Belle II 實驗中 Level-1 觸發系統之三維追蹤方法的實作與 B̅⁰ → J/ψ Λ n̅衰變的蒙地卡羅研究

Abstract

Belle II實驗位於SuperKEKB電子對撞機，是一項高亮度的B工廠實驗，旨在精確研究B介子的罕見衰變；其一級（L1）觸發系統需要即時從大量背景事件中快速篩選出物理感興趣的碰撞事例。我們為該觸發系統研發了一種新穎的三維軌跡重建（3D tracker）算法，可在中央漂移室內對帶電粒子的軌跡進行三維重建， 並將處理延遲控制在亞微秒以內。該3D重建算法利用漂移室軸向和立體層的探測訊號，通過幾何變換和線性回歸擬合計算每條軌跡的縱向頂點z（即交點位置）和極角θ。模擬結果表明，幾何算法（geo-finder）的3D追蹤相較於原有的2D追蹤，可將束流引起的背景觸發率降低約38%，同時在對|z₀|<35 cm的軌跡施加篩選時仍保留約97%的真實信號事例。此外，我們實現了一種基於查找表的算法（LUT-finder）， 透過遍歷所有立體層擊中組合以最小化軌跡擬合的χ2，進一步 提高了z₀重建精度並增強了背景抑制能力。

在物理研究方面，我們利用蒙地卡羅（Monte Carlo）模擬分析了一種罕見的重子型B介子衰變模式B̅⁰→J/ψ Λ⁰n̅（其中J/ψ →e+e−或µ+µ−，Λ⁰→ p π−）。該衰變末態包含一個粲偶素態（J/ψ）和一個中性反重子（反中子）， 屬於顏色抑制的重子衰變過程，為研究重味物理中的強子產生動力學提供了獨特的檢驗機會。我們對模擬事件採用了專為Belle II電磁量能器（ECL）開發的 多變量分析（MVA）反中子粒子鑑別算法，從電磁量能器的簇射訊號中識別中子候選體。基於相當於積分亮度200 fb−1的模擬樣本（假設該衰變的分支比約為 10−4），我們預測了此衰變模式的信號產額，並將其與通過擬合B介子質心系動量分佈（P∗ B）提取得到的信號產額進行比較。結果顯示，擬合得到的信號事件數（約67件）與預期值（約63件）在統計誤差範圍內一致，驗證了反中子鑑別方法 的有效性；同時也表明，在當前Belle II數據下觀測該衰變是可行的。

Belle II at the SuperKEKB collider is a high-luminosity e⁺e⁻ experiment designed to study rare B⁻ meson decays; its Level-1 (L1) trigger system must rapidly distinguish signal events from background in real time. To meet this challenge, we developed a novel three-dimensional (3D) track reconstruction algorithm for the Belle II L1 trigger that operates within sub-microsecond latency and uses geometric transformations with linear regression fits to determine each track’s longitudinal z-vertex and polar angle θ. By combining the drift chamber’s axial and stereo wire information, the 3D tracker greatly improves background rejection over the existing 2D trigger. In simulation, a geometry-based 3D tracking algorithm (“geo-finder”) reduced beam-induced background triggers by 38% relative to the standard 2D tracking, while retaining 97% of genuine signal tracks for vertices within |z₀| < 35 cm. Additionally, a lookup-table algorithm (“LUT-finder”) that scans all stereo hit combinations to minimize track-fit χ² further improves z₀ resolution and background suppression.

For physics analysis, we investigated the rare baryonic B⁻ meson decay B̅⁰→J/ψ Λ⁰n̅, where J/ψ → e⁺e⁻ or μ⁺μ⁻ and Λ⁰ → p π⁻. This decay mode, featuring a charmonium state (J/ψ) and a neutral anti-baryon (n̅) in the final state, is a color-suppressed channel providing a unique probe of baryonic decay dynamics. We applied a multivariate analysis (MVA)-based anti-neutron particle identification (PID) algorithm, developed for the Belle II electromagnetic calorimeter (ECL), to identify neutron-candidate clusters in simulated events. Using samples corresponding to an integrated luminosity of 200 fb⁻¹ (assuming a ∼10⁻⁴ branching fraction), we estimated the expected signal yield for this decay and compared it to the yield extracted from fits to the B⁻ meson’s center-of-mass momentum distribution (P*B). The fitted yield (67 events) agreed with the expected yield (63 events) within uncertainties, demonstrating the anti-neutron PID method’s effectiveness; this agreement suggests that this decay mode can be observed with the current Belle II dataset.