20 research outputs found

    Electromagnetic proton form factors in large Nc QCD

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    The electromagnetic form factors of the proton are obtained using a particular realization of QCD in the large Nc limit (QCD∞), which sums up the infinite number of zero-width resonances to yield an Euler's Beta function (Dual-QCD∞). The form factors F1(q2) and F2(q2), as well as GM(q2) agree very well with reanalyzed space-like data in the whole range of momentum transfer. In addition, the predicted ratio μpGE/GM is in good agreement with recent polarization transfer measurements at Jefferson Lab

    Electromagnetic nucleon form factors from QCD sum rules

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    The electromagnetic form factors of the nucleon, in the space-like region, are determined from three-point function Finite Energy QCD Sum Rules. The QCD calculation is performed to leading order in perturbation theory in the chiral limit, and to leading order in the non-perturbative power corrections. The results for the Dirac form factor, F1(q2), are in very good agreement with data for both the proton and the neutron, in the currently accessible experimental region of momentum transfers. This is not the case, though, for the Pauli form factor F2(q2), which has a soft q2-dependence proportional to the quark condensate h0|q¯q|0i

    Semileptonic charm meson decays and the matrix elements |Vcs| and |Vcd|

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    Abstract Using QCD sum rules for a two-point function involving charmed vector currents we determine the Dl3 form factor: ƒ + (0)=0.75±0.05 . This result, combined with the Dl3 decay widths, leads to a prediction for the quark mixing matrix elements |Vcs| and |Vcd|. We find: |Vcs|=0.96±0.12 and Γ( D →πl ν l )=(0.76±0.24)×10 11 [ |V cd | (0.21±0.03) ] 2 s −1 . Our estimate is reliable to the extent that employing the same technique for Kl3 decay we obtain: ƒ + (0)| Kl3 =0.96+-0.13

    Pion form factor in the Kroll-Lee-Zumino model

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    The renormalizable Abelian quantum field theory model of Kroll, Lee, and Zumino is used to compute the one-loop vertex corrections to the tree-level, Vector Meson Dominance (VMD) pion form factor. These corrections, together with the known one-loop vacuum polarization contribution, lead to a substantial improvement over VMD. The resulting pion form factor in the space-like region is in excellent agreement with data in the whole range of accessible momentum transfers. The time-like form factor, known to reproduce the Gounaris-Sakurai formula at and near the rho-meson peak, is unaffected by the vertex correction at order O\cal{O}(g_\rpp^2).Comment: Revised version corrects a misprint in Eq.(1

    Observability of an induced electric dipole moment of the neutron from nonlinear QED

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    It has been shown recently that a neutron placed in an external quasistatic electric field develops an induced electric dipole moment pIND due to quantum fluctuations in the QED vacuum. A feasible experiment which could detect such an effect is proposed and described here. It is shown that the peculiar angular dependence of pIND on the orientation of the neutron spin leads to a characteristic asymmetry in polarized neutron scattering by heavy nuclei. This asymmetry can be of the order of 10⁻³ for neutrons with epithermal energies. For thermalized neutrons from a hot moderator, one still expects experimentally accessible values of the order of 10⁻⁴. The contribution of the induced effect to the neutron scattering length is expected to be only 1 order of magnitude smaller than that due to the neutron polarizability from its quark substructure. The experimental observation of this scattering asymmetry would be the first ever signal of nonlinearity in electrodynamics due to quantum fluctuations in the QED vacuum.Facultad de Ciencias ExactasInstituto de Física La Plat

    Heavy-light quark pseudoscalar and vector mesons at finite temperature

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    The temperature dependence of the mass, leptonic decay constant, and width of heavy-light quark peseudoscalar and vector mesons is obtained in the framework of thermal Hilbert moment QCD sum rules. The leptonic decay constants of both pseudoscalar and vector mesons decrease with increasing TT, and vanish at a critical temperature TcT_c, while the mesons develop a width which increases dramatically and diverges at TcT_c, where TcT_c is the temperature for chiral-symmetry restoration. These results indicate the disappearance of hadrons from the spectral function, which then becomes a smooth function of the energy. This is interpreted as a signal for deconfinement at T=TcT=T_c. In contrast, the masses show little dependence on the temperature, except very close to TcT_c, where the pseudoscalar meson mass increases slightly by 10-20 %, and the vector meson mass decreases by some 20-30

    Chiral condensates from tau decay: a critical reappraisal

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    The saturation of QCD chiral sum rules is reanalyzed in view of the new and complete analysis of the ALEPH experimental data on the difference between vector and axial-vector correlators (V-A). Ordinary finite energy sum rules (FESR) exhibit poor saturation up to energies below the tau-lepton mass. A remarkable improvement is achieved by introducing pinched, as well as minimizing polynomial integral kernels. Both methods are used to determine the dimension d=6 and d=8 vacuum condensates in the Operator Product Expansion, with the results: {O}_{6}=-(0.00226 \pm 0.00055) GeV^6, and O_8=-(0.0053 \pm 0.0033) GeV^8 from pinched FESR, and compatible values from the minimizing polynomial FESR. Some higher dimensional condensates are also determined, although we argue against extending the analysis beyond dimension d = 8. The value of the finite remainder of the (V-A) correlator at zero momentum is also redetermined: \Pi (0)= -4 \bar{L}_{10}=0.02579 \pm 0.00023. The stability and precision of the predictions are significantly improved compared to earlier calculations using the old ALEPH data. Finally, the role and limits of applicability of the Operator Product Expansion in this channel are clarified.Comment: Replaced versio

    Strange quark condensate from QCD sum rules to five loops

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    It is argued that it is valid to use QCD sum rules to determine the scalar and pseudoscalar two-point functions at zero momentum, which in turn determine the ratio of the strange to non-strange quark condensates Rsu=R_{su} = \frac{}{} with (q=u,dq=u,d). This is done in the framework of a new set of QCD Finite Energy Sum Rules (FESR) that involve as integration kernel a second degree polynomial, tuned to reduce considerably the systematic uncertainties in the hadronic spectral functions. As a result, the parameters limiting the precision of this determination are ΛQCD\Lambda_{QCD}, and to a major extent the strange quark mass. From the positivity of RsuR_{su} there follows an upper bound on the latter: msˉ(2GeV)121(105)MeV\bar{m_{s}} (2 {GeV}) \leq 121 (105) {MeV}, for ΛQCD=330(420)MeV.\Lambda_{QCD} = 330 (420) {MeV} .Comment: Minor changes to Sections 2 and
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