Holographic thermalization in N=4 Super Yang-Mills theory at finite coupling

We investigate the behavior of energy momentum tensor correlators in holographic $\mathcal{N}=4$ super Yang-Mills plasma, taking finite coupling corrections into account. In the thermal limit we determine the flow of quasinormal modes as a function of the 't Hooft coupling. Then we use a specific model of holographic thermalization to study the deviation of the spectral densities from their thermal limit in an out-of-equilibrium situation. The main focus lies on the thermalization pattern with which the plasma constituents approach their thermal distribution as the coupling constant decreases from the infinite coupling limit. All obtained results point towards the weakening of the usual top-down thermalization pattern.Comment: 18 pages, 7 figures, v3: major revisio

Hyperfine Splitting and the Zeeman Effect in Holographic Heavy-Light Mesons

We inspect the mass spectrum of heavy-light mesons in deformed N=2 super Yang-Mills theory using the AdS/CFT correspondence. We demonstrate how some of the degeneracies of the supersymmetric meson spectrum can be removed upon breaking the supersymmetry, thus leading to the emergence of hyperfine structure. The explicit SUSY breaking scenarios we consider involve on one hand tilting one of the two fundamental D7 branes inside the internal R^6 space, and on the other hand applying an external magnetic field on the (untilted) branes. The latter scenario leads to the well-known Zeeman effect, which we inspect for both weak and strong magnetic fields.Comment: 5 pages, 1 figur

Thermalization at intermediate coupling

We use the AdS/CFT conjecture to investigate the thermalization of large-N_c N=4 Super Yang-Mills plasma in the limit of large but finite 't Hooft coupling. On the gravity side, we supplement the type IIB supergravity action by the full set of O(\alpha'^3) operators, which enables us to derive O(\lambda^{-3/2}) corrections to the emission spectrum of prompt photons in one model of holographic thermalization. Decreasing the coupling strength from the \lambda=\infty limit, we observe a qualitative change in the way the photon spectral density approaches its thermal limit as a function of the photon energy. We interpret this behavior as a sign of the thermalization pattern of the plasma shifting from top/down towards bottom/up.Comment: 5 pages, 3 figures; v2: minor corrections, added reference

Exploring nonlocal observables in shock wave collisions

We study the time evolution of 2-point functions and entanglement entropy in strongly anisotropic, inhomogeneous and time-dependent N=4 super Yang-Mills theory in the large N and large 't Hooft coupling limit using AdS/CFT. On the gravity side this amounts to calculating the length of geodesics and area of extremal surfaces in the dynamical background of two colliding gravitational shockwaves, which we do numerically. We discriminate between three classes of initial conditions corresponding to wide, intermediate and narrow shocks, and show that they exhibit different phenomenology with respect to the nonlocal observables that we determine. Our results permit to use (holographic) entanglement entropy as an order parameter to distinguish between the two phases of the cross-over from the transparency to the full-stopping scenario in dynamical Yang-Mills plasma formation, which is frequently used as a toy model for heavy ion collisions. The time evolution of entanglement entropy allows to discern four regimes: highly efficient initial growth of entanglement, linear growth, (post) collisional drama and late time (polynomial) fall off. Surprisingly, we found that 2-point functions can be sensitive to the geometry inside the black hole apparent horizon, while we did not find such cases for the entanglement entropy.Comment: 28 pp, 9 figs; v2: updated references, changed color bars in Figure 2 and Figure

Importance of drilling-related processes on the origin of borehole breakouts — Insights from LWD observations

Logging while drilling (LWD) images are widely used for the analysis of borehole stability. In this context, borehole breakouts are a crucial indication of rock failure developing when the circumferential stress around the borehole exceeds the yield value of the rock. This study investigates the impact of drilling-related processes (DRPs) on the origin of borehole breakouts. DRPs, for instance, include connections or tripping operations. For this purpose, we analyze data from 12 boreholes in different geological settings throughout the Norwegian and Danish North Sea, containing a total of 208 borehole breakouts. The extensive data acquisition of LWD offers the unique possibility to link the imaging to real-time drilling operations and to monitor anomalies of e.g., bottom hole pressure. These records allow us to connect any thermal, hydraulic, or mechanical interaction next to the borehole wall to perturbations of the stress field. This analysis resulted in an apparent strong coincidence of borehole breakouts, representing major stress perturbations, with DRPs. The causal relationship is highlighted by one order of magnitude higher occurrence of DRPs in depth sections containing breakouts. Major pressure reductions in the annulus of the borehole seem to be the most significant cause of drilling-related wellbore failures. This applies in particular to shutting off the pumps during connections, where pressure reductions of up to 16 % of the annulus pressure led to higher circumferential stresses. This process will increase the likelihood of compressive and shear failure, therefore causing borehole breakouts. These observations further open the perspective of counteracting wellbore instabilities by pressure modification. In addition to the initiation of breakouts, their temporal evolution – as seen in relogs – can also be ascribed to DRPs. This study indicates that not only plasticity but also mechanical interaction from DRPs is a key driver of the temporal growth of borehole breakouts