B介子重子性衰變至 pΛ-bar π+π−, pΛ-bar ρ以及 p-bar ΛD0 之研析與 Belle II 量能器之模擬軟體建構

Abstract

本論文主要分為三個主題來探討。在第一、二部分中我們以Belle偵 測器所收集的數據資料進行分析,研究B介子的重子性衰變。最後是介 紹Belle II升級計畫中電磁量能器的研究。 第一個數據分析研究為B介子至末態不含c夸克的重子性衰變:B+ → pΛ-barπ+π−和B+ → pΛ-barρ。以及第二個數據分析研究 B介子至末態含c夸克的重子性衰變:B− → p-barΛD(∗)0。其數據來源皆為Belle偵測器在加速器內的電子-正電子對撞束中心點所收集的事件,使用了605fb−1的數據量來進行 研究。在第一個分析中,我們發表了對B介子至末態不含c夸克的重子性四 體衰變首次量測,其量測衰變率為(5.92+0.88−0.84(stat.) ± 0.69(syst.)) × 10−6, 而其統顯著性為9.1標準差;並且發現衰變中間態B+ → pΛ-barρ,其量測衰 變率為(4.78+0.67−0.64(stat.) ± 0.60(syst.)) × 10−6而其統計顯著性為9.5標準差。在B介子至末態含c夸克的重子性衰變研究中,我們分析B− → p-bar ΛD(∗)0的衰變率,觀測到B− → p-barΛD0的衰變率為(1.43+0.28−0.25(stat.)±0.18(syst.))×10−5而其統顯著性為8.1個標準差;對於B− → p-barΛD∗0來說,以目前的數據量,尚無法證明此衰變的存在,所以我們以統計學上百分之九十的信心程度, 設定這種衰變的衰變率上限為4.8 × 10−5。 在以往量測到B介子重子性衰變 中,重子對在不變質量分布的下限衰變率增強的現象,在以上可觀測到的 衰變中仍然存在。 台灣高能實驗組負責Belle II升級計畫中電磁量能器模擬軟體的建構。 在此研究中不但提供電磁量能器的升級選項之效能評估,也建構出未 來Belle II中電磁量能器的模擬系統。其中對於電磁量能器軟體建構的部份 主要包含電磁量能器幾何形狀的描述、偵測器的模擬以及模擬資料重建。 此模擬系統將建構於Geant4模擬軟體之上。

In this thesis, there are three major topics to be addressed. Two physics analyses on the searches for the baryonic B decays at Belle. The other is the ECL upgrade study for BelleII. The first data analysis is to search for the charmless baryonic B de- cay with final state of pΛπ+π−. The second analysis is a search for the charmful baryonic B decay of B− → pΛD(∗)0. For both analyses, I use a 605fb−1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. In the charmless baryonic study, the first four-body charmless baryonic decay B+ → pΛ-barπ+π− is observed with a branching fraction (5.92+0.88-0.84(stat.) ± 0.69(syst.)) × 10−6 and a significance of 9.1 standard deviations (σ). We also observe the intermediate three-body decay B+ → pΛρ0 with a branching fraction of (4.78+0.67-0.64(stat.) ± 0.60(syst.)) × 10−6 and a significance of 9.5 σ, and find a hint of B+ → pΛ-bar f2(1270). No other intermediate three-body decay is found in B− → pΛ-bar π+π− final state study. In the charmful baryonic analysis, we study B− → p-bar ΛD(∗)0. The observed branching fraction for B− → p-bar ΛD0 is (1.43+0.28-0.25(stat.) ± 0.18(syst.)) × 10−5 with a significance of 8.1σ. No significant signal is found for B− → p-barΛD∗0 and the corresponding upper limit on branching fraction is 4.8 × 10−5 at the 90% confidence level. The com- mon feature of threshold enhancement in baryonic B decays is still observed in both of the modes that most of the signal events are found with the pΛ ̄ invariant mass peaking near threshold. The NTUHEP group is responsible for the software of ECL upgrade at Belle II. We provide not only the performance simulation for upgrade options of BelleII based on the Geant3 package which was well tested and calibrated in the present operating conditions of KEKB and Belle, but also the software implementation for BelleII. The software implementation for ECL upgrade consists of detector geometry description, detector simulation and data reconstruction. The simulation package is changed to Geant4.