arXiv subjects

K. Uno

Publications and source records attributed to K. Uno.

5 recordsLinked to original sources

arXiv2025

Aerogel RICH Counter at the Belle II Detector

We report on the design, operation, and performance of a novel proximity-focusing Ring Imaging Cherenkov (RICH) detector equipped with a multilayer focusing aerogel radiator, developed for the forward region of the Belle II spectrometer at the SuperKEKB $e^+e^-$ collider. The system achieves effective separation of charged pions, kaons, and protons across the full kinematic range of the experiment, from 0.5 GeV/c to 4 GeV/c. To date, the detector has successfully operated in data-taking, contributing to the collection and analysis of nearly 600/fb of Belle II $e^+e^-$ collision data.

hep-ex↗

arXiv2025

Search for lepton-flavor-violating tau decays to $\ell\alpha$ at Belle

We report a search for the lepton-flavor-violating decays $\tau^{\pm}\to\ell^{\pm}\alpha$~($\ell=e,\mu$), where $\alpha$ is an undetected spin-0 particle, such as an axion-like particle using $736\times10^{6}$ tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95\% confidence level: $\mathcal{B}(\tau^{\pm}\rightarrow e^{\pm}\alpha)$ $<$ $(0.4$--$6.4)\times10^{-4}$ and $\mathcal{B}(\tau^{\pm}\rightarrow \mu^{\pm}\alpha)$ $<$ $(0.2$--$3.5)\times10^{-4}$ at 95\% confidence level for an $\alpha$ mass in the range $0.0\leq m_{\alpha}\leq 1.6$~GeV/$c^{2}$.

hep-ex↗

arXiv2024

Evidence for bipolar explosions in Type IIP supernovae

Recent observations of core-collapse supernovae (SNe) suggest aspherical explosions. Globally aspherical structures in SN explosions are regarded as the key for understanding their explosion mechanism. However, the exact explosion geometries from the inner cores to the outer envelopes are poorly understood. Here, we present photometric, spectroscopic and polarimetric observations of the Type IIP SN 2021yja and discuss its explosion geometry, in comparison to those of other Type IIP SNe that show large-scale aspherical structures in their hydrogen envelopes (SNe 2012aw, 2013ej and 2017gmr). During the plateau phase, SNe 2012aw and 2021yja exhibit high continuum polarization characterized by two components with perpendicular polarization angles. This behavior can be interpreted to be due to a bipolar explosion, composed of a polar (energetic) and an equatorial (bulk) components of the SN ejecta. In such a bipolar explosion, an aspherical axis created by the polar ejecta would be dominating at early phases, while the perpendicular axis along the equatorial ejecta would emerge at late phases after the receding of the photosphere in the polar ejecta. The interpretation of the bipolar explosions in SNe 2012aw and 2021yja is also supported by other observational properties, including the time evolution of the line velocities and the line shapes in the nebular spectra. The polarization of other Type IIP SNe that show large-scale aspherical structures in the hydrogen envelope (SNe 2013ej and 2017gmr) is also consistent with the bipolar-explosion scenario, although this is not conclusive.

astro-ph.HE↗

arXiv2023

Photometry and spectroscopy of the Type Icn supernova 2021ckj: The diverse properties of the ejecta and circumstellar matter of Type Icn SNe

We present photometric and spectroscopic observations of the Type Icn supernova (SN) 2021ckj. Spectral modeling of SN 2021ckj reveals that its composition is dominated by oxygen, carbon and iron group elements, and the photospheric velocity at peak is ~10000 km/s. From the light curve (LC) modeling applied to SNe 2021ckj, 2019hgp, and 2021csp, we find that the ejecta and CSM properties of Type Icn SNe are diverse. SNe 2021ckj and 2021csp likely have two ejecta components (an aspherical high-energy component and a spherical standard-energy component) with a roughly spherical CSM, while SN 2019hgp can be explained by a spherical ejecta-CSM interaction alone. The ejecta of SNe 2021ckj and 2021csp have larger energy per ejecta mass than the ejecta of SN 2019hgp. The density distribution of the CSM is similar in these three SNe, and is comparable to those of Type Ibn SNe. This may imply that the mass-loss mechanism is common between Type Icn (and also Type Ibn) SNe. The CSM masses of SN 2021ckj and SN 2021csp are higher than that of SN 2019hgp, although all these values are within the diversity seen in Type Ibn SNe. The early spectrum of SN 2021ckj shows narrow emission lines from C II and C III, without a clear absorption component, in contrast with that observed in SN 2021csp. The similarity of the emission components of these lines implies that the emitting regions of SNe 2021ckj and 2021csp have similar ionization states, and thus suggests that they have similar properties of the ejecta and CSM, which is inferred also from the LC modeling. Taking into account the difference in the strength of the absorption features, this heterogeneity may be attributed to viewing angle effects in otherwise common aspherical ejecta.

astro-ph.HE↗

arXiv2021

Search for lepton-flavor-violating tau-lepton decays to $\ell\gamma$ at Belle

Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $\tau^{\pm}\rightarrow\ell^{\pm}\gamma$ ($\ell=e,\mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $\tau^{\pm}\rightarrow\ell^{\pm}\gamma$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $\mathcal{B}(\tau^{\pm}\rightarrow \mu^{\pm}\gamma)$ $\leq$ $4.2\times10^{-8}$ and $\mathcal{B}(\tau^{\pm}\rightarrow e^{\pm}\gamma)$ $\leq$ $5.6\times10^{-8}$, are set at 90\% confidence level.

hep-ex↗