In this article, we study the associated production of a singly charged (${\mathrm{\Delta }}^{+}$) scalar along with a $W^{+}$ boson in the newly proposed ${\mu }^{+}{\mu }^{+}$ collider (also known as $\mu \mathrm{TRISTAN}$) at $\sqrt{s}=2\mathrm{TeV}$. Such a singly charged scalar is naturally accommodated in an extremely well-motivated neutrino mass model, namely, the Type-II seesaw model. This model, besides providing a viable explanation of neutrino mass generation, also allows for lepton flavor violating (LFV) processes. Since LFV processes are not allowed in the standard model (SM), we focus on the discovery prospect of the singly charged scalar in the Type-II seesaw model at $\mu \mathrm{TRISTAN}$ through a LFV process, owing to the advantage of this process being free of any SM background. Additionally, this article also proposes a method to indicate if the underlying theory follows a normal or an inverted hierarchy depending on the distribution of lepton flavors in the final state.

Mechanisms of mesogenesis generate the baryon asymmetry and dark matter of the Universe through late-time decays of Standard Model mesons into baryons and dark matter states. Utilizing the $CP$ violation in the meson systems themselves, the resulting baryon asymmetry is directly controlled by collider observables, the $CP$ asymmetry $A_{CP}$, and the branching fraction for the meson decays. Experimental probes of these decays place strong constraints on the amount of $CP$ violation required, placing it well above observed limits in meson mixing. Additionally, strong lower bounds on the proton lifetime seemingly rule out mesogenesis mechanisms which use $D$ mesons. In this work, we propose to circumvent these constraints by “morphing” the mass of the dark sector particles using a late-time phase transition. The change in mass of the final decay products kinematically excludes meson and proton decays, relaxing the constraints on the model parameter space.

We demonstrate that discrete self-similarity (DSS), originally discovered in the asymptotically flat collapse of a massless scalar field, survives in primordial black hole (PBH) formation within an expanding cosmological background. Using fully relativistic simulations of massless scalar-field collapse in a Friedmann-Lemaître-Robertson-Walker universe, we resolve the near-critical regime and find clear log-periodic oscillations in the PBH mass-scaling relation. This result provides a proof of principle that PBHs may retain information about the critical behavior in their formation realized in the early Universe: PBH formation in scalar-field kination exhibits DSS, while radiation-fluid collapse is characterized by continuous self-similarity. The resulting mass scaling imprints characteristic log-periodic modulations in an illustrative PBH mass spectrum, which may survive when PBH formation is dominated by a sufficiently narrow range of scales, with possible consequences for PBH phenomenology. Our results show that PBHs may encode not only the amplitude of primordial perturbations, but also the nature of the matter sector and the dynamical state of the Universe at horizon reentry.

We calculate radiative-recoil contribution of order $Z^2\alpha (Z\alpha {)}^5(m/M{)}^{2}m$ to the Lamb shift in muonium. This correction is due to insertion of radiative photons in the heavy line in the two-photon exchange diagrams. Our calculations are inspired by a new round of precise $1S-2S$ and $2S-2P$ experiments currently in progress.

In various approaches to quantum gravity, spacetime geometry is understood to emerge from more fundamental discrete structures at the Planck scale. As sometimes posited, their presence could lead to dissipative effects in the smooth effective sector. In this paper, we develop the idea of nonconservation in gravity by introducing an effective cosmological model within unimodular gravity in which a varying cosmological constant arises as a consequence of dissipation. We show that this requires the incorporation of hidden degrees of freedom—termed quantum gravity defects—that act as an effective bath for the matter fields. To illustrate the viability of the framework, we study the case of an Ohmic bath inspired by the Caldeira-Leggett model for Brownian motion, leading to a diffusion equation for the matter energy density. The results show that, starting from a primordial Universe with no dark energy, dissipation can account for the generation of a small positive cosmological constant.

We study Maxwell-Chern-Simons theories on a strip $\mathcal{M}={\mathbb{R}}^{1,1}\times [0,h]$ within the Symanzik framework for quantum field theory with boundaries. We add the most general local boundary functional within the quadratic one-tangential-derivative truncation adopted throughout the paper and derive the admissible boundary conditions from locality and the variational principle, with gauge symmetry implemented through boundary Ward identities. The strip supports two boundary $U(1)$ current algebras, with opposite levels fixed by the Chern-Simons coupling $\kappa$ and by boundary orientation, while edge velocities depend on local boundary couplings. They are equal and opposite on the flip-symmetric branch, which is the natural choice for a spatially symmetric strip. The same boundary data determine the reflection coefficients for bulk modes and yield a closed determinant (scattering) representation of the finite interaction energy. In the Maxwell limit ($\kappa =0$) the interedge interaction is long ranged and produces a power-law Casimir force, whereas in Maxwell-Chern-Simons theory the topological mass $m=\kappa g^2$ generates Yukawa suppression at large separations $mh\gg 1$. The determinant form also gives analytic control of the asymptotic regimes and separates topological boundary data (levels and anomaly inflow) from dynamical bulk effects (dispersion, boundary mixing, and Casimir interaction).

We compute the radial action for massive particles along hyperboliclike geodesics in various spherically symmetric spacetimes: the standard $4D$ Schwarzschild spacetime, its $d$-dimensional generalization known as Schwarzschild-Tangherlini solutions and for the D3-brane spacetimes, showing useful resummation properties in terms of special (hypergeometric, Fox-Wright) functions in the eikonal limit and generalizing previous results valid for null geodesics. As a consequence, the scattering angle can be resummed too, and we explicitly display the resummed expressions. In addition, in the more interesting situation of a $4D$ Schwarzschild black hole spacetime, following the approach of the quantum Seiberg-Witten curves to the radial equation associated with a massive scalar field, we show that the quantum $a$ cycle (or, equivalently, the “renormalized angular momentum”) is simply related to the $\pi$ part of the radial action also in this massive case, providing fully resummed expressions. Finally, we display the explicit, expanded-form expression of the dual $a_D$ cycle, for which, however, no resummed expressions have been derived yet.

We study spontaneous scalarization of a scalar field in the magnetized Reissner-Nordström spacetime induced by parity-violating and parity-preserving interactions, represented by couplings to the electromagnetic Chern-Simons, gravitational Chern-Simons, and Gauss-Bonnet invariants, respectively. Working in the decoupling limit, we evolve scalar perturbations in the time domain and determine the critical coupling for the onset of tachyonic instability. This allows us to compare, within the same magnetized background, how the external magnetic field affects scalarization induced by parity-violating and parity-preserving interactions. We find that the magnetic field lowers the scalarization threshold in the electromagnetic and gravitational Chern-Simons channels. In the Gauss-Bonnet channel, by contrast, the magnetic field dependence splits into two branches: on the negative-$\alpha$ branch in our convention, corresponding to the standard ${\mathrm{GB}}^{+}$ branch, the magnitude of the critical coupling increases with the magnetic field, whereas on the positive-$\alpha$ branch, corresponding to ${\mathrm{GB}}^{-}$, the critical coupling decreases with the magnetic field but diverges in the limit of vanishing field. The magnetic field also modifies the late-time dynamics and gives rise to Melvin-like modes. We also present illustrative fixed-background evolutions of the scalar field with nonlinear scalar couplings, which show that nonlinear coupling terms can suppress the scalar field growth and produce bounded oscillatory behavior at the level of the scalar equation. These results show that external magnetic fields affect scalarization induced by parity-violating and parity-preserving interactions in qualitatively different ways, and reveal a pronounced asymmetry between the two Gauss-Bonnet branches.

Classical critical collapse provides a dynamical route from smooth initial data to a naked singularity, representing a sharper violation of predictability than ordinary black hole singularities. We argue that this distinction is erased by quantum backreaction. Building on the semiclassical interior analysis, where quantum self-energy of the collapsing matter generates a universal growing mode and a finite mass gap, we study the exterior naked singularity region that determines global visibility in the Einstein-scalar system. We analyze controlled exterior models in both $2+1$ and $3+1$ dimensions. In the former, smooth matching and physical boundary conditions analytically select a vacuum polarization state, whose backreaction cloaks the classically naked region by a quantum trapped branch. In the latter, numerical horizon tracing shows that near a quantum-shifted threshold the exterior develops finite-mass marginally trapped surfaces rather than a zero-mass naked endpoint. These results suggest a global quantum picture in which the Choptuik naked singularity shares the fate of an ordinary black hole singularity: quantum effects push the putative Cauchy horizon behind a quantum-generated horizon, thereby reducing the loss of predictability to the standard black hole evaporation problem.

We characterize and phenomenologically model the merger-ringdown of gravitational waves emitted by a small compact object that plunges and merges into a Kerr black hole from equatorial-eccentric inspirals. The waveforms are generated employing a time-domain Teukolsky code sourced with trajectories computed using the effective-one-body framework. We span values of the Kerr spin $a\in [-0.9,0.9]$, eccentricity at the last stable orbit (LSO) $e_{\mathrm{LSO}}\in [0,0.9]$, and relativistic anomaly ${\xi }_{\mathrm{LSO}}\in [0,2\pi ]$. We characterize the last peak of the waveform and ringdown features across the parameter space, finding that the eccentricity mainly affects the last peak features, while it has a smaller impact on the ringdown signal. In contrast, the relativistic anomaly measured at the LSO influences the morphology of the last peak in a restricted portion of the parameter space and has no impact on the ringdown part. We perform the analysis for all the spin-weighted spherical harmonic modes normally included in the SEOBNR family of models, $(\ell ,m)\in \{(2,2),(3,3),(4,4),(5,5),(2,1),(3,2),(4,3)\}$. Finally, we introduce a merger-ringdown model for SEOBE-TML, a forthcoming inspiral-merger-ringdown waveform model for eccentric spin-aligned binary black holes in the test-mass limit, whose features can be extended to comparable-mass regimes. The model also accounts for quasinormal mode mixing during the ringdown. It provides a first step toward incorporating the impact of residual eccentricity close to merger into spin-aligned effective-one-body merger-ringdown models for binary black holes.

We fully classify the phenomenology of gravitational wave emission from scaling cosmic string networks with varying tension and compute the spectral indices of the resulting stochastic backgrounds. In string compactifications, periods of varying tension occur when moduli acquire a time dependence. We present concrete examples in type IIB string theory as D3- and NS5-branes wrapping internal cycles, which become dynamical due to the effect of moduli potentials. Moreover, we use swampland constraints to derive general bounds on the allowed time variation of the effective string tension in Friedmann-Lemaitre-Robertson-Walker backgrounds and on the resulting spectral indices.

We present a model-independent, sound-horizon-free measurement of the Hubble constant $H_0$ using baryon acoustic oscillation (BAO) tracers from the Dark Energy Spectroscopic Instrument data release 2 (DESI DR2). The function reconstructions are performed using the artificial neural network method, which is a completely data-driven approach that avoids the mild $\mathrm{\Lambda }$ cold dark matter prior dependence. Our approach is based on the distance duality relation and combines three complementary observational probes, such as type Ia supernovae (SNe), cosmic chronometer, and DESI DR2 BAO—without requiring any knowledge of the sound horizon scale $r_d$ or any assumption about the absolute luminosity of SNe Ia. We obtain a joint constraint of $H_0=71.5\pm 2.2\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ at 68% confidence for 1000 bootstrap realizations and 4096 neurons, which is consistent with the tip of the red giant branch result and the SH0ES measurement within $0.6\sigma$, consistent with the Planck 2020 result within $2\sigma$. Our results favor a higher value of $H_0$ compared to the Planck cosmic microwave background inference, adding independent support for the reality of the Hubble tension.

We study the non-Gaussian features of the gravitational wave (GW) background generated by a population of inspiraling supermassive black hole (SMBH) binaries. We show that the SMBH GW amplitude distribution (GWAD) features a universal heavy power-law tail $\propto A^{-4}$, while the low-amplitude tail depends on the SMBH merger rate and the energy-loss mechanisms of the binaries. The distribution of the induced timing residuals inherits this heavy tail. As a result, the ensemble averaged statistical moments of order three and higher diverge, limiting their usefulness as measures of non-Gaussianity, and the GW background from SMBH binaries exhibits the single loud source principle, according to which the strongest signals are more likely to be caused by a small number of loud sources. We confirm that the variance-averaged Gaussian approximation accurately describes the timing residual statistics. This approximation justifies a factored likelihood structure that combines standard Gaussian-process PTA posteriors with the non-Gaussian population prior, enabling consistent incorporation of non-Gaussian effects into SMBH model inference. We provide a fast and flexible python implementation to compute the distribution of timing residuals from a given SMBH merger rate or GWAD.

The phase evolution of gravitational waves (GWs) can be modulated by the astrophysical environment surrounding the source, which provides a probe for the origin of individual binary black holes (BBHs) using GWs alone. We here study the evolving phase of the GW waveform derived from a large set of simulations of BBH mergers forming in dense stellar clusters through binary-single interactions. We uncover that a well-defined fraction of the assembled eccentric GW sources will have a notable GW phase shift induced by the remaining third object. The magnitude of the GW phase shift often exceeds conservative analytical estimates due to strong three-body interactions, which occasionally results in GW sources with clearly shifted and perturbed GW waveforms. This opens up promising opportunities for current and future GW detectors, as observing such a phase shift can identify the formation environment of a BBH, as well as help to characterise the local properties of its surrounding environment.

The midfrequency gravitational-wave window remains largely uncovered, and atom interferometry with phase-accumulating large momentum transfer is a promising candidate to fill this gap. However, in previously proposed two-photon schemes, placing atomic ensembles on separate satellites breaks common-mode laser phase noise cancellation, creating a persistent dilemma between signal amplification and noise immunity. We resolve this trade-off with a differential dual-atom-interferometer architecture whose physical layout differs fundamentally from earlier designs. Two conjugate atom interferometers are collocated on a single platform, forming the hub of a three-satellite Michelson configuration, rather than being placed on separate satellites. This structural change enables a distinct measurement scheme in which cascaded composite Bragg pulses amplify the signal coherently, while the differential readout between the two collocated interferometers suppresses common-mode laser phase noise. To the best of our knowledge, this design is the first two-photon scheme to achieve high signal amplification and robust laser phase noise rejection simultaneously. It also provides the first quantum coherent phase accumulation and readout module based on differential measurement that can be directly integrated into existing laser interferometric observatories, such as Laser Interferometer Space Antenna and TianQin, enabling a hybrid multiband gravitational-wave detector.

We study and explore the low scale implications of Yukawa unification (YU) in the minimal gauge mediated supersymmetry breaking models. We also assume nonzero nonholomorphic terms, with which the YU solutions can be accommodated in the cases with $\mu >0$. These results can be considered a direct effect from the nonholomorphic terms, but they also lead to testable low scale implications compatible with YU. We observe abundant solutions consistent with the Higgs boson mass. This constraint leads to heavy strongly interacting supersymmetric particles, while the electroweak sector can be relatively light and can be probed by several experiments. We find that the chargino can be lighter than 1 TeV and it is degenerate with the lightest neutralino in most of these solutions. Consistent solutions in this region can accommodate charginos as light as about 120 GeV, and they will likely be tested soon by the analyses over the compressed spectra. These solutions are also subjected in the lifetime analyses. Our analyses identify such light charginos as decaying in about ${10}^{-2}\mathrm{ns}$. Probing such points may need a slight improvement in the sensitivity of current analyses, and one can expect them to be tested very soon. In the same region, the stau is found to be another light supersymmetric particle, and some of the solutions can be inconsistently lighter. We find that it can weigh as light as about 600 GeV consistently, and it can be tested over its lifetime. We summarize and also exemplify our findings with five benchmark scenarios. Furthermore, the benchmark solutions also reveal that the solutions can be tested in heavy Higgs boson searches, which shape the whole Higgs boson spectrum in models.

Recent measurements of the charge-parity asymmetry in the decay $D^0\to K_S^0{K}_S^0$ by CMS Collaboration, $A_{CP}(K_S^0{K}_S^0)=(6.2\pm 3.0\pm 0.2\pm 0.8)\%$, and by LHCb, $A_{CP}(D^0\to K_S^0{K}_S^0)=(1.86\pm 1.04\pm 0.41)\%$, suggest possible deviations from Standard Model expectations, which predict asymmetries below the percent level. This singly Cabibbo-suppressed decay is particularly sensitive to new physics, as the leading amplitudes vanish in the exact U-spin symmetry limit and the process is dominated by W-exchange topologies. We investigate scalar diquark contributions to this decay, comparing color-sextet and color-triplet representations. We find that the color-sextet diquark, characterized by a symmetric color structure $(C_1^{\mathrm{NP}}=C_2^{\mathrm{NP}})$, avoids color suppression and can generate $CP$ asymmetries in the range 0.5%–1.5% for a diquark mass of order 1 TeV. In contrast, the color-triplet contribution is strongly suppressed due to destructive interference from its antisymmetric color structure. We further show that a flavor hierarchy in the sextet couplings, with ${\lambda }_{ud}>{\lambda }_{us}$, can simultaneously account for the observed deviation from the U-spin sum rule in $D^0\to K^{+}{K}^{-}$ and $D^0\to {\pi }^{+}{\pi }^{-}$ and the measured $CP$ asymmetry in $D^0\to K_S^0{K}_S^0$. These results identify color-sextet scalar diquarks as viable candidates for explaining enhanced $CP$ violation in charm decays.

Based on the effective Lagrangian approach, we investigate the $p\overline{p}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ reaction. Within this framework, we provide a dynamical explanation by analyzing its total and differential cross sections, as well as the polarization of the produced $\mathrm{\Lambda }$ hyperon. Incorporating the $t$-channel exchange of the scalar $K_0^{*}(700)$ and pseudoscalar $K$ mesons, complemented by an $s$-channel contribution from the vector excited state, we can reproduce the current experimental data fairly well in a wide energy region. Compared to the conventional $K$ and $K^{*}(892)$ mesons exchange, the $K_0^{*}(700)$ meson exchange plays a more essential role in simultaneously capturing the observed features of the total and differential cross sections. The introduction of the vector $s$-channel resonance improves the description of the spin observables. This work gives a perspective to inspect the role of $K_0^{*}(700)$ and serves as a test to search for the resonances in the reaction $p\overline{p}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ at threshold.

The ${}^1{S}_0$ and ${}^5{S}_2$ amplitudes of $J/\psi J/\psi$ scattering are determined up to 6.6 GeV from $N_f=2$ lattice QCD simulations at $m_{\pi }\approx 420\mathrm{MeV}$ and 250 MeV. We observe an attractive interaction in the ${{}^{1}S}_0$ channel, which permits the existence of a near-threshold scalar structure. This structure may correspond to $X(6200)$, but the possible left-hand-cut effect prevents us from a precise pole determination. In the ${{}^{5}S}_2$ channel, while a near-threshold repulsive interaction is observed, a resonance appears at higher energies associated with a Castillejo-Dalitz-Dyson zero ${\sqrt{s}}_{\mathrm{CDD}}\approx 6.45\mathrm{GeV}$ of the scattering amplitude. The tensor resonance has the parameters $(m_R,{\mathrm{\Gamma }}_R)=(6.543(10),0.548(34))\mathrm{GeV}$ for $m_{\pi }\approx 420\mathrm{MeV}$ and (6.538(13), 0.537(56)) GeV for $m_{\pi }\approx 250\mathrm{MeV}$, which are compatible with those of $X(6600)$ [or $X(6400)$] reported by ATLAS and CMS and also support the $2^{++}$ assignment of the recent CMS study. The difference of the ${}^1{S}_0$ and ${}^5{S}_2$ $J/\psi J/\psi$ interaction is attributed to the dominance of the quark rearrangement effects. The systematic uncertainties owing to the unphysical pion mass, the finite lattice spacing, and the finite volume should be investigated by future lattice QCD studies with more sophisticated lattice setups.

We investigate light-cone smearing induced by quantum fluctuations of gravitons and its implications for the ultraviolet structure of quantum field theory. By treating the first-order correction to Synge’s world function as an operator, we show that the retarded Green’s function is smeared by the variance of graviton fluctuations. The smearing width depends on the quantum state of gravitons: vacuum fluctuations generate a Gaussian smearing of the light cone, coherent states shift the light-cone position, and squeezed states modify the smearing width itself. We then apply the smeared Feynman propagator to one-loop self-energies in interacting scalar field theories. Within a phenomenological finite-resolution prescription that keeps the effective width nonzero, the ultraviolet-sensitive short-distance terms in the ${\varphi }^3$ bubble and ${\varphi }^4$ tadpole diagram expressions are finite. For the illustrative choice that the coarse-graining scale is the Planck length, primordial gravitons induce a relative correction of order ${10}^{-10}$ to the one-loop self-energy. Our results suggest that the quantum state of gravitons can leave a finite imprint on the causal and short-distance structure of quantum field theory.