4,375 research outputs found
Spectator Effects in Inclusive Decays of Beauty Hadrons
We evaluate the matrix elements of the four-quark operators which contribute
to the lifetimes of -mesons and the -baryon. We find that the
spectator effects are not responsible for the discrepancy between the
theoretical prediction and experimental measurement of the ratio of lifetimes
.Comment: LATTICE98. 3 pages, no figure
Note on Tests of the Factorization Hypothesis and the Determination of Meson Decay Constants
We discuss various tests of the factorization hypothesis making use of the
close relationship between semi-leptonic and factorized nonleptonic decay
amplitudes. It is pointed out that factorization leads to truely
model-independent predictions for the ratio of nonleptonic to semi-leptonic
decay rates, if in the nonleptonic decay a spin one meson of arbitrary mass or
a pion take the place of the lepton pair. Where the decay constants of those
mesons are known, these predictions represent ideal tests of the factorization
hypothesis. In other cases they may be used to extract the decay constants.
Currently available data on the decays are shown to be in excellent agreement with
the factorization results. A weighted average of the four independent values
for the QCD coefficient extracted from the data gives
suggesting that it may be equal to the Wilson coefficient evaluated
at the scale .Comment: (9 pages, ReVTeX, no figures), HD-THEP-92-3
Second Order Power Corrections in the Heavy Quark Effective Theory I. Formalism and Meson Form Factors
In the heavy quark effective theory, hadronic matrix elements of currents
between two hadrons containing a heavy quark are expanded in inverse powers of
the heavy quark masses, with coefficients that are functions of the kinematic
variable . For the ground state pseudoscalar and vector mesons, this
expansion is constructed at order . A minimal set of universal form
factors is defined in terms of matrix elements of higher dimension operators in
the effective theory. The zero recoil normalization conditions following from
vector current conservation are derived. Several phenomenological applications
of the general results are discussed in detail. It is argued that at zero
recoil the semileptonic decay rates for and receive only small second order corrections, which are unlikely
to exceed the level of a few percent. This supports the usefulness of the heavy
quark expansion for a reliable determination of .Comment: (34 pages, REVTEX, two postscript figures available upon request),
SLAC-PUB-589
Subleading Shape Functions in Inclusive B Decays
The contributions of subleading shape functions to inclusive decay
distributions of B mesons are derived from a systematic two-step matching of
QCD current correlators onto soft-collinear and heavy-quark effective theory.
At tree-level, the results can be expressed in terms of forward matrix elements
of bi-local light-cone operators. Four-quark operators, which arise at O(g^2),
are included. Their effects can be absorbed entirely into a redefinition of
other shape functions. Our results are in disagreement with some previous
studies of subleading shape-function effects in the literature. A numerical
analysis of B->X_u+l+nu decay distributions suggests that power corrections are
small, with the possible exception of the endpoint region of the charged-lepton
energy spectrum.Comment: 22 pages, 2 figures; several typos corrected; version published in
JHE
Flavor Physics in the Randall-Sundrum Model: I. Theoretical Setup and Electroweak Precision Tests
A complete discussion of tree-level flavor-changing effects in the
Randall-Sundrum (RS) model with brane-localized Higgs sector and bulk gauge and
matter fields is presented. The bulk equations of motion for the gauge and
fermion fields, supplemented by boundary conditions taking into account the
couplings to the Higgs sector, are solved exactly. For gauge fields the
Kaluza-Klein (KK) decomposition is performed in a covariant R_xi gauge. For
fermions the mixing between different generations is included in a completely
general way. The hierarchies observed in the fermion spectrum and the quark
mixing matrix are explained naturally in terms of anarchic five-dimensional
Yukawa matrices and wave-function overlap integrals. Detailed studies of the
flavor-changing couplings of the Higgs boson and of gauge bosons and their KK
excitations are performed, including in particular the couplings of the
standard W and Z bosons. A careful analysis of electroweak precision
observables including the S and T parameters and the Zbb couplings shows that
the simplest RS model containing only Standard Model particles and their KK
excitations is consistent with all experimental bounds for a KK scale as low as
a few TeV, if one allows for a heavy Higgs boson and/or for an ultra-violet
cutoff below the Planck scale. The study of flavor-changing effects includes
analyses of the non-unitarity of the quark mixing matrix, anomalous
right-handed couplings of the W bosons, tree-level flavor-changing neutral
current couplings of the Z and Higgs bosons, the rare decays t-->c(u)+Z and
t-->c(u)+h, and the flavor mixing among KK fermions. The results obtained in
this work form the basis for general calculations of flavor-changing processes
in the RS model and its extensions.Comment: 70 pages, 12 figures. v2: Incorrect treatment of phases in zero-mode
approximation corrected, and discussion of electroweak precision tests
modified. v3: Additional minor modifications and typos corrected; version
published in JHE
The emission of energetic electrons from the complex streamer corona adjacent to leader stepping
We here propose a model to capture the complexity of the streamer corona
adjacent to leader stepping and relate it to the production of energetic
electrons serving as a source of X-rays and -rays, manifesting in
terrestrial gamma-ray flashes (TGFs). During its stepping, the leader tip is
accompanied by a corona consisting of multitudinous streamers perturbing the
air in its vicinity and leaving residual charge behind. We explore the relative
importance of air perturbations and preionization on the production of
energetic run-away electrons by 2.5D cylindrical Monte Carlo particle
simulations of streamers in ambient fields of 16 kV cm and 50 kV
cm at ground pressure. We explore preionization levels between
m and m, channel widths between 0.5 and 1.5 times the
original streamer widths and air perturbation levels between 0\% and 50\% of
ambient air. We observe that streamers in preionized and perturbed air
accelerate more efficiently than in non-ionized and uniform air with air
perturbation dominating the streamer acceleration. We find that in unperturbed
air preionization levels of m are sufficient to explain
run-away electron rates measured in conjunction with terrestrial gamma-ray
flashes. In perturbed air, the production rate of runaway electrons varies from
s to s with maximum electron energies from
some hundreds of eV up to some hundreds of keV in fields above and below the
breakdown strength. In the presented simulations the number of runaway
electrons matches with the number of energetic electrons measured in alignment
with the observations of terrestrial gamma-ray flashes. Conclusively, the
complexity of the streamer zone ahead of leader tips allows explaining the
emission of energetic electrons and photons from streamer discharges.Comment: 29 pages, 11 figures, 2 table
A two-loop relation between inclusive radiative and semileptonic B-decay spectra
A shape-function independent relation is derived between the partial
B->X_u+l+nu decay rate with a cut on P_+=E_X-P_X<Delta and a weighted integral
over the normalized B->X_s+gamma photon-energy spectrum. The leading-power
contribution to the weight function is calculated at next-to-next-to-leading
order in renormalization-group improved perturbation theory, including exact
two-loop matching corrections at the scale mu_i^2 ~ m_b*Lambda_{QCD}. The
overall normalization of the weight function is obtained up to yet unknown
corrections of order [alpha_s(m_b)]^2. Power corrections from phase-space
factors are included exactly, while the remaining subleading contributions are
included at first order in 1/m_b. At this level unavoidable hadronic
uncertainties enter, which are estimated in a conservative way. The combined
theoretical accuracy in the extraction of |V_{ub}| is at the level of 5% if a
value of Delta near the charm threshold can be achieved experimentally.Comment: 22 pages, 4 figures; few comments and one reference added; version to
appear in JHE
Cancellation of Renormalon Ambiguities in the Heavy Quark Effective Theory
Recently, it has been shown that the concept of the pole mass of a heavy
quark becomes ambiguous beyond perturbation theory, because of the presence of
infrared renormalons. We argue that the predictions of heavy quark effective
theory, whose construction is based on the pole mass, are free of such
ambiguities. In the expansion of physical quantities, infrared and
ultraviolet renormalons compensate each other between coefficient functions and
matrix elements. We trace the appearance of these compensations for
current-induced exclusive heavy-to-heavy and heavy-to-light transitions, and
for inclusive decays of heavy hadrons. In particular, we show that the
structure of the heavy quark expansion is not obscured by renormalons, and none
of the predictions of heavy quark effective theory are invalidated.Comment: 26 pages LaTeX (a4), 3 figures available in separate file figures.uu,
CERN-TH.7312/9
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