微中子在暗質間接探測之應用

Abstract

暗質(dark matter, DM)，自1933年被德國天文學F. Zwicky提出用來解決后髮座星系團(Coma cluster)的亮度質量(luminous mass)與功位質量(virial mass)的不穩合以降，不斷縈繞在近代物理學的核心當中，即便粒子物理標準模型(Standard model)如此的成功，暗質卻仍做為一種在近代物理理論與實驗上皆是撲朔迷離的存在，但天文學的觀測與宇宙大尺度結構(large-scale structure)的多體模擬(N-body simulation)卻又如此強烈地支持它存有的證據。 微中子(neutrinos)的歷史則與暗質有異曲同工之妙，亦曾經被視為是神祕的存在。但我們現今對微中子已經有明朗的理解，即便對其質量、質量順序(mass ordering)、本質是狄拉克粒子(Dirac particle)或是馬約拉納粒子(Majorana particle)……等仍不明瞭，但我們至少可以明白地列出其反應方程式並測量不同味道(flavor)的散射截面積。 透過巧妙的結合，假設描述暗質反應為一帶有質量的粒子$ hi$，並可被阿貝爾規範場$U_X(1)$所描述，則$U_X(1)$與標準模型的$U(1)$規範場可以存在一非零混合參數(mixing parameter) $\varepsilon_\gamma$，透過$\varepsilon_\gamma$，我們便可以允許暗質與核子之間存在特定的反應，並透過實驗量測之間的散射截面積的直接探測(direct detection)。目前世界上最大的以低溫液態氙為靶材(target mass)的暗質實驗為坐落在加拿大的LUX，擁有目前最佳的靈敏度與成果，次世代的LUX-ZEPLINE預計更能技高一籌。除了直接量測暗質與核子的反應外，透過BEH機制(BEH mechanism)，舊稱西格斯機制(Higgs mechanism)，亦可以允許這個$U_X(1)$的粒子與標準模型的Z玻色子有一混合作用(Z mixing)，那麼$ hi$不僅能與核子有額外的作用量，並且可以衰變到微中子甚至電子和夸克(quarks)……等。這便開啟了另一類探測暗質的方式，這樣的訊號通常來自存在於銀河系的暗質暈(DM halo)或星體中的湮滅反應，並可IceCube或AMS 02這樣的探測器所觀測，這便稱做暗質間接探測(indirect detection)，做為直接探測的互補實驗。 除了以上，本論文也探討銀河暈暗質被太陽重力俘獲並大量聚集以產生足夠的煙滅量，最終可被IceCube所觀測。關於太陽重力俘獲的機制、湮滅及蒸發效應，和暗質自相作用(DM self-interaction)所扮演的角色，皆有詳細的推導與討論。最後並介紹位於南極的立方公里微中子探測器IceCube與其在間接探測中的應用，以五年內達成2σ的信心水準為基準，我們計算了其靈敏度並將其呈現在論文中。預計於2020年前IceCube將完成次世代的升級模組PINGU (Precision IceCube Next Generation Upgrade)，屆時便可將探測的能量窗口擴展至1 GeV到數個EeV之間，我們在這篇論文中也討論了關於PINGU未來的應用。

The notion of dark matter (DM) was proposed by F. Zwicky in 1933 to reconcile the inconsistency between the luminous mass and the virial mass of the Coma cluster. Since then, even the very existence of DM is strongly supported by the astrophysical observation and the N-body simulation in the large-scale structure of the Universe, its elusive nature still puzzles the theorists and experimentalists of the modern physics. Neutrinos were once considered sharing the similar ambiguity to DM in the history. However, we can measure the scattering cross sections to different flavors and known its interaction Lagrangian quite well. Despite their masses, the mass ordering, Dirac or Majarona nature of neutrino…etc. still yet known. By assuming only one kind of force mediator $ hi$ that gauges the dark sector, eg. DM, and it is de-scribed the an abelian $U_X(1)$ field. It is allowed a mixing between the $U_X(1)$ and the Standard model (SM) $U(1)$ by an undetermined parameter $\varepsilon_\gamma$. Such $\varepsilon_\gamma$ allows a non-zero DM-nucleus interaction. The experi-ments built to detect such recoil generated by the DM-nucleus scattering are called the direct detection of DM. The most sensitivity experiment is LUX and it reveals the most stringent bound on DM-nucleon cross section in the world. A next generation detector the LUX-ZEPLINE is expected to be better. In addition to the direct detection, through the BEH mechanism that makes $ hi$ acquired a mass term allows a mixing between the UX(1) and the SM Z boson field (Z mixing). In this case, not only an extra contribution to the DM-nucleus interaction mediating by $ hi$, it is possible to decay into neutrinos, electron-positron pair and quark pairs…etc. These signals from the annihilations of DMs in the galactic halo and stars provide another way to detect DM that is called DM indirect detection. IceCube and AMS 02 are capable to detect signals come from these annihilations. Besides, the DMs being captured by the solar gravity and DM self-scattering inside the Sun, and their subsequently annihilations and evaporations are discussed in detail in this thesis as well. In the last chapter we also introduce the world’s largest cubic kilometers under-ice neutrino detector IceCube in the Antarctica and its application in the DM indirect detection. We calculated the IceCube sensitivity with 2σ detection significance in five years as the benchmark and presented in the thesis. The next generation upgrade PINGU (Precision IceCube Next Generation Upgrade) will be fully deployed before 2020, it expands the energy window roughly from 1 GeV to many EeVs of IceCube. We discussed such appli-cations to PINGU detector in the future as well.