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1. Source anisotropies and pulsar timing arrays

   https://doi.org/10.1103/PhysRevD.110.123507  

   Allen, Bruce; Agarwal, Deepali; Romano, Joseph D; Valtolina, Serena (December 2024, Physical Review D)

   Pulsar timing arrays (PTAs) hunt for gravitational waves (GWs) by searching for the correlations that GWs induce in the time-of-arrival residuals from different pulsars. If the GW sources are of astrophysical origin, then they are located at discrete points on the sky. However, PTA data are often modeled, and subsequently analyzed, via a “standard Gaussian ensemble.” That ensemble is obtained in the limit of an infinite density of vanishingly weak, Poisson-distributed sources. In this paper, we move away from that ensemble, to study the effects of two types of “source anisotropy.” The first (a), which is often called “shot noise,” arises because there are $N$discrete GW sources at specific sky locations. The second (b) arises because the GW source positions are not a Poisson process, for example, because galaxy locations are clustered. Here, we quantify the impact of (a) and (b) on the mean and variance of the pulsar-averaged Hellings and Downs correlation. For conventional PTA sources, we show that the effects of shot noise (a) are much larger than the effects of clustering (b). Published by the American Physical Society2024 

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2. Circularly polarized photon emission from magnetized chiral plasmas

   https://doi.org/10.1103/PhysRevD.110.116005  

   Wang, Xinyang; Shovkovy, Igor A (December 2024, Physical Review D)

   We investigate the emission of circularly polarized photons from a magnetized quark-gluon plasma with nonzero quark-number and chiral charge chemical potentials. These chemical potentials qualitatively influence the differential emission rates of circularly polarized photons. A nonzero net electric charge density, induced by quark-number chemical potentials, enhances the overall emission of one circular polarization over the other, while a nonzero chiral charge density introduces a spatial asymmetry in the emission with respect to reflection in the transverse plane. The signs of the electrical and chiral charge densities determine which circular polarization dominates overall and whether the emission preferentially aligns with or opposes the magnetic field. Based on these findings, we propose that polarized photon emission is a promising observable for characterizing the quark-gluon plasma produced in heavy-ion collisions. Published by the American Physical Society2024 

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3. Stochastic ultralight dark matter fluctuations in pulsar timing arrays

   https://doi.org/10.1103/PhysRevD.109.055017  

   Kim, Hyungjin; Mitridate, Andrea (March 2024, Physical Review D)

   Metric perturbations induced by ultralight dark matter (ULDM) fields have long been identified as a potential target for pulsar timing array (PTA) observations. Previous works have focused on the coherent oscillation of metric perturbations at the characteristic frequency set by the ULDM mass. In this work, we show that ULDM fields source low-frequency stochastic metric fluctuations and that these low-frequency fluctuations can produce distinctive detectable signals in PTA data. Using the NANOGrav 12.5-yr dataset and synthetic datasets mimicking present and future PTA capabilities, we show that the current and future PTA observations provide the strongest probe of ULDM density within the Solar System for masses in the range of ${10}^{-18}\mathrm{eV}-{10}^{-16}\mathrm{eV}$. Published by the American Physical Society2024 

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4. Evidence for nonzero electron velocity at the tunnel exit in strong-field atomic ionization

   https://doi.org/10.1103/PhysRevResearch.6.023049  

   Ivanov, I A; Kheifets, A S; Schimmoller, A; Landsman, A S; Kim, Kyung Taec (April 2024, Physical Review Research)

   We provide some evidence for nonzero electron velocity at the tunnel exit in strong-field atomic ionization. Our investigation is based on the analysis of a suitably chosen correlation function which describes correlations between the two observables: the longitudinal electron velocity and the appearance of the photoelectron in the continuum at the end of the laser pulse. The results of the correlation function analysis that we perform are confirmed by the calculations using the quantum orbits method. Published by the American Physical Society2024 

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5. Microscopic Model for Fractional Quantum Hall Nematics

   https://doi.org/10.1103/PhysRevLett.132.236503  

   Pu, Songyang; Balram, Ajit C; Taylor, Joseph; Fradkin, Eduardo; Papić, Zlatko (June 2024, Physical Review Letters)

   Geometric fluctuations of the density mode in a fractional quantum Hall (FQH) state can give rise to a nematic FQH phase, a topological state with a spontaneously broken rotational symmetry. While experiments on FQH states in the second Landau level have reported signatures of putative FQH nematics in anisotropic transport, a realistic model for this state has been lacking. We show that the standard model of particles in the lowest Landau level interacting via the Coulomb potential realizes the FQH nematic transition, which is reached by a progressive reduction of the strength of the shortest-range Haldane pseudopotential. Using exact diagonalization and variational wave functions, we demonstrate that the FQH nematic transition occurs when the system’s neutral gap closes in the long-wavelength limit while the charge gap remains open. We confirm the symmetry-breaking nature of the transition by demonstrating the existence of a “circular moat” potential in the manifold of states with broken rotational symmetry, while its geometric character is revealed through the strong fluctuations of the nematic susceptibility and Hall viscosity. Published by the American Physical Society2024 

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