-dependent CP asymmetry analysis are reported, together with the experimental challenges and previous results. \\ Chapter 2 presents an overview of the Belle II detector, while in chapter 3 the silicon vertex detector is described in detail, with particular attention to the assembly procedures and the electrical tests. Additional measurements on the electrical behaviour of the sensors are reported at the end of the chapter.\\ In chapter 4 the SVD sensor simulation is described, including a final comparison with the measurements performed in the previous chapter.\\ Chapter 5 describes the time-dependent CP asymmetry analysis work which provides an estimation of the statistic uncertainty on the CP asymmetry parameters, obtained using Toy Monte Carlo studies.

Note: Presented on 15 03 2017
Note: MSc

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