中性希格斯粒子變味交互作用之探索

Abstract

頂夸克奇重無比,質量在弱作用的尺度上,這使得它和還未找到的希格斯 玻色子之間可能有 大的變味耦合常數.在本論文的第一部份,我們闡明了 這個常數可在LHC上直接地量測到,經 過的子過程是c(anti-c)g-->t( anti-t) A^0,而膺純量粒子A^0隨後又衰變到t anti-c或an ti-t c,因此 最後得到了令人感興趣的末態-同號頂夸克對.當實驗上要求同號輕子對 l^{+, -} l^{+,-},消失能量和兩個b-噴流之後,主要的背景會是q anti- q^primed-->W^+ t anti -t,而藉著計算噴流總數,能把背景削弱.又l^+ l^+和l^- l^-事件數的比值可表現出信號的 大小.為進一步讓信號蓋過背 景,應發展出有效的t對anti-t的加標籤法.論文的第二部份,我 們經由e^+ e^- -->nu nu t anti-c,在第二代線性對撞機上探索變味耦合常數的可能 性.在 一般的雙希格斯粒子偶對模型的架構下,全計算結果是: sqrt(s)=2 TeV時, e^+e^- -->nu nu t anti-c,nu nu anti-t c 散射截面可達9 fb.背景nu nu W^+ W^-在b-加標籤後應能 妥當處理,而背景nu nu t anti-t在信號數目仍然可觀時,尚不構成問題.信號和e^+e^- - ->nu nu W^+ W^-,nu nu ZZ都提供了機會給t-c-Higgs耦合的測量. Uniquely heavy with mass at the weak scale, the top quark may have large flavo r changing couplings to Higgs bosons that are as yet unexplored. In the first part of this thesis, we show that such couplings could be directly probed at t he LHC through the parton subprocess c(anti-c)g-->t(anti-t) A^0, where the pse udoscalar A^0 subsequently decays into t anti-c or anti-t c, giving rise to th e intriguing final state of like sign top quark pairs. After demanding l^{+,-} l^{+,-} missing energy and two b-jets, the major background turns out to be q anti-q^primed-->W^+ t anti-t, which can be partially suppressed by jet countin g. The signal can then manifest itself in the asymmetry of numbers of l^{+} l^ {+} and l^{-} l^{-} events. To further improve the signal over background, eff icient t vs anti-t tagging methods should be developed. In the second part of this thesis, we explore the possibility of detecting this coupling at the Next Linear Collider (NLC) through e^+e^- -->nu nu t anti-c . In the framework of a general two-Higgs doublet, we perform a complete calculation and find that e^ +e^- -->nu nu t anti-c,nu nu anti-t c could reach 9 fb for sqrt(s)=2 TeV. The nu nu W^+ W^- culd be manageable after b-tagging, while e^+ e^- -->nu nu t anti -c,nu nu anti-t background should not be a problem when the signal event rate is still interesting. The process, together with e^+e^- -->nu nu W^+ W^-,nu nu ZZ studies, offer the chance of measuring the t-c-Higgs coupling.