arXiv subjects

C. A. Dominguez

Publications and source records attributed to C. A. Dominguez.

At least 91 records · Page 5Linked to original sources

arXiv2005

Electromagnetic nucleon form factors from QCD sum rules

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, $F_1(q^2)$, 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 $F_2(q^2)$, which has a soft $q^2$-dependence proportional to the quark condensate $<0|\bar{q}q|0>$.

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arXiv2004

Electromagnetic proton form factors in large $N_{c}$ QCD

The electromagnetic form factors of the proton are obtained using a particular realization of QCD in the large $N_c$ limit (${QCD}_{\infty}$), which sums up the infinite number of zero-width resonances to yield an Euler's Beta function (Dual-${QCD}_{\infty}$). The form factors $F_1(q^2)$ and $F_2(q^2)$, as well as $G_M(q^2)$ agree very well with reanalyzed space-like data in the whole range of momentum transfer. In addition, the predicted ratio $μ_p G_E/G_M$ is in good agreement with recent polarization transfer measurements at Jefferson Lab.

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arXiv2003

Finite energy chiral sum rules in QCD

The saturation of QCD chiral sum rules of the Weinberg-type is analyzed using ALEPH and OPAL experimental data on the difference between vector and axial-vector correlators (V-A). The sum rules exhibit poor saturation up to current energies below the tau-lepton mass. A remarkable improvement is achieved by introducing integral kernels that vanish at the upper limit of integration. The method is used to determine the value of the finite remainder of the (V-A) correlator, and its first derivative, at zero momentum: $\barΠ(0) = - 4 \bar{L}_{10} = 0.0257 \pm 0.0003 ,$ and $\barΠ^{\prime}(0) = 0.065 \pm 0.007 {GeV}^{-2}$. The dimension $d=6$ and $d=8$ vacuum condensates in the Operator Product Expansion are also determined: $<{\cal {O}}_{6}> = -(0.004 \pm 0.001) {GeV}^6,$ and $<{\cal {O}}_{8}> = -(0.001 \pm 0.006) {GeV}^8.$

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arXiv2001

Ratio of strange to non-strange quark condensates in QCD

Laplace transform QCD sum rules for two-point functions related to the strangeness-changing scalar and pseudoscalar Green's functions $ψ(Q^2)$ and $ψ_5(Q^2)$, are used to determine the subtraction constants $ψ(0)$ and $ψ_5(0)$, which fix the ratio $R_{su}\equiv \frac{<\bar{s}s>}{<\bar{u}u>}$. Our results are $ψ(0)= - (1.06 \pm 0.21) \times 10^{-3} {GeV}^4$, $ψ_5(0)= (3.35 \pm 0.25) \times 10^{-3} {GeV}^4$, and $R_{su}\equiv \frac{<\bar{s}s>}{<\bar{u}u>} = 0.5 \pm 0.1$. This implies corrections to kaon-PCAC at the level of 50%, which although large, are not inconsistent with the size of the corrections to Goldberger-Treiman relations in $SU(3)\otimes SU(3)$.

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arXiv2001

Pion form factor in large $N_c$ QCD

The electromagnetic form factor of the pion is obtained using a particular realization of QCD in the large $N_c$ limit, which sums up the infinite number of zero-width resonances to yield an Euler's Beta function of the Veneziano type. This form factor agrees with space-like data much better than single rho-meson dominance. A simple unitarization ansatz is illustrated, and the resulting vector spectral function in the time-like region is shown to be in reasonable agreement with the ALEPH data from threshold up to about 1.3 ${GeV}^2$.

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arXiv2000

Vector Meson Dominance, Axial Anomaly and the Thermal behavior of $g_{ρωπ}(T)$

By using a thermal Finite Energy QCD Sum Rule, we are able to establish the temperature dependence of the $g_{ωρπ}(T)$ strong coupling. It turns out that this coupling decreases as a function of temperature, vanishing at the critical temperature. This corresponds to a possible deconfining phenomenological signal. This result, together with the Vector Meson Dominance (VMD) expression for the amplitude $π^{0} \to γγ$, allows us to establish that this amplitude also vanishes at the critical temperature, in agreement with previous independent analysis. This results supports, once again, the validity of VMD at finite temperature. Several posssible scenarios are discussed. However, if VMD would not hold at finite temperature, then we will not be able to find a prediction for the thermal behavior of the $π^{0} \to γγ$ amplitude.

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arXiv2000

Is there evidence for dimension-two corrections in QCD two-point functions?

The ALEPH data on the (non-strange) vector and axial-vector spectral functions, extracted from tau-lepton decays, is used in order to search for evidence for a dimension-two contribution, $C_{2 V,A}$, to the Operator Product Expansion (other than $d=2$ quark mass terms). This is done by means of a dimension-two Finite Energy Sum Rule, which relates QCD to the experimental hadronic information. The average $C_{2} \equiv (C_{2V} + C_{2A})/2$ is remarkably stable against variations in the continuum threshold, but depends rather strongly on $Λ_{QCD}$. Given the current wide spread in the values of $Λ_{QCD}$, as extracted from different experiments, we would conservatively conclude from our analysis that $C_{2}$ is consistent with zero.

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arXiv2000

Axial anomaly, vector meson dominance and $π^0 \to γγ$ at finite temperature

A thermal Finite Energy QCD Sum Rule is used to determine the temperature behaviour of the $ωρπ$ strong coupling. This coupling decreases with increasing $T$ and vanishes at the critical temperature, a likely signal for quark deconfinement. This is then used in the Vector Meson Dominance (VMD) expression for the $π^0 \to γγ$ amplitude, which is also found to vanish at the critical temperature, as expected. This result supports the validity of VMD at $T \neq 0$. However, if VMD would not hold at finite temperature, then there is no prediction for the $π^0 \to γγ$ amplitude.

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arXiv1999

QCD determination of the axial-vector coupling of the nucleon at finite temperature

A thermal QCD Finite Energy Sum Rule (FESR) is used to obtain the temperature dependence of the axial-vector coupling of the nucleon, $g_{A}(T)$. We find that $g_{A}(T)$ is essentially independent of $T$, in the very wide range $0 \leq T \leq 0.9 T_{c}$, where $T_{c}$ is the critical temperature. While $g_{A}$ at T=0 is $q^{2}$-independent, it develops a $q^{2}$ dependence at finite temperature. We then obtain the mean square radius associated with $g_{A}$ and find that it diverges at $T=T_{c}$, thus signalling quark deconfinement. As a byproduct, we study the temperature dependence of the Goldberger-Treiman relation.

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arXiv1999

Finite energy chiral sum rules in QCD

A set of well known chiral sum rules, expected to be valid in QCD, is confronted with experimental data on the vector and axial-vector hadronic spectral functions, obtained from tau-lepton decay by the ALEPH collaboration. The Das-Mathur-Okubo sum rule, the first and second Weinberg sum rules, and the electromagnetic pion mass difference sum rule are not well saturated by the data. Instead, a modified set of sum rules having additional weight factors that vanish at the end of the integration range on the real axis, is found to be precociously saturated by the data to a remarkable extent.

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arXiv1998

Chiral sum rules and duality in QCD

The ALEPH data on the vector and axial-vector spectral functions, extracted from tau-lepton decays is used in order to test local and global duality, as well as a set of four QCD chiral sum rules. These are the Das-Mathur-Okubo sum rule, the first and second Weinberg sum rules, and a relation for the electromagnetic pion mass difference. We find these sum rules to be poorly saturated, even when the upper limit in the dispersion integrals is as high as $3 GeV^{2}$. Since perturbative QCD, plus condensates, is expected to be valid for $|q^{2}| \geq \cal{O}$$(1 GeV^{2})$ in the whole complex energy plane, except in the vicinity of the right hand cut, we propose a modified set of sum rules with weight factors that vanish at the end of the integration range on the real axis. These sum rules are found to be precociously saturated by the data to a remarkable extent. As a byproduct, we extract for the low energy renormalization constant $\bar{L}_{10}$ the value $- 4 \bar{L}_{10}= 2.43 \times 10^{-2}$, to be compared with the standard value $- 4 \bar{L}_{10} = (2.73 \pm 0.12) \times 10^{-2}$. This in turn implies a pion polarizability $α_{E} = 3.7 \times 10^{-4} fm^{3}$.

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arXiv1998

Determination of the strange-quark mass from QCD pseudoscalar sum rules

A new determination of the strange-quark mass is discussed, based on the two-point function involving the axial-vector current divergences. This Green function is known in perturbative QCD up to order O(alpha_s^3), and up to dimension-six in the non-perturbative domain. The hadronic spectral function is parametrized in terms of the kaon pole, followed by its two radial excitations, and normalized at threshold according to conventional chiral-symmetry. The result of a Laplace transform QCD sum rule analysis of this two-point function is: m_s(1 GeV^2) = 155 pm 25 MeV.

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arXiv1998

Pion-Nucleon coupling at finite temperature

The pion nucleon vertex function at finite temperature is studied in the framework of: (a) the thermal (linear) sigma model to leading (one-loop) order, and (b) a thermal QCD-Finite Energy Sum Rule. Results from both methods indicate that the strength of the pion-nucleon coupling decreases with increasing T, vanishing at a critical temperature. The associated mean-square radius is a monotonically increasing function of T, diverging at the critical temperature. This is interpreted as (analytical) evidence for the quark-gluon deconfinement phase transition.

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arXiv1997

The strange-quark mass from QCD sum rules in the pseudoscalar channel

QCD Laplace transform sum rules, involving the axial-vector current divergences, are used in order to determine the strange quark mass. The two-point function is known in QCD up to four loops in perturbation theory, and up to dimension-six in the non-perturbative sector. The hadronic spectral function is reconstructed using threshold normalization from chiral symmetry, together with experimental data for the two radial excitations of the kaon. The result for the running strange quark mass, in the $\bar{MS}$ scheme at a scale of 1 ${GeV}^{2}$ is: ${\bar m}_{s}(1 GeV^{2}) = 155 \pm 25 {MeV}$.

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arXiv1997

The $ρ$ Meson and the Thermal Behavior of an Effective Hadronic Coupling Constant

Vector Meson Dominance ideas together with a Finite Energy QCD sum rule allows for the determination of the $q^{2}$- and the $T$- dependence of the effective hadronic coupling constant $g_{ρππ}$ in the space-like region. It turns out that $g_{ρππ}(q^{2},T)$ vanishes at the critical temperature $T_{c}$, independently of $q^{2}$. A comparison with a previous independent QCD determination of the electromagnetic pion form factor at finite temperature supports the validity of Vector Meson Dominance at finite temperature. We find also thet the pion radius increases with $T$, having a divergent behavior at $T_{c}$.

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arXiv1997

The strange-quark mass from QCD sum rules: An Update

Three different ways of determining the strange quark mass using QCD sum rules are reviewed. First, from a QCD sum rule determination of the up and down quark masses, together with the current algebra ratio $ m_{s}/(m_{u}+m_{d})$. Second and third, from QCD sum rules involving the strangeness changing vector and axial-vector current divergences, respectively. Present results are encompassed in the value: $m_{s} {(1 GeV)} = 150 \pm {70 MeV}$. It is argued that until direct, and precision measurements of the relevant spectral functions become available, the above error is not likely to be reduced.

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arXiv1997

Vector Meson Dominance and $g_{ρππ}$ at Finite Temperature from QCD Sum Rules

A Finite Energy QCD sum rule at non-zero temperature is used to determine the $q^2$- and the T-dependence of the $ρππ$ vertex function in the space-like region. A comparison with an independent QCD determination of the electromagnetic pion form factor $F_π$ at $T \neq 0$ indicates that Vector Meson Dominance holds to a very good approximation at finite temperature. At the same time, analytical evidence for deconfinement is obtained from the result that $g_{ρππ}(q^{2},T)$ vanishes at the critical temperature $T_c$, independently of $q^{2}$. Also, by extrapolating the $ρππ$ form factor to $q^2 = 0$, it is found that the pion radius increases with increasing $T$, and it diverges at $T=T_c$.

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arXiv1997

Violations of Local Duality in the Heavy Quark Sector

We examine the origin of possible failures of the quark-hadron local duality. In particular, we consider a correlator of two currents comprising heavy quark operators, and compare the evaluation by the Operator Product Expansion with the result obtained inserting an infinite set of hadronic states, in the N_c to infinity limit. Whereas the smeared spectral functions agree with each other, their local behaviour is different. The difference manifests itself in the form of a term O(Lambda/epsilon) (epsilon being the residual energy) that is not present in the expansion in powers of Lambda/epsilon predicted by the OPE.

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