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B. Saghai

Publications and source records attributed to B. Saghai.

At least 19 recordsLinked to original sources
arXiv2019

Orbital angular momentum of the proton and intrinsic five-quark Fock states

The orbital angular momentum ($L_q$) of the proton is studied by employing the extended constituent quark model. Contributions from different flavors, namely, up, down, strange, and charm quarks in the proton are investigated. Probabilities of the intrinsic $q\bar{q}$ pairs are calculated using a $^{3}P_{0}$ transition operator to fit the sea flavor asymmetry $I_a=\bar{d}-\bar{u}=0.118\pm0.012$ of the proton. Our numerical results lead to $L_q=0.158 \pm 0.014$, in agreement with $4/3I_a=0.157 \pm 0.016$, and consistent with findings based on various other approaches.

hep-ph↗
arXiv2017

Intrinsic light and strange quark--antiquark pairs in the proton and nonperturbative strangeness suppression

The CLAS Collaboration recently reported measured ratios of pion and kaon electroproduction cross sections from a proton target and extracted the ratios for light and strange quark--antiquark pairs, ${u\bar{u}}/ {d\bar{d}}$ and ${s\bar{s}}/ {d\bar{d}}$. Within an extended chiral constituent quark formalism, we investigate contributions to those ratios from the nonperturbative mechanism due to all possible intrinsic $|uudQ \bar{Q}\rangle$ Fock states in the proton; with $Q \bar{Q} \equiv u\bar{u},~ d\bar{d},~s\bar{s},~c\bar{c}$. Our results are compared with the CLAS data and findings from other phenomenological approaches, offering insights into the manifestations of the genuine five--quark Fock states in the proton and its relevance to interpreting the experimental data.

hep-ph↗
arXiv2016

Intrinsic charm content of the nucleon and charmness-nucleon sigma term

In the extended chiral constituent quark model, the intrinsic $c \bar{c}$ content of the nucleon is investigated. The probabilities of the quark-antiquark components in the nucleon wave functions are calculated by taking the nucleon to be admixtures of three- and five-quark components, with the relevant transitions handled {\it via} the $^{3}$P$_{0}$ mechanism. Predictions for the probability of the $c \bar{c}$ in the nucleon wave function and the charmness-nucleon sigma term are presented. Our numerical results turn out to be consistent with the predictions from various other approaches reported in the literature.

hep-ph↗
arXiv2015

Pion- and strangeness-baryon $σ$ terms in the extended chiral constituent quark model

Within an extended chiral constituent quark formalism, we investigate contributions from all possible five-quark components in the octet baryons to the pion-baryon ($σ_{πB}$) and strangeness-baryon ($σ_{s B}$) sigma terms; $B \equiv N,~Λ,~Σ,~Ξ$. The probabilities of the quark-antiquark components in the ground-state baryon octet wave functions are calculated by taking the baryons to be admixtures of three- and five-quark components, with the relevant transitions handled {\it via} the $^{3}$P$_{0}$ mechanism. Predictions for $σ_{πB}$ and $σ_{s B}$ obtained by using input parameters taken from the literature are reported. Our results turn out to be, in general, consistent with the findings via lattice QCD and chiral perturbation theory.

hep-ph↗
arXiv2015

Configuration Mixing of Quark States in Baryons

Mixing angles are used to describe the $SU(6)\otimes O(3)$ symmetry breaking in $\left[70,~1^-\right]$ multiplet in the sector of the lowest mass nucleon resonances, which are investigated extensively in constituent quark models for baryon spectroscopy. The transition amplitudes for the meson photoproduction off nucleon can also be expressed in terms of the mixing angles to take into account the configuration mixing. Those amplitudes are derived as a function of the mixing angles between $|N^2P_M{J}^-\rangle$ and $|N^4P_M{J}^-\rangle$ states, with $J$=1/2 and 3/2, for the processes $γp \to ηp,~K^+ Λ,~K^+ Σ^\circ,~K^\circ Σ^+$. The present status of our knowledge on the mixing angles between $S_{11}(1535)$ and $S_{11}(1650)$ ($θ_{S}$), as well as between $D_{13}(1520)$ and $D_{13} (1700)$ ($θ_{D}$) is reported. Since these resonances play very important role in the threshold region for both $η$ and kaon production mechanisms, they are expected to provide crucial tests of different quark models for the baryon spectroscopy.

hep-ph↗
arXiv2013

Strangeness magnetic form factor of the proton in the extended chiral quark model

Background: Unravelling the role played by nonvalence flavors in baryons is crucial in deepening our comprehension of QCD. Strange quark, a component of the higher Fock states in baryons, is an appropriate tool to investigate nonperturbative mechanisms generated by the pure sea quark. Purpose: Study the magnitude and the sign of the strangeness magnetic moment $μ_s$ and the magnetic form factor ($G_M^s$) of the proton. Methods: Within an extended chiral constituent quark model, we investigate contributions from all possible five-quark components to $μ_s$ and $G_M^s (Q^2)$ in the four-vector momentum range $Q^2 \leq 1$ (GeV/c)$^2$. Probability of the strangeness component in the proton wave function is calculated employing the $^3 P_0$ model. Results: Predictions are obtained without any adjustable parameters. Observables $μ_s$ and $G_M^s (Q^2)$ are found to be small and negative, consistent with the lattice-QCD findings as well as with the latest data released by the PVA4 and HAPPEX Collaborations. Conclusions: Due to sizeable cancelations among different configurations contributing to the strangeness magnetic moment of the proton, it is indispensable to (i) take into account all relevant five-quark components and include both diagonal and non-diagonal terms, (ii) handle with care the oscillator harmonic parameter $ω_5$ and the ${s \bar s}$ component probability.

hep-ph↗
arXiv2013

Faddeev-Chiral Unitary Approach to the $K^- d$ scattering length

Our earlier Faddeev three-body study in the $K^-$-deuteron scattering length, $A_{K^-d}$, is revisited here in the light of the recent developments in two fronts: {\it (i)} the improved chiral unitary approach to the theoretical description of the coupled $\Kbar N$ related channels at low energies, and {\it (ii)} the new and improved measurement from SIDDHARTA Collaboration of the strong interaction energy shift and width in the lowest $K^-$-hydrogen atomic level. Those two, in combination, have allowed us to produced a reliable two-body input to the three-body calculation. All available low-energy $K^-p$ observables are well reproduced and predictions for the $\Kbar N$ scattering lengths and amplitudes, $(πΣ)^\circ$ invariant-mass spectra, as well as for $A_{K^-d}$ are put forward and compared with results from other sources. The findings of the present work are expected to be useful in interpreting the forthcoming data from CLAS, HADES, LEPS and SIDDHARTA Collaborations.

hep-ph↗
arXiv2010

$η$ production off the proton in a Regge-plus-chiral quark approach

A chiral constituent quark model approach, embodying s- and u-channel exchanges,complemented with a Reggeized treatment for t-channel is presented. A model is obtained allowing data for $π^- p \to ηn$ and $γp \to ηp$ to be describe satisfactorily. For the latter reaction, recently released data by CLAS and CBELSA/TAPS Collaborations in the system total energy range $1.6 \lesssim W \lesssim 2.8$ GeV are well reproduced due to the inclusion of Reggeized trajectories instead of simple $ρ$ and $ω$ poles. Contribution from "missing" resonances is found to be negligible in the considered processes.

nucl-th↗
arXiv2010

Role of five-quark components in radiative and strong decays of the Lambda(1405) resonance

Within an extended chiral constituent quark model, three- and five-quark structure of the $S_{01}$ resonance $Λ(1405)$ is investigated. Helicity amplitudes for the electromagnetic decays ($Λ(1405) \to Λ(1116)γ$, $Σ(1194)γ$), and transition amplitudes for strong decays ($Λ(1405)\toΣ(1194)π$, $ K^{-}p$) are drived, as well as the relevant decay widths. The experimental value for the strong decay width, $Γ_{Λ(1405)\to (Σπ)^\circ}=50\pm 2$ MeV, is well reproduced with about 50% of five-quark admixture in the $Λ(1405)$. Important effects due to the configuration mixings among $Λ^{2}_{1}P_{A}$, $Λ^{2}_{8}P_{M}$ and $Λ^{4}_{8}P_{M}$ are found. In addition, transitions between the three- and five-quark components in the baryons turn out to be significant in both radiative and strong decays of the $Λ(1405)$ resonance.

nucl-th↗
arXiv2009

Combined study of gamma p to eta p and pi-p to eta n in a chiral constituent quark approach

Within a chiral constituent quark model approach, $η$-meson production on the proton {\it via} electromagnetic and hadron probes are studied. With few parameters, differential cross-section and polarized beam asymmetry for γp \to ηp$ and differential cross section for $π^- p\to ηn$ processes are calculated and successfully compared with the data in the center-of-mass energy range from threshold up to 2 GeV. The five known resonances $S_{11}(1535)$, $S_{11}(1650)$, $P_{13}(1720)$, $D_{13}(1520)$, and $F_{15}(1680)$ are found to be dominant in the reaction mechanisms in both channels. Possible roles plaied by new resonances are also investigated and in the photoproduction channel, significant contribution from $S_{11}$- and $D_{15}$-resonances, with masses around 1715 MeV and 2090 MeV, respectively, are deduced. For the so-called missing resonances, no evidence is found within the investigated reactions. The helicity amplitudes and decay widths of $N^* \to πN, ηN$ are also presented, and found consistent with the PDG values.

nucl-th↗
arXiv2008

Insights into the pi- p -> eta n reaction mechanism

A dynamical coupled-channels formalism is used to investigate the $η-$meson production mechanism on the proton induced by pions, in the total center-of-mass energy region from threshold up to 2 GeV. We show how and why studying exclusively total cross section data might turn out to be misleading in pinning down the reaction mechanism.

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arXiv2008

Coupled-channels study of the $π^{-}p \to ηn$ process

The reaction $π^{-}p \to ηn$ is investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson baryon channels included are $πN$, $ηN$, $πΔ$, $σN$, and $ρN$. The non-resonant meson-baryon interactions of the model are derived from a set of Lagrangians by using a unitary transformation method. One or two excited nucleon states in each of $S$, $P$, $D$, and $F$ partial waves are included to generate the resonant amplitudes. Data of $π^{-}p \to ηn$ reaction from threshold up to a total center-of-mass energy of about 2 GeV are satisfactorily reproduced and the roles played by the following nine nucleon resonances are investigated: $S_{11}(1535)$, $S_{11}(1650)$, $P_{11}(1440)$, $P_{11}(1710)$, $P_{13}(1720)$, $D_{13}(1520)$, $D_{13}(1700)$, $D_{15}(1675)$, and $F_{15}(1680)$. The reaction mechanism as well as the predicted $ηN$ scattering length are discussed.

nucl-th↗
arXiv2008

Study of $η$ photoproduction on the proton in a chiral constituent quark approach via one-gluon-exchange model

A formalism based on a chiral quark model ($χ$QM) approach complemented with a one-gluon exchange model, to take into account the breakdown of the $SU(6)\otimes O(3)$ symmetry, is presented. The configuration mixing of wave functions for nucleon and resonances are derived. % With few adjustable parameters, differential cross-section and polarized beam asymmetry for the $γp \to ηp$ process are calculated and successfully compared with the data in the centre-of-mass energy range from threshold up to 2 GeV. The known resonances $S_{11}(1535)$, $S_{11}(1650)$, $P_{13}(1720)$, $D_{13}(1520)$, and $F_{15}(1680)$, as well as two new $S_{11}$ and $D_{15}$ resonances are found to be dominant in the reaction mechanism. Besides, connections among the scattering amplitudes of the $χ$QM approach and the helicity amplitudes, as well as decay widths of resonances are established. Possible contributions from the so-called "missing resonances" are investigated and found to be negligible.

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arXiv2006

Dynamical coupled-channel approach to hadronic and electromagnetic production of kaon-hyperon on the proton

A dynamical coupled-channel formalism for processes $πN \to KY$ and $γN \to KY$ is presented which provides a comprehensive investigation of recent data on the $γp \to K^+ Λ$ reaction. The non-resonant interactions within the subspace $KY\oplusπN$ are derived from effective Lagrangians, using a unitary transformation method. The calculations of photoproduction amplitudes are simplified by casting the coupled-channel equations into a form such that the empirical $γN \to πN$ amplitudes are input and only the parameters associated with the $KY$ channel are determined by performing $χ^2$-fits to all of the available data for $π^- p \to K^\circΛ, K^\circΣ^\circ$ and $γp \to K^+Λ$. Good agreement between our models and those data are obtained. In the fits to $πN \to KY$ channels, most of the parameters are constrained within $\pm 20%$ of the values given by the Particle Data Group and/or quark model predictions, while for $γp \to K^+ Λ$ parameters, ranges compatible with broken $SU(6)\otimes O(3)$ symmetry are imposed. The main reaction mechanisms in $K^+ Λ$ photoproduction are singled out and issues related to newly suggested resonances $S_{11}$, $P_{13}$, and $D_{13}$ are studied. Results illustrating the importance of using a coupled-channel treatment are reported. Meson cloud effects on the $γN \to N^*$ transitions are also discussed.

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arXiv2005

Coupled channel study of $K^+Λ$ photoproduction

A coupled channel model with $γN$, $KY$ and $πN$ channels has been used to analyze the recent data of $γp \to K^+ Λ$. The non-resonant interactions within the subspace $KY \oplus πN$ are derived from effective Lagrangians using a unitary transformation method. The direct photoproduction reaction is obtained from a chiral constituent quark model with $SU(6)\otimes O(3)$ breaking. Missing baryon resonances issues are briefly discussed.

nucl-th↗
arXiv2005

Dynamical coupled-channel study of K+ Lambda photoproduction

Results for the reaction gamma p --> K+ Lambda, studied within a constituent quark model and a dynamical coupled-channel approach, are presented and compared with recent data. Issues related to the search for missing baryon resonances are briefly discussed and the role played by a third S_{11} resonance is underlined.

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