arXiv subjects

Ismail Zahed

Publications and source records attributed to Ismail Zahed.

At least 37 records · Page 2Linked to original sources

arXiv2025

Tomography of Pions and Kaons in the QCD Vacuum: Transverse Momentum Dependent Parton Distribution Functions

We evaluate the pion and kaon transverse momentum dependent parton distribution functions (TMDPDFs) in the instanton liquid model (ILM), a model of the QCD vacuum at low resolution. The relevant TMDs are factored into a constituent quark distribution times a rapidity dependent soft factor from staple-shaped Wilson lines, for fixed parton longitudinal momentum and transverse separation. The results are evolved to higher rapidities using the Collins-Soper (CS) kernel and higher resolution using renormalization group evolution. The comparison to existing extractions of pion TMDs from Drell-Yan (DY) data is briefly discussed.

hep-ph↗

arXiv2025

Pentaquarks made of light quarks and their admixture to baryons

This paper is a continuation of our studies of multiquark hadrons. The anti-symmetrization of their wavefunctions required by Fermi statistics is nontivial, as it mixes orbital, color, spin and flavor structures. In our previous papers we developed a method to find them based on the representations of the permutation group, and derived the explicit wave functions for baryons excited to the first and second shells $(L=1,2)$, tetraquarks $qq\bar q\bar q$ and hexaquarks ($6q$). Now we apply it to light pentaquarks ($qqqq\bar q$), in the S- and P-shells ($L=0,1$). Using Jacobi coordinates, one can use the hyperdistance approximation in 12-dimensional space. We further address the issue of ``unquenching" of baryons, by considering their mixing with pentaquarks, via two channels, through the addition of $σ$-like or $π$-like $\bar q q$ pairs. This mixing is central for understanding of the observed flavor asymmetry of the antiquark sea, the amount of orbital motion issue as well as other nucleon properties.

hep-ph↗

arXiv2025

D-shell mixing in light baryons and its effect on the orbital motion

The standard description of the nucleon in the non-relativistic quark model is an $1S,L=0$ state without orbital motion. Yet, there are several indications from phenomenology that an admixture of states with nonzero orbital motion maybe substantial. In this paper we focus on the ``second shell" of the nucleon excitations (D-shell), for which we give a modern description of the wave functions. We follow it by investigating what we call a ``maximal mixing" scenario, assuming a hypothetical long-range tensor force. We give the explicit wave functions for all states, before and after mixing, and re-assess many predictions such as the magnetic moments, the standard and transitional form-factors from the nucleon to $N^*$. Unexpectedly, in this scenario we can reproduce the long-puzzling features of the Roper resonance $N^*( 1440)$. But even in this extreme case, the admixture of the $1D,L=2$ state to a nucleon remains significantly smaller than expected from phenomenology.

hep-ph↗

arXiv2025

Universality and emergent effective fluid from jets and string breaking in the massive Schwinger model using tensor networks

We analyze the correlation between the energy, momentum and spatial entanglement produced by two luminal jets in the massive Schwinger model. Using tensor network methods, we show that for m/g > 1/π, in the vicinity of the strong to weak coupling transition, a nearly perfect and chargeless effective fluid behavior appears around the mid-rapidity region with a universal energy-pressure relationship. The evolution of energy and pressure is strongly correlated with the rise of the spatial entanglement entropy, indicating a key role of quantum dynamics. Some of these observations may be used to analyze high multiplicity jet fragmentation events, energy-energy and energy-charge correlators at current collider energies.

hep-ph↗

arXiv2025

Collins-Soper Kernel in the QCD Instanton Vacuum

We outline a general framework for evaluating the non-perturbative soft functions in the QCD instanton vacuum. In particular, from the soft function we derive the Collins-Soper (CS) kernel, which drives the rapidity evolution of the transverse-momentum-dependent parton distributions. The resulting CS kernel, when supplemented with the perturbative contribution, agrees well with recent lattice results and some phenomenological parameterizations. Moreover, our CS kernel depends logarithmically on the large quark transverse separation, providing a key constraint on its phenomenological parametrization. Finally, a lattice calculation can be directly compared to our generic results in Euclidean signature, thus providing a new approach for evaluating the soft function and extracting the CS kernel by analytical continuation.

hep-ph↗

arXiv2024

Quasifragmentation functions in the massive Schwinger model

We introduce the concept of the quark quasifragmentation function (qFF) using an equal-time and spatially boosted form of the Collins-Soper fragmentation function where the out-meson fragment is replaced by the current asymptotic condition. We derive the qFF for a fermion in two-dimensional quantum electrodynamics (QED2) using the Kogut-Susskind Hamiltonian after a mapping onto spin qubits in a spatial lattice with open boundary conditions. This form is suitable for quantum computations. We compute the qFF by exact diagonalization of the spin Hamiltonian. The results are compared to the qFF following from the Drell-Levy-Yan result for QED2, both at strong and weak coupling, and to two-dimensional quantum chromodynamics in the lowest Fock approximation.

hep-ph↗

arXiv2024

String-based parametrization of nucleon GPDs at any skewness: a comparison to lattice QCD

We introduce a string-based parametrization for nucleon quark and gluon generalized parton distributions (GPDs) valid at all skewness values. The conformal moments of the GPDs are expressed as sums of the spin-j nucleon A-form factor, and the skewness-dependent spin-j nucleon D-form factor. This representation, which fulfills the polynomiality condition (due to Lorentz invariance) and does not rely on model-specific assumptions, is derived from t-channel string exchanges in AdS spaces. The spin-j nucleon D-form factor is closely related to the the spin-j nucleon A-form factor. We use the Mellin moments from empirical parton distributions to model the spin-j nucleon A-form factors. Using only five Regge slope parameters, fixed from the electromagnetic and gravitational form factors, our string-based parametrization generates accurate singlet, non-singlet, isovector, and flavor-separated nucleon quark GPDs, along with symmetric nucleon gluon GPDs from their Mellin-Barnes integral representations. Our isovector nucleon quark GPD is in agreement with existing lattice data. Our string-based parametrization should facilitate the empirical extraction and global analysis of nucleon GPDs in exclusive processes, bypassing the deconvolution challenge.

hep-ph↗

arXiv2024

Parametrization of GPDs from t-channel string exchange in AdS spaces

We introduce a string-based parametrization for nucleon quark and gluon generalized parton distributions (GPDs) that is valid for all skewness. Our approach leverages conformal moments, representing them as the sum of spin-j nucleon A-form factor and skewness-dependent spin-j nucleon D-form factor, derived from t-channel string exchange in AdS spaces consistent with Lorentz invariance and unitarity. This model-independent framework, satisfying the polynomiality condition due to Lorentz invariance, uses Mellin moments from empirical data to estimate these form factors. With just five Regge slope parameters, our method accurately produces various nucleon quark GPD types and symmetric nucleon gluon GPDs through pertinent Mellin-Barnes integrals. Our isovector nucleon quark GPD is in agreement with existing lattice data, promising to improve the empirical extraction and global analysis of nucleon GPDs in exclusive processes, by avoiding the deconvolution problem at any skewness, for the first time.

hep-ph↗

arXiv2024

Quasiparton distributions in massive QED2: Toward quantum computation

We analyze the quasi-parton distributions of the lightest $η'$ meson in massive two-dimensional Quantum electrodynamics (QED2) by performing a digital quantum simulation on a classical computer (exact diagonalization). The Hamiltonian and boost operators are mapped onto spin qubits in a spatial lattice with open boundary conditions. The lowest excited state in the exact diagonalization is shown to interpolate continuously between an anomalous $η'$ state at strong coupling,and a non-anomalous heavy meson at weak coupling, with a cusp at the critical point. The boosted $η'$ state follows relativistic kinematics but with large deviations in the luminal limit. The spatial quasi-parton distribution function and amplitude for the $η'$ state are computed numerically for increasing rapidity both at strong and weak coupling, and compared to the exact light front results. The numerical results from the boosted form of the spatial parton distributions, compare fairly with the inverse Fourier transformation of the luminal parton distributions, derived in the lowest Fock space approximation. Our analysis points out some of the limitations facing the current lattice program for the parton distributions.

hep-ph↗

arXiv2024

Polarized $ϕ$ meson rates with viscous corrections at RHIC

We discuss the emission of $ϕ$ mesons with longitudinal and transverse polarization in ultra-relativistic heavy ion collisions. In the hadronic phase and in leading order in the kaon diluteness, $ϕ$ emission is isotropic and driven by the flavor singlet and octet vector spectral functions. At next to leading order, the emissivities receive a non-isotropic correction from the flavor octet spectral function, and the non-equilibrium contributions originating from the shear and bulk viscosities. Implications for the $ϕ$ meson alignment in heavy-ion collisions are discussed.

nucl-th↗

arXiv2024

Threshold photo-production of $J/Ψ$ off light nuclei

We analyze threshold photoproduction of heavy mesons off a deuteron and Helium-4, using the QCD factorization method. Assuming large skewness, the production amplitude is dominated by the leading twist-2 gluonic energy-momentum tensor (EMT). We use our recent results for the gluonic gravitational form factors of light nuclei in the impulse approximation, to estimate the differential cross sections for $J/Ψ$ production off a deuteron and Helium-4 at current electron facilities.

nucl-th↗

arXiv2024

Glue in hadrons at medium resolution and the QCD instanton vacuum

We discuss a general framework for the evaluation of the gluonic form factors in light hadrons at low momentum transfer, in the QCD instanton vacuum. At medium resolution of the order of the inverse mean instanton size, the glue is mostly localized in single or pair of pseudoparticles, and globally constrained by the fluctuations of their topological charges. These pseudoparticles trap light quarks, giving rise to emerging multiflavor 't Hooft interactions. We explicitly evaluate the gluonic scalar, pseudoscalar, energy-momentum tensor (EMT), and the leading C-odd and C-even three gluons hadronic form factors, at next to leading order (NLO) in the instanton density, including molecular clusters of like and unlike instantons. We use the results for the EMT to address the contribution of the gluons in Ji$^\prime$s mass and spin sum rules, at low resolution. When evolved, our results for the mass and spin composition of the nucleon, are shown to be in good agreement with the recently reported lattice results at higher resolution.

hep-ph↗

arXiv2024

Helium-4 gravitational form factors: exchange currents

We evaluate the leading exchange corrections to the Helium-4 gravitational form factors (GFFs) upto momenta of the order of the nucleon mass. We use both the K-harmonic method with simple pair nucleon potential, and a Jastrow trial function using the Argonne $v_{14}$ potential, to evaluate the Helium-4 GFFs. The exchange current contributions include the pair interaction, plus the seagull and the pion exchange interactions, modulo the recoil corrections. To estimate the off-shellness of the pion nucleon coupling in this momenta range, we discuss the results using either the pseudo-scalar (PS) or pseudo-vector (PV) pion-nucleon couplings. When the PV coupling is used, the pair diagram contribution is higher order in the non relativistic expansion. The results for the Helium-4 A-GFF are comparable to those given by the impulse approximation, especially for the PS coupling using both the K-Harmonic method and variational method. The exchange current contributions with the PS coupling for the charge form factor of Helium-4, yield better agreement with the existing data over a broad range of momenta, especially when the Argonne $v_{14}$ potential including the D-wave admixture is used.

nucl-th↗

arXiv2024

Pion gravitational form factors in the QCD instanton vacuum II

We revisit the hard QCD contributions to the pion gravitational form factors (GFFs), in terms of the twist-2,3 pion distribution amplitudes (DA), including novel semi-hard contributions from the instantons. The pion DAs are evaluated in the QCD instanton vacuum, and then properly evolved to higher resolution. The results are compared to our recent results from the QCD instanton vacuum, as well as Bethe-Salpeter calculations and recent lattice data. The interpolated hard and soft contributions to the pion D-form-factor, are used to derive the (gravitational) pressure and shear within the pion, with a clear delineation of their range.

hep-ph↗

arXiv2024

Pion gravitational form factors in the QCD instanton vacuum I

The pion form factors of the QCD energy-momentum tensor (EMT) are studied in the instanton liquid model (ILM) of the QCD vacuum. In this approach the breaking of conformal symmetry is encoded in the form of stronger-than-Poisson fluctuations in the number of instantons. For the trace of the EMT, it is shown that the gluonic trace anomaly term contributes half the pion mass, with the other half coming from the quark-mass-dependent sigma term. The $Q^2$ dependence of the form factors is governed by glueball and scalar meson exchanges. For the traceless EMT, the spin-0 and 2 form factors are computed at next-to-leading order in the instanton density using effective quark operators. Relations between the gluon and quark contributions to the EMT form factors are derived. The form factors are also expressed in terms of the pion light-front wave functions in the ILM. The results at the low resolution scale of the inverse instanton size are evolved to higher scales using the renormalization group equation. The ILM results compare well with those of recent lattice QCD calculations.

hep-ph↗

arXiv2024

Gravitational form factors of light nuclei: Impulse approximation

The gravitational form factors of light nuclei are evaluated up to momenta of the order of the nucleon mass, using the impulse approximation. The nucleon gravitational form factors are reduced non-relativistically, and used to derive the gravitational form factors of light nuclei. The deuteron gravitational form factors are analysed using the Reid soft core potential. The helium-4 gravitational form factors are assessed using the K-harmonics method, and compared to those following from a mean-field approximation with a Woods-Saxon potential. The importance of removing the center of mass motion for the ensuing form factors is emphasized. The mass radii of these light nuclei are extracted and compared to their charge radii counterparts. The details of their pressure and shear distributions are discussed.

nucl-th↗

arXiv2024

Threshold photoproduction of $η_c$ and $η_b$ using holographic QCD

We discuss the possibility that threshold photoproduction of $η_{c,b}$ may be sensitive to the pseudovector $1^{+-}$ glueball exchange. We use the holographic construction to identify the pseudovector glueball with the Kalb-Ramond field, minimally coupled to bulk Dirac fermions. We derive the holographic C-odd form factor and its respective charge radius. Using the pertinent Witten diagrams, we derive and analyze the differential photoproduction cross section for $η_{c,b}$ in the threshold regime, including the interference from the dual bulk photon exchange with manifest vector dominance. The possibility of measuring this process at current and future electron facilities is discussed.

hep-ph↗

arXiv2024

Photo-production of $η_{c,b}$ near Threshold

We analyze the photo-production of $η_{c,b}$ off a proton in the threshold region, in terms of C-odd gluonic correlations in the off-forward proton matrix element. Near threshold, the skewness is large leading to a production amplitude that is dominated by four C-odd twist-3 gluon GPDs. We use the QCD instanton vacuum to estimate these C-odd contributions in the proton. The results are used to estimate the differential cross sections for coherent photo-production of $η_{c,b}$ in the threshold region, at current electron facilities.

hep-ph↗