arXiv subjects

Di-Lun Yang

Publications and source records attributed to Di-Lun Yang.

61 records · Page 4Linked to original sources

arXiv2013

Holographic Photon Production with Magnetic Field in Anisotropic Plasmas

We investigate the thermal photon production from constant magnetic field in a strongly coupled and anisotropic plasma via the gauge/gravity duality. The dual geometry with pressure anisotropy is generated from the axion-dilaton gravity action introduced by Mateos and Trancancelli and the magnetic field is coupled to fundamental matters(quarks) through the D3/D7 embeddings. We find that the photon spectra with different quark mass are enhanced at large frequency when the photons are emitted parallel to the anisotropic direction with larger pressure or perpendicular to the magnetic field. However, in the opposite conditions for the emitted directions, the spectra approximately saturate isotropic results in the absence of magnetic field. On the other hand, a resonance emerges at moderate frequency for the photon spectrum with heavy quarks when the photons move perpendicular to the magnetic field. The resonance is more robust when the photons are polarized along the magnetic field. On the contrary, in the presence of pressure anisotropy, the resonance will be suppressed. There exist competing effects of magnetic field and pressure anisotropy on meson melting in the strongly coupled super Yang-Mills plasma, while we argue that the suppression led by anisotropy may not be applied to the quark gluon plasma.

hep-th↗

arXiv2013

Light Probes in a Strongly Coupled Anisotropic Plasma

We employ the gauge/gravity duality to study the jet quenching of light probes traversing a static yet anisotropic strongly coupled $\mathcal{N}=4$ super Yang-Mills plasma. We compute the stopping distance of an image jet induced by a massless source field, which is characterized by a massless particle falling along the null geodesic in the WKB approximation, in an anisotropic dual geometry introduced by Mateos and Trancancelli. At mid and large anisotropic regimes, the stopping distances of a probe traveling in the anisotropic plasma along various orientations are suppressed compared to those in an isotropic plasma especially along the longitudinal direction at equal temperature. However, when fixing the entropy density, the anisotropic values of stopping distances near the transverse directions slightly surpass the isotropic values. In general, the jet quenching of light probes is increased by the anisotropic effect in a strongly coupled and equilibrium plasma.

hep-th↗

arXiv2013

Production of Prompt Photons and Dileptons in Rapid Holographic Thermalization

We study the lightlike and timelike R-current correlators in the AdS-Vaidya spacetime, which are associated with the production of prompt photons and dileptons from a nonequilibrium, rapidly thermalizing, and strongly coupled plasma in the AdS/CFT correspondence. In particular, we focus on the photon production. We find that the amplitude of the photon-emission spectrum increases during the thermalization of the medium. Also, the time-dependent emission rate for hard photons has an oscillatory pattern and thermalizes faster than soft photons.

hep-th↗

arXiv2012

Jet Quenching and Holographic Thermalization

We employ the AdS/CFT correspondence to investigate the thermalization of the strongly-coupled plasma and the jet quenching of a hard probe traversing such a thermalizing medium.

hep-th↗

arXiv2011

$J/ψ$ Production by Magnetic Excitation of $η_c$

We compute the probability of $J/ψ$ production from the interaction between $η_c$ and the strong magnetic field generated in relativistic heavy ion collisions. The computation is first carried out in the heavy quark effective model, in which the M1 radiative transition is considered. Then we investigate the transition in the framework of non-relativistic heavy hadron chiral perturbation theory and show that the polarization of $J/ψ$ produced by this process is parallel to the direction of magnetic field and thus perpendicular to the reaction plane. The transition probability obtained in both approaches is of order $2\times 10^{-4}$.

hep-ph↗

arXiv2011

On the Role of Charmed Meson Loops in Charmonium Decays

We investigate the effect of intermediate charmed meson loops on the M1 radiative decays $J/ψ\to η_c γ$ and $ψ'\rightarrowη^{(\prime)}_cγ$ as well as the isospin violating hadronic decays $ψ'\rightarrow J/ψ\,π^0(η)$ using heavy hadron chiral perturbation theory (HH$χ$PT). The calculations include tree level as well as one loop diagrams and are compared to the latest data from CLEO and BES-III. Our fit constrains the couplings of 1S and 2S charmonium multiplets to charmed mesons, denoted $g_2$ and $g_2^\prime$, respectively. We find that there are two sets of solutions for $g_2$ and $g_2^\prime$. One set, which agrees with previous values of the product $g_2 g_2^\prime$ extracted from analyses that consider only loop contributions to $ψ'\rightarrow J/ψ\,π^0(η)$, can only fit data on radiative decays with fine-tuned cancellations between tree level diagrams and loops in that process. The other solution for $g_2$ and $g_2^\prime$ leads to couplings that are smaller by a factor of 2.3. In this case tree level and loop contributions are of comparable size and the numerical values of the tree level contributions to radiative decays are consistent with estimates based on the quark model as well as non-relativistic QCD (NRQCD). This result shows that tree level HH$χ$PT couplings are as important as the one loop graphs with charmed mesons in these charmonium decays. The couplings $g_2$ and $g_2^\prime$ are also important for the calculations of the decays of charmed meson bound states, such as the X(3872), to conventional charmonia.

hep-ph↗

arXiv2008

Phase Transitions and the Perfectness of Fluids

We calculate the ratio eta/s, the shear viscosity (eta) to entropy density (s), which characterizes how perfect a fluid is, in weakly coupled real scalar field theories with different types of phase transitions. The mean-field results of the eta/s behaviors agree with the empirical observations in atomic and molecular systems such as water, He, N, and all the matters with data available in the NIST database. These behaviors are expected to be the same in N component scalar theories with an O(N) symmetry. We speculate these eta/s behaviors are general properties of fluid shared by QCD and cold atoms. Finally, we clarify some issues regarding counterexamples of the conjectured universal eta/s bound found in Refs.[16,17].

hep-ph↗