Showing 1–8 of 8 results
Eli Fonseca, Jacob Shapiro, Ahmed Sheta, Angela Wang, Kohtaro Yamakawa
We study spinful non-interacting electrons moving in two-dimensional materials which exhibit a spectral gap about the Fermi energy as well as time-reversal invariance. Using Fredholm theory we revisit the (known) bulk topological invariant, define a new one for the edge, and show their equivalence (the bulk-edge correspondence) via homotopy.
Ahmed Sheta, Mathias Zinnen, Aline Sindel, Andreas Maier, Vincent Christlein
Finding smell references in historic artworks is a challenging problem. Beyond artwork-specific challenges such as stylistic variations, their recognition demands exceptionally detailed annotation classes, resulting in annotation sparsity and extreme class imbalance. In this work, we explore the potential of synthetic data generation to alleviate these issues and enable accurate detection of smell-related objects. We evaluate several diffusion-based augmentation strategies and demonstrate that incorporating synthetic data into model training can improve detection performance. Our findings suggest that leveraging the large-scale pretraining of diffusion models offers a promising approach for improving detection accuracy, particularly in niche applications where annotations are scarce and costly to obtain. Furthermore, the proposed approach proves to be effective even with relatively small amounts of data, and scaling it up provides high potential for further enhancements.
Ahmed Sheta, Yidi Zhao, Norman H. Christ
For an asymptotically free theory, a promising strategy for eliminating Critical Slowing Down (CSD) is naïve Fourier acceleration. This requires the introduction of gauge-fixing into the action, in order to isolate the asymptotically decoupled Fourier modes. In this article, we present our approach and results from a gauge-fixed Fourier-accelerated hybrid Monte Carlo algorithm, using an action that softly fixes the gauge links to Landau gauge. We compare the autocorrelation times with those of the pure hybrid Monte Carlo algorithm. We work on a small-volume lattice at weak coupling. We present preliminary results and obstacles from working with periodic boundary conditions, and then we present results from using fixed, equilibrated boundary links to avoid $\mathbb{Z}_3$ and other topological barriers and to anticipate applying a similar acceleration to many small cells in a large, physically-relevant lattice volume.
Ahmed Sheta, Andrew Strominger, Adam Tropper, Hongji Wei
Flat Minkowski space (M$^4$) and AdS$_4$ can both be conformally mapped to the Einstein cylinder. The maps may be judiciously chosen so that some null generators of the $\mathcal{I}^+$ boundary of M$^4$ coincide with antipodally-terminating null geodesic segments on the boundary of AdS$_4$. Conformally invariant nonabelian gauge theories in M$^4$ have an asymptotic $S$-algebra generated by a tower of soft gluons given by weighted null line integrals on $\mathcal{I}^+$. We show that, under the conformal map to AdS$_4$, the leading soft gluons are dual to light transforms of the conserved global symmetry currents in the boundary CFT$_3$. The tower of light ray operators obtained from the $SO(3,2)$ descendants of this light transform realize a full set of generators of the $S$-algebra in the boundary CFT$_3$. This provides a direct connection between holographic symmetry algebras in M$^4$ and AdS$_4$.
Daniel Kapec, Ahmed Sheta, Andrew Strominger, Chiara Toldo
Recent work has shown that loop corrections from massless particles generate $\frac{3}{2}\log T_{\text{Hawking}}$ corrections to black hole entropy which dominate the thermodynamics of cold near-extreme charged black holes. Here we adapt this analysis to near-extreme Kerr black holes. Like AdS$_2\times S^2$, the Near-Horizon Extreme Kerr (NHEK) metric has a family of normalizable zero modes corresponding to reparametrizations of boundary time. The path integral over these zero modes leads to an infrared divergence in the one-loop approximation to the Euclidean NHEK partition function. We regulate this divergence by retaining the leading finite temperature correction in the NHEK scaling limit. This "not-NHEK" geometry lifts the eigenvalues of the zero modes, rendering the path integral infrared finite. The quantum-corrected near-extremal entropy exhibits $\frac{3}{2}\log T_{\text{Hawking}}$ behavior characteristic of the Schwarzian model and predicts a lifting of the ground state degeneracy for the extremal Kerr black hole.
Daniel Kapec, Ahmed Sheta
There are two interesting classes of trapped null geodesics in any black hole spacetime: those that lie on the photon ring and those that generate the horizon. Recent work introduced a "near-ring" scaling limit that exhibits the emergent symmetries of the eikonal quasinormal mode (QNM) spectrum associated to the photon ring. This analysis was reformulated geometrically by Fransen using the Penrose limit, which produces pp-waves from geodesics. We elaborate on and generalize various aspects of this construction for the Schwarzschild black hole. We also discuss the Penrose limit onto the horizon generators. This second limit, although technically simpler, also displays emergent near-horizon symmetries that explain the equally-spaced overtones of the highly-damped QNM spectrum. In both examples, symmetry considerations distinguish the QNM solutions from the scattering states and produce overtones as descendants.
Ahmed Sheta, Yuri Levin
We consider gravitational waves from a pair of monopoles or black holes that are moving non-relativistically and are connected by a cosmic string. Shortly after the binary's formation, the connecting string straightens due the direct coupling of its motion to gravitational radiation. Afterwards, the motion of the binary can be well-approximated by a non-relativistic motion of its components that have an additional constant mutual attraction force due to the tension of the straight string that connects them. The orbit shrinks due to the gravitational radiation backreacting on the binary's components. We find that if the binary's semimajor axis $a\gg \sqrt{R_1 R_2/μ}$, its eccentricity grows on the inspiral's timescale; here $R_1$ and $R_2$ are the gravitational radii of the binary components, and $μ$ is the dimensionless tension of the string. When the eccentricity is high, it approaches unity super-exponentially. If the binary's components are monopole-antimonopole pair, this leads to the physical collision that would likely destroy the string and annihilate the monopoles when the semimajor axis is still many orders of magnitude greater than the string thickness. If the binary's components are black holes, then the eccentricity reaches its peak when $a\sim \sqrt{R_1 R_2/μ}$, and then decays according to the standard Peter's formula. The black-hole spins initially become locked to the orbital motion, but then lag behind as the inspiral proceeds. We estimate the string-tension-induced dimensionless spins just prior to the merger and find them to be $\simμ^{3/8}\ll 1$.
Ahmed Sheta
Online multiplayer games are population-dependent systems whose playability depends on the continued presence of an active player base. We propose a formal framework for reasoning about viability collapse in such systems under explicit scope conditions. The framework introduces a conditional Critical Mass Threshold $Φ$, below which queue times, match quality, or role balance render a game operationally non-viable under a fixed operational profile; an uninhabited runtime taxonomy spanning pre-launch and post-decline states; and a Nostalgia Inversion Point $ψ$, at which cultural memory exceeds active participation. We model post-peak decline using a threshold-sensitive hazard model and show how games in the modeled class can cross below viability under finite official-service horizons or bounded novelty under continuing exposure. Case studies based on public concurrent-player data are used illustratively rather than as formal validation. The contribution of the paper is not a universal law, but a formal vocabulary, a collapse model, and an empirical agenda for studying online game decline, preservation risk, and uninhabited virtual worlds.