Journal Article

Prospects for $$ {B}_{(s)}^0 $$ → π0π0 and $$ {B}_{(s)}^0 $$ → ηη modes and corresponding CP asymmetries at Tera-Z

Li, Lingfeng, Deschamps, Olivier, Wang, Yuexin, Ruan, Manqi, Zhu, Yongfeng, Descotes-Genon, Sébastien, Chen, Shanzhen

2022

Overview

The physics potential of measuring $$ {B}_{(s)}^0 $$ B s 0 → π 0 π 0 and $$ {B}_{(s)}^0 $$ B s 0 → ηη decays via four-photon final states at Tera- Z phase of CEPC or FCC-ee is investigated in this paper. We propose an electromagnetic calorimeter (ECAL) with both high energy resolution and excellent separation power to efficiently reconstruct π 0 and η from hadronic final states with high photon multiplicity. The resulting B -meson mass resolution is approximately 30 MeV, allowing 3 σ separation between B 0 and $$ {B}_s^0 $$ B s 0 . With the assistance of the b -jet tagging, the relative sensitivities to B 0 → π 0 π 0 , $$ {B}_s^0 $$ B s 0 → π 0 π 0 , B 0 → ηη , and $$ {B}_s^0 $$ B s 0 → ηη signal strengths at Tera- Z are projected as 0.45%, 4.5%, 18%, and 0.95%, respectively. Their dependence on various detector performances is also discussed. In addition, B 0 → π 0 π 0 and its two isospin-related modes are paid special attention due to their roles in the determination of the CKM angle α ( ϕ 2 ). The anticipated precisions of their branching-ratio and CP -asymmetry measurements at Tera- Z are evaluated. We show that the measurement of the time-integrated B 0 → π 0 π 0 CP asymmetry at Tera- Z is complementary to B -factory ones. The precision on α combining Z - and B -factory results reaches 0 . 4 ° , lower than the systematic uncertainties attached to isospin breaking.