Enhanced thermal photon and dilepton production in strongly coupled N=4 SYM plasma in strong magnetic field

We calculate the DC conductivity tensor of strongly coupled N=4 super-Yang-Mills (SYM) plasma in a presence of a strong external magnetic field B>>T^2 by using its gravity dual and employing both the RG flow approach and membrane paradigm which give the same results. We find that, since the magnetic field B induces anisotropy in the plasma, different components of the DC conductivity tensor have different magnitudes depending on whether its components are in the direction of the magnetic field B. In particular, we find that a component of the DC conductivity tensor in the direction of the magnetic field B increases linearly with B while the other components (which are not in the direction of the magnetic field B) are independent of it. These results are consistent with the lattice computations of the DC conductivity tensor of the QCD plasma in an external magnetic field B. Using the DC conductivity tensor, we calculate the soft or low-frequency thermal photon and dilepton production rates of the strongly coupled N=4 SYM plasma in the presence of the strong external magnetic field B>>T^2. We find that the strong magnetic field B enhances both the thermal photon and dilepton production rates of the strongly coupled N=4 SYM plasma in a qualitative agreement with the experimentally observed enhancements at the heavy-ion collision experiments.Comment: 22 pages, 7 figures, published version in JHE

Comments on the holographic shear viscosity to entropy density ratio

We revisit the membrane paradigm calculations of the holographic shear viscosity tensor of strongly coupled isotropic plasmas with Einstein gravity dual by emphasizing the fact which was overlooked in the previous literatures that the shear viscosity is a fourth-rank tensor. Using the membrane paradigm we show that depending on whether the holographic shear viscosity tensor to entropy density ratio is \eta^j_i^j_i/s or \eta^{jiji}/s or \eta_{jiji}/s, we can derive three distinct formulae for the holographic shear viscosity tensor to entropy density ratios given explicitly in terms of the background metric g_{ij}. We find that the widely studied \eta^j_i^j_i/s holographic shear viscosity tensor to entropy density ratio takes the universal value 1/(4\pi) for isotropic background metric g_{ij} but \eta^{jiji}/s and \eta_{jiji}/s holographic shear viscosity tensor to entropy density ratios take non-universal values which depend on the details of the isotropic background metric g_{ij}.Comment: 9 pages, membrane paradigm derivation of \eta/s used, applications on M2-branes and M5-branes added, the title is change

Deep Inelastic Scattering on an Extremal RN-AdS Black Hole

We consider deep inelastic scattering (DIS) on a large nucleus described as an extremal RN-AdS black hole using the holographic principle. Using the R-current correlators we determine the structure functions as a function Bjorken-x, and map it on a finite but large nucleus with fixed atomic number. The R-ratio of the nuclear structure functions exhibit strong shadowing at low-x.Comment: 7 pages, 2 figure

Holographic Approaches to DIS on a Nucleus

We consider deep inelastic scattering (DIS) on a dense nucleus described as an extremal RN-AdS black hole with holographic quantum fermions in the bulk. We find that the R-ratio (the ratio of the structure function of the black hole to proton) exhibit shadowing for $x < 0.1$, anti-shadowing for $0.1 < x < 0.3$, EMC-like effect for $0.3 0.8$ in a qualitative agreement with the experimental observation of the ratio for DIS on nucleus for all range of $x$. We also take the dilute limit of the black hole and show that its R-ratio exhibits EMC-like effect for $0.2 < x < 0.8$ and the Fermi motion for $x > 0.8$, and no shadowing is observed in the dilute limit for both bottom-up (using Thomas-Fermi approximation for the nucleon distribution inside the dilute nucleus), and top-down (considering the dilute nucleus to be a Fermi gas in AdS) approaches.Comment: Proceeding written for INT Program (INT-18-3) "Probing Nucleons and Nuclei in High Energy Collisions" (November 14, 2018

Spin Polarized Photons from Axially Charged Plasma at Weak Coupling: Complete Leading Order

In the presence of (approximately conserved) axial charge in the QCD plasma at finite temperature, the emitted photons are spin-aligned, which is a unique P- and CP-odd signature of axial charge in the photon emission observables. We compute this "P-odd photon emission rate" in weak coupling regime at high temperature limit to complete leading order in the QCD coupling constant: the leading log as well as the constant under the log. As in the P-even total emission rate in the literature, the computation of P-odd emission rate at leading order consists of three parts: 1) Compton and Pair Annihilation processes with hard momentum exchange, 2) soft t- and u-channel contributions with Hard Thermal Loop re-summation, 3) Landau-Pomeranchuk-Migdal (LPM) re-summation of collinear Bremstrahlung and Pair Annihilation. We present analytical and numerical evaluations of these contributions to our P-odd photon emission rate observable.Comment: 41 pages, 11 figures; v2: one minor typo fixed, references added, accepted for publication in PR

Spin Polarized Photons and Di-leptons from Axially Charged Plasma

Axial charge in a QCD plasma is P- and CP-odd. We propose and study P- and CP-odd observables in photon and di-lepton emissions from an axially charged QCD plasma, which may provide possible experimental evidences of axial charge fluctuation and triangle anomaly in the plasma created in heavy-ion collisions. Our observables measure spin alignments of the emitted photons and di-leptons, and are shown to be related to the imaginary part of chiral magnetic conductivity at finite frequency-momentum, which ultimately arises from the underlying triangle anomaly of the QCD plasma with a finite axial charge density. We present an exemplar computation of these observables in strongly coupled regime using AdS/CFT correspondence.Comment: 29 pages, 3 figure

Gradient Correction to Photon Emission Rate at Strong Coupling

We compute the correction to the photon emission rate in first order of shear components of fluid velocity gradients, $\sigma_{ij}$, in near-equilibrium hydrodynamic plasma at strong coupling regime, using the real-time Schwinger-Keldysh formalism in AdS/CFT correspondence. Our result is an integral of an analytic expression. We observe that the gradient correction to the photon emission rate at strong coupling is about 0.3 - 0.4 times of the equilibrium rate in units of $\sigma_{ij}/T$.Comment: 28 pages, 9 figures; references added; more discussion about the phenomenological importance of the gradient correction added in the Discussion section, published in PR

Electromagnetic radii of the nucleon in soft-wall holographic QCD

We revisit the electromagnetic form factors of the proton and neutron in the holographic soft wall model. At low momentum transfer, we show that by matching the nucleon and rho Regge slopes and fixing the nucleon anomalous dimension by the nucleon mass, a perfect match to the world average charge radii from e-p scattering (including the recent small charge radius of the proton measured by the PRad collaboration at JLab) follows. At high momentum transfer, the nucleon anomalous dimension runs up to match the hard scaling rule.Comment: 10 pages (with two columns), 3 figures, 1 tabl

Deep Inelastic Scattering on a Nucleus using Holography

We consider deep inelastic scattering (DIS) on a nucleus described using a density expansion. In leading order, the scattering is dominated by the incoherent scattering on individual nucleons distributed using the Thomas-Fermi approximation. We use the holographic structure functions for DIS scattering on single nucleons to make a non-perturbative estimate of the nuclear structure function in leading order in the density. Our results are compared to the data in the large-x regime.Comment: 6 pages, 2 figure

Jet quenching parameter of quark-gluon plasma in strong magnetic field: Perturbative QCD and AdS/CFT correspondence

We compute the jet quenching parameter $\hat q$ of QCD plasma in the presence of strong magnetic field in both weakly and strongly coupled regimes. In weakly coupled regime, we compute $\hat q$ in perturbative QCD at complete leading order (that is, leading log as well as the constant under the log) in QCD coupling constant $\alpha_s$, assuming the hierarchy of scales $\alpha_s eB\ll T^2\ll eB$. We consider two cases of jet orientations with respect to the magnetic field: 1) the case of jet moving parallel to the magnetic field, 2) the case jet moving perpendicular to the magnetic field. In the former case, we find $\hat q\sim \alpha_s^2 (eB)T\log(1/\alpha_s)$, while in the latter we have $\hat q\sim \alpha_s^2 (eB)T\log(T^2/\alpha_seB)$. In both cases, this leading order result arises from the scatterings with thermally populated lowest Landau level quarks. In strongly coupled regime described by AdS/CFT correspondence, we find $\hat q\sim \sqrt{\lambda}(eB)T$ or $\hat q\sim\sqrt{\lambda}\sqrt{eB}T^2$ in the same hierarchy of $T^2\ll eB$ depending on whether the jet is moving parallel or perpendicular to the magnetic field, respectively, which indicates a universal dependence of $\hat q$ on $(eB)T$ in both regimes for the parallel case, the origin of which should be the transverse density of lowest Landau level states proportional to $eB$. Finally, the asymmetric transverse momentum diffusion in the case of jet moving perpendicular to the magnetic field may give an interesting azimuthal asymmetry of the gluon Bremsstrahlung spectrum in the BDMPS-Z formalism.Comment: 39 pages, 2 figures; v2: two minor typos fixed, a reference adde