Re+e−R_{e^+ e^-} and an effective QCD charge

We consider the electron-positron annihilation process into hadrons $R_{e^+e^-}$ up to $\mathcal{O}(\alpha_{s}^{3})$ and we adopt the smearing method suggest by Poggio, Quinn and Weinberg to confront the experimental data with theory. As a theoretical model we use a QCD coupling constant frozen in the low energy regime, where this coupling can be parameterised in terms of an effective dynamical gluon mass ($m_g$) which is determined through Schwinger-Dyson equations. In order to find the best fit between experimental data and theory we perform a $\chi^2$ study, that, within the uncertainties of the approach, has a minimum value when $m_g/\Lambda_{QCD}$ is in the range $1.2 \, - \, 1.4$. These values are in agreement with other phenomenological determinations of this ratio and lead to an infrared effective charge $\alpha_s(0) \approx 0.7$. We comment how this effective charge may affect the global duality mass scale that indicates the frontier between perturbative and non-perturbative physics.Comment: 8 pages, 8 figures. Type and language correction. New references. Version to be publishe

QCD fixed points: Banks-Zaks scenario or dynamical gluon mass generation?

Fixed points in QCD can appear when the number of quark flavors ($N_f$) is increased above a certain critical value as proposed by Banks and Zaks (BZ). There is also the possibility that QCD possess an effective charge indicating an infrared frozen coupling constant. In particular, an infrared frozen coupling associated to dynamical gluon mass generation (DGM) does lead to a fixed point even for a small number of quarks. We compare the BZ and DGM mechanisms, their $\beta$ functions and fixed points, and within the approximations of this work, which rely basically on extrapolations of the dynamical gluon masses at large $N_f$, we verify that near the so called QCD conformal window both cases exhibit fixed points at similar coupling constant values ($g^*$). We argue that the states of minimum vacuum energy, as a function of the coupling constant up to $g^*$ and for several $N_f$ values, are related to the dynamical gluon mass generation mechanism.Comment: 9 pages, 9 figures, new references, improved discussion and small change in the title, to appear in IJMP

Medium effects in the pion pole mechanism (photon photon --> pion-zero --> neutrino-R antineutrino-L (neutrino-L antineutrino-R)) of neutron star cooling

Nuclear medium effects in the neutrino cooling of neutron stars through the exotic reaction channel \gamma \gamma --> \pi^0--> \nu_R \bar{\nu_L} (\nu_L \bar{\nu_R}) are incorporated. Throughout the paper we discuss different possibilities of right-handed neutrinos, massive left-handed neutrinos and standard massless left-handed neutrinos (reaction is then allowed only with medium modified vertices). It is demonstrated that multi-particle effects suppress the rate of this reaction channel by 6-7 orders of magnitude that does not allow to decrease existing experimental upper limit on the corresponding \pi^0\nu\bar{\nu} coupling. Other possibilities of the manifestation of the given reaction channel in differente physical situations, e.g. in the quark color superconducting cores of some neutron stars, are also discussed. We demonstrate that in the color-flavor-locked superconducting phase for temperatures T < (0.1-10) MeV (depending on the effective pion mass and the decay width) the process is feasibly the most efficient neutrino cooling process, although the absolute value of the reaction is rather small.Comment: Replaced with revised version. New appendix, many clarifying comments, corrected figs 3 and

Theoretical fits of the \delta Cephei light, radius and radial velocity curves

We present a theoretical investigation of the light, radius and radial velocity variations of the prototype $\delta$ Cephei. We find that the best fit model accounts for luminosity and velocity amplitudes with an accuracy better than $0.8\sigma$, and for the radius amplitude with an accuracy of $1.7\sigma$. The chemical composition of this model suggests a decrease in both helium (0.26 vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover, distance determinations based on the fit of light curves agree at the $0.8\sigma$ level with the trigonometric parallax measured by the Hubble Space Telescope (HST). On the other hand, distance determinations based on angular diameter variations, that are independent of interstellar extinction and of the $p$-factor value, indicate an increase of the order of 5% in the HST parallax.Comment: accepted for publication on ApJ Letter

Relating a gluon mass scale to an infrared fixed point in pure gauge QCD

We show that in pure gauge QCD (or any pure non-Abelian gauge theory) the condition for the existence of a global minimum of energy with a gluon (gauge boson) mass scale also implies the existence of a fixed point of the $\beta$ function. We argue that the frozen value of the coupling constant found in some solutions of the Schwinger-Dyson equations of QCD can be related to this fixed point. We also discuss how the inclusion of fermions modifies this property.Comment: 4 pages, Revtex - Added some clarifying comments and new reference

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La_{2-x}Sr_{x}CuO_4 point-contact junctions

We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 <= x <= 0.2. Andreev reflection features disappear at about the bulk Tc, giving no evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the Tc(x) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x <= 0.2.Comment: 4 pages, 4 eps figures, 1 table (RevTeX). Labels added to Fig. 1; Fig. 3 resized; references added; short discussion about ballistic contact regime adde

A genetic algorithm-assisted semi-adaptive MMSE multi-user detection for MC-CDMA mobile communication systems

In this work, a novel Minimum-Mean Squared-Error (MMSE) multi-user detector is proposed for MC-CDMA transmission systems working over mobile radio channels characterized by time-varying multipath fading. The proposed MUD algorithm is based on a Genetic Algorithm (GA)-assisted per-carrier MMSE criterion. The GA block works in two successive steps: a training-aided step aimed at computing the optimal receiver weights using a very short training sequence, and a decision-directed step aimed at dynamically updating the weights vector during a channel coherence period. Numerical results evidenced BER performances almost coincident with ones yielded by ideal MMSE-MUD based on the perfect knowledge of channel impulse response. The proposed GA-assisted MMSE-MUD clearly outperforms state-of-the-art adaptive MMSE receivers based on deterministic gradient algorithms, especially for high number of transmitting users

Expression of aldehyde dehydrogenase family 1, member A3 in glycogen trophoblast cells of the murine placenta

Introduction: Retinoic acid (RA) signaling is a well known regulator of trophoblast differentiation and placental development, and maternal decidual cells are recognized as the source of much of this RA. We explored possible trophoblast-derived sources of RA by examining the expression of RA synthesis enzymes in the developing mouse placenta, as well as addressed potential sites of RA action by examining the ontogeny of gene expression for other RA metabolizing and receptor genes. Furthermore, we investigated the effects of endogenous RA production on trophoblast differentiation