Analysis of $B_s\to\phi\nu\bar{\nu}$ at CEPC

TL;DR

This paper evaluates the potential of the CEPC collider to precisely measure the rare decay $B_s o \,phi \,nu\bar{\nu}$, which is sensitive to new physics and can help resolve B flavor anomalies.

Contribution

It provides updated SM predictions for the decay and demonstrates the feasibility of precise measurements at CEPC using full simulation data.

Findings

BR($B_s \to \phi \nu\bar{\nu}$) can be measured with percent-level uncertainty.

The $S/B$ ratio is approximately 1, indicating good signal-to-background separation.

Constraints on effective theory couplings can be derived from combined measurements.

Abstract

The rare $b\to s\nu\bar{\nu}$ decays are sensitive to contributions of new physics (NP) and helpful to resolve the puzzle of multiple $B$ flavor anomalies. In this work, we propose to study the $b\to s\nu\bar{\nu}$ transition at a future lepton collider operating at the $Z$ pole through the $B_s \to \phi\nu\bar{\nu}$ decay. Using the $B_s\to\phi$ decay form factors from lattice simulations, we first update the SM prediction of BR($B_s \to \phi\nu\bar{\nu})_{\mathrm{SM}}=(9.93\pm 0.72)\times 10^{-6}$ and the corresponding $\phi$ longitudinal polarization fraction $F_{L,{\mathrm{SM}}}=0.53\pm 0.04$. Our analysis uses the full CEPC simulation samples with a net statistic of $\mathcal{O}(10^9)$ $Z$ decays. Precise $\phi$ and $B_s$ reconstructions are used to suppress backgrounds. The results show that BR($B_s \to \phi\nu\bar{\nu})$ can be measured with a statistical uncertainty of $\mathcal{O}(\%)$ and an $S/B$ ratio of $\mathcal{O}(1)$ at the CEPC. The quality measures for the event reconstruction are also derived. By combining the measurement of BR($B_s \to \phi\nu\bar{\nu})$ and $F_L$, the constraints on the effective theory couplings at low energy are given.