Simulations of the 3-D WIMP Velocity Distribution for
Directional Dark Matter Detection Experiments

方向性暗物質直接偵測現象學：
重質量弱交互作用粒子三維速度分佈模擬

Chung-Lin Shan^1*

¹DM-DD, CLShan IchAG, Taiwan

* Presenter:Chung-Lin Shan, email:clshan@mail.ncku.edu.tw

Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for Galactic Dark Matter (DM). In the last (more than) three decades, more than 40 experiments have been built or are being planned to search for different WIMP candidates by direct detection of scattering signals of ambient WIMPs off target nuclei in low-background underground laboratory detectors. Among these experiments, "directional" DM detection has been proposed (more than one decade ago) to be a promising experimental strategy for discriminating WIMP signals from backgrounds by using 3-dimensional information (the recoil track and/or the head-tail sense) of (elastic) WIMP-nucleus scattering events.
In this talk, I will first review briefly the basic concept of directional DM detection experiments. Then I will present our (Monte Carlo) simulation results of the angular distribution of the 3-dimensional WIMP velocity Besides the diurnal modulation of the angular WIMP velocity distribution, the so-called "directionality" of DM signals discussed in literature, a possible "annual" modulation will also be demonstrated.


重質量弱交互作用粒子（WIMPs）至今仍是最主要的宇宙暗物質候選粒子。三十多年以來，超過四十個已正式運行及正在建設或計畫中的暗物質直接偵測實驗嘗試透過觀測 WIMP 粒子與地下低背景雜訊偵測器原子核間的（彈性）散射信號，以確認 WIMP 暗物質的存在，並給定出其（一維）速度分佈函數、質量、不同交互作用的散射截面等天文與粒子物理性質。眾多實驗中，在十多年前開始提出、且近年來逐漸獲得重要技術進展的「方向性」暗物質直接偵測實驗，則是試圖透過測量三維散射訊息（反衝軌跡、反衝方向）在極大量的背景雜訊中區分出具有（入射）方向性的暗物質－原子核散射事件。
在本報告中，我將先簡單介紹方向性暗物質直接偵測實驗的基本概念。其後，會逐一討論在不同天球座標系統裡的 WIMP 三維速度分佈模擬結果。除了「日週期變化」—即已在文獻中被提出過的暗物質信號的「方向性」—外，我也將提出另一重要且具可觀測性的「年週期變化」。

Keywords: Dark Matter, directional direct detection