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

Abstract

Abstract The physics potential of measuring $$ {B}_{(s)}^0 $$ B s 0 → π0π0 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 π0 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 B0 and $$ {B}_s^0 $$ B s 0 . With the assistance of the b-jet tagging, the relative sensitivities to B0→ π0π0, $$ {B}_s^0 $$ B s 0 → π0π0, B0→ 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, B0→ π0π0 and its two isospin-related modes are paid special attention due to their roles in the determination of the CKM angle α (ϕ2). 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 B0→ π0π0CP 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.