Searcharxiv⌕ Search

arXiv subjects

Y. Shimizu

Publications and source records attributed to Y. Shimizu.

At least 55 records · Page 3Linked to original sources
arXiv2022

Border of the Island of Inversion: Unbound states in $^{29}$Ne

The nucleus $^{29}$Ne is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder $f_{7/2}$-state, which would place $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ reactions at a beam energy of around $230$ MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction

nucl-ex↗
arXiv2021

Charge-changing cross sections for $^{42\textrm{--}51}$Ca and effect of charged-particle evaporation induced by neutron removal reaction

Charge-changing cross sections $σ_\mathrm{CC}$ for $^{42\textrm{--}51}$Ca on a carbon target at around 280~MeV/nucleon have been measured. The measured $σ_\mathrm{CC}$ values differ significantly from the previously developed calculations based on the Glauber model. However, through introduction of the charged-particle evaporation effect induced by the neutron-removal reaction in addition to the Glauber-model calculation, experimental $σ_\mathrm{CC}$ values on $^{12}$C at around 300~MeV/nucleon for nuclides from C to Fe isotopes are all reproduced with approximately 1\% accuracy. This proposed model systematically reproduces $σ_\mathrm{CC}$ data without phenomenological corrections, and can also explain experimental $σ_\mathrm{CC}$ values obtained in other energy regions.

nucl-ex↗
arXiv2021

Search for $α$ condensed states in $^{13}$C using $α$ inelastic scattering

We searched for the $α$ condensed state in $^{13}$C by measuring the $α$ inelastic scattering at $E_α = 388$ MeV at forward angles including 0 degrees. We performed the distorted-wave Born-approximation calculation with the single-folding potential and the multipole decomposition analysis to determine the isoscalar transition strengths in $^{13}$C. We found a bump structure around $E_x = 12.5$ MeV due to the isoscalar monopole ($IS0$) transition. A peak-fit analysis suggested that this bump consisted of several $1/2^-$ states. We propose that this bump is due to the mirror state of the 13.5 MeV-state in $^{13}$N, which dominantly decays to the $α$ condensed state in $^{12}$C. It was speculated that the $1/2^-$ states around $E_x = 12.5$ MeV were candidates for the $α$ condensed state, but the $3α+ n$ orthogonality condition model suggests that the $α$ condensed state is unlikely to emerge as the negative parity states. We also found two $1/2^+$ or $3/2^+$ states at $E_x = 14.5$ and 16.1 MeV excited with the isoscalar dipole ($IS1$) strengths. We suggest that the 16.1-MeV state is a possible candidate for the $α$ condensed state predicted by the cluster-model calculations on the basis of the good correspondence between the experimental and calculated level structures. However, the theoretical $IS1$ transition strength for this state is significantly smaller than the measured value. Further experimental information is strongly desired to establish the $α$ condensed state in $^{13}$C.

nucl-ex↗
arXiv2021

Disappearance of spin glass behavior in ThCr2Si2-type intermetallic PrAu2Si2

It is unexpected that a spin-glass transition, which generally occurs only in the system with some form of disorder, was observed in the ThCr2Si2-type compound PrAu2Si2 at a temperature of ~3 K. This puzzling phenomenon was later explained based on a novel dynamic frustration model that does not involve static disorder. We present the results of re-verification of the reported spin-glass behaviors by measuring the physical properties of three polycrystalline PrAu2Si2 samples annealed under different conditions. Indeed, in the sample annealed at 827 C for one week, a spin-glass transition does occur at a temperature of Tf=2.8 K as that reported previously in the literature. However, it is newly found that the spin-glass effect is actually more pronounced in the as-cast sample, and almost completely disappears in the well-annealed (at 850 C for 4 weeks) sample. The apparent sample dependence of the magnetic characteristics of PrAu2Si2 is discussed by comparing it with similar phenomena observed in the isomorphic compounds URh2Ge2 and CeAu2Si2. Our experimental results strongly suggest that the spin-glass behavior observed in the as cast and insufficient annealed samples is most likely due to the presence of small amount of crystalline impurities and/or partial site disorder on the Au and Si sublattices, and thus is not the inherent characteristic of ideal ThCr2Si2-type PrAu2Si2. The perfectly ordered PrAu2Si2 should be regarded as a paramagnetic system with obvious crystal-field effects.

cond-mat.str-el↗
arXiv2021

Measurement of the Iron Spectrum in Cosmic Rays from 10 GeV$/n$ to 2.0 TeV$/n$ with the Calorimetric Electron Telescope on the International Space Station

The Calorimetric Electron Telescope (CALET), in operation on the International Space Station since 2015, collected a large sample of cosmic-ray iron over a wide energy interval. In this Letter a measurement of the iron spectrum is presented in the range of kinetic energy per nucleon from 10 GeV$/n$ to 2.0 TeV$/n$ allowing the inclusion of iron in the list of elements studied with unprecedented precision by space-borne instruments. The measurement is based on observations carried out from January 2016 to May 2020. The CALET instrument can identify individual nuclear species via a measurement of their electric charge with a dynamic range extending far beyond iron (up to atomic number $Z$ = 40). The energy is measured by a homogeneous calorimeter with a total equivalent thickness of 1.2 proton interaction lengths preceded by a thin (3 radiation lengths) imaging section providing tracking and energy sampling. The analysis of the data and the detailed assessment of systematic uncertainties are described and results are compared with the findings of previous experiments. The observed differential spectrum is consistent within the errors with previous experiments. In the region from 50 GeV$/n$ to 2 TeV$/n$ our present data are compatible with a single power law with spectral index -2.60 $\pm$ 0.03.

astro-ph.HE↗
arXiv2021

Cosmic antihelium-3 nuclei sensitivity of the GAPS experiment

The General Antiparticle Spectrometer (GAPS) is an Antarctic balloon experiment designed for low-energy (0.1$-$0.3 GeV/$n$) cosmic antinuclei as signatures of dark matter annihilation or decay. GAPS is optimized to detect low-energy antideuterons, as well as to provide unprecedented sensitivity to low-energy antiprotons and antihelium nuclei. The novel GAPS antiparticle detection technique, based on the formation, decay, and annihilation of exotic atoms, provides greater identification power for these low-energy antinuclei than previous magnetic spectrometer experiments. This work reports the sensitivity of GAPS to detect antihelium-3 nuclei, based on full instrument simulation, event reconstruction, and realistic atmospheric influence simulations. The report of antihelium nuclei candidate events by AMS-02 has generated considerable interest in antihelium nuclei as probes of dark matter and other beyond the Standard Model theories. GAPS is in a unique position to detect or set upper limits on the cosmic antihelium nuclei flux in an energy range that is essentially free of astrophysical background. In three 35-day long-duration balloon flights, GAPS will be sensitive to an antihelium flux on the level of $1.3^{+4.5}_{-1.2}\cdot 10^{-6}\mathrm{m^{-2}sr^{-1}s^{-1}}(\mathrm{GeV}/n)^{-1}$ (95% confidence level) in the energy range of 0.11$-$0.3 GeV/$n$, opening a new window on rare cosmic physics.

hep-ph↗
arXiv2021

Probing the Symmetry Energy with the Spectral Pion Ratio

Many neutron star (NS) properties, such as the proton fraction within a NS, reflect the symmetry energy contributions to the Equation of State that dominate when neutron and proton densities differ strongly. To constrain these contributions at supra-saturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be $42 < L < 117$ MeV. This value is slightly lower but consistent with the $L$ values deduced from a recent measurement of the neutron skin thickness of $^{208}$Pb.

nucl-ex↗
arXiv2021

Two-loop scalar functions with $I$ internal lines, $I \le 5$

This article displays a proof of concept of the mixed analytical/numerical method, presented in previous publications, to compute two-loop functions with up to five massive propagators in a scalar theory having three- and four-leg vertices as the Higgs sector of the Standard Model. Several amplitudes are considered with two, three and four external legs. Some of them diverge in the UV region and we demonstrate that the method works in that case. It is shown that all these classes of amplitudes can be generated by four master topologies. Results of a numerical evaluation for some kinematics are presented, they are compared to a public code and agree well within the error bars quoted by the different programs.

hep-ph↗
arXiv2021

Quasi-free Neutron Knockout Reaction Reveals a Small $s$-orbital Component in the Borromean Nucleus $^{17}$B

A kinematically complete quasi-free $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus $^{17}$B, which had long been considered to have neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for $1s_{1/2}$ and $0d_{5/2}$ orbitals, and a surprisingly small percentage of 9(2)$\%$ was determined for $1s_{1/2}$. Our finding of such a small $1s_{1/2}$ component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in $^{17}$B. The present work gives the smallest $s$- or $p$-orbital component among known nuclei exhibiting halo features, and implies that the dominant occupation of $s$ or $p$ orbitals is not a prerequisite for the occurrence of neutron halo.

nucl-ex↗
arXiv2020

Direct Measurement of the Cosmic-Ray Carbon and Oxygen Spectra from 10 GeV$/n$ to 2.2 TeV$/n$ with the Calorimetric Electron Telescope on the International Space Station

In this paper, we present the measurement of the energy spectra of carbon and oxygen in cosmic rays based on observations with the Calorimetric Electron Telescope (CALET) on the International Space Station from October 2015 to October 2019. Analysis, including the detailed assessment of systematic uncertainties, and results are reported. The energy spectra are measured in kinetic energy per nucleon from 10 GeV$/n$ to 2.2 TeV$/n$ with an all-calorimetric instrument with a total thickness corresponding to 1.3 nuclear interaction length. The observed carbon and oxygen fluxes show a spectral index change of $\sim$0.15 around 200 GeV$/n$ established with a significance $>3σ$. They have the same energy dependence with a constant C/O flux ratio $0.911\pm 0.006$ above 25 GeV$/n$. The spectral hardening is consistent with that measured by AMS-02, but the absolute normalization of the flux is about 27% lower, though in agreement with observations from previous experiments including the PAMELA spectrometer and the calorimetric balloon-borne experiment CREAM.

astro-ph.HE↗
arXiv2020

Symmetry energy investigation with pion production from Sn+Sn systems

In the past two decades, pions created in the high density regions of heavy ion collisions have been predicted to be sensitive at high densities to the symmetry energy term in the nuclear equation of state, a property that is key to our understanding of neutron stars. In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central $^{132}$Sn+$^{124}$Sn, $^{112}$Sn+$^{124}$Sn, and $^{108}$Sn+$^{112}$Sn collisions at $E/A=270~\mathrm{MeV}$ with the S$π$RIT Time Projection Chamber. While the uncertainties of individual pion multiplicities are measured to 4\%, those of the charged pion multiplicity ratios are measured to 2\%. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron to proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data. This finding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au+Au system. These new results call for better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.

nucl-ex↗
arXiv2020

Surface localization of the dineutron in $^{11}$Li

The formation of a dineutron in the nucleus $^{11}$Li is found to be localized to the surface region. The experiment measured the intrinsic momentum of the struck neutron in $^{11}$Li via the $(p,pn)$ knockout reaction at 246 MeV/nucleon. The correlation angle between the two neutrons is, for the first time, measured as a function of the intrinsic neutron momentum. A comparison with reaction calculations reveals the localization of the dineutron at $r\sim3.6$ fm. The results also support the density dependence of dineutron formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear matter.

nucl-ex↗
arXiv2020

A two-neutron halo is unveiled in $^{29}$F

We report the measurement of reaction cross sections ($σ_R^{\rm ex}$) of $^{27,29}$F with a carbon target at RIKEN. The unexpectedly large $σ_R^{\rm ex}$ and derived matter radius identify $^{29}$F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the $2p_{3/2}$ orbital, thereby vanishing the shell closure associated with the neutron number $N = 20$. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of $^{27}$F but are challenged for $^{29}$F.

nucl-ex↗
arXiv2020

Extending the Southern Shore of the Island of Inversion to $^{28}$F

Detailed spectroscopy of the neutron-unbound nucleus $^{28}$F has been performed for the first time following proton/neutron removal from $^{29}$Ne/$^{29}$F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the $^{27}$F$^{(*)}+n$ and $^{26}$F$^{(*)}+2n$ coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the $^{28}$F ground state, with $S_n(^{28}$F$)=-199(6)$ keV, while analysis of the $2n$ decay channel allowed a considerably improved $S_n(^{27}$F$)=1620(60)$ keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of $^{28}$F. Importantly, in the case of the ground state, the reconstructed $^{27}$F$+n$ momentum distribution following neutron removal from $^{29}$F indicates that it arises mainly from the $1p_{3/2}$ neutron intruder configuration. This demonstrates that the island of inversion around $N=20$ includes $^{28}$F, and most probably $^{29}$F, and suggests that $^{28}$O is not doubly magic.

nucl-ex↗
arXiv2020

$β$-Decay Half-Lives of 55 Neutron-Rich Isotopes beyond the N=82 Shell Gap

The $β$-decay half-lives of 55 neutron-rich nuclei $^{134-139}$Sn, $^{134-142}$Sb, $^{137-144}$Te, $^{140-146}$I, $^{142-148}$Xe, $^{145-151}$Cs, $^{148-153}$Ba, $^{151-155}$La were measured at the Radioactive Isotope Beam Factory (RIBF) employing the projectile fission fragments of $^{238}$U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly-measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.

nucl-ex↗
arXiv2020

A novel approach to the computation of one-loop three- and four-point functions. I -- The real mass case

This article is the first of a series of three presenting an alternative method to compute the one-loop scalar integrals. This novel method enjoys a couple of interesting features as compared with the method closely following 't Hooft and Veltman adopted previously. It directly proceeds in terms of the quantities driving algebraic reduction methods. It applies to the three-point functions and, in a similar way, to the four-point functions. It also extends to complex masses without much complication. Lastly, it extends to kinematics more general than the one of physical e.g.\ collider processes relevant at one loop. This last feature may be useful when considering the application of this method beyond one loop using generalised one-loop integrals as building blocks.

hep-ph↗
arXiv2020

Framework for a novel mixed analytical/numerical approach for the computation of two-loop $N$-point Feynman diagrams

A framework to represent and compute two-loop $N$-point Feynman diagrams as double-integrals is discussed. The integrands are 'generalised one-loop type" multi-point functions multiplied by simple weighting factors. The final integrations over these two variables are to be performed numerically, whereas the ingredients involved in the integrands, in particular the "generalised one-loop type" functions, are computed analytically. The idea is illustrated on a few examples of scalar three- and four-point functions.

hep-ph↗