RG-improved single-particle inclusive cross sections and forward-backward asymmetry in $tbar t$ production at hadron colliders

We use techniques from soft-collinear effective theory (SCET) to derive renormalization-group improved predictions for single-particle inclusive (1PI) observables in top-quark pair production at hadron colliders. In particular, we study the top-quark transverse-momentum and rapidity distributions, the forward-backward asymmetry at the Tevatron, and the total cross section at NLO+NNLL order in resummed perturbation theory and at approximate NNLO in fixed order. We also perform a detailed analysis of power corrections to the leading terms in the threshold expansion of the partonic hard-scattering kernels. We conclude that, although the threshold expansion in 1PI kinematics is susceptible to numerically significant power corrections, its predictions for the total cross section are in good agreement with those obtained by integrating the top-pair invariant-mass distribution in pair invariant-mass kinematics, as long as a certain set of subleading terms appearing naturally within the SCET formalism is included.

NNLO corrections to B --> X_u l nu and the determination of |V_ub|

We study the impact of next-to-next-to-leading order (NNLO) QCD corrections on partial decay rates in B --> X_u l nu decays, at leading-order in the 1/m_b expansion for shape-function kinematics. These corrections are implemented within a modified form of the BLNP framework, which allows for arbitrary variations of the jet scale mu_i sim 1.5 GeV. Our analysis includes a detailed comparison between resummed and fixed-order perturbation theory, and between the complete NNLO results and those obtained in the large-$beta_0$ approximation. For the default choice mu_i=1.5 GeV used in current extractions of |V_ub| within the BLNP framework, the NNLO corrections induce significant downward shifts in the central values of partial decay rates with cuts on the hadronic variable P_+, the hadronic invariant mass, and the lepton energy. At the same time, perturbative uncertainties are reduced, especially those at the jet scale, which are the dominant ones at next-to-leading order (NLO). For higher values of mu_i and in fixed-order perturbation theory, the shifts between NLO and NNLO are more moderate. We combine our new results with known power-suppressed terms in order to illustrate the implications of our analysis on the determination of |V_ub| from inclusive decays.