more experimental and theoretical efforts are expected to understand the underlying QCD dynamics of spectator scattering and annihilation contributions.'
author:
- Qin Chang
- Junfeng Sun
- Yueling Yang
- Xiaonan Li
title: |
    Spectator Scattering and Annihilation Contributions\
    as a Solution to the ${\pi}K$ and ${\pi}{\pi}$ Puzzles\
    within QCD Factorization Approach
---

Introduction {#sec01}
============

Charmless hadronic $B$-meson decays provide a fertile ground for testing the Standard Model (SM) and exploring the source of $CP$ violation, which attract much attention in the past years. Thanks to the fruitful accomplishment of BABAR and Belle, the constraints on the sides and interior angles of the unitarity triangle significantly reduce the allowed ranges of some of the CKM elements, and many rare $B$ decays are well measured. With the successful running of LHC and the advent of Belle II at SuperKEKB, heavy flavour physics has entered a new exciting era and more $B$ decay modes will be measured precisely soon.

Recently, the evidence of pure annihilation decays $\bar{B}_{s}^{0}$ ${\to}$ ${\pi}^{+}{\pi}^{-}$ and $\bar{B}_{d}^{0}$ ${\to}$ $K^{+}K^{-}$ are firstly reported by CDF Collaboration [@CDFanni], and soon confirmed by LHCb Collaboration [@LHCbanni]. The Heavy Flavor Averaging Group (HFAG) presents their branching ratios [@HFAG] $${\cal B}(\bar{B}_{s}^{0}{\to}{\pi}^{+}{\pi}^{-})
  =(0.73{\pm}0.14){\times}10^{-6}
  