The “Taiji program in space” for space-based gravitational wave detection will open a window for observing gravitational waves in the mid-to-low-frequency range (0.1 mHz to 1 Hz). Taiji-1 satellite as the inaugural satellite, was launched on 31 August 2019, to validate several critical technologies. The radio-frequency ion micro-propulsion (RMP) as a primary propulsion system underwent in-orbit flight validation. All four thrusters successfully completed crucial performance tests. This paper will present the primary in-orbit test results. It is found that the thrust range of the RMP is $5\mathrm{\mu }\mathrm{N}$ to $52\mathrm{\mu }\mathrm{N}$. When a thrust of $50\mathrm{\mu }\mathrm{N}$, the thrust noise is approximately $0.4\mathrm{\mu }\mathrm{N}/\surd \mathrm{Hz}$ (10 mHz–1 Hz), which is 2 to 4 times that of low-thrust ($5\mathrm{\mu }\mathrm{N}$). The thrust response time is about 20 ms. The characterization of RMP in orbit has reached the expectations of Taiji-1 satellite mission. The current in-orbit performance (thrust resolution, thrust noise and lifetime) of the RMP falls short of the goals set by the “Taiji program in space” and requires further enhancement.

The trouble in detecting low mass dark matter due to its low kinetic energy can be ameliorated in the boosted dark matter framework, where a subpopulation of galactic dark matter attains very high energy after being upscattered by energetic standard model particles. However, in such a scenario the upper limits on the cross section obtained hitherto are typically large. Hence in the minimal extension of standard model where new mediators act as a portal between the dark and visible sectors, the direct detection limits for sub-GeV dark matter might lie within the exclusion region of other ground based searches of the mediator. To evade this deadlock, we allude to blazar boosted dark matter electron scattering in multiton neutrino detector Super-Kamiokande. We consider minimal models such as axionlike particle (ALP) and vector portal dark matter being upscattered by high energy blazar jet and analyse the interesting parameter reaches from Super-Kamiokande in the parameter space of the mediator, surpassing the existing constraints. Besides, this scenario exhibits stronger limits for previously unexplored ALP mediated sub-MeV dark matter search which is difficult due to associated momentum suppression.

In this article, we solve the instantaneous Bethe-Salpeter equation with Cornell potential and Coulomb potential and conduct a meticulous study of the mass spectrum and wave function of toponium. Our investigation reveals that, owing to the exceedingly heavy mass of the top quark, the mass splitting between singlet and triplet states, as well as within the triplet states, is negligible. Consequently, relativistic corrections can be safely disregarded in the study of toponium. As such, we present the nonrelativistic wave functions for $S$-wave, $P$-wave, and $D$-wave toponia and study the decays ${\eta }_t\to \gamma \gamma$, ${\eta }_t\to gg$, and $\mathrm{\Theta }\to {\ell }^{+}{\ell }^{-}$.

A stable numerical solution of the impact-parameter-dependent next-to-leading order Balitsky-Kovchegov equation is presented for the first time. The rapidity evolution of the dipole amplitude is discussed in detail. Dipole amplitude properties, such as the evolution speed or anomalous dimension behavior, are studied as a function of the impact parameter and the dipole size and compared to solutions of the impact-parameter-dependent leading-order Balitsky-Kovchegov equation with the collinearly improved kernel. The next-to-leading evolution presented here also strongly suppresses the Coulomb tails compared to the collinearly improved and leading order solutions. The origin of this behavior is explored and potential avenues to improve the next-to-leading order kernel are pointed out.

In this work, we have studied, for the first time, the impact of a realistic picture of a time dependent electric and magnetic field on the shear and bulk viscosities of the medium. Both the electric and magnetic fields are considered to be exponentially decaying with time, and the study is valid in the regime where the magnetic field strength is weak ($eB\ll T^2$). The evaluation has been done in the kinetic theory framework wherein we have solved the relativistic Boltzmann transport equation within the relaxation time approximation collision kernel. We have shown that the constant weak field results as well as the $B=0$ results in the literature can be obtained as special cases of our general results. We have observed that the shear and bulk viscosities increase with time or equivalently, increase with the decrease of the strength of the magnetic field. To connect these observations with experiments, we have calculated the thermalization time, shear viscosity to entropy ratio ($\eta /s$), and bulk viscosity to entropy ratio ($\zeta /s$).

Scenarios for secluded WIMP (weakly interactive dark matter) dark matter models have been extensively studied in simplified versions. This paper shows a complete UV realization of a secluded WIMP dark matter model with an extra Abelian gauge symmetry that includes two-component dark matter candidates, where the dark matter conversion process plays a significant role in determining the relic density in the Universe. The model contains two new unstable mediators: a dark Higgs and a dark photon. It generates Dirac neutrino masses and can be tested in future direct detection experiments of dark matter. The model is also compatible with cosmological and theoretical constraints, including the branching ratio of Standard Model particles into invisible, big bang nucleosynthesis restrictions, and the number of relativistic degrees of freedom in the early Universe, even without kinetic mixing.

The gauged $U(1{)}_{L_{\mu }-L_{\tau }}$ model is well motivated to explain the muon $g-2$ anomaly and dark matter in previous studies. However, the latest $\mathrm{NA}64\mu$ experiment has almost excluded all the parameter space for the muon $g-2$, which indicates that the light dark matter benchmark scenarios interpreting muon $g-2$ in previous studies are under significant pressure at present. In light of many recent and future experimental results, we revisit the minimal $Z^{\prime }$ portal dark matter in $U(1{)}_{L_{\mu }-L_{\tau }}$. Focusing on the phenomenology of dark matter $\chi$, we first explore the viable parameter space for light dark matter under various tight constraints. Around the $Z^{\prime }$ resonance, we find that there is still a large parameter space for $m_{\chi }\gtrsim 10\mathrm{MeV}$ via thermal freeze-out. On the other hand, the constraints on $Z^{\prime }$ above the electroweak scale are quite loose, but are less studied for dark matter. We also investigate the heavy dark matter around the TeV scale and corresponding constraints. A large part of the parameter space for dark matter is within the reach of future experiments.

We study the boosted dark matter scenario in a two-component model. We consider a neutrinophilic two-Higgs doublet model, which consists of one extra Higgs doublet and a light right-handed neutrino. This model is extended with a light ($\sim 10\mathrm{MeV}$) singlet scalar dark matter (DM) ${\varphi }_3$, which is stabilized under an extra dark $Z_2^{\mathrm{DM}}$ symmetry and can only effectively annihilate through the $CP$ even scalar $H$. Although the presence of a light scalar $H$ modifies the oblique parameters to put tight constraints on the model, the introduction of vectorlike leptons can potentially salvage the issue. The vectorlike doublet $N$ and singlet $\chi$ are also stabilized through dark $Z_2^{\mathrm{DM}}$ symmetry. The lightest vectorlike mass eigenstate (${\chi }_1\sim 100\mathrm{GeV}$) is the second DM component of the model. Individual scalar and fermionic DM candidates have $\mathrm{Higgs}/Z$ mediated annihilation, restricting the fermion DM in a narrow mass region while a somewhat broader mass region is allowed for the scalar DM. However, when two DM sectors are coupled, the annihilation channel ${\chi }_1{\chi }_1\to {\varphi }_3{\varphi }_3$ opens up. As a result, the fermionic relic density decreases, and paves the way for a broader fermionic DM mass region with underabundant relic: a region of [30–65] GeV compared to a narrower [40–50] GeV window for the single component case. On the other hand, the light DM ${\varphi }_3$ acquires significant boost from the annihilation of ${\chi }_1$, causing a dilution in the resonant annihilation of ${\varphi }_3$. This in turn increases the scalar DM relic, allowing for a smaller mass region compared to the individual case. The exact and underabundant relic is achievable in a significant parameter space of the two-component model where the total DM relic is mainly dominated by the fermionic DM contribution. The scalar DM is found to be subdominant or equally dominant ($\sim 30\%–80\%$ of total DM) with significant boost, which can be detected in experiments.

We conduct a comprehensive analysis of potential annihilation processes of light dark matter (DM) in minimal Higgs portal models near the supermassive black hole [Sagittarius ${}^{\star }$ (Sgr ${\mathrm{A}}^{\star }$)] in the Galactic Center, considering interactions between DM particles mediated by either a light scalar or pseudoscalar with couplings $c_s$ and $c_p$. Accelerated by the supermassive black hole, DM particles can reach velocities up to half the speed of light, significantly enhancing the $p$-wave annihilation cross section, allowing forbidden annihilation channels within specific mass ranges and producing unique $\gamma$-ray spectral signals. Utilizing $\gamma$-ray observation from the Fermi Large Area Telescope in the direction of Sgr ${\mathrm{A}}^{\star }$, we constrain light DM parameter in the mass range of 0.3–10  GeV. Our results indicate that the couplings $c_s$ and $c_p$ are constrained to the order of ${10}^{-5}$, corresponding to a DM annihilation cross section as low as ${10}^{-38}{\mathrm{cm}}^3/\mathrm{s}$. In the future, the Very Large Gamma-Ray Space Telescope, with a larger detection area and broader detection range from 1 MeV to 1 TeV, will enhance our ability to probe sub-GeV DM and offer the opportunity to further study the forbidden annihilation scenario.

In this work, we derive the Newman-Penrose formalism of Maxwell’s equations using two approaches: differential forms and intrinsic derivatives. Denoting $(k_{AF}{)}^{\mu }$ as $k^{\mu }$, with $k^{\mu }=(k^t,k^r,0,0)$ in spherically symmetric spacetimes, we show that the expansion in $r^{-1}$ fails to produce consistent, closed solutions due to the inability to separate Lorentz-violating (LV) phase factors, as the Lorentz-invariant (LI) null tetrad does not adapt to the LV wavefront. Moreover, with exact formal solutions, we demonstrate that the expansion is nonperturbative in the LV parameter $k^2\equiv k^t-k^r$. For $r\gg 1/k^2$, higher powers of $k^2$ dominate over lower powers, as the latter decay more rapidly with increasing $r$. Although the Coulomb mode ${\varphi }_1\sim \mathcal{O}(\ln r/r^2)$ deviates from the LI expectation $\mathcal{O}(r^{-2})$ due to LV corrections, the leading outgoing radiation mode remains unaffected, i.e., ${\varphi }_2\sim \mathcal{O}(r^{-1})$. Given the constraint $|k_{AF}|\le {10}^{-44}\mathrm{GeV}$ [L. Caloni et al., J. Cosmol. Astropart. Phys. 03 (2023) 018.], the three complex scalars ${\varphi }_a$ ($a=0$, 1, 2) still obey the peeling theorem: ${\varphi }_a\sim \mathcal{O}(r^{(a-3)})$, $a=0$, 1, 2 for large, finite distances.

We examine the vacuum stability of gauge symmetry breaking in five dimensions, compactified on the $S_1/({\mathbb{Z}}_2\times {\mathbb{Z}}_2^{\prime })$ orbifold. We consider $\mathrm{SU}(N)$, $\mathrm{Sp}(N)$, $\mathrm{SO}(2N)$, and $\mathrm{SO}(2N+1)$ theories in the bulk, and provide an exhaustive classification of possible parity assignments that lead to stable orbifolds and of the corresponding symmetry breaking patterns. We use these results in the search for viable asymptotic grand unification theories (aGUT), testing the stability criteria on models based on SU(6) and SU(8). As a result, we identify two viable aGUTs: a unique SU(6) pathway down to the Standard Model, and one SU(8) model leading to an intermediate Pati-Salam partial unification.

We discuss triple $Z^{\prime }$ boson signatures via the decay chain of $Z\to Z^{\prime }\varphi \to Z^{\prime }{Z}^{\prime }{Z}^{\prime }$, with a new light scalar $\varphi$, at future $Z$ factories such as the Circular Electron-Positron Collider and Future Circular Collider. These new bosons $\varphi$ and $Z^{\prime }$ naturally appear in models with a new $U(1)$ gauge symmetry which is spontaneously broken and introduced in various new physics scenarios. The branching ratio of $Z\to Z^{\prime }\varphi \to Z^{\prime }{Z}^{\prime }{Z}^{\prime }$ can be larger than ${10}^{-12}$, which gives $O(1)$ events at Tera-$Z$ experiments, when a product of $g_X$ (new gauge coupling) and $\zeta$ ($Z-Z^{\prime }$ mixing) is larger than around ${10}^{-6}$. We find that the search for $Z\to Z^{\prime }{Z}^{\prime }{Z}^{\prime }$ can significantly improve the current bound on a kinetic mixing parameter $\epsilon$ in the dark photon case, where $e\epsilon \gtrsim \mathcal{O}({10}^{-5})$ with $g_X=\mathcal{O}(1)$ can be explored at Tera-$Z$ experiments. We also show that a sufficiently large number of events with multilepton plus hadronic jets can be obtained in benchmark points, which cannot be realized by the usual decay of $Z$ in the standard model.

Recent measurements of the lepton flavor universality ratios $R_K^{\mu e}$ and $R_{K^{*}}^{\mu e}$ in $B\to (K,K^{*}){\mu }^{+}{\mu }^{-}(e^{+}{e}^{-})$ at LHCb align with the Standard Model predictions, necessitating a search for the complementary decay modes. In this context, we derive the angular decay distributions of the four-fold $B\to K_1(1270,1400)(\to VP){\mu }^{+}{\mu }^{-}$ decays, where $K_1$ is an axial-vector meson, $V=\rho ,K^{*}$, and $P=K$, $\pi$. Considering the weak effective Hamiltonian with vector and axial-vector new physics operators, and employing the helicity formalism, we obtain the four-dimensional differential decay distributions and extract various physical observables. These include differential branching ratios, lepton forward-backward asymmetry, forward-backward asymmetry for transversely polarized $K_1$ meson, longitudinal and transverse polarization fractions of the $K_1$ meson, and normalized angular coefficients with longitudinally and transversely polarized final state vector meson $V$, in the cascade decay $K_1\to VP$. Based on the latest global fit analysis data for all $b\to s$ transitions, we predict the results of these observables in various bins of the square of the momentum transfer $(q^2)$. Our findings indicate that these physical observables are not only sensitive to new physics but also can differentiate between various new physics scenarios in certain kinematical regions. The precise measurements of these observables in $B\to K_1(1270,1400)(\to VP){\mu }^{+}{\mu }^{-}$ decays in current and future experiments will provide opportunities for complementary searches for physics beyond the Standard Model in $b\to s{\ell }^{+}{\ell }^{-}$ decays.

We identify the matrix elements necessary to determine the leading-twist gluon generalized parton distributions (GPDs) $H_g,E_g,{\tilde{H}}_g,{\tilde{E}}_g,H_g^T,E_g^T,{\tilde{H}}_g^T,{\tilde{E}}_g^T$ in lattice QCD calculations. We present a method to achieve a Lorentz-covariant parameterization of the matrix elements in terms of a linearly independent basis of tensor structures. This parameterization is crucial for projecting lattice QCD matrix elements onto light cone distributions. For the first time, we determine the corresponding components that project onto the linear combinations of invariant amplitudes, which reduce to the different gluon GPDs in the light cone limit and enable their separation in a lattice QCD calculation for spin-0 and spin-$\frac{1}{2}$ hadrons. Hence, this work lays the foundation for the numerical determination of the gluon GPDs from first-principle lattice QCD calculations, directly advancing our understanding of the mass and spin structures and mechanical properties of the nucleon, as well as the physics underlying deeply virtual Compton scattering and deeply virtual meson production in a range of experimental processes.

We investigate how a magnetic background field influences the location and the nature of the Roberge-Weiss (RW) finite temperature transition for $N_f=2+1$ quantum chromodynamics (QCD) with physical quark masses. To that purpose, we perform numerical simulations of the finite temperature theory, discretized through stout staggered quarks and the tree-level improved Symanzik pure gauge action, considering two different values of the Euclidean temporal extent in lattice units, $N_t=6$, 8. The RW transition temperature $T_{RW}$ decreases with $eB$, in particular it follows closely the behavior of the pseudocritical QCD crossover temperature $T_{pc}$, so that $T_{RW}(eB)-T_{pc}(eB)$ is practically constant, within errors, for magnetic fields up to $eB\sim 1{\mathrm{GeV}}^2$; consistent results are found from the drop of the chiral condensate, which signals chiral symmetry restoration, leading also to the phenomenon of inverse magnetic catalysis above the transition. Moreover, we find that the magnetic field turns the RW transition from second order to first order, with a tricritical magnetic field in-between 1 and $2.4{\mathrm{GeV}}^2$, i.e., for magnetic fields substantially lower than those for which the standard QCD transition turns to first order.

We compute the leading clustering (Abelian nonglobal) logarithms, which arise in the distribution of nonglobal QCD observables when final-state partons are clustered using the $k_t$ jet algorithm, up to six loops in perturbation theory. Our calculations are based on the recently introduced formula for the analytic structure of $k_t$ clustering [Phys. Rev. D 111, 014027 (2025).]. These logarithms exhibit a pattern of exponentiation and are subsequently resummed into an exponential form. We compare this resummed result with all-orders numerical calculations.

We compute the contribution of the gluonic component of the energy-momentum tensor (EMT) to the angular momentum (AM) density in various decompositions. We use the light-front Hamiltonian technique, and a two-component formalism in light-front gauge, where the constrained degrees of freedom are eliminated. Instead of a nucleon, we consider a simple composite spin-$1/2$ state, namely a quark dressed with a gluon. We present two-dimensional light-front distributions in transverse impact parameter space and compare the different angular momentum decompositions at the density level. Incorporating also the contribution coming from the quark part of the EMT, we verify the spin sum rule for such a state.

The next-to-next-to-leading-order (NNLO) perturbative QCD (pQCD) predictions for both the decay width and the transition form factor in the ${\eta }_c\to \gamma \gamma$ process, based on nonrelativistic QCD (NRQCD), deviate from precise experimental measurements. These significant discrepancies have cast doubt on the applicability of NRQCD to charmonium processes. In this paper, we analyze the ${\eta }_c\to \gamma \gamma$ process by applying the principle of maximum conformality (PMC), a systematic method for eliminating renormalization scheme-and-scale ambiguities. The PMC renormalization scales are determined by absorbing the nonconformal $\beta$-terms that govern the behavior of the QCD running coupling via the renormalization group equation. We obtain the PMC scale $Q_{\star }=4.49m_c$ for the ${\eta }_c\to \gamma \gamma$ decay width. Even after using the PMC method, the convergence of the pQCD series is still poor, which indicates the importance of uncalculated next-to-next-to-next-to-leading-order and higher-order terms. The resulting value for ${\mathrm{\Gamma }}_{{\eta }_c\to \gamma \gamma }$ is in agreement with the Particle Data Group’s reported value of ${\mathrm{\Gamma }}_{{\eta }_c\to \gamma \gamma }=5.1\pm 0.4\mathrm{keV}$ within the bounds of uncertainties. Moreover, the transition form factor obtained using the PMC is also in good agreement with precise experimental measurements. The application of the PMC suggests a potential resolution to ${\eta }_c\to \gamma \gamma$ puzzle and supports the applicability of NRQCD to charmonium processes.

The hadronic ${\mathrm{\Sigma }}_c^{(*)}{\overline{D}}^{(*)}$ and ${\mathrm{\Sigma }}_c^{(*)}{D}^{(*)}$ interactions are revisited, with a focus on their short-range parts, motivated by a tension between the interpretations of $P_{c\overline{c}}(4312)$, $P_{c\overline{c}}(4440)$, and $P_{c\overline{c}}(4457)$ in effective field theory (EFT) frameworks and the one-boson-exchange (OBE) model. While the three states can be interpreted as ${\mathrm{\Sigma }}_c{\overline{D}}^{(*)}$ molecular states within EFT frameworks, this is not feasible in the single-channel OBE model with consistent cutoff. In this work, the possibility to reconcile OBE model with EFTs by refitting the $\rho$-, $\omega$-, and $\sigma$-exchange interaction is explored and ruled out. It is pointed out that the problem in OBE arises from the strong short-range spin-dependent one-pion-exchange (OPE) interaction and the fixed signs of other short-range interactions in OBE model also prevent the cancellation. To address this issue, the short-range subtraction strategies within the OBE model are revisited. Two subtraction schemes are explored: removing the delta-function from all interactions and eliminating it only from the pseudoscalar-meson-exchange component. These schemes favor different spin assignments for $P_{c\overline{c}}(4440)$ and $P_{c\overline{c}}(4457)$. Though solving the problem, there is no clear dynamical picture to support the subtraction schemes. We propose a new quark-exchange mechanism motivated by the Pauli principle. Different from the two subtraction schemes in OBE, the quark-exchange mechanism offers an explanation grounded in microscopic dynamics. It is shown that the spin-dependent quark-exchange interaction cancels those from OPE. The differences in the predictions for the spin, isospin, and open-charm partner states of the experimental $P_{c\overline{c}}$ states offer a way to distinguish between the subtracted OBE model and the OBE model with quark-exchange contributions.