# Measurement of CKM angle $\boldsymbol{ \phi_{3}} $ at Belle II

## TL;DR

The paper discusses the Belle II experiment's approach to precisely measure the CKM angle φ₃ using interference in B meson decays, aiming for a 1° precision with large data samples.

## Contribution

It presents the Belle II experiment's capabilities and plans for measuring φ₃ with unprecedented precision using a variety of decay modes.

## Key findings

- First collisions recorded in 2018
- Design luminosity of 8×10^{35} cm^{-2}s^{-1}
- Expected φ₃ measurement precision of 1° with 50 ab^{-1} data

## Abstract

The CKM angle $ \phi_{3} $ is the only angle of the unitarity triangle that is accessible with tree-level decays in a theoretically clean way. The Belle II experiment is a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric $ e^{+}e^{-} $ collider. The accelerator has already successfully completed the first phase of commissioning, with the first $ e^{+}e^{-} $ collisions recorded in 2018. The design luminosity of SuperKEKB is 8$ \times 10^{35}$ cm$^{-2}$s$^{-1}$ and the Belle II experiment aims to record 50 ab$ ^{-1} $ of data, a factor of 50 more than the Belle experiment. The key method to measure $ \phi_{3} $ is through the interference between $ B^{+} \to D^{0}K^{+} $ and $ B^{+} \to \overline{D}^{0}K^{+} $ decays, which occurs if the final state of the charm-meson decay is accessible to both the $ D^{0} $ and $ \overline{D}^{0} $ mesons. To achieve the best sensitivity, a large variety of $ D $ and $ B $ decay modes are required, which is possible at the Belle II experiment as almost any final state can be reconstructed, including those with photons. With the ultimate Belle II data sample of 50 ab$ ^{-1} $, a determination of $ \phi_{3} $ with a precision of 1$^{\rm o} $ or better is foreseen.