Theory of charmless hadronic B-decays

TL;DR

This paper reviews the dynamical QCD factorization approach to charmless hadronic B decays, discussing recent theoretical advancements, experimental challenges, and the implications of new measurements for understanding CP violation and decay mechanisms.

Contribution

It provides an updated overview of NNLO calculations in QCD factorization and discusses how recent experimental results inform theoretical models and potential new physics.

Findings

NNLO amplitudes show well-behaved perturbative expansion

Large B0 -> pi0 pi0 branching fraction remains a challenge

Recent measurements align better with QCDF predictions

Abstract

I summarize results and performance of the dynamical theory of charmless hadronic B decays, based on QCD factorization in the heavy quark limit. On the theoretical side, a number of NNLO (alpha_s^2) amplitudes are now available, all showing a well-behaved perturbative expansion. The large observed branching fraction in B0 -> pi0 pi0 remains a challenge, implying either a large inverse moment of the B-meson wave function or a sizable power correction (or unexpected new physics). B-factory/Belle2 analyses of B -> gamma l nu may shed light on this. On the other hand, the new Belle and LHCb measurements of A_CP(pi^+ pi^-) bring the experimental result closer to QCDF predictions, similar to what is found in B -> pi K decays, while S_pipi gives a competitive gamma-determination. I remind the reader that for vector-vector final states, a theoretical treatment of the full set of helicity amplitudes has existed and met with some success since the B-factory era, and applies equally to LHCb measurements of e.g. B_s -> phi phi.