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Abstract We present and analyze panchromatic (0.35–14μm) spectroscopy of the Type II supernova 2023ixf, including near- and mid-infrared spectra obtained 33.6 days after explosion during the plateau phase, with the James Webb Space Telescope (JWST). This is the first in a series of papers examining the evolution of SN 2023ixf with JWST during the initial 1000 days after explosion and monitoring the formation and growth of molecules and dust in ejecta and the surrounding environment. The JWST infrared spectra are overwhelmingly dominated by H lines, whose profiles reveal ejecta structures, including flat tops, blue notches, and red shoulders, unseen in the optical spectra. We characterize the nature of these structures, concluding that they likely result from a combination of ejecta geometry, viewing angle, and opacity effects. We find no evidence for the formation of dust precursor molecules such as carbon monoxide (CO), nor do we observe an infrared excess attributable to dust. These observations imply that the detections of molecules and dust in SN 2023ixf at later epochs arise either from freshly synthesized material within the ejecta or circumstellar material at radii not yet heated by the supernova at this epoch. 

Abstract We present multiwavelength observations of the Type II supernova (SN II) 2023ixf during its first 2 yr of evolution. We combine ground-based optical/near-infrared spectroscopy with Hubble Space Telescope far- and near-ultraviolet spectroscopy and James Webb Space Telescope near- and mid-infrared photometry and spectroscopy to create spectral energy distributions of SN 2023ixf at +374 and +620 days postexplosion, covering a wavelength range of ∼0.1–30μm. The multiband light curve of SN 2023ixf follows a standard radioactive decay decline rate after the plateau until ∼500 days, at which point shock-powered emission from ongoing interaction between the SN ejecta and circumstellar material (CSM) begins to dominate. This evolution is temporally consistent with 0.3–10 keV X-ray detections of SN 2023ixf and broad “boxy” spectral line emission, which we interpret to signal reprocessing of shock luminosity in a cold dense shell located between forward and reverse shocks. Using the expected absorbed radioactive decay power and the detected X-ray luminosity, we quantify the total shock-powered emission at the +374 and +620 day epochs and find that it can be explained by nearly complete thermalization of the reverse shock luminosity as SN 2023ixf interacts with a continuous, “wind-like” CSM with a progenitor mass-loss rate of $\dot{M}\approx 10^{-4}$ M_⊙yr⁻¹(v_w = 20 ± 5 km s⁻¹). Additionally, we construct multiepoch spectral models from the non-LTE radiative transfer codeCMFGENthat contain radioactive decay and shock powers as well as dust absorption, scattering, and emission. We find that models with shock powers ofL_sh = (0.5–1) × 10⁴⁰erg s⁻¹and ∼(0.5–1) × 10⁻³M_⊙of silicate dust in the cold dense shell and/or inner SN ejecta can effectively reproduce the global properties of the late-time (>300 days) UV-to-IR spectra of SN 2023ixf. 

Abstract We present JWST NIRCam (F356W and F444W filters) and MIRI (F770W) images and NIRSpec Integral Field Unit (IFU) spectroscopy of the young Galactic supernova remnant Cassiopeia A (Cas A) to probe the physical conditions for molecular CO formation and destruction in supernova ejecta. We obtained the data as part of a JWST survey of Cas A. The NIRCam and MIRI images map the spatial distributions of synchrotron radiation, Ar-rich ejecta, and CO on both large and small scales, revealing remarkably complex structures. The CO emission is stronger at the outer layers than the Ar ejecta, which indicates the re-formation of CO molecules behind the reverse shock. NIRSpec-IFU spectra (3–5.5μm) were obtained toward two representative knots in the NE and S fields that show very different nucleosynthesis characteristics. Both regions are dominated by the bright fundamental rovibrational band of CO in the two R and P branches, with strong [Arvi] and relatively weaker, variable strength ejecta lines of [Siix], [Caiv], [Cav], and [Mgiv]. The NIRSpec-IFU data resolve individual ejecta knots and filaments spatially and in velocity space. The fundamental CO band in the JWST spectra reveals unique shapes of CO, showing a few tens of sinusoidal patterns of rovibrational lines with pseudocontinuum underneath, which is attributed to the high-velocity widths of CO lines. Our results with LTE modeling of CO emission indicate a temperature of ∼1080 K and provide unique insight into the correlations between dust, molecules, and highly ionized ejecta in supernovae and have strong ramifications for modeling dust formation that is led by CO cooling in the early Universe. 

We report the first observation of the radiative decay $D_{s0}^{*}(2317{)}^{+}\to D_s^{*+}\gamma$ with a statistical significance exceeding 10 standard deviations. The signal is observed in the continuum $e^{+}{e}^{-}\to c\overline{c}$ process, using combined data samples of $980.4{\mathrm{fb}}^{-1}$ from Belle and $427.9{\mathrm{fb}}^{-1}$ from Belle II, collected at the KEKB and SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ colliders, respectively. The branching fraction ratio $\mathcal{B}\left[D_{s0}^{*}\right(2317{)}^{+}\to D_s^{*+}\gamma ]/\mathcal{B}[D_{s0}^{*}(2317{)}^{+}\to D_s^{+}{\pi }^0]$ is measured to be $[7.13\pm 0.70(\mathrm{stat})\pm 0.26(\mathrm{syst}\left)\right]\%$ . This result provides crucial discrimination between theoretical models of the $D_{s0}^{*}(2317{)}^{+}$ structure. 

We report measurements of production cross sections for ${\rho }^{+}$ , ${\rho }^0$ , $\omega$ , $K^{*+}$ , $K^{*0}$ , $\varphi$ , $\eta$ , $K_S^0$ , $f_0\left(980\right)$ , $D^{+}$ , $D^0$ , $D_s^{+}$ , $D^{*+}$ , $D^{*0}$ , and $D_s^{*+}$ in $e^{+}{e}^{-}$ collisions at a center-of-mass energy near 10.58 GeV. The data were recorded by the Belle experiment, consisting of $571{\mathrm{fb}}^{-1}$ at 10.58 GeV and $74{\mathrm{fb}}^{-1}$ at 10.52 GeV. Production cross sections are extracted as a function of the fractional hadron momentum $x_p$ . The measurements are compared to Monte Carlo generator predictions with various fragmentation settings, including those that have increased fragmentation into vector mesons over pseudoscalar mesons. The cross sections measured for light hadrons are consistent with no additional increase of vector over pseudoscalar mesons. The charmed-meson cross sections are compared to earlier measurements—when available—including older Belle results, which they supersede. They are in agreement before application of an improved initial-state radiation correction procedure that causes slight changes in their $x_p$ shapes. Published by the American Physical Society2025 

Using a data sample of $427.9{\mathrm{fb}}^{-1}$ collected by the Belle II detector at or near the $\mathrm{\Upsilon }\left(4S\right)$ and $\mathrm{\Upsilon }\left(10753\right)$ resonances, the cross sections for $e^{+}{e}^{-}\to h^{+}{h}^{-}J/\psi (h=\pi /K/p)$ at center-of-mass energies ranging from 3.8 GeV or the production threshold to $5.5/6.0/7.0\mathrm{GeV}$ have been measured via initial-state radiation. The cross sections for the processes $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ and $e^{+}{e}^{-}\to K^{+}{K}^{-}J/\psi$ are consistent with previously published results. The cross sections for these channels obtained by combining with previous Belle results are also given. The process $e^{+}{e}^{-}\to p\overline{p}J/\psi$ is investigated for the first time. The yields are small and no significant structure is observed in the cross section versus energy. Searches for vector charmoniumlike states in the $h^{+}{h}^{-}J/\psi$ systems, and for associated intermediate states in the $h^{\pm }J/\psi$ systems, are also presented. 

We report a measurement of the branching fraction for the leptonic decay $B^{+}\to {\mu }^{+}{\nu }_{\mu }$ . This work presents the first $B^{+}\to {\mu }^{+}{\nu }_{\mu }$ result using Belle II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on $1076{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at a center-of-mass energy of 10.58 GeV with the Belle and Belle II detectors at the KEKB and SuperKEKB colliders, respectively. We measure $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })=(4.4\pm 1.9\pm 1.0)\times {10}^{-7}$ , where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90% confidence level upper limit of $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })<6.7\times {10}^{-7}$ using a frequentist approach and a 90% credibility level upper limit of $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })<7.2\times {10}^{-7}$ using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type II and type III two-Higgs-doublet models. We search for stable sterile neutrinos with masses $m_N\in [0,1.5]\mathrm{GeV}$ . No signal is observed, and the resulting exclusion on the squared mixing parameter $|U_{\mu N}{|}^2$ provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ell {\nu }_{\ell }$ decays with $p_{\mu }^B>2.2\mathrm{GeV}$ , obtaining $\mathrm{\Delta }\mathcal{B}(B\to X_u\ell {\nu }_{\ell })=(2.72\pm 0.05\pm 0.29)\times {10}^{-4}$ . We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of $B\to X_u\ell {\nu }_{\ell }$ decays. 

We report the first search for the flavor-changing neutral-current decays $B\to X_s\nu \overline{\nu }$ , where $X_s$ is a hadronic system with strangeness equal to 1, in data collected with the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The data sample corresponds to an integrated luminosity of $365{\mathrm{fb}}^{-1}$ collected at the $\mathrm{\Upsilon }\left(4S\right)$ resonance and $43{\mathrm{fb}}^{-1}$ collected at a center-of-mass energy 60 MeV below resonance for estimation of $e^{+}{e}^{-}\to q\overline{q}$ continuum background. One of the $B$ mesons from the $\mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$ decay is fully reconstructed in a hadronic decay mode. The $B\to X_s\nu \overline{\nu }$ decay is reconstructed with a sum-of-exclusives approach that uses 30 $X_s$ decay modes. This approach provides high sensitivity to the inclusive decay, despite the presence of two undetected neutrinos. The search is performed in three regions of the $X_s$ mass, $M_{X_s}^{\mathrm{true}}$ , chosen to separate contributions from $K$ , $K^{*}\left(892\right)$ , and heavier strange final states. We do not observe a significant signal and set upper limits at 90% confidence level on the partial branching fractions for the regions $0.0<M_{X_s}^{\mathrm{true}}<0.6\mathrm{GeV}/c^2$ , $0.6<M_{X_s}^{\mathrm{true}}<1.0\mathrm{GeV}/c^2$ , and $1.0\mathrm{GeV}/c^2<M_{X_s}^{\mathrm{true}}$ of $2.2\times {10}^{-5}$ , $9.3\times {10}^{-5}$ , and $30.9\times {10}^{-5}$ , respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, $B(B\to X_s\nu \overline{\nu })<3.3\times {10}^{-4}$ . 

A<sc>bstract</sc> Using data samples collected by the Belle and Belle II experiments at the Υ(4S) resonance with integrated luminosities of 571 fb⁻¹and 365 fb⁻¹, respectively, we measure the pseudoscalarB-meson mass difference to bem(B⁰) −m(B⁺) = (0.495 ± 0.024 ± 0.005) MeV/c². The results are based on a simultaneous fit to the variable$${\overline{M} }_{bc}$$, which is related to theBmomentum, forB⁰andB⁺candidates; and to the energy dependence of$$\mathcal{R}=\sigma ({e}^{+}{e}^{-}\to {B}^{0}{\overline{B} }^{0}) / \sigma ({e}^{+}{e}^{-}\to {B}^{+}{B}^{-})$$, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis$$\mathcal{R}={({p}_{{B}^{0}} /{p}_{{B}^{+}})}^{3}$$, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs significantly from our measurement. We constrain$$\mathcal{R}$$in a broader energy range than covered by the direct measurement and extract the energy dependence of$$\mathcal{R}$$in the range from the$$B\overline{B }$$threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the$$B\overline{B }$$potential in the isovector channel. 

A<sc>bstract</sc> We report a search for the lepton-flavor-violating decaysτ^±→ℓ^±α(ℓ=e,μ), whereαis an undetected spin-0 particle, such as an axion-like particle using 736 × 10⁶tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energye⁺e⁻collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95% confidence level:$$\mathcal{B}$$(τ^±→ e^±α) < (0.4–6.4)×10⁻⁴and$$\mathcal{B}$$(τ^±→ μ^±α) < (0.2–3.5)×10⁻⁴at 95% confidence level for anαmass in the range 0.0≤ m_α≤1.6 GeV/c².