arXiv subjects

Shuntaro Sakai

Publications and source records attributed to Shuntaro Sakai.

At least 19 recordsLinked to original sources

arXiv2023

Spectral function of the $η'$ meson in nuclear medium based on phenomenological models

The in-medium modification of the spectral function of the $η'$ meson with and without the spatial momentum is studied with the $Tρ$ approximation by employing two phenomenological models for the $η'N$ scattering; one is called coupled channels model and the other the $N(1895)$-dominance model. In the former model, the $η'N$ scattering amplitude is calculated in the unitarized coupled-channel approach involving the $η'N$ channel, while in the latter model the $η'N$ scattering process is dominated by the $N(1895)$ resonance with the spin and parity $J^P=1/2^-$. In the \com{coupled channels model}, one single peak of the in-medium $η'$ mode appears in the spectral function and the peak position shifts to higher energies along with the increase of the nuclear density reflecting the repulsive $η'N$ scattering length of the unitarized coupled-channel amplitude. On the other hand, two branches related to the $η'$ and $N(1895)$-hole modes appear in the $N(1895)$-dominance model. In both models, the shift of the peak position and the width in the spectral function are a few tens of MeV at the normal nuclear density for the $η'$ meson at rest in the nuclear medium. Once the spatial momentum is turned on, the peak positions in the spectral function approach the energies without the nuclear medium effect. Particularly, in the $N(1895)$-dominance model, the peak strength of the $N(1895)$-hole mode gets smaller with the finite momentum and the spectral function comes to have one single peak.

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arXiv2022

Investigation of the $X(4020)$ peak in the $D^\bar D^$ and $πh_c$ channels

In this paper, the nature of $X(4020)$ observed in the vicinity of the $D^*\bar D^*$ threshold of the $πh_c$ and $D^*\bar D^*$ distributions is studied. Using a model containing one bare-particle state with the $πh_c$ and $D^*\bar D^*$ one-loop self energies, we make a fit to the $πh_c$ and $D^*\bar D^*$ distributions of the $e^+e^-\toπ^+π^-h_c$ and $e^+e^-\toπ^- D^{*+}\bar D^{*0}$ reactions reported by the BESIII Collaboration. It is found that both structures seen in the $πh_c$ and $D^*\bar D^*$ channels are described well with one resonant state which can be associated with $X(4020)$ while the obtained $X(4020)$ mass and width are found to be larger than those in the latest PDG. We evaluate the scattering length and effective range of the $S$-wave $D^*\bar D^*$ pair with spin $J=1$ related to the $X(4020)$ properties which can be tested with model calculations or future lattice QCD study. Our analysis suggests that a large part of $X(4020)$ may come from a bare-particle state originating from some short-range dynamics other than the $D^*\bar D^*$ channel.

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arXiv2020

Possible precise measurements of the $X(3872)$ mass with the $e^+e^-\toπ^0γX(3872)$ and $p\bar p\toγX(3872)$ reactions

It was recently proposed that the $X(3872)$ binding energy, the difference between the $D^0\bar D^{*0}$ threshold and the $X(3872)$ mass, can be precisely determined by measuring the $γX(3872)$ line shape from a short-distance $D^{*0}\bar D^{*0}$ source produced at high-energy experiments. Here, we investigate the feasibility of such a proposal by estimating the cross sections for the $e^+e^-\toπ^0γX(3872)$ and $p\bar p\toγX(3872)$ processes considering the $D^{*0}\bar D^{*0}D^0/\bar D^{*0}D^{*0}\bar D^0$ triangle loops. These loops can produce a triangle singularity slightly above the $D^{*0}\bar D^{*0}$ threshold. It is found that the peak structures originating from the $D^{*0}\bar D^{*0}$ threshold cusp and the triangle singularity are not altered much by the energy dependence introduced by the $e^+e^-\toπ^0D^{*0}\bar D^{*0}$ and $p\bar p\to\bar D^{*0}D^{*0}$ production parts or by considering a finite width for the $X(3872)$. We find that $σ(e^+e^-\toπ^0γX(3872)) \times {\rm Br}(X(3872)\toπ^+π^-J/ψ)$ is $\mathcal{O}(0.1~{\rm fb})$ with the $γX(3872)$ invariant mass integrated from 4.01 to 4.02 GeV and the c.m. energy of the $e^+e^-$ pair fixed at 4.23 GeV. The cross section $σ(p\bar p\toγX(3872))\times {\rm Br}(X(3872)\toπ^+π^-J/ψ)$ is estimated to be of $\mathcal{O}(10~{\rm pb})$. Our results suggest that a precise measurement of the $X(3872)$ binding energy can be done at PANDA.

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arXiv2020

Extraction of $ND$ scattering lengths from the $Λ_b\rightarrowπ^-pD^0$ decay and properties of the $Σ_c(2800)^+$

The isovector and isoscalar $ND$ $s$-wave scattering lengths are extracted by fitting to the LHCb data of the $pD^0$ invariant-mass distribution in the decay $Λ_b\rightarrowπ^-pD^0$, making use of the cusp effect at the $nD^+$ threshold. The analysis is based on a coupled-channel nonrelativistic effective field theory. We find that the real part of the isovector $ND$ scattering length is unnaturally large due to the existence of a near-threshold state with a mass around 2.8 GeV. The state is consistent with the $Σ_c(2800)^+$ resonance observed at Belle. Our results suggest that it couples strongly to the $ND$ channel in an $s$-wave, and that its quantum numbers are $J^P=1/2^-$. The strong cusp behavior at the $nD^+$ threshold can be verified using updated LHCb data.

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arXiv2020

Role of the triangle mechanism in the $Λ_b\rightarrow Λ_cπ^-f_0(980)$ reaction

We investigate the $Λ_b\toΛ_cπ^-f_0(980)$ production with a $f_0(980)$ decay into $π^+π^-$ via the $K^{*0}K^-K^+$ and $K^{*-}K^0\bar{K}^0$ triangle loops. These loops produce a peak around 1.42 GeV in the $π^-f_0(980)$ invariant mass distribution, which is the same mechanism as the one considered to explain the $a_1(1420)$ peak. In the $π^+π^-$ distribution obtained by fixing the $π^-f_0(980)$ invariant mass to some values, a clear peak of $f_0(980)$ is seen, and the $π^-f_0(980)$ distribution has a peak around $M_{π^-f_0}=1.42$ GeV, which is caused by the triangle mechanism of the $K^*\bar KK$ loop. The branching ratio of $Λ_b\rightarrowΛ_cπ^-f_0(980)$ with $f_0(980)\rightarrowπ^+π^-$ by the triangle mechanism, obtained by integrating the $π^-f_0(980)$ distribution from 1 to 1.6 GeV, is estimated to be the order $10^{-4}$. Future measurements of the $Λ_b\toΛ_cπ^-f_0(980)$ branching ratio and the $π^-f_0(980)$ invariant mass distribution predicted in this work would give further clues to clarify the nature of the $a_1(1420)$ peak.

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arXiv2020

Triangle singularity in the $B^-\to K^-π^0X(3872)$ reaction and sensitivity to the $X(3872)$ mass

We have done a study of the $B^- \to K^- π^0 X(3872)$ reaction by means of a triangle mechanism via the chain of reactions: $B^- \to K^- D^{*0}\bar D^{*0}$; $D^{*0} \to π^0 D^0$; $D^0\bar D^{*0}\to X(3872)$. We show that this mechanism generates a triangle singularity in the $π^0 X(3872)$ invariant mass for a very narrow window of the $X(3872)$ mass, around the present measured values, and show that the peak positions and the shape of the mass distributions are sensitive to the $X(3872)$ mass, such that a measurement of the reaction can serve to improve on the present values of this mass. In particular, we point out that the $X(3872)$ mass relative to the $D^0\bar D^{*0}$ threshold may be extracted from the asymmetry of the $π^0X$ line shape.

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arXiv2020

Threshold cusps and triangle singularities in hadronic reactions

The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities, and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates.

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arXiv2019

Triangle singularities in ${J/ψ\rightarrowηπ^0ϕ}$ and ${π^0π^0ϕ}$

The BESIII Collaboration recently reported the observation of the $a_0(980)^0-f_0(980)$ mixing in the isospin breaking decay $J/ψ\to ηπ^0ϕ$. In the Dalitz plot for that decay with the $η$ reconstructed from two photons, there is a band around $1.4$~GeV on the $π^0 ϕ$ distribution. In general, this peak can be due to a resonance or a kinematic effect. In this paper, we study the effects of a set of $K^*K\bar K$ triangle diagrams, and show that due to triangle singularities such diagrams can lead to a peak around 1.4~GeV in the $π^0ϕ$ invariant mass distribution. The Dalitz plot induced by such a mechanism has a feature consistent with the BESIII observation, namely events along the band accumulate at both ends close to the Dalitz plot boundary. The effect of the same mechanism on the $J/ψ\to π^0π^0ϕ$ and $J/ψ\to ηπ^0 K^+K^-$ decays are also investigated. We suggest to take more data for the $J/ψ\to ηπ^0ϕ\to ηπ^0 K^+ K^-$ and check whether the structure around $1.4$~GeV persists for the $K^+K^-$ invariant mass away from the $ϕ$ mass region. This is crucial for understanding whether the band is due to triangle singularities or due to a resonance. Were it the latter, the band should remain while it would not if it is due to the former.

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arXiv2019

Decays of $P_c$ into $J/ψN$ and $η_cN$ with heavy quark spin symmetry

We investigate the consequences of heavy quark spin symmetry (HQSS) on hidden-charm pentaquark $P_c$ states. As has been proposed before, assuming the $P_c(4440)$ and the $P_c(4457)$ as $S$-wave $\bar{D}^*Σ_c$ molecules, seven hadronic molecular states composed of $\bar{D}Σ_c$, $\bar{D}Σ_c^*$, $\bar{D}^*Σ_c$, and $\bar{D}^*Σ_c^*$ can be obtained, with the $\bar{D}Σ_c$ molecule corresponding to the $P_c(4312)$. These seven states can decay into $J/ψN$ and $η_c N$, and we use HQSS to predict ratios of partial widths of the $S$-wave decays. For the decays into $J/ψN$, it is found that among all six $P_c$ molecules with spin $1/2$ or $3/2$, at least four states decay much more easily into the $J/ψN$ than the $P_c(4312)$, and two of them couple dominantly to the $\bar D^*Σ_c^*$. While no significant peak around the $\bar{D}^*Σ_c^*$ threshold is found in the $J/ψp$ distribution, these higher $P_c$ states either are produced with lower rates or some special production mechanism for the observed $P_c$ states might play an important role, such as an intricate interplay between the production of pentaquarks and triangle singularities.

hep-ph↗

arXiv2017

Effect of the final state interaction of $η' N$ on the $η'$ photoproduction off nucleon

We investigate the $η'$ photoproduction off the nucleon with a particular interest in the effect of the final-state interaction (FSI) of the $η'$ meson and nucleon $(η'N)$ based on three-flavor linear sigma model. We find an enhancement in the cross section of the $η'$ photoproduction near the $η'N$-threshold energy owing to the $η'N$ FSI. With the $η'$ meson at forward angles, the energy dependence near the $η'N$ threshold is well reproduced with the $η'N$ FSI. The cross section at backward angles can also be a good probe to investigate the strength of the $η'N$ interaction.

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arXiv2017

Investigation of the $η'N$ system using the linear sigma model

In this paper, we investigate the $η'N$ system using the three-flavor linear sigma model including the effect of flavor SU(3) symmetry breaking. The $η'N$ bound state is also found in the case including flavor symmetry breaking and coupling with the $ηN$ and $πN$ channels. The $η'N$ interaction becomes more attractive with the inclusion of flavor symmetry breaking, which causes mixing between the singlet and octet scalar mesons. The existence of such a bound state would have some impact on the $η'$-nucleus system, which is of interest from both theoretical and experimental viewpoints.

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arXiv2016

Theoretical study of photoproduction of $η^{\prime} N$ bound state on deuteron target with forward proton emission

Possibilities of observing a signal of an $η^{\prime} n$ bound state are investigated by considering photoproductions of the $η$ and $η^{\prime}$ mesons on a deuteron target with forward proton emission. For this purpose, we take the $η^{\prime} n$ interaction from the linear sigma model with a coupling to $ηn$, in which an $s$-wave $η^{\prime} n$ bound state can be dynamically generated, and we fix the $γp \to ηp$ and $η^{\prime} p$ scattering amplitudes so as to reproduce the experimental cross sections with forward proton emission. By using these $γp \to η^{( \prime )} p$ and $η^{( \prime )} n \to η^{( \prime )} n$ amplitudes, we calculate cross sections of the $γd \to ηn p$ and $η^{\prime} n p$ reactions with forward proton emission in single and $η^{(\prime )}$-exchange double scattering processes. As a result, we find that the signal of the $η^{\prime} n$ bound state can be seen below the $η^{\prime} n$ threshold in the $ηn$ invariant mass spectrum of the $γd \to ηn p$ reaction and is comparable with the contribution from the quasifree $η^{\prime}$ production above the $η^{\prime} n$ threshold. We also discuss the behavior of the signal of the $η^{\prime} n$ bound state in several experimental conditions and model parameters.

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arXiv2016

Possible relevance of the softening of the sigma meson to $η$ decay into 3$π$ in the nuclear medium

We investigate the role of the softening of the scalar-isoscalar (sigma) meson in the $η\rightarrowπ^+π^-π^0$ and $3π^0$ decay widths in the symmetric nuclear medium using a linear sigma model. Our calculation shows that these decay widths in the nuclear medium increase by up to a factor of four to ten compared with those in the free space mainly depending on the mass of the sigma meson in the free space, which is an input parameter of the model. The enhancements are considerable even at a half of the normal nuclear density. Thus, the $η$ decay into $3π$ could be a possible new probe for the chiral restoration in the nuclear medium. We find that the density dependence of the $η\rightarrow3π^0$ decay is moderate in comparison with that of $η\rightarrowπ^+π^-π^0$, although the former width is greater than the latter one at a given density. This is because the softening of the sigma meson causes cancellation of the terms appearing from the Bose symmetry in the $η\rightarrow3π^0$ decay. The difference between the density dependences should be helpful for experimental confirmation of the findings of the present study.

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arXiv2015

Possible medium effects on η-π^0 mixing and η\rightarrow π^0 π^+ π^- decay in the asymmetric nuclear matter

It is known that the decay process η\rightarrowπ^0π^+π^- in free space is possible due to the isospin-symmetry breaking in Quantum Chromodynamics (QCD), i.e., the small mass difference between u and d quarks: The small width is intimately related with the mixing property between ηand π^0 mesons for which the chiral anomaly play a role. In asymmetric nuclear matter such as heavy nuclei, isospin-symmetry breaking is large and it is expected that the mixing property of the mesons changes significantly and the above-mentioned decay width of the ηcreated in such a medium may be enhanced. This is an intriguing possibility to revealing a medium effect on a hadron in nuclei. We apply the in-medium chiral perturbation theory to estimate the modification of the η-π^0 mixing angle and the partial width of ηπ^0π^+π^- in asymmetric nuclear matter as a function of the isospin asymmetry. We find that these quantities can be greatly enhanced in neutron-rich matter, which should be deteactable in experiment.

hep-ph↗

arXiv2015

The $η$ decay into 3$π$ in asymmetric nuclear medium

We explore how the $η$-$π^0$ mixing angle and the $η$ meson decay into $π^{+}π^{-}π^0$ and 3$π^{0}$ are modified in the nuclear medium on the basis of the in-medium chiral effective theory with the isospin asymmetry $α$ varied, where $α\equiv δρ/ρ$ with $δρ=ρ_n-ρ_p$ and $ρ=ρ_n+ρ_p$. We find that the larger the isospin asymmetry $δρ$ and smaller the total density $ρ$, the more enhanced the mixing angle. We show that the decay width in the nuclear medium has an additional density dependence which cannot be renormalized into that of the mixing angle: The additional term originates from the vertex proportional to a low energy constant $c_1$, which only comes into play in the nuclear medium but not in the free space. It turns out that the resultant density effect on the decay widths overwhelms that coming from the isospin asymmetry, and the higher the $ρ$, the more enhanced the decay widths; the width for the $π^{+}π^{-}π^0$ decay is enhanced with a factor two to three at the normal density $ρ_0$ with a minor increase due to $δρ$, while that for the 3$π^0$ decay shows only a small increase of around 10 percent even at $ρ_0$. We mention the possible relevance of the partial restoration of chiral symmetry to the unexpected density effect on the decay widths in the nuclear medium.

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arXiv2014

The $η'N$ interaction from a chiral effective model and $η'$-$N$ bound state

The $η'$ mass reduction in the nuclear medium is expected from the degeneracy of the pseudoscalar-singlet and octet mesons when chiral symmetry is manifest. In this study, we investigate the $η'N$ 2body interaction which is the foundation of the in-medium $η'$ properties using the linear sigma model as a chiral effective model. The $η'N$ interaction in the linear sigma model comes from the scalar meson exchange with U$_A$(1) symmetry effect and is found to be fairly strong attraction. Moreover, the $ηN$ transition is included in our calculation, and is important for the imaginary part of the $η'$-optical potential. The transition amplitude of $η'N$ to the $ηN$ channel is relatively small compared to that of elastic channel. From the analysis of the $η'N$ 2body system, we find a $η'N$ bound state with the binding energy $12.3-3.3i$MeV. We expect that this strongly attractive two body interaction leads to a deep and attractive optical potential.

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arXiv2014

In-medium η' mass and η'N interaction in vacuum in the linear sigma model

We investigate the η'N two-body interaction in the context of the η' meson mass modification in the nuclear medium. It has been argued in several articles that the masses of η' and the other pseudoscalar mesons (π, K, η) should degenerate in the chiral-symmetric phase. It is expected that the reduction of the mass difference between eta and eta' would take place in the nuclear matter if one assumes that the decrease of the quark condensate at the normal nuclear density occurs with partial restoration of chiral symmetry. At low density, the in-medium self-energy giving the mass modification by the medium effect can be obtained by the η'N two-body T matrix. Thus, we also estimate the η'N interaction strength in vacuum with the linear sigma model which involves the effect of partial restoration of chiral symmetry. In the view of the linear sigma model, we find that the η'N interaction is attractive and generated through the sigma meson exchange.We expect that the interaction is enough strong and for the existence of a bound state of the η'N system.

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arXiv2013

In medium eta' mass and eta' N interaction based on chiral effective theory

The in-medium eta' mass and the eta'N interaction are investigated in an effective theory based on the linear realization of the SU(3) chiral symmetry. We find that a large part of the eta' mass is generated by the spontaneous breaking of chiral symmetry through the U_A (1) anomaly. As a consequence of this observation, the eta' mass is reduced in nuclear matter where chiral symmetry is partially restored. In our model, the mass reduction is found to be 80MeV at the saturation density. Estimating the eta' N interaction based on the same effective theory, we find that the eta' N interaction in the scalar channel is attractive sufficiently to form a bound state in the eta' N system with a several MeV binding energy. We discuss the origin of attraction by emphasizing the special role of the sigma meson in the linear sigma model for the mass generation of eta' and N.

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