22 research outputs found
What two models may teach us about duality violations in QCD
Though the operator product expansion is applicable in the calculation of
current correlation functions in the Euclidean region, when approaching the
Minkowskian domain, violations of quark-hadron duality are expected to occur,
due to the presence of bound-state or resonance poles. In QCD finite-energy sum
rules, contour integrals in the complex energy plane down to the Minkowskian
axis have to be performed, and thus the question arises what the impact of
duality violations may be. The structure and possible relevance of duality
violations is investigated on the basis of two models: the Coulomb system and a
model for light-quark correlators which has already been studied previously. As
might yet be naively expected, duality violations are in some sense "maximal"
for zero-width bound states and they become weaker for broader resonances whose
poles lie further away from the physical axis. Furthermore, to a certain
extent, they can be suppressed by choosing appropriate weight functions in the
finite-energy sum rules. A simplified Ansatz for including effects of duality
violations in phenomenological QCD sum rule analyses is discussed as well.Comment: 17 pages, 6 figures; version to appear in JHE
Second order QCD corrections to inclusive semileptonic b \to Xc l \bar \nu_l decays with massless and massive lepton
We extend previous computations of the second order QCD corrections to
semileptonic b \to c inclusive transitions, to the case where the charged
lepton in the final state is massive. This allows accurate description of b \to
c \tau \bar \nu_\tau decays. We review techniques used in the computation of
O(\alpha_s^2) corrections to inclusive semileptonic b \to c transitions and
present extensive numerical studies of O(\alpha_s^2) QCD corrections to b \to c
l \bar \nu_l decays, for l =e, \tau.Comment: 30 pages, 4 figures, 5 table
Charm-loop effect in and
We calculate the long-distance effect generated by the four-quark operators
with -quarks in the decays. At the lepton-pair
invariant masses far below the -threshold, , we use
OPE near the light-cone. The nonfactorizable soft-gluon emission from
-quarks is cast in the form of a nonlocal effective operator. The matrix elements of this operator are calculated from the QCD
light-cone sum rules with the -meson distribution amplitudes. As a
byproduct, we also predict the charm-loop contribution to
beyond the local-operator approximation. To describe the charm-loop effect at
large , we employ the hadronic dispersion relation with contributions, where the measured amplitudes
are used as inputs. Matching this relation to the result of QCD calculation
reveals a destructive interference between the and
contributions. The resulting charm-loop effect is represented as a
-dependent correction to the Wilson coefficient .
Within uncertainties of our calculation, at below the charmonium region
the predicted ratio is for , but can reach as much as 20% for , the
difference being mainly caused by the soft-gluon contribution.Comment: A few comments added, version to appear in JHE
Factorization at Subleading Power and Irreducible Uncertainties in Decay
Using methods from soft-collinear and heavy-quark effective theory, a
systematic factorization analysis is performed for the
photon spectrum in the endpoint region . It is proposed that, to all orders in , the spectrum obeys a
novel factorization formula, which besides terms with the structure
familiar from inclusive decay
distributions contains "resolved photon" contributions of the form and . Here and
are new soft and jet functions, whose form is derived. These
contributions arise whenever the photon couples to light partons instead of
coupling directly to the effective weak interaction. The new contributions
appear first at order and are related to operators other than
in the effective weak Hamiltonian. They give rise to
non-vanishing corrections to the total decay rate, which cannot be
described using a local operator product expansion. A systematic analysis of
these effects is performed at tree level in hard and hard-collinear
interactions. The resulting uncertainty on the decay rate defined with a cut
GeV is estimated to be approximately . It could be
reduced by an improved measurement of the isospin asymmetry to
the level of . We see no possibility to reduce this uncertainty further
using reliable theoretical methods.Comment: 63 pages, 11 Figures, Journal Versio
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe
Averages of b-hadron, c-hadron, and tau-lepton properties as of 2018 Heavy Flavor Averaging Group (HFLAV)
This paper reports world averages of measurements of b-hadron, c-hadron, and
τ
-lepton properties obtained by the Heavy Flavour Averaging Group using results available through September 2018. In rare cases, significant results obtained several months later are also used. For the averaging, common input parameters used in the various analyses are adjusted (rescaled) to common values, and known correlations are taken into account. The averages include branching fractions, lifetimes, neutral meson mixing parameters,
C
P
violation parameters, parameters of semileptonic decays, and Cabibbo–Kobayashi–Maskawa matrix elements
Results from a large sample of MRPC-strip prototypes for the ALICE TOF detector
The MRPC (multi resistive plate chamber) strip is the basic element of the ALICE time-of-flight detector. A test of a large sample of MRPC-strip prototypes corresponding to 1.2% of the full detector was carried out during the autumn of 2002 at the CERN proton syncroton facility. This paper summarizes the main results obtained in terms of uniformity of response for all the tested channels