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1. Kinetic simulations underestimate the effects of waves during magnetic reconnection

   https://doi.org/10.1103/PhysRevResearch.6.L042072  

   Ng, J; Yoo, J; Chen, L-J; Bessho, N; Ji, H (December 2024, Physical Review Research)

   Collisionless plasma systems are often studied using fully kinetic simulations, where protons and electrons are treated as particles. Due to their computational expense, it is necessary to reduce the ion-to-electron mass ratio $m_i/m_e$or the ratio between plasma and cyclotron frequencies in simulations of large systems. In this Letter we show that when electron-scale waves are present in larger-scale systems, numerical parameters affect their amplitudes and effects on the larger system. Using lower-hybrid drift waves during magnetic reconnection as an example, we find that the ratio between the wave electric field and the reconnection electric field scales as $\sqrt{m_i/m_e}$, while the phase relationship is also affected. The combination of these effects means that the anomalous drag that contributes to momentum balance in the reconnection region can be underestimated by an order of magnitude. The results are relevant to the coupling of electron-scale waves to ion-scale reconnection regions, and other systems such as collisionless shocks. Published by the American Physical Society2024 

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2. Interaction Renormalization and Validity of Kinetic Equations for Turbulent States

   https://doi.org/10.1103/PhysRevLett.133.244002  

   Rosenhaus, Vladimir; Falkovich, Gregory (December 2024, Physical Review Letters)

   We consider turbulence of waves interacting weakly via four-wave scattering (sea waves, plasma waves, spin waves, etc.). In the first order in the interaction, a closed kinetic equation has stationary solutions describing turbulent cascades. We show that the higher-order terms generally diverge both at small (IR) and large (UV) wave numbers for direct cascades. The analysis up to the third order identifies the most UV-divergent terms. To gain qualitative analytic control, we sum a subset of the most UV divergent terms, to all orders, giving a perturbation theory free from UV divergence, showing that turbulence becomes independent of the dissipation scale when it goes to zero. On the contrary, the IR divergence (present in the majority of cases) makes the effective coupling parametrically larger than the naive estimate and growing with the pumping scale $L$(similar to anomalous scaling in fluid turbulence). In such cases, the kinetic equation does not describe wave turbulence even of arbitrarily small level at a given $k$if $kL$is large enough that is the cascade is sufficiently long. We show that the character of strong turbulence is determined by whether the effective four-wave interaction is enhanced or suppressed by collective effects. The enhancement possibly signals that strong turbulence is dominated by multiwave bound states (solitons, shocks, cusps), similar to confinement in quantum chromodynamics. Published by the American Physical Society2024 

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3. Propagation of strong electromagnetic waves in tenuous plasmas

   https://doi.org/10.1103/PhysRevResearch.6.043213  

   Sobacchi, Emanuele; Iwamoto, Masanori; Sironi, Lorenzo; Piran, Tsvi (November 2024, Physical Review Research)

   We study the propagation of electromagnetic waves in tenuous plasmas, where the wave frequency ${\omega }_0$is much larger than the plasma frequency ${\omega }_{\mathrm{P}}$. We show that in pair plasmas, nonlinear effects are weak for $a_0\ll {\omega }_0/{\omega }_{\mathrm{P}}$, where $a_0$is the wave strength parameter. In electron-proton plasmas, a more restrictive condition must be satisfied, namely, either $a_0\ll 1/{\omega }_{\mathrm{P}}{\tau }_0$, where ${\tau }_0$is the duration of the radiation pulse, or $a_0\ll 1$. We derive the equations that govern the evolution of the pulse in the weakly nonlinear regime. Our results have important implications for the modeling of fast radio bursts. We argue that (i) millisecond duration bursts with a smooth profile must be produced in a proton-free environment, where nonlinear effects are weaker, and (ii) propagation through an electron-proton plasma near the source can imprint a submicrosecond variability on the burst profile. Published by the American Physical Society2024 

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4. Quark counting, Drell-Yan West, and the pion wave function

   https://doi.org/10.1103/PhysRevC.110.L042201  

   Alberg, Mary; Miller, Gerald A (October 2024, Physical Review C)

   The relation between the pion's quark distribution function, $q\left(x\right)$, its light-front wave function, and the elastic charge form factor, $F\left({\mathrm{\Delta }}^2\right)$, is explored. The square of the leading-twist pion wave function at a special probe scale, ${\zeta }_H$, is determined using models and Poincaré covariance from realistic results for $q\left(x\right)$. This wave function is then used to compute form factors with the result that the Drell-Yan-West and quark counting relationships are not satisfied. A new relationship between $q\left(x\right)$and $F\left({\mathrm{\Delta }}^2\right)$is proposed. Published by the American Physical Society2024 

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5. Universal density shift coefficients for the thermal conductivity and shear viscosity of a unitary Fermi gas

   https://doi.org/10.1103/PhysRevResearch.6.L042021  

   Li, Xiang; Huang, J; Thomas, J E (October 2024, Physical Review Research)

   We measure universal temperature-independent density shifts for the thermal conductivity ${\kappa }_T$and shear viscosity $\eta$, relative to the high temperature limits, for a normal phase unitary Fermi gas confined in a box potential. We show that a time-dependent kinetic theory model enables extraction of the hydrodynamic transport times ${\tau }_{\eta }$and ${\tau }_{\kappa }$from the time-dependent free decay of a spatially periodic density perturbation, yielding the static transport properties and density shifts, corrected for finite relaxation times. Published by the American Physical Society2024 

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