Benchmarks for the New-Physics Search through CP Violation in B^0->pi^0 K_S

TL;DR

This paper establishes precise Standard-Model benchmarks for CP asymmetries in B^0->pi^0 K_S decays using isospin relations, aiding the search for new physics through future lattice-QCD improvements.

Contribution

It provides a theoretically robust, small-error prediction of CP asymmetries in B^0->pi^0 K_S, serving as a benchmark for new-physics searches and analyzing current experimental discrepancies.

Findings

Current data shows a discrepancy with the Standard Model prediction.

A modified electroweak penguin with a large CP-violating phase can explain the discrepancy.

Future lattice-QCD progress can improve the precision of the benchmark predictions.

Abstract

Using isospin relations, we predict the Standard-Model correlation between S_{pi^0 K_S}=(sin 2beta)_{pi^0 K_S} and A_{pi^0 K_S}, the mixing-induced and direct CP asymmetries of B^0-> pi^0 K_S. The calculation uses flavour SU(3) only to fix the isospin-3/2 amplitude through the B^\pm->pi^\pm pi^0 branching ratio, and thus has a small irreducible theoretical error. It can reach percent level precision thanks to expected future lattice-QCD progress for the calculation of the relevant SU(3)-breaking form-factor ratio, and serves as a benchmark for new-physics searches. We obtain an interesting picture in the A_{pi^0 K_S} - S_{pi^0 K_S} plane, where the current experimental data show a discrepancy with the Standard Model, and comment on the direct CP asymmetries of B^0->pi^-K^+ and B^+ -> pi^0 K^+. A modified electroweak penguin with a large new CP-violating phase can explain the discrepancy and allows us to accommodate also the corresponding data for other b->s penguin-dominated decays.