2,666 research outputs found
Lifetimes of Heavy-Flavour Hadrons -- Whence and Whither?
A theoretical treatment for the weak decays of heavy-flavour hadrons has been
developed that is genuinely based on QCD. Its methodology as it applies to
total lifetimes and the underlying theoretical issues are sketched. Predictions
are compared with present data. One discrepancy emerges: the beauty baryon
lifetime appears to be significantly shorter than expected. The ramifications
of those findings are analyzed in detail.Comment: 15 pages, no figures, LATEX, two references added and new information
concerning a lower charm content in B decays incorporate
Heavy Quark Expansion and Preasymptotic Corrections to Decay Widths in the 't Hooft Model
We address nonperturbative power corrections to inclusive decay widths of
heavy flavor hadrons in the context of the 't Hooft model (two-dimensional QCD
at N_c->oo), with the emphasis on the spectator-dependent effects sensitive to
the flavor of the spectator. The summation of exclusive widths is performed
analytically using the `t Hooft equation. We show that the 1/m_Q expansion of
both the Weak Annihilation and Pauli Interference widths coincides with the OPE
predictions, to the computed orders. Violation of local duality in the
inclusive widths is quantified, and the new example is identified where the OPE
prediction and the actual effect are completely saturated by a single final
state. The qualitative aspects of quark hadronization emerging from the
analysis in the 't Hooft model are discussed. Certain aspects of summation of
spectator-independent hadronic weak decay widths are given in more detail,
which were not spelled out previously. We also give some useful details of the
1/m_Q expansion in the 't~Hooft model.Comment: 54 pages, 8 figures in the text. Version to be published in Phys.
Rev. D. A number of typos are corrected and relevant references added.
Clarifications in Conclusions, Appendices 2.1 and 3 are adde
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
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
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
Analytical Heavy Quark Expansion in the 't Hooft Model
We present a number of exact relations for the heavy quark limit and develop
an analytical 1/m_Q expansion for heavy mesons in the 't Hooft model. Among the
new results are relation 3\mu_\pi^2=\bar\Lambda^2 - m_{sp}^2+\beta^2, 1/m_Q
corrections to the decay constants, to the kinetic expectation values and
1/m_Q^2 nonperturbative corrections to the B->D amplitude at zero recoil. The
properties of the IW functions are addressed and the small velocity sum rules
are verified.Comment: 22 pages, plain LaTeX (no figures
Enhanced subleading structure functions in semileptonic B decay
The charged lepton spectrum in semileptonic decay near
maximal lepton energy receives important corrections from subleading structure
functions that are formally suppressed by powers of but are
enhanced by numerical factors. We investigate the series of higher order terms
which smear over a region of width near the
endpoint the contributions proportional to times (i)
the matrix element of the chromomagnetic operator, and (ii) four-quark
operators. These contribute to the total rate at the few percent level, but
affect the endpoint region much more significantly. Implications for the
determination of are discussed.Comment: 12 page
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
Experimental limits on New Physics from charm decay
Recent measurements in the charm sector are reviewed, concentrating on
results which are sensitive to New Physics effects. The scope of the
presentation includes D0-D0bar mixing searches, a CPT / Lorentz invariance
study, and a range of searches for rare and forbidden decays. Results from the
BaBar, Belle, CDF, CLEO, and FOCUS collaborations are presented, including an
important first observation.Comment: 14 pages, 5 tables, 17 figures. Invited talk presented at the 21st
International Symposium On Lepton And Photon Interactions At High Energies
(LP03) 11-16 August 2003, Batavia, Illinois. To appear in the proceedings of
the symposiu
Theoretical Uncertainties in Gamma_sl(b->u)
I review the existing theoretical uncertainties in relating the semileptonic
decay width in b->u transitions to the underlying Kobayashi-Maskawa mixing
element |V_ub|. The theoretical error bars are only a few percent in |V_ub|,
with uncertainties from the impact of the nonperturbative effects nearly
negligible.Comment: 12 pages, LaTeX; discussion of 1/m_b^3 effects is expanded.
Contribution to Workshop on the Derivation of |V_cb| and |V_ub|: Experimental
Status and Theory Uncertainties, CERN, May 28 - June 2, 199
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