修改Georgi-Machacek 模型中希格斯玻色子特性之研究

Abstract

質量125GeV 希格斯玻色子在大型強子對撞機中被發現是目前標準模型最大的成功，然而目前實驗數據還無法排除標準模型外新的希格斯粒子存在的可能性。目前已有許多希格斯拓展模型被研究並個別給出了不同的預測。在本篇論文中，我們專注於類似Georgi-Machacek (GM) 模型但是放寬了希格斯位能要滿足custodial symmetry 的要求。在這稱為” 修改”Georgi-Machacek (MGM) 模型的架構中，中性希格斯粒子衰變到W 玻色子對和Z 玻色子對的比率λWZ 就算在電弱參數 ρ ≡ m2W/m2Z cos2 θW 等於1 的條件下仍可以和原始的GM 模型有完全不同的預測。 在傳統的GM模型下，在custodial symmetry 的要求下兩個電單荷希格斯玻色子只能分別耦合到費米子和W±Z 且彼此不會有混合。在MGM 模型下，我們發現兩個電單荷希格斯玻色子在tree level 可以藉由混合導致同時有費米子和W±Z 的耦合。擁有此種特性的電單荷希格斯玻色子並不存在於大多數的希格斯拓展模型，且給出了在大型強子對撞機下尋找帶電希格斯粒子的新手段。我們提議此種粒子的特性可藉由pp → jjH±,H± → tb 這個過程來顯現。藉由在MGM 模型下進行13TeV 對撞機分析來獲得模型參數的靈敏度(sensitivity)，並假設新粒子質量在200GeV 到1TeV 之間。我們發現質量介於300 到400GeV 之間的帶電希格斯粒子能在三重態的真空期望值介於80 到100GeV 的區間內擁有超過5倍標準差的顯著度。此種過程也擁有比現有的研究更好的靈敏度，且能給出同時能耦合到費米子及W±Z 的帶電希格斯粒子存在的直接證據。

The discovery of the 125 GeV Higgs boson is a great success of the Standard Model (SM). However, the possibility of ”exotic” Higgs bosons beyond the SM has not been ruled out by current data. There are many Higgs extension models which provide different predictions of additional Higgs bosons. In this thesis, we studied a model which has the same number of Higgs sectors as the Georgi-Machacek (GM) model but without the custodial symmetry in the Higgs potential. In this model, so called the Modified Georgi-Machacek (MGM) model, the predictions of the ratio λWZ of Higgs→ ZZ and Higgs→ WW can be totally different than those predicted in the original GM model even with the electroweak parameter ρ ≡ m2W/m2Z cos2 θW equal to one. In original GM model, due to the custodial symmetry, the singly charged Higgs bosonsH±3 and H±5 couple to fermions andW∓Z separately and there’s no mixing between them. In MGM model, we have shown that in general the two physical singly charged Higgs bosons can mix with each through a mixing angle δ, resulting in two mass eigenstates couple to both W±Z and fermions at tree level. Such a singly charged Higgs boson does not exist in many popular Higgs extended models which provides a new way of charged Higgs searches at the Large Hadron Collider (LHC). We proposed that the properties of singly charged Higgs bosons with both couplings to fermions and W∓Z can be revealed through the process pp → jjH±,H± → tb 1 which depends on both couplings. We perform a collider analysis at the collision energy √s = 13 TeV with the integrated luminosity of 3 ab−1 in MGM model to obtain the sensitivities to the mixing angle δ and the triplet vacuum expectation value (VEV) in the mass range from 200 GeV to 1 TeV. We find that the charged Higgs boson with mass within 300∼400 GeV can be discovered in 5σ region with the triplet VEV 80∼100 GeV. It also shows that our proposed search has better sensitivity to both couplings than current searches. This would provide direct evidence for a charged Higgs boson that couples to fermions and W±Z.