尋找 Belle II 中稀有 B0 → ϕ γ 衰變

Abstract

本研究針對稀有味改變中性流（FCNC）輻射衰變 $B^0 \to \phi \gamma$ 進行了靈敏度分析。該衰變在標準模型（SM）中受到高度壓制，因此成為探測超越標準模型之新物理與長程強子效應（例如 $\omega-\phi$ 混合）的極佳途徑。本分析基於 SuperKEKB 非對稱能量 $e^+e^-$ 對撞機上 Belle II 實驗所生成之模擬數據進行開發，對應於 $\Upsilon(4S)$ 共振態下 $362\text{ fb}^{-1}$ 的積分亮度。研究中採用提升決策樹（BDT）來壓制佔主導地位的連續區 $q\bar{q}$ 背景，並套用經最佳化的雙光子不變質量區間，以否決（veto）來自 $\pi^0$ 與 $\eta$ 衰變的背景光子。訊號產額係透過對束流能量約束質量（$M_{\rm bc}$）、能量差（$\Delta E$）以及螺旋角餘弦（$\cos\theta_H$）進行三維未分箱最大概似擬合（unbinned maximum likelihood fit）來萃取。擬合框架的穩健性已透過大量的 Toy 與 Gsim 虛擬數據集進行驗證，並評估了主要的系統性誤差。利用 $\text{CL}_s$ 方法與 Asimov 數據集，在 90\% 信心水準下，分支比的預期上限被決定為 $1.30 \times 10^{-7}$。將靈敏度投影至 Belle II 最終目標積分亮度 $50\text{ ab}^{-1}$ 時，指出預期上限可達 $\mathcal{O}(10^{-9})$ 數量級，此結果已逼近包含 $\omega-\phi$ 混合貢獻在內之標準模型預期基準。

We present a sensitivity study for the rare flavor-changing neutral current (FCNC) radiative decay $B^0 \to \phi \gamma$. This decay is highly suppressed in the Standard Model (SM), making it an excellent probe of both physics beyond the Standard Model and long-distance hadronic effects, such as $\omega-\phi$ mixing. The analysis is developed based on simulated data corresponding to an integrated luminosity of 362 $fb^{-1}$ at the $\Upsilon(4S)$ resonance, generated for the Belle II experiment at the SuperKEKB asymmetric-energy $e^+e^-$ collider. A Boosted Decision Tree (BDT) is employed to suppress the dominant continuum $q\bar{q}$ background. An optimized diphoton invariant mass window is applied to veto background photons originating from $\pi^0$ and $\eta$ decays. The signal yield is extracted through a three-dimensional unbinned maximum likelihood fit to the beam-energy constrained mass ($M_{\rm bc}$), the energy difference ($\Delta E$), and the cosine of the helicity angle ($\cos\theta_H$). The robustness of the fit framework is validated using extensive Toy and Gsim ensembles, and major systematic uncertainties are evaluated.Using the $\text{CL}_s$ method with an Asimov dataset, the expected upper limit on the branching fraction is determined to be $1.30 \times 10^{-7}$ at the 90\% confidence level. A sensitivity projection to the ultimate Belle II integrated luminosity of $50\text{ ab}^{-1}$ indicates that the expected upper limit reaches the $\mathcal{O}(10^{-9})$ level. Approaching the Standard Model expectation, including the $\omega-\phi$ mixing contribution.