2,863 research outputs found
On The Expected Photon Spectrum in B -> X_s + gamma and Its Uses
Measuring the photon energy spectrum in radiative B decays provides essential
help for gaining theoretical control over semileptonic B transitions. The
hadronic recoil mass distribution in B -> X_u \ell\nu promises the best
environment for determining |V_ub|. The theoretical uncertainties are largest
in the domain of low values of the lepton pair mass q^2. Universality relations
allow to describe this domain reliably in terms of the photon spectrum in B ->
X_s + \gamma. A method is proposed to incorporate 1/m_b corrections into this
relation. The low-E_\gamma tail in radiative decays is important in the context
of extracting |V_ub|. We argue that CLEO's recent fit to the spectrum
underestimates the fraction of the photon spectrum below 2 GeV. Potentially
significant uncertainties enter in the theoretical evaluation of the integrated
end-point lepton spectrum or the B -> X_u \ell\nu width with a too high value
of the lower cut on q^2 in alternative approaches to |V_ub|.Comment: 24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical
predictions are improved and the estimate for the decay fraction revised. The
theoretical expectations for the decay fraction and the spectrum itself are
given on the plot
The Two Roads to "Intrinsic Charm" in B Decays
We describe two complementary ways to show the presence of higher order
effects in the 1/m_Q expansion for inclusive B decays that have been dubbed
"Intrinsic Charm". Apart from the lessons they can teach us about QCD's
nonperturbative dynamics their consideration is relevant for precise
extractions of |V_{cb}|: for they complement the estimate of the potential
impact of 1/m_Q^4 contributions. We draw semiquantitative conclusions for the
expected scale of Weak Annihilation in semileptonic B decays, both for its
valence and non-valence components.Comment: 17 pages, 3 figure
Bloch-Nordsieck cancellations beyond logarithms in heavy particle decays
We investigate the one-loop radiative corrections to the semileptonic decay
of a charged particle at finite gauge boson mass. Extending the Bloch-Nordsieck
cancellation of infrared logarithms, the subsequent non-analytic terms are also
found to vanish after eliminating the pole mass in favor of a mass defined at
short distances. This observation justifies the operator product expansion for
inclusive decays of heavy mesons and implies that infrared effects associated
with the summation of the radiative corrections are suppressed by at least
three powers of the mass of the heavy decaying particle.Comment: LATEX, 7 pages, one figure appended as uu-encoded ps-file,
MPI-PhT/94-1
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
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
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
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
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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