arXiv subjects

H. Yamazaki

Publications and source records attributed to H. Yamazaki.

At least 19 recordsLinked to original sources

arXiv2026

Direct measurement of the in-medium $\eta^{\prime}$ mass spectrum through the $\gamma\gamma$ decay channel

We measured the invariant mass of the $\eta^{\prime}(958)$ meson through the $\eta^{\prime}\to\gamma\gamma$ decay channel to search for possible character change of $\eta^{\prime}$ in a nuclear medium. The measured invariant-mass spectra were analyzed by fitting realistic spectral functions. An enhancement was observed in the lower tail of the $\eta^{\prime}$ mass peak for low-momentum ($P_{\gamma\gamma} < 1$~$\mathrm{GeV/c}$) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above 3.7$\sigma$. The spectral fitting indicates a mass reduction of $57.5^{+~5.7}_{-27.8}$~$\mathrm{MeV/c}^{2}$. Our result provides the first evidence for an in-medium spectral modification of the $\eta^{\prime}$ meson obtained through a direct measurement of its invariant mass.

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arXiv2026

Comprehensive measurement of $\eta^\prime$ photoproduction off the proton at $E_\gamma < 2.4$ $\mathrm{GeV}$

For the spectroscopy of nucleon resonances at the total energies from the $\eta^\prime$-meson production threshold to $2.32$ $\mathrm{GeV}$, photon beam asymmetries of the reaction $\gamma p \to \eta^\prime p$ were measured together with total and differential cross sections by analyzing the two decay modes $\eta^\prime \to \gamma \gamma$ and $\pi^0 \pi^0 \eta$. New constraints for amplitude decomposition were given by the first-time result of photon beam asymmetries at $E_\gamma > 1.84$ $\mathrm{GeV}$ and the most precise data of differential cross sections to date at extremely backward $\eta^\prime$ angles. The possibility of a larger coupling constant of the $\eta^\prime$-nucleon system to the $N(2250)$ resonance was implied in the partial wave analyses using the present data.

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arXiv2025

Decay study of the most neutron-deficient Zn isotopes with the Warsaw Optical TPC detector

Results of decay studies of nuclei in the vicinity of 54Zn, which is the most neutron-deficient isotope of zinc and undergoes ground-state two-proton radioactivity (2p), are presented. The measurements were performed with a gaseous time projection chamber with optical readout which allowed us to record tracks of protons emitted in the decays. A new method of data analysis was used to reconstruct energies and emission angles of low-energy protons that were stopped within the active volume of the chamber. Half-lives and branching ratios for \b{eta}-delayed proton emission channels were determined for 56Zn, 55Zn, and 55Cu. The \b{eta}-delayed emission of two protons for 55Zn was observed for the first time. Five events of 2p radioactivity of 54Zn were detected and reconstructed. The distribution of the opening angle between momenta of the two protons is consistent with the findings published in [Ascher et al. PRL 107, 102502 (2011)]. The combination of all results suggests a flat angular distribution, in contrast to the one measured for 45Fe.

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arXiv2023

New determination of the branching ratio of the structure dependent radiative $K^{+} \to e^{+} ν_{e} γ$ decay

The branching ratio of the structure dependent (SD) radiative $K^{+} \to e^{+} ν_{e} γ$ decay relative to that of the $K^+\rightarrow e^+ ν_{e} (γ)$ decay including the internal bremsstrahlung (IB) process ($K_{e2(γ)}$) has been measured in the J-PARC E36 experiment using plastic scintillator/lead sandwich detectors. In the analysis, the effect of IB was also taken into account in the SD radiative decay as $K_{e2γ(γ)}^{\rm SD}$. By combining the new data with the previously reported result of the E36 experiment using a CsI(Tl) calorimeter after revision for the IB correction for $K_{e2γ(γ)}^{\rm SD}$, a new value $Br(K_{e2γ(γ)}^{\rm SD})/Br(K_{e2(γ)})=1.20\pm0.07$ has been determined, which is consistent with a recent lattice QCD calculation, but larger than the expectation of Chiral Perturbation Theory (ChPT) at order $O(p^4)$ and the previous KLOE value. Also, using the method to relate form factor and branching ratio described in the KLOE paper, the present result is consistent with the form factor prediction based on a gauged nonlocal chiral quark model, but larger than ChPT at order $O(p^6)$.

hep-ex↗

arXiv2022

Coherent photoproduction of the neutral pion and eta meson on the deuteron at incident energies below 1.15 GeV

Coherent photoproduction of the neutral pion and eta meson on the deuteron, $γ{d}${$\to$}$π^0η{d}$, has been experimentally studied at incident photon energies ranging from the reaction threshold to 1.15 GeV. The total cross section demonstrates a rapid rise below 1 GeV. The data are underestimated by the existing theoretical calculations based on quasi-free $π^0 η$ photoproduction on the nucleon followed by deuteron coalescence. At the same time, the data are rather well reproduced by the calculations taking into account the final-state interaction. We have also measured for the first time the differential cross sections: the $π^0 η$ invariant-mass distribution $dσ/dM_{πη}$, the $π^0 d$ invariant-mass distribution $dσ/dM_{πd}$, the $ηd$ invariant-mass distribution $dσ/dM_{ηd}$, and the distribution over the deuteron emission angle $dσ/dΩ_d$ in the overall center-of-mass frame. The measured cross section $dσ/dΩ_d$ does not exhibit strongly backward-peaking behavior predicted by the calculations. At all incident energies, an increase in $dσ/dM_{ηd}$ near the $ηd$ threshold is observed, which indicates a bound or virtual $ηd$ state resulting from a strong attraction between $η$ and a deuteron. The possibilities of using coherent $π^0η$ photoproduction on a nucleus to study the $η$-nuclear interaction are also discussed.

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arXiv2022

Differential cross sections and photon beam asymmetries of $η$ photoproduction on the proton at $E_γ$ = 1.3-2.4 GeV

We have carried out exclusive measurements for the photoproduction of an $η$ meson from a proton target with an egg-shaped calorimeter made of BGO crystals (BGOegg) and forward charged-particle detectors at the SPring-8 LEPS2 beamline. The differential cross sections and photon beam asymmetries of the $γp \to ηp$ reaction are measured in a center-of-mass energy ($W$) range of $1.82$-$2.32$ GeV and a polar angle range of $-1.0 < \cos{θ^η_{\mathrm{c.m.}}} < 0.6$. The reaction is identified by selecting a proton and two $γ$'s produced by an $η$-meson decay. The kinematic fit method was employed to select the reaction candidate with the confidence level larger than $1$\%. A bump structure at $W$ = $2.0$-$2.3$ GeV in the differential cross section is confirmed at extremely backward $η$ polar angles, where the existing data are inconsistent with each other. This bump structure is likely associated with high-spin resonances that couple with $s\bar{s}$ quarks. The results of the photon beam asymmetries in a wide $η$ polar angle range are new for the photon beam energies exceeding $2.1$ GeV. These results are not reproduced by the existing partial wave analyses. They provide an additional constraint to nucleon resonance studies at high energies.

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arXiv2022

Measurement of structure dependent radiative $K^{+} \rightarrow e^{+} νγ$ decays using stopped positive kaons

The structure dependent radiative $K^+ \rightarrow e^+ νγ$ ($K_{e2γ}^{\rm SD^+}$) decay was investigated with stopped positive kaons. The $e^+$ momentum spectra containing 574$\pm$30 $K_{e2γ}^{\rm SD^+}$ events with a $K^+ \rightarrow μ^+ ν$ ($K_{\mu2}$) background of 28$\pm$19 events were measured with and without a photon in coincidence and analyzed with Monte Carlo simulations for acceptance and detector response to extract the ratio of the branching ratio of the $K_{e2γ}^{\rm SD^+}$ decay and the $K^+ \rightarrow e^+ ν$ decay including the internal bremsstrahlung process ($K_{e2(γ)}$). A value of $Br(K_{e2γ}^{\rm SD^+}) / Br(K_{e2(γ)}) = 1.12\pm0.07_{\rm stat} \pm 0.04_{\rm syst}$ was obtained. This indicates a partial branching ratio, $Br(K_{e2γ}^{\rm SD^+}, ~{p_e>200~{\rm MeV}/c}, ~{E_γ>10~{\rm MeV}})/Br(K_{\mu2}) =(1.85 \pm 0.11_{\rm{stat}} \pm 0.07_{\rm{syst}}) \times10^{-5}$, which is $25\%(\sim$2.5$σ)$ higher than the previous experimental result.

hep-ex↗

arXiv2021

Resonance-like structure near the $ηd$ threshold in the $γ{d}${$\to$}$π^0η{d}$ reaction

To investigate the interaction between the nucleon $N$ and nucleon resonance $N(1535)1/2^-$, the $ηd$ threshold structure connected to the isoscalar $S$-wave $N$-$N(1535)1/2^-$ system has been experimentally studied in the $γ{d}${$\to$}$π^0η{d}$ reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the $ηd$ threshold over the three-body phase-space contribution in the $ηd$ invariant-mass distribution. An analysis incorporating the known isovector resonance $\mathcal{D}_{12}$ with a spin-parity of $2^+$ in the $π^0d$ channel reveals the existence of a narrow isoscalar resonance-like structure with $1^-$ in the $ηd$ system. Using a Flatté parametrization, the mass is found to be $2.427_{-0.006}^{+0.013}$ GeV, close to the $ηd$ threshold, and the width is $\left(0.029_{-0.029}^{+0.006}{\rm\ GeV}\right)+\left(0.00_{-0.00}^{+0.41}\right) p_ηc$, where $p_η$ denotes the $η$ momentum in the rest frame of the $ηd$ system. The observed structure would be attributed to a predicted isoscalar $1^-$ $ηNN$ bound state from $ηNN$ and $πNN$ coupled-channel calculation, or an $ηd$ virtual state owing to strong $ηd$ attraction.

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arXiv2020

Search for $η'$ bound nuclei in the $^{12}{\rm C}(γ,p)$ reaction with simultaneous detection of decay products

We measured missing mass spectrum of the $^{12}{\rm C}(γ,p)$ reaction for the first time in coincidence with potential decay products from $η'$ bound nuclei. We tagged an ($η+p$) pair associated with the $η'N\toηN$ process in a nucleus. After applying kinematical selections to reduce backgrounds, no signal events were observed in the bound-state region. An upper limit of the signal cross section in the opening angle $\cosθ^{ηp}_{lab}<-0.9$ was obtained to be 2.2 nb/sr at the 90$\%$ confidence level. It is compared with theoretical cross sections, whose normalization ambiguity is suppressed by measuring a quasifree $η'$ production rate. Our results indicate a small branching fraction of the $η'N\toηN$ process and/or a shallow $η'$-nucleus potential.

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arXiv2019

$ωN$ scattering length from $ω$ photoproduction on the proton near the threshold

Photoproduction of the $ω$ meson on the proton has been experimentally studied near the threshold. The total cross sections are determined at incident energies ranging from 1.09 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length $a_{ωp}$ and effective range $r_{ωp}$ between the $ω$ meson and proton are estimated from the shape of the total cross section as a function of the incident photon energy: $a_{ωp} = \left(-0.97^{+0.16_{\rm stat}}_{-0.16_{\rm stat}}{}^{+0.03_{\rm syst}}_{-0.00_{\rm syst}}\right)+i \left(0.07^{+0.15_{\rm stat}}_{-0.14_{\rm stat}}{}^{+0.17_{\rm syst}}_{-0.09_{\rm syst}}\right)$ fm and $r_{ωp}=\left(+2.78^{+0.68_{\rm stat}}_{-0.54_{\rm stat}}{}^{+0.11_{\rm syst}}_{-0.13_{\rm syst}}\right)+i\left(-0.01^{+0.46_{\rm stat}}_{-0.50_{\rm stat}}{}^{+0.07_{\rm syst}}_{-0.00_{\rm syst}}\right)$ fm, resulting in a repulsive force. The real and imaginary parts for $a_{ωp}$ and $r_{ωp}$ are determined separately for the first time. A small $P$-wave contribution does not affect the obtained values.

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arXiv2019

Interplay between nuclear shell evolution and shape deformation revealed by magnetic moment of 75Cu

Exotic nuclei are characterized by a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus, may vary due to nuclear force and excess neutrons, in a phenomenon called shell evolution. This effect could be counterbalanced by collective modes causing deformations of the nuclear surface. Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus 75Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieving large spin alignment via a two-step reaction scheme that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in 75Cu are, to a large extent, of single-particle nature on top of a correlated 74Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework, we consider whether and how the double magicity of the 78Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes.

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arXiv2018

Non-strange Dibaryon Resonances Observed in the $γd\to π^0π^0 d$ Reaction

Coherent double neutral-pion photoproduction on the deuteron, $γ{d}${$\to$}$π^0π^0{d}$, has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the $γ{d}$ center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free $π^0π^0$ production with deuteron coalescence. In $π^0d $ invariant-mass distributions, a clear peak is observed at $2.14{\pm}0.01$ GeV$/c^2$ with a width of $0.09{\pm}0.01$ GeV$/c^2$. The spin-parity of this state is restricted to $1^+$, $2^+$ or $3^-$ from the angular distributions of the two $π^0$s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV$/c^2$. The $2^+$ assignment is consistent with the theoretically predicted ${\cal{D}}_{12}$ state, and also with the energy dependence of the $π{d}$ partial-wave amplitude $^3\!P_2$ for the $π^{\pm}d${$\to$}$π^{\pm}d$ and $π^+d${$\to$}${pp}$ reactions.

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arXiv2017

Neutral Kaon Spectrometer 2

A large-acceptance spectrometer, Neutral Kaon Spectrometer 2 (NKS2), was newly constructed to explore various photoproduction reactions in the gigaelectronvolt region at the Laboratory of Nuclear Science (LNS, currently ELPH), Tohoku University. The spectrometer consisted of a dipole magnet, drift chambers, and plastic scintillation counters. NKS2 was designed to separate pions and protons in a momentum range of less than 1 GeV/$c$, and was placed in a tagged photon beamline. A cryogenic H$_{2}$/D$_{2}$ target fitted to the spectrometer were designed. The design and performance of the detectors are described. The results of the NKS2 experiment on analyzing strangeness photoproduction data using a 0.8--1.1 GeV tagged photon beam are also presented.

physics.ins-det↗

arXiv2017

A study of event mixing for two-pion Bose-Einstein correlations in the γp -> π^0 π^0 p reaction

We develop an event mixing technique with a missing mass consistency (MMC) cut and a pion energy (PE) cut for the measurement of two-pion Bose-Einstein correlations (BEC) in reactions with only two identical pions produced in the final state. A multi mixing mode that allows one event to be mixed with multiple other events is proposed for the purpose of improving the statistics of mixed samples, and compared with the single mixing mode which requires one original event can be mixed only once. Numerical tests using the γp -> π^0 π^0 p events are used to testify its ability to measure two-pion correlations. To improve the mixing speed, the optimal MMC cut window is also explored via simulations.

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arXiv2017

First measurement of coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV

The total cross sections were measured for coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV for the first time. No clear resonance-like behavior is observed in the excitation function for $W_{γd}=2.38$--2.61 GeV, where the $d^*(2380)$ dibaryon resonance observed at COSY is expected to appear. The measured excitation function is consistent with the existing theoretical calculation for this reaction. The upper limit of the total cross section is found to be $0.034$~$μ$b for the dibaryon resonance at $W_{γd}=2.37$~GeV (90\% confidence level) in the $γd \to π^0π^0 d$ reaction.

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arXiv2016

An event mixing technique for Bose-Einstein correlations of two pions in photoproduction around 1 GeV

We have developed an event mixing technique to observe Bose-Einstein correlations (BEC) between two identical neutral pions produced in photo-induced reactions in the non-perturbative QCD energy region. It is found that the missing-mass consistency cut and the pion-energy cut are essential for the event mixing method to effectively extract BEC observables. A Monte Carlo (MC) simulation is used to validate these constraints and confirms the efficiency of this method. Our work paves the way for similar BEC studies at lower energies where the multiplicity of emitted bosons is limited.

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arXiv2016

New activation cross section data on longer lived radio-nuclei produced in proton induced nuclear reaction on zirconium

In the frame of a systematic study of charged particle production routes of medically relevant radionuclei, the excitation function for indirect production of $^{178m}$Ta through $^{nat}$Hf($α$,xn)$^{178-178m}$Ta nuclear reaction was measured for the first time up to 40 MeV. In parallel, the side reactions $^{nat}$Hf($α$,x)$^{179,177,176,175}$W, $^{183,182,178g,177,176,175}$Ta, $^{179m,177m,175}$Hf were also assessed. Stacked foil irradiation technique and $γ$-ray spectrometry were used. New experimental cross section data for the $^{nat}$Ta(d,xn)$^{178}$W reaction are also reported up to 40 MeV. The measured excitation functions are compared with the results of the ALICE-IPPE, and EMPIRE nuclear reaction model codes and with the TALYS 1.4 based data in the TENDL-2013 library. The thick target yields were deduced and compared with yields of other charged particle ((p,4n), (d,5n) and ($^3$He,x)) production routes for $^{178}$W.

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arXiv2015

3D cut-cell modelling for high-resolution atmospheric simulations

Owing to the recent, rapid development of computer technology, the resolution of atmospheric numerical models has increased substantially. With the use of next-generation supercomputers, atmospheric simulations using horizontal grid intervals of O(100) m or less will gain popularity. At such high resolution more of the steep gradients in mountainous terrain will be resolved, which may result in large truncation errors in those models using terrain-following coordinates. In this study, a new 3D Cartesian coordinate non-hydrostatic atmospheric model is developed. A cut-cell representation of topography based on finite-volume discretization is combined with a cell-merging approach, in which small cut-cells are merged with neighboring cells either vertically or horizontally. In addition, a block-structured mesh-refinement technique is introduced to achieve a variable resolution on the model grid with the finest resolution occurring close to the terrain surface. The model successfully reproduces a flow over a 3D bell-shaped hill that shows a good agreement with the flow predicted by the linear theory. The ability of the model to simulate flows over steep terrain is demonstrated using a hemisphere-shaped hill where the maximum slope angle is resolved at 71 degrees. The advantage of a locally refined grid around a 3D hill, with cut-cells at the terrain surface, is also demonstrated using the hemisphere-shaped hill. The model reproduces smooth mountain waves propagating over varying grid resolution without introducing large errors associated with the change of mesh resolution. At the same time, the model shows a good scalability on a locally refined grid with the use of OpenMP.

math.NA↗