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荷電レプトンフレーバー非保存過程探索のための実験感度に関する研究

大石, 航 OISHI, Koh オオイシ, コウ 九州大学

2021.03.24

概要

荷電レプトンフレーバー(CLFV)を破る過程の探索は、素粒子物理学の標準モデル(SM)でも説明できない物理を探る上で重要な役割を果たす。ミューオン・電子変換過程は、原子軌道に束縛されたミューオンが、ミューオン数と電子数保存に反して 105 MeV の単一電子に崩壊する過程であり、最も研究されている CLFV チャンネルの一つである。COMET(COherent Muon-to-Electron Transition)実験は、日本の J-PARC でアルミニウム中の変換過程を段階的に探索する。Phase-II実験では、最新の記録を 10000 倍向上する 10−17 という優れた感度を実現し、いくつかの標準模型を超える(BSM)理論が示唆する発生確率に到達することを目指す。
 九州大学のグループは、105MeV 電子の運動量とエネルギーを高分解能で測定するために、ストローチューブトラッカーと電磁カロリメータ(ECAL)からなる StrECAL 検出器システムの開発に貢献した。ストローチューブトラッカーは、エネルギー損失を最小限に抑えるために真空中でも動作可能なストロー状の極薄ガス検出器を何層も用い、磁場中での粒子軌道を測定する。ECAL は望ましいエネルギー測定性能を持つ LYSO 無機シンチレーション結晶を採用している。
 本論文では、Phase-II 実験感度の実現可能性を確認するため、現実的な StrECAL の性能に基づいたシミュレーションによる研究について述べる。性能評価のために、東北大学 ELPH において StrECAL 検出器の試作機を製作し、電子ビームを用いた試験を行った。ストロー管は 100 m 程度の空間分解能を示し、ECAL は 105MeV/c において、エネルギー分解能 3.91±0.07%、時間分解能 0.54±0.12 nsec、位置分解能 7.65±0.07mm を示し、すべての条件を満たしていた。
シミュレーションとデータ解析のための COMET 公式のソフトウェアフレームワークを用いて、観測された StrECAL の性能を再現することに成功し、また一次陽子ビームから検出器応答に至るまでの Phase-II 事象をシミュレートした。再構成アルゴリズムも機械学習技術を用いて開発し、いくつかのクオリティカットも含めて最適化した。その結果、信号電子の運動量分解能は 190keV/c、全検出効率は 72.5%であり、再構成されたイベントの内、シグナルイベント純度は 97.6%であった。
 評価された解析性能に基づき、標準の測定時間窓を 600 から 1200 nsec とした場合、Phase-II の実験アクセプタンスは 0.034 と見積もられた。これは 1 年間のデータ取得で 1.4×10−17 の単一事象測定感度(Single Event Sensitivity)が達成されることを意味する。系統誤差は全体で 11.6%と評価したが、そのうち再構成と解析の寄与は 1.2%のみである。また、異なる時間窓設定に対しても安定した性能を示した。

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