arXiv subjects

S. Sakai

Publications and source records attributed to S. Sakai.

At least 19 recordsLinked to original sources

arXiv2025

Mass enhancement and metal-nonmetal transition driven by d-f hybridization in perovskites La1-xPrxCuO3

We report the large electron-mass enhancement and the metal to nonmetal transition upon the Pr doping in perovskite-type La1-xPrxCuO3. With increasing the Pr content x around 0.6, the LaCuO3-type three-dimensional structure with trivalent Cu ions changes to the quasi-one-dimensional structure with nearly divalent Cu ions, which accompanies significant changes in the electronic properties. Based on the resistivity, optical conductivity, specific heat measurements and the first-principles calculations, we discuss the formation of a nearly localized nonmetallic state stabilized by the hybridization between Cu 3d, O 2p, and Pr 4f orbitals in the quasi-one-dimensional lattice. The present perovskite-type cuprates offer a unique opportunity to explore novel quantum phases of correlated electrons in low-dimensional lattice, where the spin/charge/orbital degrees of freedom of A- and B-site ions are entangled.

cond-mat.str-el↗

arXiv2025

Performance studies of the CE-65v2 MAPS prototype structure

With the next upgrade of the ALICE inner tracking system (ITS3) as its primary focus, a set of small MAPS test structures have been developed in the 65 nm TPSCo CMOS process. The CE-65 focuses on the characterisation of the analogue charge collection properties of this technology. The latest iteration, the CE-65v2, was produced in different processes (standard, with a low-dose n-type blanket, and blanket with gap between pixels), pixel pitches (15, 18, 22.5 $\mu$m), and pixel arrangements (square or staggered). The comparatively large pixel array size of $48\times24$ pixels in CE-65v2 allows the uniformity of the pixel response to be studied, among other benefits. The CE-65v2 chip was characterised in a test beam at the CERN SPS. A first analysis showed that hit efficiencies of $\geq 99\%$ and spatial resolution better than 5 $\mu$m can be achieved for all pitches and process variants. For the standard process, thanks to larger charge sharing, even spatial resolutions below 3 $\mu$m are reached, in line with vertex detector requirements for the FCC-ee. This contribution further investigates the data collected at the SPS test beam. Thanks to the large sensor size and efficient data collection, a large amount of statistics was collected, which allows for detailed in-pixel studies to see the efficiency and spatial resolution as a function of the hit position within the pixels. Again, different pitches and process variants are compared.

physics.ins-det↗

arXiv2024

The Galactic Center in Color: Measuring Extinction with High-Proper-Motion Stars

The Milky Way's central parsec is a highly extinguished region with a population of high-proper-motion stars. We have tracked 145 stars for $\sim$10 years at wavelengths between 1 and 4 microns to analyze extinction effects in color-magnitude space. Approximately $30\%$ of this sample dims and reddens over the course of years, likely from the motion of sources relative to an inhomogeneous screen of dust. We correct previous measurements of the intrinsic variability fraction for differential extinction effects, resulting in a reduced stellar variability fraction of $34\%$. The extinction variability sub-sample shows that the extinguishing material has sub-arcsecond scales, much smaller variations than previously reported. The observed extinction events imply a typical cross-section of 500 AU and a density of around $3 \times 10^{4} \ \mathrm{atoms/cm^{3}}$ for the extinguishing material, which are consistent with measurements of filamentary dust and gas at the Galactic Center. Furthermore, given that the stars showing extinction variability tend to be more highly reddened than the rest of the sample, the extinction changes are likely due to material localized to the Galactic Center region. We estimate the relative extinction between 1 and 4 microns as, $\mathrm{A}_{\mathrm{H}}:\mathrm{A}_{\mathrm{K'}}:\mathrm{A}_{\mathrm{L'}} = 1.67 \pm 0.05:1:0.69 \pm 0.03$. Our measurement of extinction at longer wavelengths -- L' (3.8 $\mu$m) -- is inconsistent with recent estimations of the integrated extinction towards the central parsec. One interpretation of this difference is that the dust variations this experiment is sensitive to -- which are local to the Galactic Center -- are dominated by grains of larger radius than the foreground.

astro-ph.GA↗

arXiv2023

Hyperuniformity in two-dimensional periodic and quasiperiodic point patterns

We study hyperuniform properties in various two-dimensional periodic and quasiperiodic point patterns. Using the histogram of the two-point distances, we develop an efficient method to calculate the hyperuniformity order metric, which quantifies the regularity of the hyperuniform point patterns. The results are compared with those calculated with the conventional running average method. To discuss how the lattice symmetry affects the order metric, we treat the trellis and Shastry-Sutherland lattices with the same point density as examples of periodic lattices, and Stampfli hexagonal and dodecagonal quasiperiodic tilings with the same point density as examples of quasiperiodic tilings. It is found that the order metric for the Shastry-Sutherland lattice (Stampfli dodecagonal tilings) is smaller than the other in the periodic (quasiperiodic) tiling, meaning that the order metric is deeply related to the lattice symmetry. Namely, the point pattern with higher symmetry is characterized by the smaller order metric when their point densities are identical. Order metrics for several other quasiperiodic tilings are also calculated.

cond-mat.stat-mech↗

arXiv2023

Performance of the electromagnetic and hadronic prototype segments of the ALICE Forward Calorimeter

We present the performance of a full-length prototype of the ALICE Forward Calorimeter (FoCal). The detector is composed of a silicon-tungsten electromagnetic sampling calorimeter with longitudinal and transverse segmentation (FoCal-E) of about 20$X_0$ and a hadronic copper-scintillating-fiber calorimeter (FoCal-H) of about 5$\lambda_{\rm int}$. The data were taken between 2021 and 2023 at the CERN PS and SPS beam lines with hadron (electron) beams up to energies of 350 (300) GeV. Regarding FoCal-E, we report a comprehensive analysis of its response to minimum ionizing particles across all pad layers. The longitudinal shower profile of electromagnetic showers is measured with a layer-wise segmentation of 1$X_0$. As a projection to the performance of the final detector in electromagnetic showers, we demonstrate linearity in the full energy range, and show that the energy resolution fulfills the requirements for the physics needs. Additionally, the performance to separate two-showers events was studied by quantifying the transverse shower width. Regarding FoCal-H, we report a detailed analysis of the response to hadron beams between 60 and 350 GeV. The results are compared to simulations obtained with a Geant4 model of the test beam setup, which in particular for FoCal-E are in good agreement with the data. The energy resolution of FoCal-E was found to be lower than 3% at energies larger than 100 GeV. The response of FoCal-H to hadron beams was found to be linear, albeit with a significant intercept that is about factor 2 larger than in simulations. Its resolution, which is non-Gaussian and generally larger than in simulations, was quantified using the FWHM, and decreases from about 16% at 100 GeV to about 11% at 350 GeV. The discrepancy to simulations, which is particularly evident at low hadron energies, needs to be further investigated.

physics.ins-det↗

arXiv2023

Measurement of the neutrino-oxygen neutral-current quasielastic cross section using atmospheric neutrinos in the SK-Gd experiment

We report the first measurement of the atmospheric neutrino-oxygen neutral-current quasielastic (NCQE) cross section in the gadolinium-loaded Super-Kamiokande (SK) water Cherenkov detector. In June 2020, SK began a new experimental phase, named SK-Gd, by loading 0.011% by mass of gadolinium into the ultrapure water of the SK detector. The introduction of gadolinium to ultrapure water has the effect of improving the neutron-tagging efficiency. Using a 552.2 day data set from August 2020 to June 2022, we measure the NCQE cross section to be 0.74 $\pm$ 0.22(stat.) $^{+0.85}_{-0.15}$ (syst.) $\times$ 10$^{-38}$ cm$^{2}$/oxygen in the energy range from 160 MeV to 10 GeV, which is consistent with the atmospheric neutrino-flux-averaged theoretical NCQE cross section and the measurement in the SK pure-water phase within the uncertainties. Furthermore, we compare the models of the nucleon-nucleus interactions in water and find that the Binary Cascade model and the Liege Intranuclear Cascade model provide a somewhat better fit to the observed data than the Bertini Cascade model. Since the atmospheric neutrino-oxygen NCQE reactions are one of the main backgrounds in the search for diffuse supernova neutrino background (DSNB), these new results will contribute to future studies - and the potential discovery - of the DSNB in SK.

hep-ex↗

arXiv2023

Stellar Populations in the Central 0.5 pc of Our Galaxy III: The Dynamical Sub-structures

We measure the 3D kinematic structures of the young stars within the central 0.5 parsec of our Galactic Center using the 10 m telescopes of the W.~M.~Keck Observatory over a time span of 25 years. Using high-precision measurements of positions on the sky, and proper motions and radial velocities from new observations and the literature, we constrain the orbital parameters for each young star. Our results show two statistically significant sub-structures: a clockwise stellar disk with 18 candidate stars, as has been proposed before, but with an improved disk membership; a second, almost edge-on plane of 10 candidate stars oriented East-West on the sky that includes at least one IRS 13 star. We estimate the eccentricity distribution of each sub-structure and find that the clockwise disk has <$e$> = 0.39 and the edge-on plane has <$e$> = 0.68. We also perform simulations of each disk/plane with incompleteness and spatially-variable extinction to search for asymmetry. Our results show that the clockwise stellar disk is consistent with a uniform azimuthal distribution within the disk. The edge-on plane has an asymmetry that cannot be explained by variable extinction or incompleteness in the field. The orientation, asymmetric stellar distribution, and high eccentricity of the edge-on plane members suggest that this structure may be a stream associated with the IRS 13 group. The complex dynamical structure of the young nuclear cluster indicates that the star formation process involved complex gas structures and dynamics and is inconsistent with a single massive gaseous disk.

astro-ph.GA↗

arXiv2021

Field-induced multiple metal-insulator crossovers of correlated Dirac electrons of perovskite CaIrO$_3$

The interplay between electron correlation and topology of relativistic electrons may lead to a new stage of the research on quantum materials and emergent functions. The emergence of various collective electronic orderings/liquids, which are tunable by external stimuli, is a remarkable feature of correlated electron systems, but has rarely been realized in the topological semimetals with high-mobility relativistic electrons. Here, we report that the correlated Dirac electrons with the Mott criticality in perovskite CaIrO$_3$ show unconventional field-induced successive metal-insulator-metal crossovers in the quantum limit accompanying a giant magnetoresistance (MR) with MR ratio of 3,500 % (18 T and 1.4 K). The analysis shows that the insulating state originates from the collective electronic ordering such as charge/spin density wave promoted by electron correlation, whereas it turns into the quasi-one-dimensional metal at higher fields due to the field-induced reduction of chemical potential, highlighting the highly field-sensitive character of correlated Dirac electrons.

cond-mat.str-el↗

arXiv2020

Search for a Variation of the Fine Structure around the Supermassive Black Hole in Our Galactic Center

Searching for space-time variations of the constants of Nature is a promising way to search for new physics beyond General Relativity and the standard model motivated by unification theories and models of dark matter and dark energy. We propose a new way to search for a variation of the fine-structure constant using measurements of late-type evolved giant stars from the S-star cluster orbiting the supermassive black hole in our Galactic Center. A measurement of the difference between distinct absorption lines (with different sensitivity to the fine structure constant) from a star leads to a direct estimate of a variation of the fine structure constant between the star's location and Earth. Using spectroscopic measurements of 5 stars, we obtain a constraint on the relative variation of the fine structure constant below $10^{-5}$. This is the first time a varying constant of Nature is searched for around a black hole and in a high gravitational potential. This analysis shows new ways the monitoring of stars in the Galactic Center can be used to probe fundamental physics.

astro-ph.GA↗

arXiv2020

$J/ψ\to γππ$, $γπ^0η$ reactions and the $f_0(980)$ and $a_0(980)$ resonances

We study the $J/ψ\to γπ^+ π^-$, $γπ^0 η$ reactions from the perspective that they come from the $J/ψ\to ϕ(ω) π^+ π^-, ρ^0π^0 η$ reactions, where the $ρ^0$, $ω$, and $ϕ$ get converted into a photon via vector meson dominance. Using models successfully used previously to study the $J/ψ\to ω(ϕ) ππ$ reactions, we make determinations of the invariant mass distributions for $π^+ π^-$ in the regions of the $f_0(500)$, $f_0(980)$, and for $π^0 η$ in the region of the $a_0(980)$. The integrated differential widths lead to branching ratios below present upper bounds, but they are sufficiently large for future check in updated facilities.

hep-ph↗

arXiv2019

The $χ_{cJ}$ decay to $ϕK^* \bar K, ϕh_1(1380)$ testing the nature of axial vector meson resonances

We perform a theoretical study of the $χ_{cJ} \to ϕK^* \bar K \to ϕKπ\bar K$ reaction taking into account the $K^* \bar K$ final state interaction, which in the chiral unitary approach is responsible, together with its coupled channels, for the formation of the low lying axial vector mesons, in this case the $h_1(1380)$ given the selection of quantum numbers. Based on this picture we can easily explain why in the $χ_{c0}$ decay the $h_1(1380)$ resonance is not produced, and, in the case of $χ_{c1}$ and $χ_{c2}$ decay, why a dip in the $K^+ π^0 K^-$ mass distribution appears in the 1550-1600 MeV region, that in our picture comes from a destructive interference between the tree level mechanism and the rescattering that generates the $h_1(1380)$ state. Such a dip is not reproduced in pictures where the nominal $h_1(1380)$ signal is added incoherently to a background, which provides support to the picture where the resonance appears from rescattering of vector-pseudoscalar components.

hep-ph↗

arXiv2019

Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors -- with Applications from Black Holes to Planets

We propose a new approach to Bayesian prior probability distributions (priors) that can improve orbital solutions for low-phase-coverage orbits, where data cover less than approximately 40% of an orbit. In instances of low phase coverage such as with stellar orbits in the Galactic center or with directly-imaged exoplanets, data have low constraining power and thus priors can bias parameter estimates and produce under-estimated confidence intervals. Uniform priors, which are commonly assumed in orbit fitting, are notorious for this. We propose a new observable-based prior paradigm that is based on uniformity in observables. We compare performance of this observable-based prior and of commonly assumed uniform priors using Galactic center and directly-imaged exoplanet (HR 8799) data. The observable-based prior can reduce biases in model parameters by a factor of two and helps avoid under-estimation of confidence intervals for simulations with less than about 40% phase coverage. Above this threshold, orbital solutions for objects with sufficient phase coverage such as S0-2, a short-period star at the Galactic center with full phase coverage, are consistent with previously published results. Below this threshold, the observable-based prior limits prior influence in regions of prior dominance and increases data influence. Using the observable-based prior, HR 8799 orbital analyses favor lower eccentricity orbits and provide stronger evidence that the four planets have a consistent inclination around 30 degrees to within 1-sigma. This analysis also allows for the possibility of coplanarity. We present metrics to quantify improvements in orbital estimates with different priors so that observable-based prior frameworks can be tested and implemented for other low-phase-coverage orbits.

astro-ph.EP↗

arXiv2019

$Λ_b$ decays into $Λ_c^\ell\barν_\ell$ and $Λ_c^π^-$ $[Λ_c^*=Λ_c(2595)$ \& $Λ_c(2625)]$ and heavy quark spin symmetry

We study the implications for $Λ_b \to Λ_c^*\ell\barν_\ell$ and $Λ_b \to Λ_c^*π^-$ $[Λ_c^*=Λ_c(2595)$ and $Λ_c(2625)]$ decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity $Λ_c(2595)$ and $Λ_c(2625)$ resonances as HQSS partners, with total angular momentum--parity $j_q^P=1^-$ for the light degrees of freedom, we find that the ratios $Γ(Λ_b\rightarrowΛ_c(2595)π^-)/Γ(Λ_b\rightarrowΛ_c(2625)π^-)$ and $Γ(Λ_b\rightarrow Λ_c(2595) \ell \barν_\ell)/ Γ(Λ_b\rightarrowΛ_c(2625) \ell \barν_\ell)$ agree, within errors, with the experimental values given in the Review of Particle Physics. We discuss how future, and more precise, measurements of the above branching fractions could be used to shed light into the inner HQSS structure of the narrow $Λ_c(2595)$ odd-parity resonance. Namely, we show that such studies would constrain the existence of a sizable $j^P_q=0^-$ component in its wave-function, and/or of a two-pole pattern, in analogy to the case of the similar $Λ(1405)$ resonance in the strange sector, as suggested by most of the approaches that describe the $Λ_c(2595)$ as a hadron molecule. We also investigate the lepton flavor universality ratios $R[Λ_c^*] = {\cal B}(Λ_b \to Λ_c^* τ\,\barν_τ)/{\cal B}(Λ_b \to Λ_c^* μ\,\barν_μ)$, and discuss how $R[Λ_c(2595)]$ may be affected by a new source of potentially large systematic errors if there are two $Λ_c(2595)$ poles.

hep-ph↗

arXiv2019

Theoretical description of the $\boldsymbol{J/ψ\to η(η') h_1(1380)}$, $\boldsymbol{J/ψ\to η(η') h_1(1170)}$ and $\boldsymbol{J/ψ\to π^0 b_1(1235)^0}$ reactions

We have made a study of the $J/ψ\to η' h_1, ηh_1$ (with $h_1$ being $h_1(1170)$ and $h_1(1380)$) and $J/ψ\to π^0 b_1(1235)^0$ assuming the axial vector mesons to be dynamically generated from the pseudoscalar-vector meson interaction. We have taken the needed input from previous studies of the $J/ψ\to ϕππ, ωππ$ reactions. We obtain fair agreement with experimental data and provide an explanation on why the recent experiment on $J/ψ\to η' h_1(1380), h_1(1380) \to K^{*+} K^- +c.c.$ observed in the $K^+ K^- π^0$ mode observes the peak of the $h_1(1380)$ at a higher energy than its nominal mass.

hep-ph↗

arXiv2018

Considerations on the Schmid theorem for triangle singularities

We investigate the Schmid theorem, which states that if one has a tree level mechanism with a particle decaying to two particles and one of them decaying posteriorly to two other particles, the possible triangle singularity developed by the mechanism of elastic rescattering of two of the three decay particles does not change the cross section provided by the tree level. We investigate the process in terms of the width of the unstable particle produced in the first decay and determine the limits of validity and violation of the theorem. One of the conclusions is that the theorem holds in the strict limit of zero width of that resonance, in which case the strength of the triangle diagram becomes negligible compared to the tree level. Another conclusion, on the practical side, is that for realistic values of the width, the triangle singularity can provide a strength comparable or even bigger than the tree level, which indicates that invoking the Schmid theorem to neglect the triangle diagram stemming from elastic rescattering of the tree level should not be done. Even then, we observe that the realistic case keeps some memory of the Schmid theorem, which is visible in a peculiar interference pattern with the tree level.

hep-ph↗

arXiv2018

$τ^- \to ν_τ M_1 M_2$, with $M_1, M_2$ pseudoscalar or vector mesons

We perform a calculation of the $τ^- \to ν_τ M_1 M_2$, with $M_1, M_2$ either pseudoscalar or vector mesons using the basic weak interaction and angular momentum algebra to relate the different processes. The formalism also leads to a different interpretation of the role played by $G$-parity in these decays. We also observe that, while $p$-wave $M_1 M_2$ production is compatible with chiral perturbation theory and experiment, $VP$ and $VV$ $p$-wave production is clearly incompatible with experiment and we develop the formalism also in this case. We compare our results with experiment and make predictions for unmeasured decays, and we show the value of these reactions, particularly if the $M_1 M_2$ mass distribution is measured, as a tool to learn about the meson-meson interaction and the nature of some resonances, coupling to two mesons, which are produced in such decays.

hep-ph↗

arXiv2018

Anomalous enhancement of the isospin-violating $Λ(1405)$ production by a triangle singularity in $Λ_c\rightarrowπ^+π^0π^0Σ^0$

The decay of $Λ_c^+$ into $π^+π^0Λ(1405)$ with the $Λ(1405)$ decay into $π^0Σ^0$ through a triangle diagram is studied. This process is initiated by $ Λ_c^+ \to π^+ \bar{K}^*N $, then the $\bar{K}^*$ decays into $\bar{K} π$ and $\bar{K} N$ produce the $Λ(1405)$ through a triangle loop containing $\bar{K}^*N\bar{K}$ which develops a singularity around $1890$ MeV. This process is prohibited by the isospin symmetry, but the decay into this channel is enhanced by the contribution of the triangle diagram, which is sensitive to the mass of the internal particles. We find a narrow peak in the $π^0Σ^0$ invariant mass distribution, which originates from the $Λ(1405)$ amplitude, but is tied to the mass differences between the charged and neutral $\bar{K}$ or $N$ states. The observation of the unavoidable peak of the triangle singularity in the isospin-violating $Λ(1405)$ production would provide further support for hadronic molecular picture of the $Λ(1405)$ and further information on the $\bar{K} N $ interaction.

hep-ph↗

arXiv2018

Triangle singularity enhancing isospin violation in $\bar B_s^0 \to J/ψπ^0 f_0(980)$

We perform calculations for the $\bar B_s^0 \to J/ψπ^0 f_0(980)$ and $\bar B_s^0 \to J/ψπ^0 a_0(980)$ reactions, showing that the first one is isospin-suppressed while the second one is isospin-allowed. The reaction proceeds via a triangle mechanism, with $\bar B_s^0 \to J/ψK^* \bar K +c.c.$, followed by the decay $K^* \to Kπ$ and a further fusion of $K\bar K$ into the $f_0(980)$ or $a_0(980)$. We show that the mechanism develops a singularity around the $π^0 f_0(980)$ or $π^0 a_0(980)$ invariant mass of 1420 MeV where the $π^0 f_0$ and $π^0 a_0$ decay modes are magnified and also the ratio of $π^0 f_0$ to $π^0 a_0$ production. Using experimental information for the $\bar B_s^0 \to J/ψK^* \bar K +c.c.$ decay, we are able to obtain absolute values for the reactions studied which fall into the experimentally accessible range. The reactions proposed and the observables evaluated, when contrasted with actual experiments should be very valuable to obtain information on the nature of the low lying scalar mesons.

hep-ph↗