正負電子對撞機上的暗光子研究

Abstract

摘要 標準模型(Standard Model)是描述基本粒子(elementary particles)間的強相互作用(strong interaction)與弱相互作用(weak interaction)的最佳理論。然而，它並不能解釋至今為止所有觀察到的現象，如暗物質(Dark matter)問題。除此之外，還有一些從實驗上測量到的異常(anomalies)，比如緲子(muon)的異常磁矩(anomalous magnetic moment)與標準模型的偏差。因此，我們需要標準模型以外的物理(Beyond Standard Model)來解釋這些問題。現在有許多模型嘗試去解決以上所提到的問題，暗光子(Dark Photon)是其中一種解決異常磁矩的方法，並且可能連結輕暗物質的門徑。 在本論文中，我們擴展標準模型使其包含新的U(1)_(A^')的規範群(gauge group)。在此規範群下的規範粒子(gauge particles)所扮演的角色就是暗光子。這個額外的規範群會和標準模型下的U(1)_Y規範玻色子(gauge boson)作可重整化(renormalizable)的動力學混合(kinetic mixing)。在經過簡單的介紹暗光子後，我們會著重暗光子此 e^+ e^-⟶γA^('*)⟶γμ^+ μ^- 過程下的效應。這個過程還包含了e^+ e^-⟶γ(γ^*,Z^*)⟶γμ^+ μ^- 的標準模型貢獻。為了找出暗光子的效應，我們將會使用一些方法將標準模型背景的貢獻與暗光子的貢獻分開。透過未來的環狀對撞機如Circular Electron Position Collider (CEPC) 及 Future Circular Collider (FCC-ee)，我們找到動力學混合參數在比較高的暗光子質量下的限制(constrained)。最後，我們會將結果與其他已經被完成的方法與實驗做個比較。

Abstract The Standard Model (SM) is the best theory of describing the strong and electroweak interactions of elementary particles. However, it cannot explain all the phenomenon observed today, like Dark matter problem. There are also some anomalies observed from experiments, such as the deviation of anomalous magnetic moment of the muon, (g-2)_μ from SM prediction. Therefore, we need physics beyond Standard Model (BSM) to explain those problems. There are many models trying to solve the above mentioned problems and anomalies. Dark photon is one of the ways to address the (g-2)_μ anomaly and also can provide possible light dark matter portal. In this thesis, we extend SM to include a new U(1)_(A^') gauge group. The gauge particle of this gauge group plays the role of a dark photon A^'. This extra U(1)_(A^') will have kinetic mixing with the SM U(1)_Y gauge boson which is renormalizable. After a brief review of the basic materials about dark photon, we focus on the dark photon effects on the process e^+ e^-⟶γA^('*)⟶γμ^+ μ^-. This process has SM contributions, e^+ e^-⟶γ(γ^*,Z^*)⟶γμ^+ μ^-. To find out the dark photon effects, the SM background contributions need to be separated. We develop strategies to isolate Dark photon effects. We show that using future circular collider like Circular Electron Positron Collider (CEPC) and Future Circular Collider (FCC-ee), the kinetic mixing parameter can be severely constrained in a large range for dark photon mass. Finally, we will compare the result with different methods and colliders which have been done before.