Measurement of branching fractions and direct $CP$ asymmetries for $B \to K\pi$ and $B\to\pi\pi$ decays at Belle II

TL;DR

This paper reports precise measurements of branching fractions and direct CP asymmetries in B meson decays at Belle II, testing the Standard Model's predictions through an isospin sum rule with results consistent with expectations.

Contribution

First measurements of specific B decay branching fractions and CP asymmetries at Belle II, providing new tests of the Standard Model's isospin sum rule.

Findings

Sum rule result is -0.03 ± 0.13 ± 0.04, consistent with zero.

Measured branching fractions and CP asymmetries agree with Standard Model predictions.

Data sample contains 387 million B meson pairs.

Abstract

We report measurements of the branching fractions and direct $\it{CP}$ asymmetries of the decays $B^0 \to K^+ \pi^-$, $B^+ \to K^+ \pi^0$, $B^+ \to K^0 \pi^+$, and $B^0 \to K^0 \pi^0$, and use these for testing the standard model through an isospin-based sum rule. In addition, we measure the branching fraction and direct $\it{CP}$ asymmetry of the decay $B^+ \to \pi^+\pi^0$ and the branching fraction of the decay $B^0 \to \pi^+\pi^-$. The data are collected with the Belle II detector from $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance produced by the SuperKEKB asymmetric-energy collider and contain $387\times 10^6$ bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain $-0.03 \pm 0.13 \pm 0.04$ for the sum rule, in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations.