3 research outputs found
Electroweak Absorptive Parts in NRQCD Matching Conditions
Electroweak corrections associated with the instability of the top quark to
the next-to-next-to-leading logarithmic (NNLL) total top pair threshold cross
section in e+e- annihilation are determined. Our method is based on absorptive
parts in electroweak matching conditions of the NRQCD operators and the optical
theorem. The corrections lead to ultraviolet phase space divergences that have
to be renormalized and lead to NLL mixing effects. Numerically, the corrections
can amount to several percent and are comparable to the known NNLL QCD
corrections.Comment: 17 pages, revtex4, 4 postscript figures included; minor changes in
text and references, title modified in printed versio
On Electroweak Matching Conditions for Top Pair Production at Threshold
We determine the real parts of electroweak matching conditions relevant for
top quark pair production close to threshold in e+e- annihilation at
next-to-next-to-leading logarithmic (NNLL) order. Numerically the corrections
are comparable to the NNLL QCD corrections.Comment: 12 pages, revtex4, 1 postscript figure included; minor changes in
text and references, version published in Phys. Rev.
Phase Space Matching and Finite Lifetime Effects for Top-Pair Production Close to Threshold
The top-pair production cross section close to threshold in
collisions is strongly affected by the small lifetime of the top
quark. Since the cross section is defined through final states containing the
top decay products, a consistent definition of the cross section depends on
prescriptions how these final states are accounted for the cross section.
Experimentally, these prescriptions are implemented for example through cuts on
kinematic quantities such as the reconstructed top quark invariant masses. As
long as these cuts do not reject final states that can arise from the decay of
a top and an anti-top quark with a small off-shellness compatible with the
nonrelativistic power-counting, they can be implemented through imaginary phase
space matching conditions in NRQCD. The prescription-dependent cross section
can then be determined from the optical theorem using the forward
scattering amplitude. We compute the phase space matching conditions associated
to cuts on the top and anti-top invariant masses at next-to-next-to-leading
logarithmic (NNLL) order and partially at next-to-next-to-next-to-leading
logarithmic (NLL) order in the nonrelativistic expansion and, together
with finite lifetime and electroweak effects known from previous work, analyze
their numerical impact on the cross section. We show that the phase
space matching contributions are essential to make reliable NRQCD predictions,
particularly for energies below the peak region, where the cross section is
small. We find that irreducible background contributions associated to final
states that do not come from top decays are strongly suppressed and can be
neglected for the theoretical predictions.Comment: 62 pages, 21 figure