2,809 research outputs found
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
Theoretical Update of the Semileptonic Branching Ratio of B Mesons
We reconsider the prediction of the semileptonic branching ratio of B mesons,
using a recent calculation of the radiative corrections with account for finite
quark masses in nonleptonic decays and taking into account 1/m_b^2 corrections.
For the semileptonic branching ratio we obtain B_SL =(11.8\pm 1.6)% using pole
quark masses and B_SL = (11.0\pm 1.9)% using running MS-bar quark masses. The
uncertainty is dominated by unknown higher order perturbative corrections. We
conclude that the present accuracy of the theoretical analysis does not allow
to state a significant disagreement with the experimental results. However, our
re-analysis of the decay b->ccs yields an increase of (35\pm 11)% due to
next-to-leading order corrections including mass dependent terms, which further
emphasizes the problem of the average charm quark content of the final states
in B decays. Abstract of the erratum: Some of the numerical results presented
in our paper PLB 342 (1995) 362 are affected by an error in the computer
program and need to be revised. The numerical changes are, however, marginal.
We take the opportunity to incorporate the complete results for the quark mass
dependence of the radiative corrections to the subprocess b->ccs and enlarge
the discussion of the average charm quark content in the final state, giving
the results in two different renormalization schemes and adding a figure with
charm quark content plotted vs. the semileptonic branching ratio.Comment: 14 pages latex plus one PS figure, uses epsf.sty and a4wide.sty;
Erratum to published version appende
A Vademecum on Quark-Hadron Duality
We present an elementary introduction to the problem of quark-hadron duality
and its practical limitations, in particular as it concerns local duality
violation in inclusive B meson decays. We show that the accurate definition of
duality violation elaborated over the recent years allows one to derive
informative constraints on violations of local duality. The magnitude of
duality violation is particularly restricted in the total semileptonic widths.
This explains its strong suppression in concrete dynamical estimates. We
analyze the origin of the suppression factors in a model-independent setting,
including a fresh perspective on the Small Velocity expansion. A new
potentially significant mechanism for violation of local duality in
\Gamma_sl(B) is analyzed. Yet we conclude that the amount of duality violation
in \Gamma_sl(B) must be safely below the half percent level, with realistic
estimates being actually much smaller. Violation of local duality in
\Gamma_sl(B) is thus far below the level relevant to phenomenology. We also
present a cautionary note on the B->D^* decay amplitude at zero recoil and show
that it is much more vulnerable to violations of quark-hadron duality than
\Gamma_sl(B). A critical review of some recent literature is given. We point
out that the presently limiting factor in genuinely model-independent
extraction of V_cb is the precise value of the short-distance charm quark mass.
We suggest a direct and precise experimental check of local quark-hadron
duality in semileptonic B->X_c l\nu decays.Comment: 48 pages, 4 figures; LaTe
Precision Studies of Duality in the 't Hooft Model
We address numerical aspects of local quark-hadron duality using the example
of the exactly solvable 't Hooft model, two-dimensional QCD with N_c -->
infinity. The primary focus of these studies is total semileptonic decay widths
relevant for extracting |V_{cb}| and |V_{ub}|. We compare the exact
channel-by-channel sum of exclusive modes to the corresponding rates obtained
in the standard 1/m_Q expansion arising from the Operator Product Expansion. An
impressive agreement sets in unexpectedly early, immediately after the
threshold for the first hadronic excitation in the final state. Yet even at
higher energy release it is possible to discern the seeds of duality-violating
oscillations. We find the ``Small Velocity'' sum rules to be exceptionally well
saturated already by the first excited state. We also obtain a convincing
degree of duality in the differential distributions and in an analogue of
R_{e^+e^-}(s). Finally, we discuss possible lessons for semileptonic decays of
actual heavy quarks in QCD.Comment: 45 pages, 16 eps figures include
Four-fermion heavy quark operators and light current amplitudes in heavy flavor hadrons
We introduce and study the properties of the "color-straight" four-quark
operators containing heavy and light quark fields. They are of the form (\bar
b\Gamma_b b)(\bar q\Gamma_q q) where both brackets are color singlets. Their
expectation values include the bulk of the nonfactorizable contributions to the
nonleptonic decay widths of heavy hadrons. The expectation values of the
color-straight operators in the heavy hadrons are related to the momentum
integrals of the elastic light-quark formfactors of the respective heavy
hadron. We calculate the asymptotic behavior of the light-current formfactors
of heavy hadrons and show that the actual decrease is 1/(q^2)^3/2 rather than
1/q^4. The two-loop hybrid anomalous dimensions of the four-quark operators and
their mixing (absent in the first loop) are obtained. Using plausible models
for the elastic formfactors, we estimate the expectation values of the
color-straight operators in the heavy mesons and baryons. Improved estimates
will be possible in the future with new data on the radiative decays of heavy
hadrons. We give the Wilson coefficients of the four-fermion operators in the
1/m_b expansion of the inclusive widths and discuss the numerical predictions.
Estimates of the nonfactorizable expectation values are given.Comment: 51 pages. The case of flavor-singlet operators is added for the
two-loop anomalous dimension
Four-Quark Mesons in Non-leptonic B Decays--Could They Resolve Some Old Puzzles?
We point out that non-leptonic B decays driven by b-->ccbar s should provide
a favourable environment for the production of hidden charm diquark-antidiquark
bound states that have been suggested to explain the resonances with masses
around 4 GeV recently observed by BaBar and BELLE. Studying their relative
abundances in non-leptonic B decays can teach us novel lessons about their
structure and the strong interactions. Through their decay into psi they can
provide a natural explanation of the excess of B-->psi X observed for p_psi < 1
GeV. Other phenomenological consequences are mentioned as well.Comment: 6 pages, 2 figures, revte
Phenomenological Analysis of D Meson Lifetimes
The QCD-based operator-product-expansion technique is systematically applied
to the study of charmed meson lifetimes. We stress that it is crucial to take
into account the momentum of the spectator light quark of charmed mesons,
otherwise the destructive Pauli-interference effect in decays will lead
to a negative decay width for the . We have applied the QCD sum rule
approach to estimate the hadronic matrix elements of color-singlet and
color-octet 4-quark operators relevant to nonleptonic inclusive decays. The
lifetime of is found to be longer than that of because the latter
receives a constructive -exchange contribution, whereas the hadronic
annihilation and leptonic contributions to the former are compensated by the
Pauli interference. We obtain the lifetime ratio
, which is larger than some earlier theoretical
estimates, but still smaller than the recent measurements by CLEO and E791.Comment: 14 pages, 3 figure
Heavy Quark Lifetimes, Mixing and CP Violation
This paper emphasizes four topics that represent some of the year's
highlights in heavy quark physics. First of all, a review is given of charm
lifetime measurements and how they lead to better understanding of the
mechanisms of charm decay. Secondly, the CLEO collaboration's new search for
charm mixing is reported, which significantly extends the search for new
physics in that sector. Thirdly, important updates in Bs mixing are summarized,
which result in a new limit on the mass difference, and which further constrain
the unitarity triangle. Finally, the first efforts to measure CP violation in
the B system are discussed. Results are shown for the CDF and ALEPH
measurements of sin(2beta), as well as the CLEO branching fraction measurements
of B-->Kpi,pipi, which have implications for future measurements of alpha.Comment: 25 pages, 15 figures. Talk given at the XIX International Symposium
on Lepton and Photon Interactions, Stanford University, August 9-14, 199
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
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
- …