In the next decade, the proposed Line Emission Mapper (LEM) telescope concept is poised to revolutionize Galactic and extragalactic x-ray sensitivity. The instruments aboard LEM feature unprecedented eV scale energy resolution and an effective area of $1600{\mathrm{cm}}^2$ at 0.5 keV. Such features are ideally suited to explore decaying dark matter candidates that predict X-ray signals, including axion-like particles and sterile neutrinos. We present the first forecast of LEM sensitivity to dark matter decays and find sensitivity to lifetimes beyond $\sim {10}^{32}\mathrm{s}$ in the keV range, surpassing current limits by several orders of magnitude. Notably, our results show that LEM will be the first ever instrument to probe such long dark matter lifetimes in any mass range for any decay channel.

The large number of $\tau$ leptons available at the Belle II experiment makes it possible to study their properties and the extent of their compositeness. We propose a strategy that relies on three observables defined in terms of elements of the polarization density matrix of the produced $\tau$ pairs, which we obtain via quantum tomography. We find that these observables can explore values of the magnetic dipole moment down to $6.3\times {10}^{-4}$, which is 2 orders of magnitude better than the current experimental limit, constrain the electromagnetic radius below $4.3\times {10}^{-3}\mathrm{fm}$, and exclude an electric dipole moment larger than $1.7\times {10}^{-17}\mathrm{e}\mathrm{cm}$. The quoted limits are at the 95% confidence level (CL) and are obtained for a benchmark integrated luminosity of $1{\mathrm{ab}}^{-1}$ and a Monte Carlo estimate of the statistical sensitivity.

The chirality-flipping operators of light fermions are currently poorly constrained by experimental analyses due to the lack of interference with standard model (SM) amplitudes in traditional observables. In this work, we propose to investigate the semileptonic scalar and tensor four-fermion operators of electron and quarks through the transverse double spin asymmetry (DSA) at Electron-Ion Collider, where both the electron and proton beams could be highly transversely polarized. Due to the chirality-flipping nature of these operators, we demonstrate that their interference with the SM results in an unsuppressed contribution to the DSA, and could lead to nontrivial azimuthal $\cos 2\varphi$ and $\sin 2\varphi$ distributions that are linearly dependent on their Wilson coefficients. This new method has the potential to significantly improve the current constraints on these scalar and tensor four-fermion operators without relying on theoretical assumptions about other types of new physics effects, particularly for the tensor-type operator of the $u$ quark. Additionally, our findings indicate that both the real and imaginary parts of these operators can be simultaneously constrained and offer a new opportunity for probing potential $CP$-violation effects. However, it is important to note that these results would be sensitive to the quark transversity distributions, which are currently poorly constrained by the experimental data, but could be significantly improved at the upcoming Electron-Ion Collider. Therefore, our work opens up a new avenue to utilize this new spin asymmetry for exploring the new physics effects from the scalar/tensor four-fermion operators.

We calculate the four-top-quark operator contributions to Higgs production via gluon fusion in the Standard Model effective field theory. The four-top operators enter for the first time via two-loop diagrams. Owing to their chiral structure they contain ${\gamma }_5$, so special care needs to be taken when using dimensional regularization for the loop integrals. We use two different schemes for the continuation of ${\gamma }_5$ to $D$ space-time dimensions in our calculations and present a mapping for the parameters in the two schemes. This generically leads to an interplay of different operators, such as four-top operators, chromomagnetic operators, or Yukawa-type operators at the loop level. We validate our results by examples of matching onto UV models.

In this paper, we use the charged-current Higgs boson production process at future electron-proton colliders, $e^{-}p\to Hj{\nu }_e$, with the subsequent decay of the Higgs boson into a $b\overline{b}$ pair, to probe the Standard Model effective field theory with dimension-six operators involving the Higgs boson and the bottom quark. The study is performed for two proposed future high-energy electron-proton colliders, the Large Hadron Electron Collider (LHeC) and the Future Circular Collider (FCC-he) at the center-of-mass energies of 1.3 TeV and 3.46 TeV, respectively. Constraints on the $CP$-even and $CP$-odd $Hb\overline{b}$ couplings are derived by analyzing the simulated signal and background samples. A realistic detector simulation is performed and a multivariate technique using the gradient boosted decision trees algorithm is employed to discriminate the signal from background. Expected limits are obtained at 95% confidence level for the LHeC and FCC-he assuming the integrated luminosities of 1, 2 and $10{\mathrm{ab}}^{-1}$. We find that using $1{\mathrm{ab}}^{-1}$ of data, the $CP$-even and $CP$-odd $Hb\overline{b}$ couplings can be constrained with accuracies of the order of ${10}^{-3}$ and ${10}^{-2}$, respectively, and a significant region of the unprobed parameter space becomes accessible.

The $D{D}^{*}$ scattering phase shifts in the $T_{cc}^{+}=cc\overline{u}\overline{d}$ channel are extracted from lattice QCD for five different charm quark masses and a fixed light-quark mass corresponding to $m_{\pi }\simeq 280\mathrm{MeV}$. The phase shifts are analyzed employing two approaches: effective range expansion and Lippmann-Schwinger equation derived in the effective field theory. In the latter case, the results imply an attraction at short range parametrized by contact terms and a slight repulsion at long range mediated by one-pion exchange with $m_{\pi }>m_{D^{*}}-m_D$. The poles in the amplitude across the complex energy plane are extracted and their trajectories are discussed as the charm quark mass is varied. Two complex conjugate poles corresponding to a resonance below threshold are found for $m_c$ close to the physical value. They turn into a pair of virtual states at the largest $m_c$ studied. With further increasing $m_c$, one virtual pole representing $T_{cc}^{+}$ is expected to move towards the two-body threshold and turn into a bound state. The light-quark mass dependence of the $T_{cc}^{+}$ pole is briefly discussed using the data on $D{D}^{*}$ scattering from other lattice collaborations.

We consider the quantum chromodynamics (QCD) Kondo effect for a single heavy quark in quark matter composed of light quarks with chiral symmetry breaking. Introducing several spinor structures in QCD Kondo condensates, i.e., particle-projected condensate, antiparticle-projected condensate, and normal condensate without projection, we calculate the attractive energy gained by the heavy quark within the mean-field approximation in the path-integral formalism. We show that the normal condensate is favored at low density and the particle-projected condensate is favored at high density, when the light quark has a nonzero mass. We interpret such a density-dependent transition between the two condensates in terms of the Kondo resonances.

We explore the consequences of imposing robust thermodynamic constraints arising from perturbative quantum chromodynamics (QCD) when inferring the dense-matter equation-of-state (EOS). We find that the termination density, up to which the EOS modeling is performed in an inference setup, strongly affects the constraining power of the QCD input. This sensitivity in the constraining power arises from EOSs that have a specific form, with drastic softening immediately above the termination density followed by a strong stiffening. We also perform explicit modeling of the EOS down from perturbative-QCD densities to construct a new QCD likelihood function that incorporates additional perturbative-QCD calculations of the sound speed and is insensitive to the termination density, which we make publicly available.

During galactic supernova (SN) explosions, a large amount of feebly interacting particles (FIPs) may be produced. In this work we analyze electrophilic FIPs with masses in the MeV-range that escape from SN and decay into electron-positron pairs, causing an exotic leptonic injection. This contribution adds up to known components, leading to an unexpected excess of x-ray fluxes generated by inverse-Compton scattering of the injected particles on low-energy photon backgrounds. For the first time in the context of FIPs, we use XMM-Newton x-ray measurements to obtain the strongest and most robust bounds on electrophilic FIPs produced by SN in our Galaxy.

We explore quantum corrections of electrically charged black holes subject to vacuum polarization effects of fermion fields in QED. Solving this problem exactly is challenging so we restrict to perturbative corrections that one can obtain using the heat kernel expansion in the one-loop effective action for electrons. Starting from the corrections originally computed by Drummond and Hathrell, we solve the full semiclassical Einstein-Maxwell system of coupled equations to leading order in Planck’s constant and find a new electrically charged, static black hole solution. To probe these quantum corrections, we study electromagnetic and gravitational (axial) perturbations on this background and derive the coupled system of Regge-Wheeler master equations that govern the propagation of these waves. In the classical limit, our results agree with previous findings in the literature. We finally compare these results with those that one can obtain by working out the Euler-Heisenberg effective action. We find again a new electrically charged static black hole spacetime and derive the coupled system of Regge-Wheeler equations governing the propagation of axial electromagnetic and gravitational perturbations. Results are qualitatively similar in both cases. We briefly discuss some challenges found in the numerical computation of the quasinormal mode frequency spectra when quantum corrections are included.

The backreaction of quantum fields in their vacuum state results in equilibrium structures that surpass the Buchdahl compactness limit. Such backreaction is encapsulated in the vacuum expectation value of the renormalized stress-energy tensor (RSET). In previous works we presented analytic approximations to the RSET, obtained by dimensional reduction, available in spherical symmetry, and showed that the backreaction-generated solutions described ultracompact fluid spheres with a negative mass interior. Here, we derive a novel approximation to the RSET that does not rely on dimensional reduction, but rather on a reduction of the differential order. This approximation also leads to regular stars surpassing the Buchdahl limit. We conclude that this is a consequence of the negative energies associated with the Boulware vacuum which, for sufficiently compact fluid spheres, makes the Misner-Sharp mass negative near the center of spherical symmetry. Our analysis provides further cumulative evidence that quantum vacuum polarization is capable of producing new forms of stellar equilibrium with robust properties across different analytical approximations to the RSET.

In this study, we first establish that gravity models incorporating matter-related terms, such as $f({\mathcal{L}}_{\mathrm{m}})$, $f(g_{\mu \nu }{T}^{\mu \nu })$, and $f(T_{\mu \nu }{T}^{\mu \nu })$, into the usual matter Lagrangian density ${\mathcal{L}}_{\mathrm{m}}$, are equivalent to general relativity (GR) with nonminimal matter interactions. Through the redefinition ${\mathcal{L}}_{\mathrm{m}}+f\to {\mathcal{L}}_{\mathrm{m}}^{\mathrm{tot}}$, these models are exactly GR, yet the usual material field $T_{\mu \nu }$ and its accompanying partner, namely, the modification field $T_{\mu \nu }^{\text{mod}}$, engage in nonminimal interactions. Specifically, ${\nabla }^{\mu }{T}_{\mu \nu }=-Q_{\nu }=-{\nabla }^{\mu }{T}_{\mu \nu }^{\text{mod}}$, where $Q_{\nu }$ is the interaction kernel that governs the rate of energy transfer. Our focus narrows on the specific model of $f(T_{\mu \nu }{T}^{\mu \nu })$, known as energy-momentum squared gravity (EMSG), where the usual material field $T_{\mu \nu }$ is accompanied by an energy-momentum squared field (EMSF), $T_{\mu \nu }^{\mathrm{emsf}}$, along with a sui generis nonminimal interaction between them. We demonstrate that a particular $T_{\mu \nu }^{\mathrm{emsf}}$ can be introduced by removing $\frac{{\partial }^2{\mathcal{L}}_{\mathrm{m}}}{\partial g^{\mu \nu }\partial g^{\sigma \epsilon }}$ (the new term emerging in models that incorporate scalars formed from $T_{\mu \nu }$), thanks to the freedom in determining the interaction kernel, but this approach compromises the Lagrangian formulation of EMSG. Additionally, we address the ambiguities regarding the perfect fluid stemming from this new term. We show the proper way of calculating this term for a perfect fluid, revealing that it is indeed nonzero, contrary to common assumption in the literature. Finally, we reexamine cosmological models within the realm of EMSG, offering new insights into the applicability and interpretation of our findings in EMSG and similar theoretical frameworks.

The determination of the polarization modes of gravitational waves (GWs) and their dispersion relations is a crucial task for scrutinizing the viability of extended theories of gravity. A tool to investigate the polarization states of GWs is the well-known formalism developed by Eardley, Lee, and Lightman [Phys. Rev. D 8, 3308 (1973).] which uses the Newman-Penrose (NP) coefficients to determine the polarization content of GWs in metric theories of gravity. However, if the speed of GWs is smaller than the speed of light, the number of NP coefficients is greater than the number of polarizations. To overcome this inconvenience we use the Bardeen formalism to describe the six possible polarization modes of GWs considering general dispersion relations for the modes. The definition of a new gauge-invariant quantity enables an unambiguous description of the scalar longitudinal polarization mode. We apply the formalism to general relativity, scalar-tensor theories, $f(R)$ gravity, and a wide class of quadratic gravity. To obtain a bridge between theory and experiment, we derive an explicit relation between a physical observable (the derivative of the frequency shift of an electromagnetic signal), and the gauge-invariant variables. From this relation, we find an analytical formula for the pulsar timing rms response to each polarization mode. To estimate the sensitivity of a single pulsar timing, we focus on the case of a dispersion relation of a massive particle. The sensitivity curves of the scalar longitudinal and vector polarization modes change significantly depending on the value of the effective mass. The detection (or absence of detection) of the polarization modes using the pulsar timing technique has decisive implications for alternative theories of gravity. Finally, investigating a cutoff frequency in the pulsar timing band can lead to a more stringent bound on the graviton mass than that presented by ground-based interferometers.

A general formalism to find the density profile of a slowly rotating stellar object in modified Einstein gravity is presented. We derive a general form of the modified Lane-Emden equation in the presence of rotation and a general form of its possible solutions under the slow rotation approximation for a wide class of modified Einstein gravity theories.

In some extensions of the standard model of particle physics, the values of the fundamental coupling constants vary in space and time. Some observations of quasars hint at time and spatial variation of the fine structure constant $\alpha$. Here, the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model (which posits the existence of a scalar field driving evolution in the fundamental electric charge $e$) is tested against quasar and Planck satellite cosmic microwave background (CMB) data. In this model, variations in $e$ are coupled to the matter density through a factor ${\zeta }_{\mathrm{m}}{M}_{\mathrm{pl}}^2/\overline{\omega }$, where ${\zeta }_m$ is the fractional electromagnetic contribution to nucleon rest masses, $\overline{\omega }$ is the energy-scale squared of new (BSBM) physics, and $M_{\mathrm{pl}}$ is the Planck energy scale. Simulations conducted here do not support claims that the electrostatic contribution to ${\zeta }_m$ is completely shielded. Other common approximations used in BSBM modeling are found to be adequate. Principal components of the CMB data with respect to variations in $\alpha$ are used to obtain constraints of ${\zeta }_{\mathrm{m}}{M}_{\mathrm{pl}}^2/\overline{\omega }\lesssim 9.3\times {10}^{-9}$ for a massless field. A forecast anticipating the promise of the Simons Observatory CMB experiment shows that SO will be sensitive to values of ${\zeta }_{\mathrm{m}}{M}_{\mathrm{pl}}^2/\omega \ge 2.2\times {10}^{-9}$, significantly improving on the uncertainty in $e$-variation from quasar spectra alone.

The compelling evidence for the detection of the stochastic gravitational wave background by NANOGrav imposes constraints on the mass of compact cores of ultra-light dark matter, also known as “solitons”, surrounding supermassive black holes found at the centers of large galaxies. The strong dynamical friction between the rotating black holes and the solitons competes with gravitational emission, resulting in a suppression of the characteristic strain in the nHz frequency range. Our findings robustly rule out ultralight dark matter particles with masses ranging from $1.3\times {10}^{-21}\mathrm{eV}$ to $1.4\times {10}^{-20}\mathrm{eV}$ condensing into solitons around supermassive black holes.

The equation of state dependence of a neutron star’s astrophysical features is key to our understanding of isospin asymmetric and dense matter. There exists a series of almost equation of state independent relations reported in the literature, called quasiuniversal relations, that are used to determine neutron star radii and moments of inertia from x-ray and gravitational wave signals. Using sets of equations of state constrained by multimessenger astronomy measurements and nuclear-physics theory, we discuss quasiuniversal relations in the context of future gravitational wave detectors Cosmic Explorer and Einstein Telescope, and the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays.. We focus on relations that involve the moment of inertia $I$, the tidal deformability $\mathrm{\Lambda }$, and the compactness $C$: $C(\mathrm{\Lambda })$, $I(\mathrm{\Lambda })$, and $I(C)$. The quasiuniversal fits and their associated errors are constructed with three different microphysics approaches which include state of the art nuclear physics theory and astrophysical constraints. Gravitational-wave and x-ray signals are simulated with the sensitivity of the next generation of detectors. Equation of state inference on those simulated signals is compared to determine if it will offer a better precision on the extraction of a neutron star’s macroscopic parameters than quasiuniversal relations. We confirm that the relation $I(\mathrm{\Lambda })$ offers a more pronounced universality than relations involving the compactness regardless of the equation of state set. We show that detections with the third generation of gravitational wave detectors and future x-ray detectors will be sensitive to the fit error marginalization technique. We also find that the sensitivity of those detectors will be sufficient in that using full equation of state distributions leads to significantly better precision on extracted parameters than quasiuniversal relations. We also note that nuclear physics theory offers a more pronounced equation of state invariance of quasiuniversal relations than current astrophysical constraints.

We study MeV-scale electrophilic feebly interacting particles (FIPs), that may be abundantly produced in supernova explosions, escape the star and decay into electrons and positrons. This exotic injection of leptons in the Milky Way leaves an imprint in both photon and cosmic ray fluxes. Specifically, positrons lose energy and annihilate almost at rest with background electrons, producing photons with 511 keV energy. In addition, electrons and positrons radiate photons through bremsstrahlung emission and upscatter the low-energy galactic photon fields via the inverse Compton process generating a broad emission from x-ray to $\gamma$ ray energies. Finally, electrons and positrons are directly observable in cosmic ray experiments. In order to describe the FIP-induced lepton injection in full generality, we use a model-independent parametrization which can be applied to a host of FIPs such as axionlike particles, dark photons and sterile neutrinos. Theoretical predictions are compared to experimental data to robustly constrain FIP-electron interactions with an innovative multimessenger analysis.

We measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam-sputtered ${\mathrm{SiO}}_2$ and ${\mathrm{Ta}}_2{\mathrm{O}}_5$) with and without ${\mathrm{TiO}}_2$ mixed into the ${\mathrm{Ta}}_2{\mathrm{O}}_5$ layers on sapphire disks between 6 and 77 K. The measured loss angle exhibited a temperature dependence, and the local maximum was found at approximately 20 K. This maximum was $7.0\times {10}^{-4}$ (with ${\mathrm{TiO}}_2$) and $7.7\times {10}^{-4}$ (without ${\mathrm{TiO}}_2$), although the previous measurement for the coating on sapphire disks showed almost no temperature dependence [Phys. Rev. D 74, 022002 (2006)]. We evaluated the coating thermal noise in KAGRA and discussed future investigation strategies.