arXiv subjects

Ismail Zahed

Publications and source records attributed to Ismail Zahed.

At least 109 records · Page 6Linked to original sources

arXiv2018

Diffractive Vector Photoproduction using Holographic QCD

We discuss diffractive photon-production of vector mesons in holographic QCD. At large $\sqrt{s}$, the QCD scattering amplitudes are reduced to the scattering of pair of dipoles exchanging a closed string or a pomeron. We use the holographic construction in AdS$_5$ to describe both the intrinsic dipole distribution in each hadron, and the pomeron exchange. Our results for the heavy meson photon-production are made explicit and compared to some existing experiments.

hep-ph↗

arXiv2018

Dense Instanton-Dyon Liquid Model: Diagrammatics

We revisit the instanton-dyon liquid model in the confined phase by using a non-linear Debye-Huckel (DH) resummation for the Coulomb interactions induced by the moduli, followed by a cluster expansion. The organization is shown to rapidly converge and yields center symmetry at high density. The dependence of these results on a finite vacuum angle are also discussed. We also formulate the hypernetted chain (HCN) resummation for the dense instanton-dyon liquid and use it to estimate the liquid pair correlation functions in the DH limit. At very low temperature, the dense limit interpolates between chains and rings of instanton-anti-instanton-dyons and a bcc crystal, with strong topological and magnetic correlations.

hep-ph↗

arXiv2018

Are there monopoles in the quark-gluon plasma?

Monopole-like objects have been identified in multiple lattice studies, and there is now a significant amount of literature on their importance in phenomenology. Some analytic indications of their role, however, are still missing. The 't Hooft-Polyakov monopoles, originally derived in the Georgi-Glashow model, are an important dynamical ingredient in theories with extended supersymmetry ${\cal N} = 2,\,4$, and help explain the issues related with electric-magnetic duality. There is no such solution in QCD-like theories without scalar fields. However, all of these theories have instantons and their finite-$T$ constituents known as instanton-dyons (or instanton-monopoles). The latter leads to semiclassical partition functions, which for ${\cal N} = 2,\,4$ theories were shown to be identical ("Poisson dual") to the partition function for monopoles. We show how, in a pure gauge theory, the semiclassical instanton-based partition function can also be Poisson-transformed into a partition function, interpreted as the one of moving and rotating monopoles.

hep-ph↗

arXiv2017

Rotating Dirac fermions in a magnetic field in 1+2,3 dimensions

We consider the effects of an external magnetic field on rotating fermions in 1+2,3 dimensions. The dual effect of a rotation parallel to the magnetic field causes a net increase in the fermionic density by centrifugation, which follows from the sinking of the particle lowest Landau level in the Dirac sea for free Dirac fermions. In 1+d = 2n dimensions, this effect is related to the chiral magnetic effect in 2n-2 dimensions. This phenomenon is discussed specifically for both weak and strong inter-fermion interactions in 1+2 dimensions. For QCD in 1+3 dimensions with Dirac quarks, we show that in the strongly coupled phase with spontaneously broken chiral symmetry, this mechanism reveals itself in the form of an induced pion condensation by centrifugation. We use this observation to show that this effect causes a shift in the chiral condensate in leading order, and to discuss the possibility for the formation of a novel pion super-fluid phase in present heavy ion collisions at collider energies.

hep-ph↗

arXiv2017

Pion Condensation by Rotation in a Magnetic field

We show that the combined effects of a rotation plus a magnetic field can cause charged pion condensation. We suggest that this phenomenon may yield to observable effects in current heavy ion collisions at collider energies, where large magnetism and rotations are expected in off-central collisions.

hep-ph↗

arXiv2017

Higher order string effects and the properties of the Pomeron

We revisit the description of the Pomeron within the effective string theory of QCD. Using a string duality relation, we show how the static potential maps onto the high-energy scattering amplitude that exhibits the Pomeron behavior. Besides the Pomeron intercept and slope, new additional terms stemming from the higher order string corrections are shown to affect both the growth of the nucleon's size at high energies and its profile in impact parameter space. The stringy description also allows for an odderon that only disappears in critical dimension. Some of the Pomeron's features that emerge within the effective string description can be studied at the future EIC collider.

hep-ph↗

arXiv2017

Hydrodynamical corrections to electromagnetic emissivities in QCD

We provide a general framework for the derivation of the hydrodynamical corrections to the QCD electromagnetic emissivities in a viscous fluid. Assuming that the emission times are short in comparison to the fluid evolution time, we show that the leading corrections in the fluid gradients are controlled by the bulk and shear tensors times pertinent response functions involving the energy-momentum tensor. In a hadronic fluid phase, we explicit these contributions using spectral functions. Using the vector dominance approximation, we show that the bulk viscosity correction to the photon rate is sizable, while the shear viscosity is negligible for about all frequencies. In the partonic phase near the transition temperature we provide an assessment of the viscous corrections to the photon and dilepton emissions, using a non-perturbative quark-gluon plasma with soft thermal gluonic corrections in the form of operators of leading mass dimension. Again, the thermal bulk viscosity corrections are found to be larger than the thermal shear viscosity corrections at all energies for both the photon and dilepton in the partonic phase.

hep-ph↗

arXiv2017

Regimes of the Pomeron and its Intrinsic Entropy

We suggest that the perturbative and non-perturbative descriptions of the Pomeron can be viewed as complementary descriptions of different phases in the Pomeron phase diagram, with a phase boundary where the proper description of the produced systems are "string balls". Their intrinsic entropy is calculated and turned out to be the same, as the recently reported perturbative entanglement entropy. The distribution of large multiplicities stemming from the string balls is also wide, with its moments close to those reported for hadrons in $pp$ collisions at the LHC. At low-x, the quantum string is so entangled that sufficiently weak string self-attraction can cause it to turn to a string ball dual to a black hole. We suggest that low-x saturation occurs when the density of wee-strings reaches the Bekenstein bound, with a proton size that freezes with increasing rapidity. Some of these observations maybe checked at the future eIC.

hep-ph↗

arXiv2017

Direct Photon Elliptic Flow at BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC)

We use an event-by-event hydrodynamical description of the heavy-ion collision process with Glauber initial conditions to calculate the thermal emission of photons. The photon rates in the hadronic phase follow from a spectral function approach and a density expansion, while in the partonic phase they follow from the AMY perturbative rates. The calculated photon elliptic flows are lower than those reported recently by both the ALICE and PHENIX collaborations.

nucl-th↗

arXiv2017

Heavy and Strange Holographic Baryons

We extend the $D4$-$D8$ holographic construction to include three chiral and one heavy flavor, to describe heavy-light baryons with strangeness and their exotics. At strong coupling, the heavy meson always binds to the bulk instanton in the form of a flavor zero mode in the fundamental representation. We quantize the ensuing bound states using the collective quantization method, to obtain the spectra of heavy and strange baryons with both explicit and hidden charm and bottom. Our results confirm the existence of two low-lying charmed penta-quark states with $\frac 12^-,\frac 32^-$ assignments, and predict many new ones with both charm and bottom. They also suggest a quartet of low-lying neutral $Ω_c^0$ with assignments $\frac 12^\pm ,\frac 32^\pm$ that are heavier than the quintuplet of neutral $Ω_c^0$ recently reported by LHCb.

hep-ph↗

arXiv2017

Heavy Baryons and their Exotics from Instantons in Holographic QCD

We use a variant of the $D4$-$D8$ construction that includes two chiral and one heavy meson, to describe heavy-light baryons and their exotics as heavy mesons bound to a flavor instanton in bulk. At strong coupling, the heavy meson is shown to {\it always bind} in the form of a flavor instanton zero mode in the fundamental representation. The ensuing instanton moduli for the heavy baryons exhibits both chiral and heavy quark symmetry. We detail how to quantize it, and derive model independent mass relations for heavy bayons with a single-heavy quark in leading order, in overall agreement with the reported baryonic spectra with one charm or bottom. We also discuss the low-lying masses and quantum assignments for the even and odd parity states, some of which are yet to be observed. We extend our analysis to double-heavy pentaquarks with hidden charm and bottom. In leading order, we find a pair of doube-heavy iso-doublets with $IJ^π=\frac 12 {\frac 12}^-, \frac 12 {\frac 32}^-$ assignments for all heavy flavor combinations. We also find five new Delta-like pentaquark states with $IJ^π=\frac 32 {\frac 12}^-, \frac 32 {\frac 32}^-, \frac 32 {\frac 52}^-$ assignments for both charm and bottom.

hep-ph↗

arXiv2017

Interplay of Reggeon and photon in pA collisions

We discuss the effects of the electromagnetic interaction in high-energy proton collisions with nuclei of large Z at strong coupling $λ=g^2N_c$. Using the holographic dual limit of large $N_c>λ\gg 1$, we describe the Reggeon exchange as a twisted surface and show that it gets essentially modified by the electromagnetic interaction.

nucl-th↗

arXiv2016

Disorder in the Sachdev-Yee-Kitaev Model

We give qualitative arguments for the mesoscopic nature of the Sachdev-Yee-Kitaev (SYK) model in the holographic regime with $q^2/N\ll 1$ with $N$ Majorana particles coupled by antisymmetric and random interactions of range $q$. Using a stochastic deformation of the SYK model, we show that its characteristic determinant obeys a viscid Burgers equation with a small spectral viscosity in the opposite regime with $q/N=1/2$, in leading order. The stochastic evolution of the SYK model can be mapped onto that of random matrix theory, with universal Airy oscillations at the edges. A spectral hydrodynamical estimate for the relaxation of the collective modes is made.

hep-th↗

arXiv2016

Heavy-Light Mesons in Chiral AdS/QCD

We discuss a minimal holographic model for the description of heavy-light and light mesons with chiral symmetry, defined in a slab of AdS space. The model consists of a pair of chiral Yang-Mills and tachyon fields with specific boundary conditions that break spontaneously chiral symmetry in the infrared. The heavy-light spectrum and decay constants are evaluated explicitly. In the heavy mass limit the model exhibits both heavy-quark and chiral symmetry and allows for the explicit derivation of the one-pion axial couplings to the heavy-light mesons.

hep-ph↗

arXiv2016

Holographic Heavy-Light Chiral Effective Action

We propose a variant of the $D4$-$D8$ construction to describe the low energy effective theory of heavy-light mesons, interacting with the lowest lying pseudoscalar and vector mesons. The heavy degrees of freedom are identified with the $D8_L$-$D8_H$ string low energy modes, and are approximated near the world volume of $N_f-1$ light $D8_L$ branes, by fundamental vector field valued in $U(N_f-1)$. The effective action follows from the reduction of the bulk D-brane Born-Infeld (DBI) and Chern-Simons (CS) actions, and is shown to exhibit both chiral and heavy-quark symmetry. The action interpolates continuously between the $U(N_f)$ case with massless mesons, and the $U(N_f-1)$ case with heavy-light mesons. The heavy-light meson radial spectrum is Regge-like. The one-pion and two-pion couplings to the heavy-light multiplets are evaluated. The partial widths for the charged decays $G\rightarrow H+π$ are shown to be comparable to the recently reported full widths for both the charm and bottom mesons.

hep-ph↗

arXiv2016

Heavy Exotic Molecules

We briefly review the formation of pion-mediated heavy-light exotic molecules with both charm and bottom, under the general strictures of chiral and heavy quark symmetries. The charm isosinglet exotic molecules with $J^{PC}=1^{++}$ binds, which we identify as the reported neutral $X(3872)$. The bottom isotriplet exotic with $J^{PC}=1^{+-}$ binds, and is identified as a mixed state of the reported charged exotics $Z^+_b(10610)$ and $Z^+_b(10650)$. The bound bottom isosinglet molecule with $J^{PC}=1^{++}$ is a possible neutral $X_b(10532)$ to be observed.

hep-ph↗

arXiv2016

CP violation during the electroweak sphaleron transitions

We suggest a specific semiclassical background field, the so called pure gauge sphaleron explosion, to evaluate the magnitude of the CP violation stemming from the standard phase of the CKM matrix. We use it to evaluate the matrix elements of some next-to-leading order effective CP-violating operators suggested in the literature. We also derive the scale dependence of the corresponding coefficients. Finally, we discuss the expected magnitude of the CP violation in the cold electroweak scenario.

hep-ph↗

arXiv2016

Heavy Exotic Molecules with Charm and Bottom

We revisit the formation of pion-mediated heavy-light exotic molecules with both charm and bottom and their chiral partners under the general strictures of both heavy-quark and chiral symmetry. The chiral exotic partners with good parity formed using the $(0^+, 1^+)$ multiplet are about twice more bound than their primary exotic partners formed using the $(0^-,1^-)$ multiplet. The chiral couplings across the multiplets $(0^\pm, 1^\pm)$ cause the chiral exotic partners to unbind, and the primary exotic molecules to be about twice more bound, for $J\leq 1$. Our multi-channel coupling results show that only the charm isosinglet exotic molecules with $J^{PC}=1^{++}$ binds, which we identify as the reported neutral $X(3872)$. Also, the bottom isotriplet exotic with $J^{PC}=1^{+-}$ binds, which we identify as a mixture of the reported charged exotics $Z^+_b(10610)$ and $Z^+_b(10650)$. The bound isosinglet with $J^{PC}=1^{++}$ is suggested as a possible neutral $X_b(10532)$ not yet reported.

hep-ph↗