A bstract The computation of bounce action in a phase transition involves solving partial differential equations and may introduce numerical uncertainties. Deriving characteristic temperatures and transition parameters often involves differentiating or integrating the action, which can amplify residual numerical fluctuations. In this work, we fit the action curve as a function of temperature to mitigate the uncertainties inherent in the calculation of the phase transition parameters. We find that, after extracting a factor, the sixth-order polynomial yields an excellent fit for the action in the high temperature approximated potential. In a realistic model, the singlet extension of the Standard Model, this method performs satisfactorily across most of the parameter space after trimming the fitting data. It not only enhances the accuracy of phase transition calculations but also systematically reduces computation time and facilitates error estimation, particularly in models involving multiple scalar fields. Based on this approach, we discussed the possibility of using multiple neural networks to predict the action curve from model parameters.

Cosmological first-order phase transitions are a well-motivated source of stochastic gravitational waves (GWs), but most predictions are made based on the highly idealized model of perfectly spherical vacuum bubbles, neglecting thermal fluctuations. In this work we use (3+1)-dimensional lattice simulations of a scalar model with thermal initial conditions to quantify how thermal fluctuations distort bubble profiles and modify the resulting GW spectrum. We find that thermal fluctuations can strongly break spherical symmetry at early times, allowing even an isolated bubble to emit GWs. In multi-bubble simulations, thermal fluctuations systematically reshape the spectrum, suppressing the infrared part while enhancing and broadening the high-k tail. We further provide an analytical estimate for the ultraviolet regime of the GW spectrum, which is in good agreement with our lattice results and suggests that this regime is dominated by thermal fluctuations. These effects could leave observable imprints in future GW searches.

In this work, we demonstrate the complete process of using space-based gravitational wave detectors to measure properties of the stochastic gravitational wave background arising from a first-order electroweak phase transition. Based on frequency-domain simulations of the Taiji mission, including instrumental noise and astrophysical foregrounds, we perform parameter inference using both the Fisher information matrix and Bayesian Markov Chain Monte Carlo sampling. We show how the reconstructed spectrum constrains the macroscopic parameters of the phase transition, and further how these constraints map onto the underlying particle-physics parameters in a singlet-extended Standard Model. Our results demonstrate that the Higgs cubic and quartic self-couplings can be significantly constrained using gravitational wave observations, despite limitations arising from parameter degeneracy.

Scalar-mediated interactions may exist among neutrinos, dark matter particles, or between the two. Double β-decay experiments provide a powerful tool to probe such exotic interactions. Using ^{136}Xe double β-decay data from PandaX-4T, we perform the first direct spectral search in the energy range of 20 to 2800 keV, setting the most stringent limits to date on scalar-mediated neutrino self-interactions for mediator masses below 2  MeV/c^{2}. These results place significant constraints on models invoking such interactions to alleviate the Hubble tension. Assuming the same scalar also mediates dark matter self-interactions, constraints on the dark matter-scalar interactions can be placed in conjunction with cosmological constraints.

Crystal electromagnetic calorimeters (ECALs) are essential for high-precision measurements of electrons and photons in particle physics experiments. However, the conventional design, in which long crystal bars point radially toward the interaction region and lack longitudinal segmentation, is incompatible with the three-dimensional shower imaging required by Particle Flow Approach (PFA). We propose a novel perspective on crystal ECAL design to address this limitation. The key innovation is a geometric reconfiguration in which crystal bars are oriented to face the interaction region and arranged orthogonally in adjacent longitudinal layers. This layout achieves fine spatial segmentation of energy deposits by correlating measurements of orthogonal crystal bars. An interleaved structure of regular and inverted trapezoidal modules is incorporated to maximize structural uniformity and detector hermeticity. This design is engineered to preserve the excellent intrinsic energy resolution of crystal ECALs while simultaneously providing the detailed three-dimensional shower imaging essential for PFA. Simulation results confirm the feasibility of achieving excellent energy resolution of 1.14%/√(E)⊕ 0.44%. Consequently, the proposed design repositions crystal ECAL as a foundational component for PFA-oriented detector systems at facilities such as the Circular Electron Positron Collider (CEPC), offering a new technical pathway to advance the physics goals of future colliders.

The continuous spectrum of double beta decay (ββ) provides a sensitive probe to test the predictions of the standard model and to search for signatures of new physics beyond it. We present a comprehensive analysis of the ^{136}Xe ββ spectrum utilizing 39.1±0.7  kg·yr of ^{136}Xe exposure from the PandaX-4T experiment. The analysis yields the most precise measurement to date of the ^{136}Xe two-neutrino double beta decay (2νββ) half-life, (2.14±0.05)×10^{21}  years, the uncertainty of which is reduced by a factor of 2 compared to our previous result. We measure the parameter ξ_{31}^{2ν}, defined as the ratio between the subleading and leading components of the ^{136}Xe 2νββ nuclear matrix element, to be 0.59_{-0.38}^{+0.41}, which is consistent with theoretical predictions. We also search for Majoron-emitting modes of ^{136}Xe ββ, establishing the most stringent limit for the spectral index n=7.

This corrects the article DOI: 10.1103/rtnh-jn8s.

Based on 6.1  fb^{-1} of e^{+}e^{-} annihilation data collected at center-of-mass energies from 4.600 to 4.843 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis of Λ_{c}^{+}→Λπ^{+}η is performed, and branching fractions and decay asymmetry parameters of intermediate processes are determined. The process Λ_{c}^{+}→Λa_{0}(980)^{+} is observed for the first time, and evidence for the pentaquark candidate Σ(1380)^{+} decaying into Λπ^{+} is found with statistical significance larger than 3σ with mass and width fixed to theoretical predictions. The branching fraction product B[Λ_{c}^{+}→Λa_{0}(980)^{+}]B[a_{0}(980)^{+}→π^{+}η] is determined to be (1.05±0.16_{stat}±0.05_{syst}±0.07_{ext})%, which is larger than theoretical calculations by 1-2 orders of magnitude. Here the third (external) systematic is from B(Λ_{c}^{+}→Λπ^{+}η). Finally, we precisely obtain the absolute branching fraction B(Λ_{c}^{+}→Λπ^{+}η)=(1.94±0.07_{stat}±0.11_{syst})%.

In this study, we investigate the relativistic effects of Earth on Hong-Ou-Mandel (HOM) interference experiments conducted in a terrestrial laboratory. Up to the second order, we calculate the relativistic time delay from the null geodesic equation (particle perspective), and the phase shift, along with the associated effective time delay, from the Klein-Gordon equation (wave perspective). Since gravity influences both the temporal and spatial parts of the phase shift, these time delays differ and predict different coincidence probabilities. The previous HOM experiment conducted on a rotating platform shows that the wave perspective can explain the experimental results. We further explore the frame-dragging and redshift effects within an arbitrarily oriented rectangular interferometer in two distinct cases, finding that both effects can be amplified by increasing the number of light loops. Additionally, we emphasize that the next-leading order Sagnac effect, due to the gravitational acceleration, is comparable to the Thomas precession, geodetic effect, and Lense-Thirring effect. To detect the leading-order Sagnac effect and the redshift effect caused by the gravitational acceleration within the current experimental precision, we determine the number of loops photons should travel in the interferometer. Furthermore, we suggest using the difference between any two HOM patterns with different effective time delays as a probe to detect the influence of relativistic effects on quantum systems.

We perform a search of double beta decay of 136Xe to the excited state, 0_1^+ , of 136Ba (2νββ- 0_1^+ ), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2νββ- 0_1^+ is observed, resulting in a lower limit on half-life of 7.5 × 1022 yr at the 90

The Circular Electron-Positron Collider (CEPC), a proposed next-generation Higgs factory, provides new opportunities to explore physics beyond the Standard Model (SM). With its clean electron-positron collision environment and the ability to collect large samples of Higgs, W, and Z bosons, the CEPC enables precision measurements and searches for new physics. This white paper outlines the CEPC's discovery potential, including studies of exotic decays of the Higgs, Z, and top quarks, dark matter and dark sector phenomena, long-lived particles, supersymmetry, and neutrino-related signatures. Advanced detector technologies and reconstruction techniques, such as one-to-one correspondence reconstruction and jet origin identification, significantly improve sensitivity to rare and weakly interacting processes. The CEPC is particularly well suited to probe the electroweak phase transition and test models of electroweak baryogenesis and dark sector interactions. In addition, global fit analyses highlight the CEPC's complementary role in constraining a wide range of new physics scenarios. These features position the CEPC as a powerful tool for exploring the next frontier in fundamental particle physics in the post-Higgs discovery era.

We present a novel constraint on light dark matter utilizing 1.54 metric ton/year of data acquired from the PandaX-4T dual-phase xenon time projection chamber. This constraint is derived through detecting electronic recoil signals resulting from the interaction with solar-enhanced dark matter flux. Low-mass dark matter particles, lighter than a few MeV/c^{2}, can scatter with the thermal electrons in the Sun. Consequently, with higher kinetic energy, the boosted dark matter component becomes detectable via contact scattering with xenon electrons, resulting in a few keV energy deposition that exceeds the threshold of PandaX-4T. We calculate the expected recoil energy in PandaX-4T considering the Sun's acceleration with heavy mediators and the detection capabilities of the xenon detector. The first experimental search results using the xenon detector yield the most stringent upper limits cross section of 3.51×10^{-39}  cm^{2} at 0.08  MeV/c^{2} for a solar boosted dark matter mass ranging from 0.02 to 10  MeV/c^{2}, achieving a 23-fold improvement compared with earlier experimental studies.

Axionlike particles (ALPs) and dark photons (DPs) are viable dark matter particle candidates. We have searched for possible ALP/DP signals in the PandaX-4T liquid xenon detector using 440 kg·yr of data. A binned likelihood fit is constructed to search for possible mono-energetic peaks induced by the absorption processes between ALPs/DPs and atomic electrons of xenon. A detailed temporal model of decays associated with xenon isotopes is introduced to constrain the number of background events. No signal excess over background expectations is observed, and we have established the most stringent exclusion limits for most ALP/DP masses across the range of 150 keV/c^2 to 1 MeV/c^2. The improvement is particularly significant within the mass range of 150-400 keV/c^2, with the average factor of 3.5 compared to previous results.

We study the regularization of a spin-1/2 fieldin the vacuum state in de Sitter space. We find that the 2nd order adiabatic regularization is sufficient to remove all UV divergences for the spectral stress tensor, as well as for the power spectrum. The regularized vacuum stress tensors of the massive field is maximally symmetric with the energy density remaining negative, and behaves as a “negative" cosmological constant. In the massless limit it reduces smoothly to the zero stress tensor of the massless field, and there is no trace anomaly. We also perform the point-splitting regularization in coordinate space, and obtain the analytical, regularized correlation function and stress tensor, which agree with those from the adiabatic regularization. In contrast, the 4th order regularization is an oversubtraction, and changes the sign of the vacuum energy density. In the massless limit the 4th order regularized auto-correlation becomes singular and the regularized stress tensor does not reduce to the zero stress tensor of the massless field. These difficulties tell that the 4th order regularization is inadequate for the spin-1/2 massive field.

In this Letter, we report the dark matter search results from the commissioning run (Run0) and the first science run (Run1) of the PandaX-4T experiment. The two datasets were processed with a unified procedure, with the Run1 data treated blindly. The data processing is improved compared to previous work, unifying the low-level signal reconstruction in a wide energy range up to 120 keV. With a total exposure of 1.54  tonne·year, no significant excess of nuclear recoil events is found. The lowest 90% confidence level exclusion on the spin-independent cross section is 1.6×10^{-47}  cm^{2} at a dark matter mass of 40  GeV/c^{2}. Our results represent the most stringent constraint for a dark matter mass above 100  GeV/c^{2}.

We derive a compact analytical expression for the growth factor Υ, which characterizes how the gravitational wave spectrum sourced by sound waves evolves in a universe with a generic expansion history. Assuming the dominant energy density scales as ρ∝ a^-3(1+w), we obtain Υ=2[1-y^3(w-1)/2]/3(1-w), where y = a(t)/a(t_s) is the ratio of the scale factor at a later time t to that at t_s when gravitational wave production from sound waves starts. This general result reduces to known forms in radiation-and matter-dominated eras, thereby extending previous formulas to a broader class of cosmological backgrounds. The derivation assumes only that the source is stationary, making Υ a universal factor that captures gravitational wave growth in various scenarios-not limited to phase transitions.

Nuclear β decay, a sensitive probe of nuclear structure and weak interactions, has become a precision test bed for physics beyond the Standard Model, driven by recent advances in spectrometric techniques. Here we introduce tomographic β-γ spectroscopy (TBGS) of nuclear β decay, a method that detects the energies of β, γ, and internal conversion electrons while simultaneously reconstructing the energy deposition vertices. Using the PandaX-4T detector operated as a TBGS, we obtain a precise and unbiased decay scheme of ^214Pb, a key background isotope in searches for dark matter and Majorana neutrinos. For the first time, transitions of ^214Pb to both the ground and excited states of ^214Bi are measured concurrently, revealing discrepancies in branching ratios of up to 4.7σ relative to previous evaluations. Combined with state-of-the-art theoretical spectral-shape calculations, these results establish a new benchmark for background modelling in rare-event searches and highlight the potential of TBGS as a versatile tool for fundamental physics and nuclear applications.

In recent years, the prospect of detecting gravitational waves sourced from a strongly first-order cosmological phase transition has emerged as one of the most exciting frontiers of gravitational wave astronomy. Cosmological phase transitions are an essential ingredient in the Standard Model of particle cosmology, and help explain the mechanism for creation of matter in the early Universe, provide insights into fundamental theories of physics, and shed light on the nature of dark matter. This underscores the significance of developing robust end-to-end tools for determining the resulting gravitational waves from these phase transitions. In this article we present PhaseTracer2, an improved version of the C++ software package PhaseTracer, designed for mapping cosmological phases and transitions in Standard Model extensions of multiple scalar fields. Building on the robust framework of its predecessor, PhaseTracer2 extends its capabilities by including new features crucial for a more comprehensive analysis of cosmological phase transitions. It can calculate more complex properties, such as the bounce action through the path deformation method or an interface with BubbleProfiler, thermodynamic parameters, and gravitational wave spectra. Its applicability has also been broadened via incorporating the dimensionally reduced effective potential for models obtained from DRalgo, as well as calculations in the MSbar and OS-like renormalisation schemes. This modular, flexible, and practical upgrade retains the speed and stability of the original PhaseTracer, while significantly expanding its utility.

We develop a framework based on the full one-loop finite-temperature effective potential model, within which the bubble wall velocity is calculated using the local thermal equilibrium (LTE) approximation, and the kinetic energy fraction K is computed directly. In cosmological phase transitions, these quantities play a critical role in determining the resulting gravitational wave signals. Using the xSM as a benchmark model, we compute the peak gravitational wave spectra under different methods for determining the wall velocity and the kinetic energy fraction K, and compare these results to those obtained using the commonly employed bag model. Within the scanned parameter space, we find: (1) Deflagration is the most prevalent mode of fluid motion.(2) Gravitational wave spectra based on the full effective potential with LTE-derived wall velocity and integrated K can differ significantly from those using the bag model with fitted K. In the deflagration regime, discrepancies reach up to 48% in peak frequency and 90% in amplitude. (3) The bag model provides a good approximation to the full equation of state in many cases. Notably, in deflagration scenarios with input wall velocity, the gravitational wave spectra obtained from the bag model more closely resemble the LTE-based results than those derived using the full potential with this input wall velocity (https://github.com/bwlte2025/bubblewall_LTE).

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term 12ζ(Aμ;μ)2. We obtain the vacuum stress tensor, which consists of the transverse, longitudinal, temporal, and GF parts, and each contains various UV divergences. By the minimal subtraction rule, we regularize each part of the stress tensor to its pertinent adiabatic order. The transverse stress tensor is regularized to the 0th adiabatic order, while the longitudinal, temporal, and GF stress tensors are regularized to the 2nd adiabatic order. The resulting total regularized vacuum stress tensor is convergent and maximally symmetric, has a positive energy density, and respects the covariant conservation, and thus, it can be identified as the cosmological constant that drives the de Sitter inflation. Under the Lorenz condition Aμ;μ=0, the regularized Stueckelberg stress tensor reduces to the regularized Proca stress tensor that contains only the transverse and longitudinal modes. In the massless limit, the regularized Stueckelberg stress tensor becomes zero, and is the same as that of the Maxwell field with the GF term, and no trace anomaly exists. If the order of adiabatic regularization were lower than our prescription, some divergences would remain. If the order were higher, say, under the conventional 4th-order regularization, more terms than necessary would be subtracted off, leading to an unphysical negative energy density and the trace anomaly simultaneously.