arXiv subjects

Kazuki Yoshida

Publications and source records attributed to Kazuki Yoshida.

At least 19 recordsLinked to original sources

arXiv2026

Strong Evidence for Three-$\alpha$ Clustering in the Ground State of $^{12}\mathrm{C}$

The ground state of $^{12}\mathrm{C}$ has often been approximated by a mean-field picture. This conventional view has been challenged by recent nuclear theories suggesting non-negligible $\alpha$-cluster formation, but experimental evidence remains inconclusive. Here, we show that existing $^{12}\mathrm{C}(p,p\alpha)^{8}\mathrm{Be}$ data provide direct evidence for a pronounced $\alpha$ cluster formation in the ground state of $^{12}\mathrm{C}$. We analyze the data with distorted-wave impulse approximation using $\alpha$ preformation amplitudes from an unrestricted $3\alpha$ cluster model and harmonic-oscillator-based models. The results show that the former reproduces the measured cross sections, whereas the latter underestimate them by more than an order of magnitude. Thus, contrary to conventional expectations, the data support a nearly fully developed three-$\alpha$ cluster structure in the ground state of $^{12}\mathrm{C}$.

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arXiv2026

Spectroscopic factors as a probe of nuclear shape in $^{44}$S via one-neutron knockout reaction

Background: Neutron-rich nucleus $^{44}$S lies in the region where traditional $N=28$ shell closure weakens, leading to the emergence of shape coexistence and large-amplitude collective motion (LACM). Understanding the nature and degree of shape mixing in this nucleus remains an important and fascinating problem. Purpose: We investigate the manifestation of shape fluctuations in $^{44}$S and examine how the electric transitions and the spectroscopic factors from one-neutron knockout reactions can serve as probes of shapes mixing. Method: The antisymmetrized molecular dynamics combined with the generator coordinate method (AMD+GCM) is used to study the structure of $^{44}$S and $^{43}$S. Calculations are performed by using Gogny effective interactions with two different parameter sets, D1S and D1M, to explore the interaction dependence of shape mixing. Monopole and quadrupole transition strengths and spectroscopic factors are evaluated. The cross sections for the $^{44}$S$(p,pn)^{43}$S reaction are calculated within the distorted wave impulse approximation (DWIA). Results: The calculations reveal a strong interaction dependence of shape fluctuation in $^{44}$S. The structural differences obtained from D1S and D1M interactions produce distinct patterns of the electric transitions, the spectroscopic factors, and the cross sections for $^{44}$S$(p,pn)^{43}$S knockout reaction. Conclusion: The population of $3/2^-$ and $7/2^-$ states of $^{43}$S is particularly sensitive to the underlying shape fluctuation in $^{44}$S. Thus, the measurement of $^{44}$S$(p,pn)^{43}$S reaction can provide a direct experimental probe.

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arXiv2026

Establishing the $^{40}$Ca$(p,p \alpha)$ reaction at 392 MeV under quasi-free scattering conditions

The $(p,p \alpha)$ reaction offers a direct means to probe preformed $\alpha$-cluster structures in nuclei under quasi-free scattering conditions. Previous studies around 100 MeV provided valuable insights into $\alpha$ clustering, but quantitative comparison with microscopic cluster wave functions remained limited due to strong distortion effects. At higher energies, the reaction mechanism becomes simpler and the distorted-wave impulse approximation (DWIA) provides a more reliable framework for quantitative analysis. In the present work, the $^{40}$Ca$(p,p\alpha)$ reaction was measured at an incident energy of 392 MeV using the high-resolution Grand Raiden and LAS spectrometers at RCNP. Despite the small cross section in this energy region, the achieved resolution allowed clear separation of the ground and excited states of the residual $^{36}$Ar nucleus, and corresponding momentum distributions were extracted. DWIA calculations using a Woods-Saxon $\alpha + ^{36}$Ar bound-state wave function yielded an experimental spectroscopic factor of $ S_{\mathrm{FAC}}^{\mathrm{WS}} = 0.51 \pm 0.05 $, consistent with the previous result at 101.5 MeV $(0.52 \pm 0.23 )$. This agreement demonstrates that the reaction mechanism is well described across a wide energy range. The present study establishes the feasibility of high-precision $(p,p\alpha)$ measurements at several hundred MeV and highlights their potential as a quantitative probe of $\alpha$ clustering in medium-mass nuclei, forming the basis for systematic studies in both stable and unstable systems.

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arXiv2025

Description of nucleon transfer reactions at intermediate energies within the impulse picture

Background: At intermediate energies, transfer reactions are suppressed because the momentum-matching condition is difficult to satisfy. In the standard distorted wave Born approximation (DWBA), a high momentum component of the transferred particle is required to match the large momentum transfer. Purpose: We investigate the applicability of the distorted wave impulse approximation (DWIA) for describing ($p,d$) transfer reactions at intermediate energies by performing a comparative study with the standard DWBA. DWIA, which has been successful for knockout reactions, is expected to provide an alternative reaction mechanism at this energy region. Methods: Both DWBA and DWIA formalisms are applied to the $^{16}$O($p,d$){}$^{15}$O reaction at 200~MeV. In DWBA, the reaction is described as a neutron pickup, while in DWIA, it is treated as a quasi-elastic scattering from a preformed deuteron cluster in the target. Results: The DWBA calculation is in good agreement with the experimental data, reproducing both the angular distribution and the absolute magnitude of the cross section with a reasonable spectroscopic factor. In contrast, the DWIA calculation, while qualitatively reproducing the trend of the angular distribution, severely underestimates the cross section by about two orders of magnitude. Conclusions: Our findings suggest that conventional DWBA provides a more suitable description for the $^{16}$O($p,d$){}$^{15}$O reaction at 200~MeV. The failure of DWIA in this case, unlike its success in knockout reactions, raises open questions about its applicability to transfer reactions. This motivates the need for systematic investigations to delineate the applicability of both reaction mechanisms under various conditions.

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arXiv2024

Chirality in $(\vec{p},2p)$ reactions induced by proton helicity

It is shown that longitudinally polarized protons can be used to induce chirality in the final states of the $(\vec{p},pN)$ reaction at intermediate energies, when there exist three final-state particles with non-coplanar momentum vectors. The analyzing power $A_z$ is proposed as a measure of this effect. Theoretical descriptions to obtain $A_z$ based on an intuitive picture as well as a distorted wave impulse approximation are presented, showing that the helicity of incident protons is coupled to the chirality of the orbital motion of a single-particle wave function, resulting in the chirality of the final states and a large $A_z$ value.

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arXiv2024

Reaction mechanism of quasi-free knockout processes in exotic RI beam era

The quasi-free nucleon knockout reaction has been revealed the single-particle nature of nuclei. Thanks to the advances in experimental techniques and reaction theory, various new aspects of nuclei are being revealed by knockout reactions. In this article, we review the basic concept of the quasi-free knockout reaction, and recent achievements in the SEASTAR project using the MINOS system. We also present our new findings on the low-energy nucleon knockout reaction and the $\alpha$ knockout reaction. The combination of the (microscopic) structure theory, reaction theory and experiments will be the key to a complete understanding of the $\alpha$ formation and its universality in the coming decades. Noble clusters, e.g., $d$, $t$, $^{3}$He, etc. are in the scope of the ONOKORO project. The implementation of the two (and more) nucleon correlation in the reaction theory is essential to connect the properties of such clusters and the reaction observables. A new framework, CDCCIA, is introduced for this purpose, which will also be applicable to the two-nucleon knockout reactions, e.g., $(p,3p)$, $(p,2pn)$, and $(p,p2n)$.

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arXiv2024

Ab initio informed 20Ne(p, p$\alpha$)16O reaction elucidates the emergence of alpha clustering from chiral potentials

We report on the first \textit{ab initio} informed $\alpha$ knock-out reaction in the intermediate-mass region, with the aim to probe the underlying chiral potential and its impact on the emergence of alpha clustering in this mass region. The theoretical predictions of the $\alpha+^{16}$O clustering in the $^{20}$Ne ground state, based on the \textit{ab initio} symmetry-adapted no-core shell model, yield a triple differential cross section for $^{20}$Ne(p, p$\alpha$)$^{16}$O that is in a remarkable agreement with the data. This allows us to examine predictions of surface and in-medium $\alpha$-cluster features from a chiral potential and to compare these to the successful antisymmetrized molecular dynamics approach.

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arXiv2024

Significance of the refraction effect on the $p$-$d$ elementary process in the ($p$,$pd$) reaction

The proton-induced deuteron knockout reaction, ($p$,$pd$), is one of the interests in the studies for probing the deuteron-like $p$-$n$ correlation in nuclei. According to a recent study of the inclusive deuteron-induced reaction, $(d,d'x)$, the refraction effect of the deuteron has a significant effect on the elementary process, nucleon-deuteron ($N$-$d$) binary scattering inside a nucleus, of the reaction. In the paper, it is shown that proper treatment of the local $N$-$d$ relative momentum in the elementary process is crucial in $(d,d'x)$ reactions at $100$ MeV and below. In the present work, we investigate the deuteron refraction effect in the exclusive ($p$,$pd$) reactions. We also discuss the incident energy dependence of the refraction effect. The refraction effect on the $p$-$d$ elementary process is taken into account by the local semiclassical approximation to the distorted waves. The results are compared with those obtained with the asymptotic momentum approximation, which is standardly applied to the distorted wave impulse approximation framework. It is shown that the refraction effect drastically changes the energy sharing distribution of the $^{16}$O($p$,$pd$)$^{14}$N reaction at 101.3 MeV and gives a better agreement with experimental data. In contrast, it is confirmed that the effect is negligibly small at 250 MeV. We have clarified that the deuteron refraction effect is significant in the $^{16}$O($p$,$pd$)$^{14}$N reaction at 101.3 MeV and the experimental data are well reproduced. The refraction effect plays a significant role in both the shape and magnitude of the ($p$,$pd$) cross section, while the effect is negligible at 250 MeV.

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arXiv2024

Difference in peripherality of the inclusive (p, p'x) and (d, d'x) reactions and its implications for phenomenological reaction model

Previous studies have revealed the importance of introducing surface correction into a phenomenological model for inclusive (n, n'x) and (p, p'x) reactions. These findings have contributed significantly to the improvement of nuclear data evaluation. However, the necessity for the surface correction in an inclusive (d, d'x) reaction has hardly been investigated. The purpose of this study is to investigate the difference in the peripherality of the (p, p'x) and (d, d'x) reactions by a theoretical analysis using a quantum mechanical model, and to obtain a theoretical basis on the surface correction in the (d, d'x) reaction. The energy spectra and their radial distributions for the (p, p'x) and (d, d'x) reactions are calculated by the one-step semiclassical distorted wave model. The radial distribution of the energy spectra for the (d, d'x) reaction is shifted toward the outer region of the nucleus compared to the (p, p'x) reaction. Based on this finding, we consider a larger surface correction into a phenomenological model for the (d, d'x) reaction than that for the (p, p'x) reaction, and calculated values reproduce the experimental (d, d'x) spectra well. The peripherality of the (d, d'x) reaction is more prominent than that of the (p, p'x) reaction. The stronger surface correction thus should be introduced for the (d, d'x) reaction than for the (p, p'x) reaction.

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arXiv2024

Three-body analysis reveals the significant contribution of minor $^{5}$He $s$-wave component in $^{6}$Li$(p,2p)^{5}$He cross section

$^6$Li is usually treated as an $\alpha+p+n$ three-body system, and the validity of this picture is important for understanding $^6$Li reactions. The ($p$,$2p$) reaction is a powerful method to study the structure of valence nucleons in $^6$Li. Recently, the new experimental data of the $^{6}$Li($p$,$2p$)$^{5}$He reaction have been obtained and should be analyzed. We investigate the $^{6}$Li($p$,$2p$)$^{5}$He reaction using the $\alpha+p+n$ three-body wave function of $^6$Li and study the validity of this model. We calculate the $^6$Li wave function by using Gaussian expansion method, and the function is used to obtain the relative wave function between $p$ and $^5$He. We combine the relative wave function with the distorted wave impulse approximation. Our results reproduce the experimental data of the triple differential cross section within about 10\% difference in the absolute values, and contributions from both $p$- and $s$-wave states of $^5$He in $^6$Li are found to be important. We can qualitatively understand the $^{6}$Li($p$,$2p$)$^{5}$He reaction by describing $^6$Li with the three-body model. Contribution from the $s$-wave component is important in reproducing the experimental data in the zero recoil-momentum region.

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arXiv2023

Systematic analysis of the nuclear absorption effect on the cross section of the knockout reaction

Recent studies on nucleon and alpha knockout reactions have shown that the distorted-wave impulse approximation (DWIA) is a simple and accurate method to describe these reactions. As it has been argued for decades, the nuclear absorption is one of the most important ingredients of the DWIA calculation. In this work, we systematically investigate the absorption effects on the cross sections of the nucleon and alpha knockout reactions. To do this, we calculate the ratio of the cross sections of the DWIA and plane-wave impulse approximation (PWIA) and examine its dependence on the mass number and single-particle orbital of the knocked-out particles. We will discuss the specific characteristics of the absorption effect for each reaction.

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arXiv2023

Description of inclusive $(d,d^{\prime}x)$ reaction with the semiclassical distorted wave model

The description of deuteron-induced inclusive reactions has been an important subject in direct nuclear reaction studies and nuclear data science. For proton-induced inclusive processes, the semiclassical distorted wave model (SCDW) is one of the most successful models based on quantum mechanics. We improve SCDW for deuteron-induced inclusive processes and clarify the importance of the proper treatment of the kinematics of the deuteron inside a nucleus. The double differential cross section (DDX) of the inclusive deuteron-emission process $(d,d^{\prime}x)$ is described by one-step SCDW. The changes in the kinematics due to the distortion effect, the refraction effect, is taken into account by the local semiclassical approximation (LSCA). The calculated DDXs of $(d,d^{\prime}x)$ reasonably reproduce experimental data in the small energy-transfer region and at forward and middle angles with some exceptions. The angular distributions of $(d,d^{\prime}x)$ are improved by including the refraction effect. The proper treatment of the changes in the kinematics of the deuteron inside a nucleus is necessary in describing the ($d$,$d'x$) reaction. The effect of the changes on the DDX of $(d,d^{\prime}x)$ is significant compared to on the proton-induced inclusive process $(p,p^{\prime}x)$ because of the stronger distortion effect on the deuteron.

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arXiv2023

Effective polarization in proton-induced $α$ knockout reactions

The effective polarization of the residual nucleus in the proton-induced $α$ knockout reaction is investigated within the distorted wave impulse approximation framework. The strong absorption of the emitted $α$ particle results in strong selectivity on the reaction "position" depending on the third component of the single-particle orbital angular momentum of the $α$ particle inside a nucleus, hence on the spin direction of the reaction residue. This is caused by a mechanism that is similar to the Maris effect, the effective polarization in the proton-induced proton knockout reactions. However, as a distinct feature of the effective polarization in the $α$ knockout process, the spin degrees of freedom of the reacting particles play no role. The $α$ knockout process with complete kinematics can be a useful polarization technique for the residual nucleus, without actively controlling the spin of the proton.

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arXiv2022

Importance of the deuteron breakup in the deuteron knockout reaction

An isoscalar $pn$ pair is expected to emerge in nuclei that have similar proton and neutron numbers and it may be a candidate for a deuteron ``cluster.'' There is, however, no experimental evidence for it. The purpose of this paper is to construct a new reaction model for the ($p,pd$) reaction including the deuteron breakup in the elementary process and the deuteron reformation by the final-state interactions (FSIs). How these processes contribute to the observables of the reaction is investigated. The distorted wave impulse approximation is extended in twofold. The elementary processes of the ($p,pd$), i.e., the $p$-$d$ elastic scattering and $d(p,p)pn$ reaction, are described with an impulse picture employing a nucleon-nucleon effective interaction. The three-body scattering waves in the final state of the ($p,pd$) reaction are calculated with the continuum-discretized coupled-channels method. The triple differential cross section (TDX) of the ($p,pd$) reaction is calculated with the new model. The elementary processes are described reasonably well with the present model. As for the ($p,pd$) reaction, the deuteron reformation can either increase or decrease the TDX height depending on the interference between the elastic and breakup channel of deuteron, while the \textit{back-coupling} effect always decreases it. It is shown that the deuteron reformation significantly changes the TDX of the ($p,pd$) reaction through the interference. It is important to include this process to quantitatively discuss the ($p,pd$) cross sections in view of the deuteron formation in nuclei. For more quantitative discussion regarding the experimental data, further improvement will be necessary.

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arXiv2022

$α$ knockout reaction as a new probe for $α$ formation in $α$-decay nuclei

The $α$-decay phenomennon has been studied for more than a century but the mechanism of the $α$ particle formation and its tunneling process have not yet been fully understood. As an alternative to the $α$-decay lifetime measurement, we propose the proton-induced $α$ knockout reaction, ($p$,$pα$), as a new probe for the surface $α$ formation probability of $α$-decay nuclei. The $^{210,212}$Po($p$,$pα$)$^{206,208}$Pb reaction is described by the distorted-wave impulse approximation framework. It shows that the $^{212}$Po/$^{210}$Po ratio of the $α$ knockout cross sections agrees with that of the surface $α$ formation probabilities determined by lifetime measurements. The result implies that the ($p$,$pα$) cross section is a direct probe for the surface $α$ formation probability, which is an essential quantity to lead us to a complete understanding of the $α$-decay phenomenon.

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arXiv2021

Unexpectedly enhanced $α$-particle preformation in $^{48}$Ti probed by the $(p,pα)$ reaction

The formation of $α$ particle on nuclear surface has been a fundamental problem since the early age of nuclear physics. It strongly affects the $α$ decay lifetime of heavy and superheavy elements, level scheme of light nuclei, and the synthesis of the elements in stars. However, the $α$-particle formation in medium-mass nuclei has been poorly known despite its importance. Here, based on the $^{48}{\rm Ti}(p,pα)^{44}{\rm Ca}$ reaction analysis, we report that the $α$-particle formation in a medium-mass nucleus $^{48}{\rm Ti}$ is much stronger than that expected from a mean-field approximation, and the estimated average distance between $α$ particle and the residue is as large as 4.5 fm. This new result poses a challenge of describing four nucleon correlations by microscopic nuclear models.

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arXiv2020

Proton induced deuteron knockout reaction as a probe of an isoscalar proton-neutron pair in nuclei

The isoscalar $pn$ pair is expected to emerge in nuclei having the similar proton and neutron numbers but there is no clear experimental evidence for it. We aim to clarify the correspondence between the $pn$ pairing strength in many-body calculation and the triple differential cross section (TDX) of proton-induced deuteron knockout ($p,pd$) reaction on $^{16}$O. The radial wave function of the isoscalar $pn$ pair with respect to the center of $^{16}$O is calculated with the energy density functional (EDF) approach and is implemented in the distorted wave impulse approximation (DWIA) framework. The $pn$ pairing strength $V_0$ in the EDF calculation is varied and the corresponding change in the TDX is investigated. A clear $V_0$ dependence of the TDX is found for the $^{16}$O($p,pd$)$^{14}$N($1_2^+$) at $101.3$ MeV. The nuclear distortion is found to make the $V_0$ dependence stronger. Because of the clear $V_0$-TDX correspondence, the ($p,pd$) reaction will be a promising probe for the isoscalar $pn$ pair in nuclei. For quantitative discussion, further modification of the description of the reaction process will be necessary.

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arXiv2019

Toward a reliable description of ${(p,pN)}$ reactions in the distorted-wave impulse approximation

Background: Proton-induced nucleon knockout $(p,pN)$ reactions have been successfully used to study the single-particle nature of stable nuclei in normal kinematics with the distorted-wave impulse approximation (DWIA) framework. Recently, these reactions have been applied to rare-isotope beams at intermediate energies in inverse kinematics to study the quenching of spectroscopic factors. Purpose: Our goal is to investigate the effects of various corrections and uncertainties within the standard DWIA formalism on the $(p,pN)$ cross sections. The consistency of the extracted reduction factors between DWIA and other methods is also evaluated. Method: We analyze the $(p,2p)$ and $(p,pn)$ reactions data measured at the R$^3$B/LAND setup at GSI for carbon, nitrogen, and oxygen isotopes in the incident energy range of 300--450 MeV/u. Cross sections and reduction factors are calculated by using the DWIA method. The transverse momentum distribution of the $^{12}$C($p$,$2p$)$^{11}$B reaction is also investigated. Results: We have found that including the nonlocality corrections and the Møller factor affects the cross sections considerably. The proton-neutron asymmetry dependence of reduction factors extracted by the DWIA calculation is very weak and consistent with those given by other reaction methods and \textit{ab initio} structure calculations. Conclusions: The results found in this work provide a detailed investigation of the DWIA method for $(p,pN)$ reactions at intermediate energies. They also suggest that some higher-order effects, which is essential for an accurate cross-section description at large recoil momentum, is missing in the current DWIA and other reaction models.

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