Polarization effects in the search for dark vector boson at e⁺e^- colliders

Fei-Fan Lee¹, Guey-Lin Lin², Quang Nhat Vo^2*

¹Department of Physics, Jimei University, 361021, Xiamen, Fujian province, China
²Institute of Physics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan

* Presenter:Quang Nhat Vo, email:voqnhat@gmail.com

We argue that the search for dark vector boson through e⁺e^- → Z_dγ can determine the Lorentz structure of Z_dl⁺l^- couplings with the detection of leptonic decays Z_d → l⁺l^-. We assume a general framework that the dark vector boson interacts with ordinary fermions through vector and axial-vector couplings. As a consequence of Ward-Takahashi identity, Z_d is transversely polarized in the limit m_Zd<<√s. On the other hand, the fraction of longitudinal Z_d is non-negligible for m_Zd comparable to √s. Such polarization effects can be analyzed through angular distributions of final-state particles in Z_d decays. Taking l^± ≡ μ^±, we study the correlation between Z_d angle relative to e^- beam direction in e⁺e^- CM frame and μ^- angle relative to the boost direction of Z_d in Z_d rest frame. This correlation is shown to be useful for probing the Lorentz structure of Z_dl⁺l^- couplings. We discuss the measurement of such correlation in Belle II detector, taking into account the detector acceptance and energy resolution.

Keywords: dark matter, parity violation effects, asymmetry parameter