We study top-antitop pair production in proton-proton collisions within the Standard Model effective field theory (SMEFT) at dimension six. We focus on the top chromomagnetic operator $C_{tG}$ together with the four-quark operators relevant for unpolarized $t\overline{t}$ production. We analyze the top-quark single-differential transverse-momentum ($p_T$) and rapidity ($y$) distributions and the double-differential distributions in $p_T$ and $y$ ($p_T\times y$) at 13 TeV, and we present projections for 13.6 TeV using the same binning. Our highest-order setup combines next-to-next-to-leading-order (NNLO) Standard Model (SM) predictions with approximate NNLO SM-SMEFT interference corrections. We find that higher-order QCD effects are essential for a stable SMEFT interpretation, yielding substantially more robust bounds and milder parameter degeneracies. The strongest sensitivity is obtained for $C_{tG}$. In the combined highest-order analysis of the 13-TeV results and 13.6-TeV projections, we obtain sensitivity to effective scales up to 3.9 TeV. Our results show that higher-order differential top-pair production provides a precise and theoretically controlled probe of the top chromomagnetic interaction.

We investigate the boost-invariant expansion of a recently developed first-order spin hydrodynamic framework in which the spin chemical potential is treated as a leading-order hydrodynamic variable. Considering a symmetric energy-momentum tensor and a separately conserved spin tensor, we derive the coupled evolution equations for the medium temperature and the independent components of the spin chemical potential in the presence of both viscous and spin-diffusive transport coefficients. For a boost-invariant system, only the magneticlike components of the spin chemical potential survive, and their evolution is shown to depend sensitively on the spin transport coefficients. The transverse spin components decay more rapidly due to spin dissipation, while the longitudinal component survives for a longer duration. We further demonstrate that the evolution of the spin degrees of freedom modifies the temperature profile of the expanding medium. Using the resulting temperature profiles, we calculate thermal dilepton production rates from quark-antiquark annihilation. We find that the presence of spin dynamics enhances the dilepton yield relative to standard dissipative hydrodynamics, with the magnitude of the enhancement depending on the spin transport coefficients. Within the simplified hydrodynamic evolution considered here, our results qualitatively indicate that thermal dileptons can possibly provide an indirect probe of spin dynamics and spin transport in the quark-gluon plasma.

We study the use of higher-order resummation for transverse observables to achieve $\mathrm{NNLO}+\mathrm{PS}$ matching within the geneva framework. In particular, we embed $q_T$ resummation for color-singlet production at ${\mathrm{N}}^3\mathrm{LL}$ obtained via soft-collinear effective theory and implemented in the library scetlib within geneva. We also study for the first time the use of the generalized $N$-jettiness variable in parton shower matching, and achieve the resummation of the one-jettiness defined with transverse measures up to ${\mathrm{NLL}}^{\prime }$ accuracy. As a case study, we use these resummed calculations to construct a geneva $\mathrm{NNLO}+\mathrm{PS}$ generator for Higgs boson production in heavy-quark annihilation (with beauty or charm quarks in the initial state). The use of transverse measures facilitates the matching to showers ordered in transverse momentum, and opens the door to possible future extensions of this approach to the production of color singlets in association with final-state jets.

I present a unified model for single pion production in photo-, electro-, and neutrino-nucleon interactions that is applicable across the broad kinematic range in the GeV regime relevant to accelerator-based neutrino experiments. The model incorporates vector and axial-vector transition form factors for excited nucleon states with masses up to 2 GeV, together with nonresonant background contributions, within a meson dominance framework that respects quantum chromodynamic constraints and preserves unitarity. This construction ensures the correct asymptotic behavior at large momentum transfer ($Q^2$) while providing a consistent description of the resonance and transition regions. At very low $Q^2$, the implementation of the conserved vector current and partially conserved axial current relations leads to reliable predictions and helps address challenges encountered in current neutrino data analyses. The unified framework enables a global analysis that combines the available electron, photon, pion, and neutrino scattering data. This comprehensive approach allows detailed studies of nucleon structure in the resonance region and provides important constraints on weak interaction form factors, where modern neutrino-nucleon data remain limited. By determining all model parameters simultaneously with their correlated uncertainties, the approach establishes a robust, data-driven foundation for precision neutrino interaction modeling in support of next-generation neutrino oscillation measurements.

The generalized second law (GSL) of black hole thermodynamics asserts the monotonic increase of the generalized entropy combining the black hole area and the entropy of quantum fields outside the horizon. Modern proofs of the GSL rely on information-theoretic methods and are typically formulated using algebras of observables defined on the event horizon together with a vacuum state invariant under horizon symmetries, inducing a geometric modular flow. In this work, we formulate a dual version of the generalized second law from the perspective of asymptotic observers at future null infinity, who do not have access to the black hole area. Our approach exploits the dependence of the second law on the choice of algebra of observables and of a reference state invariant under suitable symmetries, in close analogy with open quantum thermodynamics. Using algebraic quantum field theory and modular theory, we analyze several physically motivated vacuum states, including the Hartle-Hawking state and two classes of regularized vacua. We show that, at null infinity, the monotonic quantity governing an irreversible evolution is no longer the generalized entropy but rather a thermodynamic potential constructed from asymptotic observables. Depending on the chosen vacuum, this potential takes the form of the free energy or of a generalized grand potential built from the Bondi mass and additional (angular) mode-dependent chemical potentials. The resulting inequalities define a dual generalized second law at future null infinity, which can be consistently combined with the standard GSL involving variations of the black hole area.

Black hole thermodynamics provides a unique setting in which general relativity, quantum field theory, and statistical mechanics converge. In semiclassical gravity, this interplay culminates in the generalized second law (GSL), whose modern proofs rely on information-theoretic techniques applied to algebras of observables defined on null hypersurfaces. These proofs exhibit close structural parallels with the thermodynamics of open quantum systems governed by Markovian dynamics. In this work, we draw parallels between the dynamics of quantum fields in regions bounded by nonexpanding causal horizons and the thermodynamics of quantum systems weakly coupled to equilibrium reservoirs. We introduce a dictionary relating late-time boundary conditions to the choice of reservoir, vacuum states to fixed points of the dynamics, and modular Hamiltonians to thermodynamic potentials. Building on results from a companion paper on dual generalized second laws at future null infinity [A. Rignon-Bret and M. Vilatte, companion paper, Phys. Rev. D 114, 065012 (2026)], we show that additional terms appearing in the associated thermodynamic potentials admit a natural interpretation as work contributions. We demonstrate that certain nonthermal vacuum states at null infinity allow for the operation of autonomous thermal engines and enable work extraction from the radiation. Extending the analysis to the Unruh vacuum in Schwarzschild and Kerr backgrounds, we obtain generalized grand-potential-type laws incorporating graybody effects and angular momentum fluxes. Altogether, our results clarify the thermodynamic description of black hole dynamics and place it within the broader framework of open quantum thermodynamics.

The spin-weighted spheroidal functions are the eigenfunctions of the angular Teukolsky equation. They are a generalization of the widely used spin-weighted spherical functions, and are extremely important in the area of black-hole perturbation theory. Like other special functions, they have an inherent phase ambiguity and need to be phase fixed to be uniquely defined. Clearly, such a phase choice does not have a direct physical impact. But, without specifying phase choices, meaningful comparison between results reported in different works become more difficult. To date, possible phase choices for the spin-weighted spheroidal functions have received little attention. Here, we clearly define and extensively explore two useful phase fixing schemes, and we propose that the spherical-limit phase-fixing scheme be adopted as the default phase-fixing scheme for the spin-weighted spheroidal functions.

Recent gravitational wave (GW) detections showing signatures of eccentricity and spin precession underscore the need to model binary black holes (BBHs) possessing these features simultaneously. Most efforts over the past fifteen years to model spinning BBHs and their corresponding GWs have relied on heuristically twisting waveforms from nonprecessing systems. This approach is based on empirical observations rather than first principles. This article aims to model the dynamics of spinning and eccentric BBHs from a first-principles approach using the post-Newtonian (PN) approximation within general relativity. Building on the already-existing 1.5PN solution, we construct an analytical solution for the time evolution of the relative separation vector, the individual black hole spin vectors, and the orbital angular momentum vector at 2PN order for BBHs with arbitrary spins and eccentricity. Such a solution is not fully 2PN accurate in that the tiny orbital timescale fluctuations in the solutions for the spins are only leading 1.5PN order accurate, instead of 2PN. However, it is shown that our new 2PN solution is still an order of magnitude improvement over the earlier 1.5PN solution, underlining the subdominant nature of the neglected next-to-leading-order oscillations in the spin solutions.

Wormholes are fascinating alternatives to black hole geometries. In this paper, we have studied a special case of wormhole solution in the context of $R=0$ spacetime. Our approximate analytical calculations show that, under a strictly static axial gravitational perturbation of this spacetime, the magnetic-type tidal Love number (for $\ell =2$) vanishes if we keep the solution of the master equation up to linear order in the regularization parameter of the geometry. Moreover, imposing physically viable boundary conditions on the master variable, we have corroborated our result with the numerical solutions. We have further extended our analysis toward the case of a Damour-Solodukhin wormhole, which also yields a similar conclusion.

We explore the gravitational-wave phenomenology of equal-mass inspiralling boson-star binaries using numerical relativity simulations. In particular, we characterize the waveform differences between binary boson-star and black-hole systems across (i) the early inspiral, by matching our waveforms to post-Newtonian expressions, (ii) merger, and (iii) late ringdown, by extracting the quasinormal mode frequencies of the remnants. We find that boson-star binaries exhibit the largest deviations from comparable binary black-hole systems during the late inspiral and merger phases. Remarkably, for a subset of these equal-mass boson-star binaries (with certain phase offsets in the scalar-field profiles), we identify the excitation of subdominant odd $m$-multipoles in the gravitational-wave emission, absent in equal-mass nonspinning black-hole binaries. Despite differences in the phenomenology of binary boson-star and black-hole signals, injections of some boson-star signals into detector noise exhibit degeneracy with current waveform approximants. Building on these results, we demonstrate how inspiral-merger-ringdown consistency tests can overcome these degeneracies.

In this paper, nonperturbative lattice simulations are used to study Higgs preheating and the associated gravitational wave (GW) background after the inflaton $Z_2$ symmetry is broken during inflation. This symmetry breaking generates both trilinear and quartic inflaton-Higgs interactions during preheating. The quartic inflaton-Higgs coupling is characterized by $q_{\varphi h}\equiv {\lambda }_{\varphi h}/{\lambda }_{\varphi }$, while the trilinear interaction enters jointly through $q_{\varphi h}$ and $q_{\epsilon }\equiv m_{\varphi }/(\sqrt{{\lambda }_{\varphi }}{\varphi }_0)$. The Higgs self-coupling parameter $q_h\equiv {\lambda }_h/{\lambda }_{\varphi }$ determines the onset of backreaction through the effective mass induced by Higgs self-interactions. Our simulations show that efficient preheating requires both a sufficiently broad resonance band and delayed backreaction. For ${\lambda }_{\varphi }={10}^{-13}$, the viable parameter region is approximately $q_{\varphi h}>10$, $q_h<{10}^3$, and $q_{\epsilon }<{10}^{-5}$. Smaller $q_{\epsilon }$ keeps the system in a quartic-dominated regime and suppresses the rapid drift of resonance bands, while smaller $q_h$ delays the end of preheating by weakening self-interaction-induced backreaction. The amplified Higgs inhomogeneities source GW through the transverse-traceless part of the anisotropic stress tensor. The lattice results show that the GW spectrum grows rapidly during parametric resonance, broadens through rescattering, and saturates in the nonlinear stage. At late times, the spectrum develops a broad peak with amplitude ${\mathrm{\Omega }}_{\mathrm{gw}}\sim {10}^{-6}$ at production. After redshifting to the present day, the peak frequency is $f\sim {10}^9\mathrm{Hz}$ with present-day amplitude ${\mathrm{\Omega }}_{\mathrm{gw},0}{h}^2\sim {10}^{-10}$. These results suggest that high-frequency GW from Higgs preheating may be detectable by future resonant-cavity detectors.

We present a blind time-delay cosmography measurement of the Hubble-Lemaître constant $H_0$ based on the quadruply imaged quasar SDSSJ1433 + 6007. Our analysis combines deep Hubble Space Telescope imaging, extended time-delay monitoring from the Wendelstein and Maidanak Observatories, and spatially resolved stellar kinematics from the Keck Cosmic Web Imager and Reionization Mapper. We build a robust lens model to reconstruct the mass distribution and high-signal-to-noise kinematic maps to break the mass-sheet degeneracy, explicitly accounting for the lens galaxy’s oblateness, rotation, and anisotropy. Furthermore, we constrain the external convergence (${\kappa }_{\mathrm{ext}}$) by characterizing the line-of-sight environment using wide-field photometry from the Dark Energy Spectroscopic Instrument (DESI) Legacy Survey data release 10. We incorporate these constraints into our joint lensing and dynamical model, running multiple iterations to estimate random and systematic uncertainties. Accounting for maximal flexibility of the mass-sheet transformation, and assuming a flat $\mathrm{\Lambda }$ cold dark matter cosmology and an ${\mathrm{\Omega }}_{\mathrm{m},0}$ prior from DESI data release 2, we infer $H_0={73.2}_{-4.7}^{+4.8}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ (a 6.5% precision) and an internal mass-sheet parameter ${\lambda }_{\mathrm{int}}=1.12_{-0.06}^{+0.05}$. Notably, ${\lambda }_{\mathrm{int}}$ is $2\sigma$ away from unity for this system, highlighting the importance of treating it as a free parameter. Our $H_0$ measurement is consistent with the result from our 2025 milestone paper, and it will be included in our next hierarchical analysis to improve the overall precision. Moving forward, the comprehensive pipeline demonstrated herein establishes a robust framework that can be readily applied to future strongly lensed systems to further refine cosmological constraints.

Recent detections of very-high-energy [(VHE) $\gtrsim 100\mathrm{GeV}$] photons from afterglows like gamma-ray burst (GRB) 221009A demand models that treat jet angular structure, circumburst stratification, and inverse Compton radiation self-consistently. We present a numerical afterglow framework in which a Gaussian structured jet decelerates as an adiabatic forward shock in a wind-stratified medium ($\rho \propto r^{-2}$), and compute broadband synchrotron and synchrotron self-Compton (SSC) emission. Our approach aims to connect a wind-stratified environment with a Gaussian angular jet profile and to explore SSC emission to predict VHE afterglow detectability with the Cherenkov Telescope Array (CTA). We find that the TeV peak time and peak flux are primarily governed by jet geometry (ratio of ${\theta }_v/{\theta }_c$), together with wind-driven blast-wave deceleration and jet microphysics. In particular, variations in the jet kinetic energy ($E_k$), wind density parameter ($A_{\star }$), and the ratio of microphysical parameters (${\epsilon }_e$ and ${\epsilon }_B$) can change the TeV flux by orders of magnitude. Using the CTAO North differential sensitivity at 250 GeV, we simulate $4\times {10}^4$ wind-Gaussian afterglows and find that only $\sim 10\%$ satisfy our detectability criterion, primarily for bursts with high $E_k$, moderate to high $A_{\star }$, high ${\epsilon }_e$, and low ${\epsilon }_B$, and near on-axis jet geometries (${\theta }_v/{\theta }_c\lesssim 2$). Finally, fitting the multiband afterglow of GRB 221009A from radio to TeV, we obtain a mildly off-axis, SSC-dominated afterglow in a wind-driven medium that reproduces the observed broadband temporal evolution. The inferred parameters favor strong TeV SSC emission, implying that GRBs similar to GRB 221009A should be detectable by CTA.

Astrophysical formation channels of stellar-mass binary black holes (sBBHs) can induce significant orbital eccentricities in their early inspiral. We analyze the implications on the stochastic gravitational-wave background (SGWB) from unresolved sBBHs, which can be detected with the Laser Interferometer Space Antenna (LISA). We develop an improved SGWB model for the case of an idealized Dirac-delta eccentricity distribution, and extend it to the more astrophysical case of a thermal distribution. Using a fully Bayesian framework, we find that, if all binaries have a high initial eccentricity $e_0\gtrsim 0.9$ at an orbital frequency of $f_{\mathrm{orb}}={10}^{-4}\mathrm{Hz}$, the resulting SGWB can be robustly distinguished from a background of quasicircular sBBHs. For a thermal eccentricity distribution, the SGWB is consistent with a circular model when binaries form at $f_{\mathrm{orb}}={10}^{-5}\mathrm{Hz}$, but leads to significant systematic biases if formation occurs at $f_{\mathrm{orb}}={10}^{-4}\mathrm{Hz}$. We also show that, when eccentricity is properly accounted for, environmental effects such as dynamical friction can be distinguished from vacuum evolution, but only for sufficiently dense environments with gas densities $\rho \gtrsim {10}^{-7}\mathrm{g}{\mathrm{cm}}^{-3}$. Finally, we show that a LISA detection of the sBBH SGWB would place an upper bound on the maximum eccentricity of the sBBH population in the band of ground-based detectors, with direct implications for template modeling and data analysis. Our results highlight the importance of incorporating eccentricity in SGWB modeling to enable accurate astrophysical interpretation of LISA observations.

Primordial black holes (PBHs) in the asteroid-mass window (${10}^{17}–{10}^{22}\mathrm{g}$) can account for all of the dark matter without violating any observational constraint, yet are routinely dismissed as fine-tuned. I put that dismissal to the test by applying three complementary sensitivity measures uniformly across a broad landscape: three noninflationary PBH production mechanisms, six classes of inflationary PBH models, and seven particle dark matter benchmarks, all evaluated against the same observable target. Three distinct naturalness universality classes emerge, determined entirely by the analytic structure of the abundance map rather than by the nature of the dark matter candidate. Biased-domain-wall PBHs, in their least model-dependent (free-$V_b$) form, have the same low sensitivity, $\mathrm{\Delta }=4.5$, as off-resonance weakly interacting massive particles and freeze-in particles ($\mathrm{\Delta }=2$), a sensitivity that, because it is constant over the entire parameter space of the construction, also coincides trivially with its own Wilson-normalized average within that parameter space (Section Definition and conventions), an equivalence that concerns only the space over which $\mathrm{\Delta }$ is computed and is not a naturalness statement about the construction as a whole; a further reduction to $\mathrm{\Delta }=2$ is possible only under the additional, independently motivated but not required, assumption that the domain-wall bias is generated by Planck-suppressed operators; early matter-domination PBHs occupy an intermediate tier alongside coannihilating weakly interacting massive particles (WIMPs), unified by a structural identity in which the sensitivity measure equals the logarithm of the ratio of the formation scale to the matter–radiation equality scale; first-order phase transition PBHs, once the more accurate super-exponential collapse probability is used in place of the single-exponential approximation, instead belong to the same highly sensitive tier as resonant WIMP annihilation and single-field inflationary collapse, for a structurally distinct reason; single-field ultraslow-roll inflationary collapse is severely tuned for a distinct reason: a double exponential in which the power spectrum amplitude is itself exponentially sensitive to the inflaton potential coefficients, on top of the exponential collapse sensitivity of the abundance map. My main conclusion is that the claim that PBH dark matter is generically fine-tuned conflates the worst case with a landscape spanning every naturalness tier. The Barbieri-Giudice sensitivity computed here and the Wilson-normalized measure of Iovino and Riotto answer distinct and mutually consistent questions about the same construction, a distinction I clarify within the two-layer decomposition.

Motivated by the observed horizon-scale polarization variability in multiepoch Event Horizon Telescope (EHT) data, we develop a time-dependent general relativistic polarized ray-tracing framework to study how localized magnetic reconnection near a black hole imprints itself on synchrotron polarization. We model the reconnection phenomenologically as a transient magnetic bubble. While keeping the underlying spacetime, emission prescription, and polarization transport scheme unchanged, the bubble weakens the local ordered field, reorganizes its geometry, induces a conservative local kinematic response, and relaxes through a finite magnetic-memory prescription. We explore how the key bubble parameters control the recovery of the magnetic field strength, the spatial spreading of the disturbed region, the persistence of the post-reconnection imprint, and the resulting polarization signatures. Perturbations dominated by different magnetic field components produce distinct polarized morphologies, electric vector position angle (EVPA) evolution, and Stokes-$(Q,U)$ trajectories: azimuthal perturbations favor outward EVPA shifts, radial perturbations favor inward shifts, and vertical perturbations mainly suppress the polarized intensity. For the illustrative $\mathrm{M}{87}^{*}$-like cases, the phase evolution of the ${\beta }_2$ polarization coefficient further traces how localized magnetic reconfiguration contributes to the global EVPA twist of the ring. These results show that horizon-scale polarization variability can provide a sensitive diagnostic of localized magnetic restructuring and that the present framework offers a physically transparent way to interpret time-dependent polarimetric signatures in future EHT observations.

Recent observations by IceCube and KM3Net of PeV-scale ultra-high-energy (UHE) neutrinos, together with detections of TeV-PeV photons from various sources such as the Crab Nebula, the Galactic Center, and gamma-ray burst by ground-based observatories including Tibet $\mathrm{AS}\gamma$, MAGIC, Carpet-3, and LHAASO, point to the existence of extreme astrophysical environments capable of accelerating particles to ultra-high energies. These findings motivate investigations of possible connections between UHE neutrinos and photons in such environments. Theoretically, dense regions surrounding compact objects can efficiently produce UHE neutrinos. In this work, we calculate the production of UHE photons from neutrino-nucleon interactions, and note that if these interactions occur in the outer, optically thin regions of dense environments, the resulting photons could potentially be observed. In our model, an incident neutrino scatters off a nucleon, generating secondary partons that hadronize into pions and subsequently decay into UHE photons. We calculate the resulting photon energy spectra and find that for incident (anti)neutrinos with energies above 1 PeV, the probability of producing photons with energies exceeding 1 TeV is greater than 13%. As a concrete application, we show that this mechanism can quantitatively account for the preburst TeV photons observed in GRB 221009A, providing a natural explanation for both their energies and lead times. These findings establish a plausible mechanism linking UHE neutrino events to gamma-ray observations, providing new insights into hadronic processes in extreme astrophysical environments and supporting multimessenger astronomy studies.

We study induced (stimulated) scattering of linearly polarized, strong electromagnetic waves in pair plasmas, which is crucial for understanding the propagation of fast radio bursts (FRBs). Magnetars are the most promising progenitors of FRBs, and FRBs propagate through the magnetar wind and successfully escape before being significantly scattered. We revisit the steady-state solution of linearly polarized electromagnetic waves in pair plasmas with arbitrary amplitude, and demonstrate that the nonlinearity is characterized by the nonlinearity parameter $a_0{\omega }_{pe}/{\omega }_0$ rather than the dimensionless amplitude $a_0$, where ${\omega }_{pe}$ is the electron plasma frequency and ${\omega }_0$ is the wave frequency in the frame in which the plasma is at rest when the wave fields vanish. We follow the time evolution of the steady-state solution for the linear regime $a_0{\omega }_{pe}/{\omega }_0\ll 1$ by performing one-dimensional particle-in-cell simulations, and show that the conventional linear analysis of induced scattering assuming $a_0\ll 1$ is applicable even for $a_0>1$ when the Lorentz boost due to the plasma motion in the incident wave is considered. The saturation level is controlled by $a_0{\omega }_0/{\omega }_{pe}$, which corresponds to the ratio of the wave energy to the plasma energy, and the incident wave is hardly scattered for $a_0{\omega }_0/{\omega }_{pe}\gg 1$. We discuss the application of our results to FRBs.

Higher-order Laguerre-Gaussian (LG) laser modes have been proposed to reduce test-mass thermal noise in laser interferometric gravitational-wave detectors, owing to their more homogeneous intensity profiles compared to the currently employed fundamental Gaussian beam. However, LG beams such as the ${\mathrm{LG}}_{3,3}$ mode suffer significant beam quality degradation in Fabry-Perot arm cavities in gravitational-wave detectors with realistic state-of-the-art mirror surface figure errors, due to scattering into degenerate modes of the same order, which are resonantly enhanced by shared cavity resonance conditions. In this work, we investigate an alternative “doughnut-shaped” ${\mathrm{LG}}_{0,\ell }$-like mode, specifically the ${\mathrm{LG}}_{0,6}$ mode, and demonstrate strategies to improve its performance. These include the introduction of a tailored circular mirror mask with antireflective coating in the central region, which selectively increases the losses of parasitic degenerate modes while minimally impacting the ${\mathrm{LG}}_{0,6}$ mode due to its limited overlap with the masked area. We further assess the marginal benefits of anticipated improvements in mirror surface figure errors and the potential reduction of cavity finesse. We demonstrate that these strategies can reduce the average contrast defect by more than 2 orders of magnitude and lower the mode loss by nearly a factor of 5, achieving performance at or below the typical values observed in current detectors. This work opens up new research and development pathways for employing ${\mathrm{LG}}_{0,\ell }$-type modes that achieve significant thermal noise reduction while maintaining beam quality and optical performance comparable to current gravitational-wave interferometers.

AlGaAs/GaAs coatings are being considered as coating candidates for gravitational-wave detectors. In this paper we investigate the birefringence properties of this crystalline semiconductor material by modulating the optical illumination on the mirror coating and monitoring the induced birefringence. While the measured low-frequency birefringence values align with previous studies, we observed a frequency-dependent behavior in the illumination-to-birefringence coupling, characterized by a pole increasing with illumination intensity and a gain at zero frequency (DC gain) decreasing with illumination intensity. We developed a generic theoretical model based on a master equation to characterize the measurement results by considering photon-induced electric fields and electro-optic effects. This model can fit the frequency and intensity dependencies of the induced birefringence. Additionally, this model predicts a generation-recombination noise (GR noise) will be observable in the coating birefringence. While the presented measurement cannot predict the exact level of GR noise, for the frequency band and spot sizes relevant for gravitational-wave detectors we expect GR noise to be white below the pole frequency, scale with power the same way laser shot noise does, and for fixed power be independent of spot size.