449 research outputs found
Color suppressed contributions to the decay modes B_{d,s} -> D_{s,d} D_{s,d}, B_{d,s} -> D_{s,d} D^*_{s,d}, and B_{d,s} -> D^*_{s,d} D^*_{s,d}
The amplitudes for decays of the type , have no
factorizable contributions, while , and have relatively small factorizable contributions
through the annihilation mechanism. The dominant contributions to the decay
amplitudes arise from chiral loop contributions and tree level amplitudes which
can be obtained in terms of soft gluon emissions forming a gluon condensate. We
predict that the branching ratios for the processes ,
and are all
of order , while ,
and are of
order . We obtain branching ratios for two 's in
the final state of order two times bigger.Comment: 15 pages, 4 figure
Violation of quark-hadron duality and spectral chiral moments in QCD
We analyze the spectral moments of the V-A two-point correlation function. Using all known short-distance constraints and the most recent experimental data from tau decays, we determine the lowest spectral moments, trying to assess the uncertainties associated with the so-called violations of quark-hadron duality. We have generated a large number of 'acceptable' spectral functions, satisfying all conditions, and have used them to extract the wanted hadronic parameters through a careful statistical analysis. We obtain accurate values for the ChPT couplings L10 and C87, and a realistic determination of the dimension six and eight contributions in the operator product expansion, O6=(-5.4^{+3.6}_{-1.6})*10^-3 GeV^6 and O8=(-8.9^{+12.6}_{-7.4})*10^-3 GeV^8, showing that the duality-violation effects have been usually underestimated in previous literature
Massive Pions, Anomalies and Baryons in Holographic QCD
We consider a holographic model of QCD, obtained by a very simple
modification of the original construction, which describes at the same time the
pion mass, the QCD anomalies and the baryons as topological solitons. We study
in detail its phenomenological implications in both the mesonic and baryonic
sectors and compare with the observations.Comment: 31 pages, 2 figures; v2: Version published in Nucl. Phys.
Masses and couplings of vector mesons from the pion electromagnetic, weak, and \pi\gamma transition form factors
We analyse the pion electromagnetic, charged-current, and
transition form factors at timelike momentum transfers ,
GeV, using a dispersion approach. We discuss in detail the propagator
matrix of the photon-vector-meson system and define certain reduced amplitudes,
or vertex functions, describing the coupling of this system to final states. We
then apply the derived analytic expressions to the analysis of the recent
, , and data. We find the reduced amplitudes for the coupling of the
photon and vector mesons to two pseudoscalars to be constant, independent of
, in the range considered, indicating a "freezing" of the amplitudes for
GeV. The fit to the form factor data leads to the following values of
the Breit-Wigner resonance masses m_{\rho^-}=775.3\pm 0.8 MeV,
m_{\rho^0}=773.7\pm 0.6 MeV and m_\omega=782.43\pm 0.05 MeV, where the errors
are only statistical.Comment: revtex, 23 page
Issues in determining alpha_s from hadronic tau decay and electroproduction data
We discuss some key issues associated with duality-violating and
non-perturbative OPE contributions to the theoretical representations of light
quark current-current two-point functions and relevant to precision
determinations of alpha_s from hadronic tau decay and electroproduction
cross-section data. We demonstrate that analyses with an explicit
representation of duality-violating effects are required to bring theoretical
errors in such extractions under control, motivating the accompanying paper in
these proceedings, which presents the results of such an analysis.Comment: 5 pages, 4 figures. Prepared for the Proceedings of the International
Workshop on e+e- collisions from Phi to Psi (PHIPSI11), Sep. 19-22, 2011,
BINP, Novosibirsk, Russi
Remark on the perturbative component of inclusive -decay
In the context of the inclusive -decay, we analyze various forms of
perturbative expansions which have appeared as modifications of the original
perturbative series. We argue that analytic perturbation theory, which combines
renormalization-group invariance and -analyticity, has significant merits
favoring its use to describe the perturbative component of -decay.Comment: 5 pages, ReVTEX, 2 eps figures. Revised paper includes clarifying
remarks and corrected references. To be published in Phys. Rev.
Griffiths-McCoy singularities in random quantum spin chains: Exact results through renormalization
The Ma-Dasgupta-Hu renormalization group (RG) scheme is used to study
singular quantities in the Griffiths phase of random quantum spin chains. For
the random transverse-field Ising spin chain we have extended Fisher's
analytical solution to the off-critical region and calculated the dynamical
exponent exactly. Concerning other random chains we argue by scaling
considerations that the RG method generally becomes asymptotically exact for
large times, both at the critical point and in the whole Griffiths phase. This
statement is checked via numerical calculations on the random Heisenberg and
quantum Potts models by the density matrix renormalization group method.Comment: 4 pages RevTeX, 2 figures include
The effect of rare regions on a disordered itinerant quantum antiferromagnet with cubic anisotropy
We study the quantum phase transition of an itinerant antiferromagnet with
cubic anisotropy in the presence of quenched disorder, paying particular
attention to the locally ordered spatial regions that form in the Griffiths
region. We derive an effective action where these rare regions are described in
terms of static annealed disorder. A one loop renormalization group analysis of
the effective action shows that for order parameter dimensions the rare
regions destroy the conventional critical behavior. For order parameter
dimensions the critical behavior is not influenced by the rare regions,
it is described by the conventional dirty cubic fixed point. We also discuss
the influence of the rare regions on the fluctuation-driven first-order
transition in this system.Comment: 6 pages RevTe
Phase separation in supersolids
We study quantum phase transitions in the ground state of the two dimensional
hard-core boson Hubbard Hamiltonian. Recent work on this and related models has
suggested ``supersolid'' phases with simultaneous diagonal and off-diagonal
long range order. We show numerically that, contrary to the generally held
belief, the most commonly discussed ``checkerboard'' supersolid is
thermodynamically unstable. Furthermore, this supersolid cannot be stabilized
by next near neighbour interaction. We obtain the correct phase diagram using
the Maxwell construction. We demonstrate the ``striped'' supersolid is
thermodynamically stable and is separated from the superfluid phase by a
continuous phase transition.Comment: 4 pages, 4 eps figures, include
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