4,429 research outputs found
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) () 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
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 for at the
next-to-leading order of 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 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 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 -renormalized
mass is governed by an infrared renormalon singularity, which leads to an
ambiguity of order 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
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