Ultrasoft contribution to heavy-quark pair production near threshold

We compute the third-order correction to the heavy-quark current correlation function due to the emission and absorption of an ultrasoft gluon. Our result supplies a missing contribution to top-quark pair production near threshold and the determination of the bottom quark mass from QCD sum rules.Comment: 10 pages, LaTe

Spectator scattering at NLO in non-leptonic B decays: Tree amplitudes

We compute the 1-loop (alpha_s^2) correction to hard spectator scattering in non-leptonic B decay tree amplitudes. This forms part of the NNLO contribution to the QCD factorization formula for hadronic B decays, and introduces a new rescattering phase that corrects the leading-order result for direct CP asymmetries. Among the technical issues, we discuss the cancellation of infrared divergences, and the treatment of evanescent four-quark operators. The infrared finiteness of our result establishes factorization of spectator scattering at the 1-loop order. Depending on the values of hadronic input parameters, the new 1-loop correction may have a significant impact on tree-dominated decays such as B -> pi pi.Comment: 28 pages, 5 figures, LaTe

Electroweak non-resonant NLO corrections to e+ e- -> W+ W- b bbar in the t tbar resonance region

We analyse subleading electroweak effects in the top anti-top resonance production region in e+ e- collisions which arise due to the decay of the top and anti-top quarks into the W+ W- b bbar final state. These are NLO corrections adopting the non-relativistic power counting v ~ alpha_s ~ sqrt(alpha_EW). In contrast to the QCD corrections which have been calculated (almost) up to NNNLO, the parametrically larger NLO electroweak contributions have not been completely known so far, but are mandatory for the required accuracy at a future linear collider. The missing parts of these NLO contributions arise from matching coefficients of non-resonant production-decay operators in unstable-particle effective theory which correspond to off-shell top production and decay and other non-resonant irreducible background processes to t tbar production. We consider the total cross section of the e+ e- -> W+ W- b bbar process and additionally implement cuts on the invariant masses of the W+ b and W- bbar pairs.Comment: LaTeX, 33 pages, 6 figure

Top quark production near threshold

The present theoretical status of top quark pair production near threshold at (future) $e^+ e^-$ ($\mu^+\mu^-$) colliders is summarized.Comment: 8 pages, LaTeX, Talk presented at the High Energy Physics International Euroconference on Quantum Chromodynamics (QCD'99), Montpellier, France, 7-13 July 199

Infrared Divergence and Twist-3 Distribution Amplitudes in QCD Factorization For B→PPB \to PP

Since b quark mass is not asymptotically large, chirally enhanced corrections which arise from twist-3 wave functions may be important in B decays. We thus evaluate the hadronic matrix elements with the final light pseudoscalar mesons described by leading twist and twist-3 distribution amplitudes. We find that chirally enhanced corrections can be included consistently in the framework of QCD factorization only if the twist-3 distribution amplitudes are symmetric. We then give explicit expressions of $a_i^p$ for $B \to \pi\pi$ at the next-to-leading order of $\alpha_s$ including chirally enhanced corrections. We also briefly discuss the divergence appeared in the hard spectator contributions.Comment: 12 pages, 3 figures, A revised version to appear in Phys. Lett.

Third-order non-Coulomb correction to the S-wave quarkonium wave functions at the origin

We compute the third-order correction to the S-wave quarkonium wave functions |\psi_n(0)|^2 at the origin from non-Coulomb potentials in the effective non-relativistic Lagrangian. Together with previous results on the Coulomb correction and the ultrasoft correction computed in a companion paper, this completes the third-order calculation up to a few unknown matching coefficients. Numerical estimates of the new correction for bottomonium and toponium are given.Comment: 12 pages, v2: matches published version, missing factors in eq. (9), (29) adde

Heavy-to-light B meson form factors at large recoil energy -- spectator-scattering corrections

We complete the investigation of loop corrections to hard spectator-scattering in exclusive B meson to light meson transitions by computing the short-distance coefficient (jet-function) from the hard-collinear scale. Adding together the two coefficients from matching QCD to SCET_I to SCET_II, we investigate the size of loop effects on the ratios of heavy-to-light meson form factors at large recoil. We find the corrections from the hard and hard-collinear scales to be of approximately the same size, and significant, but the perturbative expansions appear to be well-behaved. Our calculation provides a non-trivial verification of the factorization arguments. We observe considerable differences between the predictions based on factorization in the heavy-quark limit and current QCD sum rule calculations of the form factors. We also include the hard-collinear correction in the B -> pi pi tree amplitudes, and find an enhancement of the colour-suppressed amplitude relative to the colour-allowed amplitude.Comment: 55 pages, LaTeX, uses axodraw.st

Multipole-expanded soft-collinear effective theory with non-abelian gauge symmetry

In position space the interaction terms of soft-collinear effective theory must be multipole-expanded to obtain interaction terms with homogeneous scaling behaviour. In this note we provide a manifestly gauge-invariant formulation of the theory after this expansion in the presence of non-abelian gauge fields, extending our previous result. We give the effective Lagrangian (including the Yang-Mills Lagrangian for collinear and ultrasoft gluons) and heavy-to-light transition currents to second order in the power expansion, paying particular attention to the field redefinitions that lead to the gauge symmetries of the effective Lagrangian.Comment: 11 pages, LaTe

Top quark production near threshold and the top quark mass

We consider top-anti-top production near threshold in $e^+ e^-$ collisions, resumming Coulomb-enhanced corrections at next-to-next-to-leading order (NNLO). We also sum potentially large logarithms of the small top quark velocity at the next-to-leading logarithmic level using the renormalization group. The NNLO correction to the cross section is large, and it leads to a significant modification of the peak position and normalization. We demonstrate that an accurate top quark mass determination is feasible if one abandons the conventional pole mass scheme and if one uses a subtracted potential and the corresponding mass definition. Significant uncertainties in the normalization of the $t\bar{t}$ cross section, however, remain.Comment: 14 pages, LaTeX, 2 figures included via epsf.st

More on ambiguities in the pole mass

The relation between the pole quark mass and the $\overline{MS}$-renormalized mass is governed by an infrared renormalon singularity, which leads to an ambiguity of order $\Lambda_{QCD}$ in the definition of the pole mass. We use the renormalization group and heavy quark effective theory to determine the exact nature of this singularity up to an overall normalization. In the abelian gauge theory, the normalization is computed partially to next-to-leading order in the flavour expansion.Comment: LaTeX, 10 pages, Figures appended as uu-encoded file, UM-TH-94-3