arXiv subjects

S. Mohammad Zebarjad

Publications and source records attributed to S. Mohammad Zebarjad.

7 recordsLinked to original sources

arXiv2022

Chiral Nonet Mixing in $πη$ Scattering

The generalized linear sigma model for mixing among two- and four-quark components of scalar (and psedudosclar) mesons below and above 1 GeV is applied to the $πη$ channel in which the isovector scalars $a_0(980)$ and $a_0(1450)$ are probed. In the leading order, the model parameters have been previously fixed by various low-energy experimental data, and then applied to $ππ$ and $πK$ channels in which the properties of the light and broad $σ$ and $κ$ mesons are extracted in agreement with estimates reported in the literature. With the same parameters fixed in the leading order, in the present work the prediction of the model for the $πη$ scattering amplitude in the elastic region is given and unitarized with the K-matrix method. The poles of the unitarized scattering amplitude, which determine the mass and decay width of $a_0(980)$ and $a_0(1450)$ are computed. It is found that the model predicts an isovector scalar state below 1 GeV, with mass 984 $\pm$ 6 MeV and decay width 108 $\pm$ 30 MeV which is a clear signal for the $a_0(980)$. The $a_0$ pole extracted in this work, further supports the plausibility of the mixing patterns for scalar mesons predicted by this model according to which there is a significant underlying mixing among scalars below and above 1 GeV, with those below 1 GeV being generally of four-quark nature while those above 1 GeV being overall closer to quark-antiquark states. Predictions for various scattering lengths as well as for properties of $a_0(1450)$ are also presented.

hep-ph↗

arXiv2018

Mixing among lowest-lying scalar mesons and scalar glueball

Scalar glueball is implemented in single nonet linear sigma model (SNLSM) which basically includes lowest lying scalar and pseudoscalar mesons. Our new version of SNLSM involves mixing among scalar matter fields and glueball field which is enforced by scale symmetry considerations and the associated anomaly. Performing iterative Monte Carlo simulations, it is found that among the three candidates of scalar glueball, i.e., $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, only $f_0(1500)$ is predominately a glueball state with the mass prediction of $1.566 \pm 0.046$ GeV and the other two are predominately quarkonium. The $ππ$, $πK$ and $πη$ scatterings are reinvestigated in the presence of scalar glueball and it is found that the overall behavior of the real part of the K-matrix unitarized $ππ$ scattering amplitude is more compatible with observed data compared with the case of SNLSM without glueball. We have also presented the predictions of the model for the masses and decay widths of the scalars obtained from the poles of the K-matrix unitarized $ππ$, $πK$ and $πη$ scattering amplitudes. Moreover the $ππ$ scattering phase shift predicted by our model is compared with the prediction of generalized linear sigma model (GLSM) which contains two nonets of scalar mesons and two nonets of pseudoscalar mesons (a quark-antiquark nonet and a four-quark nonet). Despite the fact that at this stage our model lacks the second meson nonet above $1$ GeV, its prediction for the $ππ$ scattering phase shift is close to the prediction of GLSM for $\sqrt{s}<1.1$ GeV and in better agreement with experimental data for $\sqrt{s}>1.1$ GeV in comparison with GLSM.

hep-ph↗

arXiv2016

Chiral Nonet Mixing in $πK$ Scattering

The underlying mixing of quark components of scalar mesons is probed in $πK$ scattering within a generalized linear sigma model that contains two scalar meson nonets and two pseudoscalar meson nonets (a quark-antiquark and a four-quark). In the leading order of this model, all free parameters have been previously fixed using the mass spectra and several low-energy parameters known from experiment and consistent predictions have been made. As other predictions of the model, in the present work the isospins $I$= 1/2, 3/2 and $J$=0 projection of $πK$ scattering amplitude (as well as phase shifts) are computed and compared with experiment. In the $I$=1/2 channel, it is shown that within the uncertainties of the model parameters a good agreement with experimental data up to an energy of about 1 GeV is obtained, whereas in the $I$=3/2 channel there is a better agreement with experiment which extends to about 1.4 GeV. The effect of final state interactions of $πK$ in the $I$=1/2 channel is approximated by the K-matrix method and the poles of the unitarized scattering amplitude are found. It is shown that the model predicts a light and broad kappa resonance with a mass and decay width of 670-770 MeV and 640-750 MeV consistent with other prior works. Moreover, the scattering lengths in the $I$=1/2, 3/2 are also computed and shown to qualitatively agree with experiment. The overall predictions presented here further support previous findings that the scalar mesons below and above 1 GeV have substantial underlying mixings and that those below 1 GeV have dominant four-quark substructures while those above 1 GeV are closer to conventional $P$-wave quark-antiquark states.

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arXiv2015

Two-body decay widths of lowest lying and next-to-lowest lying scalar and pseudoscalar mesons in generalized linear sigma model

Two-body decay widths of lowest lying and next-to-lowest lying scalar and pseudoscalar mesons are studied in Generalized Linear Sigma Model (GLSM) of low-energy QCD. This model which considers mixing between "two quark" and "four quark" chiral nonets has been employed to investigate various decays and scatterings in low energy region of QCD. In this paper, $Γ[f_0(980) \rightarrow K \bar{K}]$ and $Γ[a_0(980) \rightarrow K \bar{K}]$ are obtained and it is shown that two-body decay widths of lowest lying mesons are well predicted by this model while for the next-to-lowest lying mesons, only some of the decay widths agree with the experimental results. We have compared the predicted decay widths in GLSM with the results obtained in single nonet linear sigma model (SNLSM) to indicate that chiral nonet mixing greatly improves the predictions of SNLSM for decay widths.

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arXiv2013

Charm Mass Determination from QCD Charmonium Sum Rules at Order alpha_s^3

We determine the MS-bar charm quark mass from a charmonium QCD sum rules analysis. On the theoretical side we use input from perturbation theory at O(alpha_s^3). Improvements with respect to previous O(alpha_s^3) analyses include (1) an account of all available e+e- hadronic cross section data and (2) a thorough analysis of perturbative uncertainties. Using a data clustering method to combine hadronic cross section data sets from different measurements we demonstrate that using all available experimental data up to c.m. energies of 10.538 GeV allows for determinations of experimental moments and their correlations with small errors and that there is no need to rely on theoretical input above the charmonium resonances. We also show that good convergence properties of the perturbative series for the theoretical sum rule moments need to be considered with some care when extracting the charm mass and demonstrate how to set up a suitable set of scale variations to obtain a proper estimate of the perturbative uncertainty. As the final outcome of our analysis we obtain m_c(m_c) = 1.282 \pm 0.006_stat \pm 0.009_syst \pm 0.019)_pert \pm 0.010_alpha \pm 0.002_GG GeV. The perturbative error is an order of magnitude larger than the one obtained in previous O(alpha_s^3) sum rule analyses.

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arXiv2008

Heavy Quark Vacuum Polarization Function at O(alpha_s^2) and O(alpha_s^3)

We determine the full mass and $q^2$ dependence of the heavy quark vacuum polarization function $Π(q^2)$ and its contribution to the total $e^+e^-$ cross section at ${\cal O}(α_s^2)$ and ${\cal O}(α_s^3)$ in perturbative QCD. We use known results for the expansions of $Π(q^2)$ at high energies, in the threshold region and around $q^2=0$, conformal mapping and the Padé approximation method. From our results for $Π(q^2)$ we determine numerically at ${\cal O}(α_s^3)$ the previously unknown non-logarithmic contributions in the high-energy expansion at order $(m^2/q^2)^i$ for $i=0,1$ and the coefficients in the expansion around $q^2=0$ at order $q^{2n}$ with $n\ge 2$. We also determine at ${\cal O}(α_s^2)$ the previously unknown ${\cal O}(v^0)$ constant term in the expansion of $Π(q^2)$ in the threshold region, where $v$ is the quark velocity. Our method allows for a quantitative estimate of uncertainties and can be systematically improved once more information in the three kinematic regions becomes available by future multi-loop computations. For the contributions to the total $e^+e^-$ cross section at ${\cal O}(α_s^2)$ we confirm results obtained earlier by Chetyrkin, Kühn and Steinhauser.

hep-ph↗

arXiv1996

Derivation of the Lamb Shift using an Effective Field Theory

We rederive the $O(α^5)$ shift of the hydrogen levels in the non-recoil ($m_e/m_P \to 0$) limit using Nonrelativistic QED (NRQED), an effective field theory developed by Caswell and Lepage (Phys. Lett. 167B, 437 (1986)). Our result contains the Lamb shift as a special case. Our calculation is far simpler than traditional approaches and has the advantage of being systematic. It also clearly illustrates the need to renormalize (or ``match'') the coefficients of the effective theory beyond tree level.

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