arXiv subjects
Publications and source records attributed to Nasrallah F. Nasrallah.
A method used previously to calculate the masses of the vector mesons is extended to the calculation of the nucleon resonances. The method is based on the choice of integration kernels which eliminate the unknown parts of the spectrum. We obtain remarkably stable results in a wide range of R, the radius of the integration contour in the complex plane. Agreement with experiment is good.
We present a calculation of the masses of the isovector mesons ( vector, scalar and pseudoscalar including the established recurrences) using a new method of finite energy QCD sum rules. The method is based on the idea of choosing a suitable integration kernel which minimizes the occurring integral over the cut in the complex energy (squared) plane. We obtain remarkably stable results in a wide range R, where R is the radius of the integration contour. The sum rule predictions agree with the experimental values within the expected accuracy showing that QCD describes single resonances.
A new QCD calculation of the mass of the nucleon is presented. It makes use of a polynomial kernel in the dispersion integrals tailored to practically eliminate the contribution of the unknown 1=2+ and 1=2- continuum. This approach avoids the arbitrariness and instability attached to the Borel kernel used in previous sum rules calculations. Our method yields stable results for the nucleon mass and coupling. For standard values of the condensates, the prediction of the nucleon mass in the chiral limit is mN = (830+/-50)MeV. With the pion-nucleon sigma-term given by chiral perturbation theory and the strange sigma-term estimated by the Zweig rule we get mN = (990+/-50)MeV.
QCD sum rules using polynomial kernels are used to evaluate the strong part of the proton-neutron mass difference DeltaM_np in a model independent fashion. The result for the mass difference turns out to depend sensitively on the value of the four quark condensate <(qq)^2> and reproduces the experimental value of DeltaM_np for <(qq)^2>~ 2 ^2.
The strange quark mass is determined from a new QCD Finite Energy Sum Rule (FESR) optimized to reduce considerably the systematic uncertainties arising from the hadronic resonance sector. As a result, the main uncertainty in this determination is due to the value of $Λ_{QCD}$. The correlator of axial-vector divergences is used in perturbative QCD to five-loop order, including quark and gluon condensate contributions, in the framework of both Fixed Order (FOPT), and Contour Improved Perturbation Theory (CIPT). The latter exhibits very good convergence, leading to a remarkably stable result in the very wide range $s_0 = 1.0 - 4.0 {GeV}^2$, where $s_0$ is the radius of the integration contour in the complex energy (squared) plane. The value of the strange quark mass in this framework at a scale of 2 GeV is $m_s(2 {GeV}) = 95 \pm 5 (111 \pm 6) {MeV}$ for $Λ_{QCD} = 420 (330) {MeV}$, respectively.