663 research outputs found
Decay of pseudoscalars into lepton pairs and large-Nc QCD
The counterterm combination that describes the decay of pseudoscalar mesons
into charged lepton pairs at lowest order in chiral perturbation theory is
considered within the framework of QCD in the limit of a large number of
colours Nc. When further restricted to the lowest meson dominance approximation
to large-Nc QCD, our results agree well with the available experimental data.Comment: 5 pages, 2 figure
Right handed currents and FSI phases in B^0 --> \phi K^{*0}
We consider possible effects of New Physics (NP) on the angular distributions
of the decay B^0 --> \phi K^{*0}, showing how these effects depend on the
nature of nonstandard interactions. In a general framework based on
factorization, we show that triple products can be used to probe the chirality
of NP currents. In this analysis we take into account the presence of
non-vanishing strong phases, which is motivated by recent experimental
evidence. It is seen that the observability of right-handed NP is strongly
dependent on the relation between the relative magnitude of these phases and
the ratio of Standard Model and NP scales. As an application we estimate the
expected values of relevant observables in a particular Left Right Symmetric
Model.Comment: 16 pages, no figures. Comments and references added, conclusions
unchanged. Final version to appear in Phys. Rev.
Quantum Loops in the Resonance Chiral Theory: The Vector Form Factor
We present a calculation of the Vector Form Factor at the next-to-leading
order in the 1/N_C expansion, within the framework of Resonance Chiral Theory.
The calculation is performed in the chiral limit, and with two dynamical quark
flavours. The ultraviolet behaviour of quantum loops involving virtual
resonance propagators is analyzed, together with the kind of counterterms
needed in the renormalization procedure. Using the lowest-order equations of
motion, we show that only a few combinations of local couplings appear in the
final result. The low-energy limit of our calculation reproduces the standard
Chiral Perturbation Theory formula, allowing us to determine the resonance
contribution to the chiral low-energy couplings, at the next-to-leading order
in 1/N_C, keeping a full control of their renormalization scale dependence.Comment: 27+1 pages, 9 figure
Band and filling controlled transitions in exactly solved electronic models
We describe a general method to study the ground state phase diagram of
electronic models on chains whose extended Hubbard hamiltonian is formed by a
generalized permutator plus a band-controlling term. The method, based on the
appropriate interpretation of Sutherland's species, yields under described
conditions a reduction of the effective Hilbert space. In particular, we derive
the phase diagrams of two new models; the first one exhibits a band-controlled
insulator-superconductor transition at half-filling for the unusually high
value ; the second one is characterized by a filling-controlled
metal-insulator transition between two finite regions of the diagram.Comment: 5 pages, REVTEX, 2 eps figure
High field magnetic resonant properties of beta'-(ET)2SF5CF2SO3
A systematic electron spin resonance (ESR) investigation of the low
temperature regime for the (ET)2SF5CF2SO3 system was performed in the frequency
range of ~200-700 GHz, using backward wave oscillator sources, and at fields up
to 25 T. Newly acquired access to the high frequency and fields shows
experimental ESR results in agreement with the nuclear magnetic resonance (NMR)
investigation, revealing evidence that the transition seen at 20 K is not of
conventional spin-Peierls order. A significant change of the spin resonance
spectrum in beta'-(ET)2SF5CF2SO3 at low temperatures, indicates a transition
into a three-dimensional-antiferromagnetic (3D AFM) phase.Comment: 4 pages, 7 figures, minor grammatical change
Nonleptonic decays and the Skyrme model
Nonleptonic decay branching ratios are estimated by means of the
QCD enhanced effective weak Hamiltonian supplemented by the SU(3) Skyrme model
used to estimate the nonperturbative matrix elements. The model has only one
free parameter, namely the Skyrme charge , which is fixed through the
experimental values of the octet-decuplet mass splitting and the
axial coupling constant . The whole scheme is equivalent to that which
works well for the nonleptonic hyperon decays. The ratios of calculated
amplitudes are in agreement with experiment. However, the absolute values are
about twice too large if short-distance corrections and only ground
intermediate states are included.Comment: 4 pages, 1 figure, 1 table, version to appear in Phys.Rev.
On the peak in the far-infrared conductivity of strongly anisotropic cuprates
We investigate the far-infrared and submillimeter-wave conductivity of
electron-doped La_(2-x)Ce_xCuO_4 tilted 1 degree off from the ab-plane. The
effective conductivity measured for this tilt angle reveals an intensive peak
at finite frequency (\nu ~ 50 cm{-1}) due to a mixing of the in-plane and
out-of-plane responses. The peak disappears for the pure in-plane response and
transforms to the Drude-like contribution. Comparative analysis of the mixed
and the in-plane contributions allows to extract the c-axis conductivity which
shows a Josephson plasma resonance at 11.7 cm{-1} in the superconducting state.Comment: 4 pages, 4 figures include
Optical conductivity of one-dimensional doped Hubbard-Mott insulator
We study the optical response of a strongly correlated electron system near
the metal-insulator transition using a mapping to the sine-Gordon model. With
semiclassical quantization, the spectral weight is distributed between a Drude
peak and absorption lines due to breathers. We calculate the Drude weight, the
optical gap, and the lineshape of breather absorption.Comment: 4 pages, 2 EPS figures, REVTEX 4, a final versio
Hadronic light-by-light scattering contribution to the muon g-2: an effective field theory approach
The hadronic light-by-light contribution to a_{mu}, the anomalous magnetic
moment of the muon, is discussed from the point of view of an effective
low-energy theory. As an application, the coefficient of the leading logarithm
arising from the two-loop graphs involving two anomalous vertices is computed,
and found to be positive. This corresponds to a positive sign for the pion-pole
contribution to the hadronic light-by-light correction to a_{mu}, and to a
sizeable reduction of the discrepancy between the present experimental value of
a_{mu} and its theoretical counterpart in the standard model.Comment: 4 pages, 1 figure. v2: published versio
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