3,364 research outputs found
Hadronic Spectral Moments in Semileptonic B Decays With a Lepton Energy Cut
We compute the first two moments of the final hadronic invariant mass in
inclusive semileptonic B decay, in the presence of a cut on the charged lepton
energy. These moments may be measured directly by experiments at the
Upsilon(4S) using the neutrino reconstruction technique, which requires such a
cut. Measurement of these moments will place constraints on the nonperturbative
parameters \bar\Lambda and \lambda_1, which are relevant for extracting the
quark masses m_b and m_c, as well as the CKM angle V_cb. We include terms of
order \alpha_s^2\beta_0 and 1/m_b^3 in the operator product expansion, and use
the latter to estimate the theoretical uncertainty in the extraction of
\bar\Lambda and \lambda_1.Comment: 13 pages, 5 figures, REVTe
The Mass Definition in Hqet and a New Determination of V
Positive powers of the mass parameter in a physical quantity calculated with
the help of heavy quark effective theory originate from a Wilson coefficient in
the matching of QCD and HQET Green function. We show that this mass parameter
enters the calculation as a well--defined running current mass. We further
argue that the recently found ill--definition of the pole mass, which is the
natural expansion parameter of HQET, does not affect a phenomenological
analysis which uses truncated perturbative series. We reanalyse inclusive
semileptonic decays of heavy mesons and obtain the quark mass
where the error
is almost entirely due to scale--uncertainties. We also obtain
and
where the errors come
from the uncertainty in the kinetic energy of the heavy quark inside the meson,
in the experimental branching ratios, in QCD input parameters, and
scale--uncertainties.Comment: 21 p., 5 figs, all style files incl., TUM-T31-56/R (Sec. 2 revised,
phenomenological results unchanged
Proposal for a Precision Measurement of |Vub|
A new method for a precision measurement of the CKM matrix element |Vub| is
discussed, which combines good theoretical control with high efficiency and a
powerful discrimination against charm background. The resulting combined
theoretical uncertainty on |Vub| is estimated to be 10%.Comment: 4 pages, 2 figures, RevTe
Radiatively corrected shape function for inclusive heavy hadron decays
We discuss the non-perturbative and the radiative corrections to inclusive B
decays from the point of view known from QED corrections to high energy e^+ e^-
processes. Here the leading contributions can be implemented through the so
called ``radiator function'' which corresponds to the shape function known in
heavy hadron decays. In this way some new insight into the origin of the shape
function is obtained. As a byproduct, a parameterization of the radiatively
corrected shape function is suggested which can be implemented in Monte Carlo
studies of inclusive heavy hadron decays.Comment: LaTeX, uses a4, graphicx and psfrag, 10 pages. The complete paper is
also available at http://www-ttp.physik.uni-karlsruhe.de/Preprints
Four-quark Operators Relevant to B Meson Lifetimes from QCD Sum Rules
At the order of 1/m_b^3, the B meson lifetimes are controlled by the hadronic
matrix elements of some four-quark operators. The nonfactorizable magnitudes of
these four-quark operator matrix elements are analyzed by QCD sum rules in the
framework of heavy quark effective theory. The vacuum saturation for
color-singlet four-quark operators is justified at hadronic scale, and the
nonfactorizable effect is at a few percent level. However for color-octet
four-quark operators, the vacuum saturation is violated sizably that the
nonfactorizable effect cannot be neglected for the B meson lifetimes. The
implication to the extraction of some of the parameters from B decays is
discussed. The B meson lifetime ratio is predicted as
\tau(B^-)/\tau(B^0)=1.09\pm 0.02. However, the experimental result of the
lifetime ratio \tau(\Lambda_b)/\tau(B^0) still cannot be explained.Comment: 20 pages, latex, 6 figures, discussion on non-factorizable effect of
the four-quark condensate added, to appear in Phys. Rev. D57 (1998
The Pole Mass of The Heavy Quark. Perturbation Theory and Beyond
The key quantity of the heavy quark theory is the quark mass . Since
quarks are unobservable one can suggest different definitions of . One of
the most popular choices is the pole quark mass routinely used in perturbative
calculations and in some analyses based on heavy quark expansions. We show that
no precise definition of the pole mass can be given in the full theory once
non-perturbative effects are included. Any definition of this quantity suffers
from an intrinsic uncertainty of order \Lam /m_Q. This fact is succinctly
described by the existence of an infrared renormalon generating a factorial
divergence in the high-order coefficients of the series; the
corresponding singularity in the Borel plane is situated at . A
peculiar feature is that this renormalon is not associated with the matrix
element of a local operator. The difference \La \equiv M_{H_Q}-m_Q^{pole} can
still be defined in Heavy Quark Effective Theory, but only at the price of
introducing an explicit dependence on a normalization point : \La (\mu
). Fortunately the pole mass {\em per se} does not appear in
calculable observable quantities.Comment: 22 pages, Latex, 6 figures (available upon request), TPI-MINN-94/4-T,
CERN-TH.7171/94, UND-HEP-94-BI
Heavy Quark Distribution Function in QCD and the ACM Model
We show that the phenomenological \ACM ansatz is consistent with QCD through
order in the description of B\ra l\bar \nu_l+X_u and B\ra \gamma
+X_s transitions, including their energy spectra and differential
distributions. This suggests a concrete realization for the QCD distribution
function, which we call the ``Roman'' function. On the other hand the \ACM
model description of the end-point domain in B\ra l\bar \nu_l + X_c is
incompatible with QCD: a different distribution function enters the description
of b\ra c decays as compared to the transitions to the massless quarks. Both
observations -- the validity of the {\ACM}-like description for heavy-to-light
transitions and the emergence of the new distribution function in the b\ra c
case -- are in contradiction to a recent claim in the literature. The intrinsic
limitation of the \ACM model could reveal itself in different values of the
effective quark mass from fits of the and decays.Comment: 15 pages, Latex, 2 figures are included (as 2 appended postscript
files), CERN-TH.7159/94, TPI-MINN-94/2-T, UND-HEP-94-BIG02 (a few comments on
the literature are added
On the Determination of from Inclusive Semileptonic Decay Spectra
We propose a model independent method to determine from the energy
spectrum of the charged lepton in inclusive semileptonic decays. The method
includes perturbative QCD corrections as well as nonperturbative ones.Comment: LaTeX, 19 pages, 8 figures appended after \end{document} as
uu-encoded and compressed .eps files, uses epsf, Technion-PH-94/9,
CERN-TH.7308/9
Large electroweak penguin contribution in B -> K pi and pi pi decay modes
We discuss about a possibility of large electroweak penguin contribution in B
-> K pi and pi pi from recent experimental data. The experimental data may be
suggesting that there are some discrepancies between the data and theoretical
estimation in the branching ratios of them. In B -> K pi decays, to explain it,
a large electroweak penguin contribution and large strong phase differences
seem to be needed. The contributions should appear also in B -> pi pi. We show,
as an example, a solution to solve the discrepancies in both B -> K pi and B ->
pi pi. However the magnitude of the parameters and the strong phase estimated
from experimental data are quite large compared with the theoretical
estimations. It may be suggesting some new physics effects are including in
these processes. We will have to discuss about the dependence of the new
physics. To explain both modes at once, we may need large electroweak penguin
contribution with new weak phases and some SU(3) breaking effects by new
physics in both QCD and electroweak penguin type processes.Comment: 23 pages, 9 figure
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