Speaker
Description
As one of the hypothetical principles in the Standard Model (SM), lepton flavor universality (LFU) should be measured with a precision as high as possible such that the physics violating this principle can be fully tested. The $Z$ factories at the future $e^-e^+$ colliders provide a great opportunity to perform this task because of their large statistics and high reconstruction efficiencies for $B$ mesons at $Z$ pole. These advantages are further strengthened if $\tau$ leptons are produced via the $B$-meson decays as a third-generation flavor. In this paper, we present a systematic study on the LFU test in different operation scenarios of the future $Z$ factories. The goal is two-fold. Firstly, we study the sensitivities of measuring the LFU-violating observables which have involved the $b\to c \tau \nu$ transitions, namely $R_{J/\psi}$, $R_{D_s}$, $R_{D_s^\ast}$ and $R_{\Lambda_c}$, using the mode of muonic $\tau$ decays. For this purpose, the strategies of event reconstruction are developed on the basis of the charged-track information. Secondly, we interpret the projected sensitivities in the SM effective field theory (with an assumption that the LFU-violating physics occurs to the third generation exclusively), by combining the LFU tests with $b\to c \tau \nu$ and the measurements of the $b\to s \tau^-\tau^+$ and $b\to s \nu \nu$ transitions. We show that the limits on the LFU-violating energy scale can be pushed up to $\sim 10~\rm TeV$ for $\sim \mathcal O(1)$ Wilson coefficients.
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