Search

We propose that the non-observation of WIMPs may be explained by dark matter primarily annihilating to the darker concealed sector while coupling to the standard model with only minimal strength. To demonstrate this scenario, we focus on the WIMP dark matter candidate from a $U(1)_x$ hidden sector, which couples more strongly to another concealed $U(1)_c$ sector than to the standard model. We explore two possible cases for the evolution of dark particles among hidden sectors: (1) The WIMP annihilates efficiently and achieves the observed relic density with the assistance of the concealed sector. (2) The WIMP transforms into another type of dark matter within the concealed sector and attains the observed relic density. Annihilating to the darker explains why WIMPs have remained undetected, and all WIMP models will continue to hold interest., Comment: 5 pages, 3 figures

The evolution of superconducting quantum processors is driven by the need to reduce errors and scale for fault-tolerant computation. Reducing physical qubit error rates requires further advances in the microscopic modeling and control of decoherence mechanisms in superconducting qubits. Piezoelectric interactions contribute to decoherence by mediating energy exchange between microwave photons and acoustic phonons. Centrosymmetric materials like silicon and sapphire do not display piezoelectricity and are the preferred substrates for superconducting qubits. However, the broken centrosymmetry at material interfaces may lead to piezoelectric losses in qubits. While this loss mechanism was predicted two decades ago, interface piezoelectricity has not been experimentally observed in superconducting devices. Here, we report the observation of interface piezoelectricity at an aluminum-silicon junction and show that it constitutes an important loss channel for superconducting devices. We fabricate aluminum interdigital surface acoustic wave transducers on silicon and demonstrate piezoelectric transduction from room temperature to millikelvin temperatures. We find an effective electromechanical coupling factor of $K^2\approx 2 \times 10^{-5}\%$ comparable to weakly piezoelectric substrates. We model the impact of the measured interface piezoelectric response on superconducting qubits and find that the piezoelectric surface loss channel limits qubit quality factors to $Q\sim10^4-10^8$ for designs with different surface participation ratios and electromechanical mode matching. These results identify electromechanical surface losses as a significant dissipation channel for superconducting qubits, and show the need for heterostructure and phononic engineering to minimize errors in next-generation superconducting qubits.

We study the mass spectra and radiative decays of $D_{s0}^*(2317)$ and $D_{s1}^{\prime}(2460)$ in an unquenched framework. In addition to coupled channel effects between the $c\bar{s}$ cores and $D^{(*)}K$ channels, $D^{(*)}K$-$D^{(*)}K$ self interactions are also considered in this work and we succeed to reproduce their mass spectra. Furthermore, we study the radiative decays of the $D_{s0}^*(2317)$ and $D_{s1}^{\prime}(2460)$ by simultaneously including the compound structures of conventional $c\bar{s}$ cores and $D^{(*)}K$ components. We also calculate their bottom analogs with heavy quark symmetry. Our study offers useful insights into the important unquenched effects in the formation of $D_{s0}^*(2317)$, $D_{s1}^{\prime}(2460)$ and the bottom counterparts., Comment: 18 pages, 3 tables, 7 figures

The mass transfer process is prevalent during the inspiral phase of compact binary systems. Our study focuses on systems comprising low-mass white dwarfs, particularly in neutron star-white dwarf binaries and double white dwarf binaries, where a stable mass transfer process occurs at low frequencies. By analyzing the evolution of gravitational wave frequencies in the presence of mass transfer within quasi-circular orbits, we derive an analytical expression for the time-dependent frequency across different frequency bands and the waveforms emitted by compact binaries. Considering gravitational waves emitted by compact binaries in the $1\thicksim10$ mHz band, based on the Fisher analysis, we find that the mass transfer rate can be measured as accurately as $10^{-7} M_\odot/\text{year}$ by space-based gravitational-wave detectors with a signal-to-noise ratio of the order of $10^3$. Including the mass transfer effect in the waveforms provides a new possibility to measure the individual masses of double white dwarf binaries. The relative error of measured white dwarf masses can be down to the order of $0.01$.

Silicon nanomechanical resonators display ultra-long lifetimes at cryogenic temperatures and microwave frequencies. Achieving quantum control of single-phonons in these devices has so far relied on nonlinearities enabled by coupling to ancillary qubits. In this work, we propose using atomic forces to realize a silicon nanomechanical qubit without coupling to an ancillary qubit. The proposed qubit operates at 60 MHz with a single-phonon level anharmonicity of 5 MHz. We present a circuit quantum acoustodynamics architecture where electromechanical resonators enable dispersive state readout and multi-qubit operations. The combination of strong anharmonicity, ultrahigh mechanical quality factors, and small footprints achievable in this platform could enable quantum-nonlinear phononics for quantum information processing and transduction.

We study the Lamb shift by considering the steady-state heat current through two coupled two-level atoms, which, respectively, interact with a heat reservoir at a certain temperature. It is found that the Lamb shift significantly alters the energy levels. In particular, it is shown that the heat current will approach an upper bound if the Lamb shift isn't considered, while the heat current will break the upper bound if the Lamb shift is considered. This finding can deepen our understanding of Lamb shift in the quantum thermodynamic field.

The galactic 511~keV photon signal can be fully explained by the decaying dark matter generated through the freeze-in mechanism. The explanation of the 511~keV signal requires an extremely tiny coupling between the decaying dark matter and $e^+e^-$ pair and thus cannot be generated via direct freeze-in from standard model particles. We construct models involving two $U(1)$ hidden sectors, one of which couples directly to the standard model, the other couples directly to the first hidden sector while couples indirectly to the standard model. The decaying dark photon dark matter, which explains the 511~keV signal, is generated via a two-step freeze-in process. In the models we study, the freeze-in mechanism generates the entire dark matter relic density, and thus any types of additional dark matter components produced from other sources are unnecessary. The two-$U(1)$ model remains a strong candidate for explaining the 511 keV signal consistent with various dark matter density profiles., Comment: 26 pages, 3 figures

In non-central heavy-ion collisions, the quark-gluon plasma (QGP) encounters the most intense magnetic field ever produced in nature, with a strength of approximately 10$^{19 - 20}$ Gauss. Recent lattice-QCD calculations reveal that the QGP exhibits paramagnetic properties at high temperatures. When an external strong magnetic field is applied, it generates an anisotropic squeezing force density that competes with pressure gradients resulting from the purely QGP geometric expansion. In this study, we employ (3+1)-dimensional ideal hydrodynamics simulations to estimate the paramagnetic squeezing effect of this force density on the anisotropic expansion of QGP in non-central Pb+Pb collisions at the Large Hadron Collider (LHC). We consider both up-to-date magnetic susceptibility and various magnetic field profiles in this work. We find that the impact of rapidly decaying magnetic fields is insignificant, while enduring magnetic fields produce a strong force density that diminishes the momentum anisotropy of the QGP by up to 10% at the intial stage, leaving a visible imprint on the elliptic flow $v_{2}$ of final charged particles. Our results provide insights into the interplay between magnetic fields and the dynamics of QGP expansion in non-central heavy-ion collisions., Comment: 11 pages, 8 figures, published in Phys. Rev. C 110, 014902 (2024)

In this paper, we investigate the quasi-two-body decays $B_c \to K_0^{*}(1430,1950) D_{(s)} \to K \pi D_{(s)}$ within the perterbative QCD (PQCD) framework. The S-wave two-meson distribution amplitudes (DAs) are introduced to describe the final state interactions of the $K\pi$ pair, which involve the time-like form factors and the Gegenbauer polynomials. In the calculations, we adopt two kinds of parameterization schemes to describe the time-like form factors: One is the relativistic Breit-Wigner (RBW) formula, which is usually more siutable for the narrow resonances, and the other is the LASS line shape proposed by the LASS Collaboration, which includes both the resonant and nonresonant components. We find that the branching ratios and the direct CP violations for the decays $B_c \to K_0^{*}(1430) D_{(s)}$ obtained from those of the quasi-two-body decays $B_c \to K_0^{*}(1430) D_{(s)} \to K \pi D_{(s)}$ under the narrow width approximation (NWA) can be consistent well with the previous PQCD results calculated in the two-body framework by assuming that $K^*_0(1430)$ being the lowest lying $\bar q s$ state, which is so-called scenario II (SII). We conclude that the LASS parameterization is more siutable to describe the $K_0^{*}(1430)$ than the RBW formula, and the nonresonant components play an important role in the branching ratios of the decays $B_c \to K_0^{*}(1430) D_{(s)} \to K \pi D_{(s)}$. In view of the large difference between the decay width measurements for the $K_0^{*}(1950)$ given by BaBar and LASS collaborations, we calculate the branching ratios and the CP violations for the quasi-two-body decays $B_c \to K_0^{*}(1950) D_{(s)} \to K \pi D_{(s)}$ by using two values, $\Gamma_{K^*_0(1950)}=0.100\pm0.04$ GeV and $\Gamma_{K^*_0(1950)}=0.201\pm0.034$ GeV, besides the two kinds of parameterizations for the resonance $K^*_0(1950)$., Comment: 16 pages, 3 figures

Dark matter (DM) constitutes the predominant portion of matter in our universe. Despite compelling evidence, the precise characteristics of DM remain elusive. Among the leading DM candidates are weakly interacting massive particles, which may aggregate into steep concentrations around the central black holes of galaxies, forming dense spikes. Employing Schwarzschild geometry, we assess the density and velocity distribution of DM within such spikes. Through variations in black hole masses and dark halo parameters, we identify universal features in the density profile of DM and fit them with Gaussian distributions. Additionally, we investigate the impact of dynamical friction on gravitational waves (GWs) generated by extreme-mass-ratio inspirals (EMRIs) within DM spikes. Our findings uncover phase shifts in the time-domain waveform, potentially providing significant insights for the GW-based detection of DM in galactic centers., Comment: 15 pages, 5 figures

In this work, we estimate the relativistic corrections to the energy spectrum of hydrogen resulting from the full one-photon-exchange interaction using a highly precise numerical method. In the frame of the effective Schrodinger-like equation, which is derived exactly from the Bethe-Salpeter equation in quantum electrodynamics, we express the effective potential corresponding to the full one-photon-exchange interaction in terms of eight scalar functions. Unlike the usual calculations performed in the references, where the effective potential is expanded in terms of momenta order by order, we retain the exact momentum dependence in the effective potential to estimate its corrections to the energy spectrum of hydrogen using a highly precise numerical method. We also use the same numerical method to double-check the results in the non-relativistic case to ensure accuracy. We discuss the comparison of the numerical results with those obtained using the usual bound-state perturbative theory. Our calculations suggest that it is possible to accurately account for all the relativistic contributions using this method. It would be interesting to extent these calculations to positronium, muonic hydrogen, and cases involving nuclear structure and radiative corrections., Comment: 2 figures

The performance of superconducting quantum circuits is primarily limited by dielectric loss due to interactions with two-level systems (TLS). State-of-the-art circuits with engineered material interfaces are approaching a limit where dielectric loss from bulk substrates plays an important role. However, a microscopic understanding of dielectric loss in crystalline substrates is still lacking. In this work, we show that boron acceptors in silicon constitute a strongly coupled TLS bath for superconducting circuits. We discuss how the electronic structure of boron acceptors leads to an effective TLS response in silicon. We sweep the boron concentration in silicon and demonstrate the bulk dielectric loss limit from boron acceptors. We show that boron-induced dielectric loss can be reduced in a magnetic field due to the spin-orbit structure of boron. This work provides the first detailed microscopic description of a TLS bath for superconducting circuits, and demonstrates the need for ultrahigh purity substrates for next-generation superconducting quantum processors.

Subsystems of a composite system in a pure state generally exist in mixed states and undergo changes with the overall state. This phenomenon arises from the coherence of the entire system and represents a crucial distinction between quantum and classical systems. Such a quantum property can enhance the work of an Otto heat engine, where two coupled qubits serve as the working substance, allowing situations in which negative work output initially occurred to now yield positive work. We utilize the imagery of Maxwell's demon to explain the reason for positive work in this Otto cycle, attributing it to the increased coherence after the mutual measurement of the two subsystems. Conversely, the quantum measurement-erase cycle typically outputs negative work, attributed to the decoherence of the instrument during the measurement process., Comment: 8 pages, 4 figures

Few-shot class-incremental learning (FSCIL) aims at recognizing novel classes continually with limited novel class samples. A mainstream baseline for FSCIL is first to train the whole model in the base session, then freeze the feature extractor in the incremental sessions. Despite achieving high overall accuracy, most methods exhibit notably low accuracy for incremental classes. Some recent methods somewhat alleviate the accuracy imbalance between base and incremental classes by fine-tuning the feature extractor in the incremental sessions, but they further cause the accuracy imbalance between past and current incremental classes. In this paper, we study the causes of such classification accuracy imbalance for FSCIL, and abstract them into a unified model bias problem. Based on the analyses, we propose a novel method to mitigate model bias of the FSCIL problem during training and inference processes, which includes mapping ability stimulation, separately dual-feature classification, and self-optimizing classifiers. Extensive experiments on three widely-used FSCIL benchmark datasets show that our method significantly mitigates the model bias problem and achieves state-of-the-art performance., Comment: 8 pages (not including references and checklist)

The structure of data organization is widely recognized as having a substantial influence on the efficacy of machine learning algorithms, particularly in binary classification tasks. Our research provides a theoretical framework suggesting that the maximum potential of binary classifiers on a given dataset is primarily constrained by the inherent qualities of the data. Through both theoretical reasoning and empirical examination, we employed standard objective functions, evaluative metrics, and binary classifiers to arrive at two principal conclusions. Firstly, we show that the theoretical upper bound of binary classification performance on actual datasets can be theoretically attained. This upper boundary represents a calculable equilibrium between the learning loss and the metric of evaluation. Secondly, we have computed the precise upper bounds for three commonly used evaluation metrics, uncovering a fundamental uniformity with our overarching thesis: the upper bound is intricately linked to the dataset's characteristics, independent of the classifier in use. Additionally, our subsequent analysis uncovers a detailed relationship between the upper limit of performance and the level of class overlap within the binary classification data. This relationship is instrumental for pinpointing the most effective feature subsets for use in feature engineering., Comment: 98 pages, 69 figures

In this study, we conducted a detailed analysis of the core parameter of Warm Higgs Inflation (WHI) $-$ the dissipation coefficient ($Q$). As a crucial parameter in the warm inflation process, $Q$ exerts profound influences on the entire evolutionary process. By meticulously deriving the relationships between various quantities and $Q$, we successfully circumvented the common preconceptions regarding strong and weak dissipation, laying the foundation for a more accurate exploration of their interconnections. Taking into account the constraints imposed by Cosmic Microwave Background, we observed that the dissipation coefficient $Q$ remains at extremely low levels throughout the entire warm inflation process, i.e., $Q \ll 1$. This observation indicates that WHI falls under the category of weakly dissipative warm inflation. Despite being weakly dissipative, $Q$ still plays a crucial role in the evolution of temperature, energy, and other quantities, highlighting its significance and non-negligibility. We delved deeper into the impact of the primordial power spectrum on the dissipation coefficient $Q$ during the warm inflation process, discovering that the dependency is not significant. Consequently, this naturally leads to the unobtrusive dependence of the gravitational wave power spectrum on $Q$. Finally, we found that gravitational waves generated by WHI hold the potential for verification in future observational experiments, especially through the SKA100 experiment. These findings provide a theoretical support for a more profound understanding of the early evolution of the universe., Comment: 16 pages, 5 figures

We investigate the role of the electromagnetic interaction in the formation and decay of the $X(3872)$. The binding properties of the $X(3872)$ are studied by assuming the molecular nature and considering the $S$-$D$ wave mixing, isospin breaking, and coupled channel effects, and in particular the correction from the electromagnetic interaction. The radiative decays can better reflect the difference between the charged and neutral $D\bar D^*$ components, since the electromagnetic interaction explicitly breaks the isospin symmetry. We further study the radiative decay widths with the obtained wave functions for different $D\bar D^*$ channels. We also explore other similar hidden-charm molecular states. The electromagnetic interaction can make the molecule tighter. Our result of the radiative decay width for $X(3872)\rightarrow \gamma J/\psi$ is in agreement with the experiment. The branching ratio $R_{\gamma\psi}$ is less than 1 in our framework, which supports the Belle and BESIII measurements., Comment: 17 pages, 8 figures, 7 tables

We conduct a comprehensive study on the sub-GeV millicharge dark matter produced through the freeze-in mechanism. We discuss in general the mixing mechanism, encompassing both kinetic mixing and mass mixing, between the $U(1)_X$ hidden sector and the standard model, which can generate millicharge carried by the dark fermions from the hidden sector. We discuss in depth how such millicharge is generated, and clarify several misunderstandings regarding this subject in the literature. Without employing an effective field theory approach, where the photon field directly mixed with the additional $U(1)$, we analyze a general renormalizable model and investigate the complete evolution of the hidden sector particles. Due to the substantial self-interactions among hidden sector particles, the evolution of the hidden sector temperature plays a crucial role, which is addressed concurrently with the number densities of hidden sector particles by solving a set of coupled Boltzmann equations. We thoroughly examine eight benchmark models from six distinct cases. Some of our key findings from the analysis of these benchmark models may be generalizable and applicable to broader freeze-in scenarios. We also explore the possibility that the $\mathcal{O}$(keV) $U(1)_X$ dark photon is a viable dark matter candidate, even though it can contribute at most $\sim 5\%$ to the total observed dark matter relic density., Comment: 55 pages, 13 figures

The overhead to construct a logical qubit from physical qubits rapidly increases with the decoherence rate. Current superconducting qubits reduce dissipation due to two-level systems (TLSs) by using large device footprints. However, this approach provides partial protection, and results in a trade-off between qubit footprint and dissipation. This work introduces a new platform using phononics to engineer superconducting qubit-TLS interactions. We realize a superconducting qubit on a phononic bandgap metamaterial that suppresses TLS-mediated phonon emission. We use the qubit to probe its thermalization dynamics with the phonon-engineered TLS bath. Inside the phononic bandgap, we observe the emergence of non-Markovian qubit dynamics due to the Purcell-engineered TLS lifetime of 34 $\mu s$. We discuss the implications of these observations for extending qubit relaxation times through simultaneous phonon protection and miniaturization., Comment: 13 pages, 10 figures

Smith-Purcell radiation (SPR) occurs when an electron skims above a spatial grating, but the fixed momentum compensation from the static grating imposes limitations on the emission wavelength. It has been discovered that a temporally periodic system can provide energy compensation to generate light emissions in free space. Here, we introduce temporal SPR (t-SPR) emerging from a time grating and propose a generalized t-SPR dispersion equation to predict the relationship between radiation frequency, direction, electron velocity, modulation period, and harmonic orders. Compared to conventional SPR, t-SPR can: 1) Provide a versatile platform for manipulating SPR emission through temporal modulation (e.g., period, amplitude, wave shape). 2) Exhibit strong robustness to the electron-grating separation, alleviating the constraints associated with extreme electron near-field excitation. 3) Introduce additional energy channels through temporal modulation, enhancing and amplifying emission., Comment: 6 pages, 3 figures

This study aims at modeling the universal failure in preventing the outbreak of COVID-19 via real-world data from the perspective of complexity and network science. Through formalizing information heterogeneity and government intervention in the coupled dynamics of epidemic and infodemic spreading; first, we find that information heterogeneity and its induced variation in human responses significantly increase the complexity of the government intervention decision. The complexity results in a dilemma between the socially optimal intervention that is risky for the government and the privately optimal intervention that is safer for the government but harmful to the social welfare. Second, via counterfactual analysis against the COVID-19 crisis in Wuhan, 2020, we find that the intervention dilemma becomes even worse if the initial decision time and the decision horizon vary. In the short horizon, both socially and privately optimal interventions agree with each other and require blocking the spread of all COVID-19-related information, leading to a negligible infection ratio 30 days after the initial reporting time. However, if the time horizon is prolonged to 180 days, only the privately optimal intervention requires information blocking, which would induce a catastrophically higher infection ratio than that in the counter-factual world where the socially optimal intervention encourages early-stage information spread. These findings contribute to the literature by revealing the complexity incurred by the coupled infodemic-epidemic dynamics and information heterogeneity to the governmental intervention decision, which also sheds insight into the design of an effective early warning system against the epidemic crisis in the future., Comment: This version contains the full-resolution figures for the paper DOI: 10.1007/s11071-023-08427-5

Influence maximization (IM) aims to select a small number of nodes that are able to maximize their influence in a network and covers a wide range of applications. Despite numerous attempts to provide effective solutions in ordinary networks, higher-order interactions between entities in various real-world systems are not usually taken into account. In this paper, we propose a versatile meta-heuristic approach, hyper genetic algorithm (HGA), to tackle the IM problem in hypergraphs, which is based on the concept of genetic evolution. Systematic validations in synthetic and empirical hypergraphs under both simple and complex contagion models indicate that HGA achieves universal and plausible performance compared to baseline methods. We explore the cause of the excellent performance of HGA through ablation studies and correlation analysis. The findings show that the solution of HGA is distinct from that of other prior methods. Moreover, a closer look at the local topological features of the seed nodes acquired by different algorithms reveals that the selection of seed nodes cannot be based on a single topological characteristic, but should involve a combination of multiple topological features to address the IM problem.

We perform the potential analysis for the holographic Schwinger effect in spinning Myers-Perry black holes. We compute the potential between the produced pair by evaluating the classical action of a string attaching on a probe D3-brane sitting at an intermediate position in the AdS bulk. It turns out that increasing the angular momentum reduces the potential barrier thus enhancing the Schwinger effect, consistent with previous findings obtained from the local Lorentz transformation. In particular, these effects are more visible for the particle pair lying in the transversal plane compared with that along the longitudinal orientation. In addition, we discuss how the Schwinger effect changes with the shear viscosity to entropy density ratio at strong coupling under the influence of angular momentum., Comment: to be published in Chinese Physics C

Hypernetwork is a useful way to depict multiple connections between nodes, making it an ideal tool for representing complex relationships in network science. In recent years, there has been a marked increase in studies on hypernetworks, however, the comparison of the difference between two hypernetworks has been given less attention. This paper proposes a hyper-distance-based method (HD) for comparing hypernetworks. This method takes into account high-order information, such as the high-order distance between nodes. The experiments carried out on synthetic hypernetworks have shown that HD is capable of distinguishing between hypernetworks generated with different parameters, and it is successful in the classification of hypernetworks. Furthermore, HD outperforms current state-of-the-art baselines to distinguish empirical hypernetworks when hyperedges are disrupted.

In this work, we investigate the semileptonic decays of $B_{c}$ meson to $\eta_{c}(1S,2S,3S)$, $\psi(1S,2S,3S)$ and $X(3872)$ within the framework of covariant light-front quark model (CLFQM). We combine the helicity amplitudes via the corresponding form factors to obtain the branching ratios of the semileptonic decays $B_{c}\to \eta_{c}(1S, 2S, 3S)\ell\nu_{\ell}$, $B_{c}\to \psi(1S, 2S, 3S))\ell\nu_{\ell}$ and $B_{c}\to X(3872)\ell\nu_{\ell}$ with $\ell=e,\mu,\tau$. In view of the $R_{J/\Psi}$ anomaly released by the LHCb collaboration, it is necessary to calculate the ratios $R_X$ with $X=\psi(1S,2S,3S),\eta_c(1S,2S,3S),X(3872)$ systematically, which are helpful to check the lepton flavor universality (LFU). Furthermore, we also take into account another two physical observables, one is the longitudinal polarization fraction $f_{L}$ and the other is the forward-backward asymmetry $A_{FB}$, which can provide new clues to understand the $R_{J /\Psi}$ anomaly. Such theoretical predictions are necessary and interesting, which can be tested in the future LHCb experiments., Comment: 23 pages, 4 figures