在8 TeV質心能量的質子對撞實驗中尋找頂夸克透過Z玻色子及魅夸克（或上夸克）衰變的事件

Abstract

在標準模型中，按照GIM機制的預測，以Z玻色子為媒介之跨世代衰變反應是被抑止的，在頂夸克的例子中，其反應率遠低於目前之測量範圍，但在其它模型中，該反應率可以目前之測量範圍。 我們使用了緊湊渺子線圈在2012年紀錄的質心能量為8 TeV之質子對撞數據，總通量約為19.5 fb-1，以尋找頂夸克衰變至Z玻色子與魅夸克(或上夸克)的事件。 在大強子對撞機實驗中，大多數頂夸克係成對產生，故我們以tt ‾→Wb+Zq之輕子性衰變為尋找標的，即W玻色子衰變至電子或渺子及對應的微中子，Z玻色子衰變為電子或渺子對。我們使用了b簇射(jet)標記，以配對衰變至Wb的頂夸克，以反b簇射標記配對衰變至Zq的頂夸克。在此選擇條件下，背景之主要組成為Ztt ‾以及WZ反應。 我們以b簇射標記法進行數據導向之背景分析，預測將觀測3.08±0.85±0.76個背景事件，與蒙地卡羅法之預測一致。 在數據中僅觀察到一個滿足選擇條件的事件，符合標準模型之預測；我們計算出t→Zq之衰變佔頂夸克衰變的百分率在95%之信心水準下不超過0.06%。

In the standard model, cross-generation interactions mediated by Z bosons, known as the flavor-changing neutral current (FCNC), are highly suppressed. For top quark FCNC decay, the cross-section is far below the experimental limit. However, other models predict much higher cross-sections, and some of them are even within reach. Therefore, using the data sample corresponding to an integrated luminosity of 19.5 fb^−1 proton-proton collisions at a center-of-mass energy 8TeV , collected with the CMS detector at the LHC, we performed the search with the decay chain t‾t to Wb+Zq, where W decays to a charged lepton and a neutrino and Z decays to two charged leptons. The data-driven analysis using b-tagging method is performed with the estimated background being 3.08±0.85±0.76, which is consistent with the estimation of Monte-Carlo method. One event is observed in the data which is consistent with the expected background, and the upper limit of the branching fraction for t to Z+q is calculated as 0.06% at the 95% confidence level.