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Satoshi X. Nakamura

Publications and source records attributed to Satoshi X. Nakamura.

At least 19 recordsLinked to original sources
arXiv2026

Two nearby states in the $X(3872)$ region: Resolving the radiative-decay ratio tension with $η_{c2}$

Recently, LHCb reported the radiative-decay ratio ${\cal R}^{ψγ}\equiv {\cal B}[X(3872)\to ψ'γ]/{\cal B}[X(3872)\to J/ψγ]=1.67\pm 0.25$ extracted from $B^+\to K^+(J/ψγ, ψ'γ)$. This result differs markedly ($\sim4.6σ$) from the BESIII value obtained from $e^+e^-\to γ(J/ψγ, ψ'γ)$, ${\cal R}^{ψγ}=-0.04\pm 0.28$. Such a significant tension suggests that more than one state in the $X(3872)$ region contributes to the processes. We therefore propose a two-state scenario: a shallow $D^{*0}\bar{D}^0$ bound state with $J^{PC}=1^{++}$ and a $2^{-+}$ charmonium candidate, $η_{c2}$, slightly above the $D^{*0}\bar{D}^0$ threshold. We show that this hypothesis consistently describes these ratios along with other branching fractions and lineshapes across multiple processes. By contrast, fits without the $η_{c2}$ component fail to reproduce the radiative-ratio data. We also predict helicity-angle distributions that motivates the future experiments to test the two-state hypothesis and search for the so-far missing $η_{c2}$.

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arXiv2025

Coupled-channel analysis for vector charmonia and their nature

High-precision $e^+e^-\to c\bar{c}$ data (20 final states) from the BESIII and Belle in $\sqrt{s}=3.75-4.7$ GeV are analyzed with a semi three-body unitary coupled-channel model. Vector charmonium poles are extracted from the amplitudes obtained from the fit. We find well-known $ψ$ states listed in the PDG, and also several states near open-charm thresholds. The compositeness of the near-threshold poles suggests that $ψ(4040)$ could mainly consist of a $D^*\bar{D}^*$-molecule component, rather than a conventionally accepted quark-model $ψ(3S)$ state. Also, $ψ(4230)$ and $ψ(4360)$ might be substantial mixtures of $D_1(2420)\bar{D}$, $D_1(2420)\bar{D}^*$, $D_s^*\bar{D}_s^*$, and $c\bar{c}$ components.

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arXiv2023

Strange hidden-charm pentaquark poles from $B^-\to J/ψΛ\bar{p}$

Recent LHCb data for $B^-\to J/ψΛ\bar{p}$ show a clear peak structure at the $Ξ_c\bar{D}$ threshold in the $J/ψΛ$ invariant mass ($M_{J/ψΛ}$) distribution. The LHCb's amplitude analysis identified the peak with the first hidden-charm pentaquark with strangeness $P_{ψs}^Λ(4338)$. We conduct a coupled-channel amplitude analysis of the LHCb data by simultaneously fitting the $M_{J/ψΛ}$, $M_{J/ψ\bar{p}}$, $M_{Λ\bar{p}}$, and $\cosθ_{K^*}$ distributions. Rather than the Breit-Wigner fit employed in the LHCb analysis, we consider relevant threshold effects and a unitary $Ξ_c\bar{D}$-$Λ_c\bar{D}_s$ coupled-channel scattering amplitude from which $P_{ψs}^Λ$ poles are extracted for the first time. In our default fit, the $P_{ψs}^Λ(4338)$ pole is almost a $Ξ_c \bar{D}$ bound state at $( 4338.2\pm 1.4)-( 1.9\pm 0.5 )\,i$ MeV. Our default model also fits a large fluctuation at the $Λ_c\bar{D}_s$ threshold, giving a $Λ_c\bar{D}_s$ virtual state, $P_{ψs}^Λ(4255)$, at $4254.7\pm 0.4$ MeV. We also found that the $P_{ψs}^Λ(4338)$ peak cannot solely be a kinematical effect, and a nearby pole is needed.

hep-ph↗
arXiv2023

$X$ and $Z_{cs}$ in $B^+\to J/ψϕK^+$ as $s$-wave threshold cusps and alternative spin-parity assignments to $X(4274)$ and $X(4500)$

Recent LHCb's amplitude analysis on $B^+\to J/ψϕK^+$ suggests the existence of exotic $X$ and $Z_{cs}$ hadrons, based on an assumption that Breit-Wigner resonances describe all the peak structures. However, all the peaks and also dips in the spectra are located at relevant meson-meson thresholds where threshold kinematical cusps might cause such structures. This points to the importance of an independent amplitude analysis with due consideration of the kinematical effects, and this is what we do in this work. Our model fits well $J/ψϕ$, $J/ψK^+$, and $K^+ϕ$ invariant mass distributions simultaneously, demonstrating that all the $X$, $Z_{cs}$, and dip structures can be well described with the ordinary $s$-wave threshold cusps. Spin-parity of the $X(4274)$ and $X(4500)$ structures are respectively $0^-$ and $1^-$ from our model, as opposed to $1^+$ and $0^+$ from the LHCb's. With all relevant threshold cusps considered, the number of fitting parameters seems to be significantly reduced. The LHCb data requires $D_s^{(*)}\bar{D}^{*}$ scattering lengths in our model to be consistent with zero, disfavoring $D_s^{(*)}\bar{D}^{*}$ molecule interpretations of $Z_{cs}(4000)$ and $Z_{cs}(4220)$ and, via the SU(3) relation, being consistent with previous lattice QCD results.

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arXiv2022

$X$ structures in $B^+\to J/ψϕK^+$ as one-loop and double-triangle threshold cusps

The LHCb data on $B^+\to J/ψϕK^+$ show four peaks and three dips in the $J/ψϕ$ invariant mass distribution, and the peaks are interpreted as $X(4140)$, $X(4274)$, $X(4500)$ and $X(4685)/X(4700)$ resonance contributions. Interestingly, all the peaks and dips are located at (or close to) $D^*_{s}\bar{D}^{(*)}_{s}$, $D_{s0}^*(2317)\bar{D}^{(*)}_{s}$, $D_{s1}(2536)\bar{D}^{(*)}_{s}$, and $ψ'ϕ$ thresholds. These coincidences suggest a close connection between the structures and the thresholds, which however has not been seriously considered in previous theoretical studies on the $X$ structures. In fact, if we utilize this connection and interpret the $X$ structures as common $s$-wave threshold cusps, we face a difficulty: $X(4274)$ and $X(4500)$ have spin-parity that conflict with the experimentally determined ones. In this work, we introduce double triangle mechanisms that cause threshold cusps significantly sharper than the ordinary one-loop ones of the same spin-parity. We demonstrate that all the $X$ and dip structures are well described by a combination of one-loop and double-triangle threshold cusps, thereby proposing a novel interpretation of the $X$ and dip structures.

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arXiv2021

Novel description of $P_c(4312)^+$, $P_c(4380)^+$, and $P_c(4457)^+$ with double triangle cusps

We propose a novel scenario for the peak structures, usually interpreted as hidden charm pentaquark ($P_c$) contributions, in the LHCb's $Λ_b^0\to J/ψp K^-$ data. The key idea is to utilize leading or lower-order singularities from double triangle mechanisms. The singularities cause anomalous threshold cusps, which are significantly more singular than the ordinary ones, at the $Σ_c^{(*)}\bar{D}^{(*)}$ threshold. We demonstrate that the double triangle amplitudes interfere with other common mechanisms to create peak structure that fit well the $P_c(4312)^+$, $P_c(4380)^+$, and $P_c(4457)^+$ peaks in the data. This picture is completely different from commonly used ones such as hadron molecules and compact pentaquarks. Meanwhile, $P_c(4440)^+$ is included in the proposed model as a resonance with width and strength significantly smaller than previously estimated. The proposed model can (partly) explain the current data for other processes where $P_c^+$ signals are expected such as: no $P_c^+$ signals in the GlueX $J/ψ$ photoproduction data; a possible signal only from $P_c(4440)^+$ in the LHCb's $Λ_b^0\to J/ψp π^-$ data.

hep-ph↗
arXiv2021

$P_c(4312)^+$, $P_c(4380)^+$, and $P_c(4457)^+$ as double triangle cusps

Understanding the nature of the hidden charm pentaquark(like) signals in the LHCb data for $Λ_b^0\to J/ψp K^-$ is a central problem of hadron spectroscopy. We propose a scenario completely different from previous ones such as hadron molecules and compact pentaquarks. We identify relevant double triangle mechanisms with leading or lower-order singularities. The associated anomalous threshold cusps at the $Σ_c^{(*)}\bar{D}^{(*)}$ thresholds are significantly more singular than the ordinary ones. Then we demonstrate that the double triangle amplitudes reproduce the peak structures of $P_c(4312)^+$, $P_c(4380)^+$, and $P_c(4457)^+$ in the LHCb data, through an interference with other common mechanisms. Only the $P_c(4440)^+$ peak is due to a resonance with width and strength significantly smaller than previously estimated. $P_c^+$ signals are expected in other processes and the proposed model (partly) explains the current data such as: the GlueX $J/ψ$ photoproduction data with no $P_c^+$ signals; the LHCb $Λ_b^0\to J/ψp π^-$ data with a possible signal only from $P_c(4440)^+$. The double triangle singularity is now a possible option to interpret resonancelike structures near thresholds in general.

hep-ph↗
arXiv2021

An $X(3872)$-like peak in $B\to (J/ψπ^+π^-) Kπ$ due to triangle singularity

Triangle mechanisms for $B^0\to (J/ψπ^+π^-) K^+π^-$ are studied. Experimentally, an $X(3872)$ peak has been observed in this process. When the final $(J/ψπ^+π^-)π$ invariant mass is around the $D^*\bar D^*$ threshold, one of the triangle mechanisms causes a triangle singularity and generates a sharp $X(3872)$-like peak in the $J/ψπ^+π^-$ invariant mass distribution. The Breit-Wigner mass and width fitted to the spectrum are 3871.68 MeV (a few keV above the $D^{*0}\bar{D}^0$ threshold) and $\sim$0.4 MeV, respectively.These Breit-Wigner parameters hardly depends on a choice of the model parameters. Comparing with the precisely measured $X(3872)$ mass and width, $3871.69\pm 0.17$ MeV and $< 1.2$ MeV, the agreement is remarkable. When studying the $X(3872)$ signal from this process, this non-resonant contribution has to be understood in advance. We also study a charge analogous process $B^0\to (J/ψπ^0π^-) K^+π^0$. A similar triangle singularity exists and generates an $X^-(3876)$-like peak.

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arXiv2020

Triangle singularity appearing as an $X(3872)$-like peak in $B\to (J/ψπ^+π^-) Kπ$

We consider a triangle diagram for $B^0\to (J/ψπ^+π^-) K^+π^-$ where an $X(3872)$ peak has been observed experimentally. We demonstrate that a triangle singularity inherent in the triangle diagram creates a sharp peak in the $J/ψπ^+π^-$ invariant mass distribution when the final $(J/ψπ^+π^-)π$ invariant mass is at and around the $D^*\bar D^*$ threshold. The position and width of the peak is 3871.68 MeV (a few keV above the $D^{*0}\bar{D}^0$ threshold) and $\sim$0.4 MeV, respectively, in perfect agreement with the precisely measured $X(3872)$ mass and width: $3871.69\pm 0.17$ MeV and $< 1.2$ MeV. This remarkable agreement is virtually parameter-free. The result indicates that the considered mechanism has to be understood in advance when separating an $X(3872)$-pole contribution from $B^0\to (J/ψπ^+π^-) K^+π^-$ data; the separation yields an $X(3872)π$ lineshape that could be used to determine the $X(3872)$ mass. We suggest a method to set a constraint on the triangle mechanism by analyzing a charge analogous process $B^0\to (J/ψπ^0π^-) K^+π^0$ where a similar triangle singularity generates an $X^-(3876)$-like peak.

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arXiv2020

Neutron-neutron scattering length from $π^+$ photoproduction on the deuteron

We discuss the possibility of extracting the neutron-neutron scattering length $a_{nn}$ and effective range $r_{nn}$ from cross section data ($d^2σ/dM_{nn}/dΩ_π$), as a function of the $nn$ invariant mass $M_{nn}$, for $π^+$ photoproduction on the deuteron ($γd\to π^+nn$). The analysis is based on a $γd\to π^+nn$ reaction model in which realistic elementary amplitudes for $γp\to π^+n$, $NN\to NN$, and $πN\to πN$ are built in. We show that $M_{nn}$ dependence (lineshape) of a ratio $R_{\rm th}$, $d^2σ/dM_{nn}/dΩ_π$ normalized by $dσ/dΩ_π$ for $γp\toπ^+ n$ and the nucleon momentum distribution inside the deuteron, at the kinematics with $θ_π=0^\circ$ and $E_γ\sim 250$ MeV is particularly useful for extracting $a_{nn}$ and $r_{nn}$ from the corresponding data $R_{\rm exp}$. It is found that $R_{\rm exp}$ with 2% error, resolved into the $M_{nn}$ bin width of 0.04 MeV (corresponding to the $p_π$ bin width of 0.05 MeV$/c$), can determine $a_{nn}$ and $r_{nn}$ with uncertainties of $\pm 0.21$ fm and $\pm 0.06$ fm, respectively, for the case of $a_{nn}=-18.9$ fm and $r_{nn}=2.75$ fm. The requirement of such narrow bin widths indicates that the momenta of the incident photon and the emitted $π^+$ have to be measured with high resolutions. This can be achieved by utilizing virtual photons of very small $Q^2$ from electron scattering at Mainz MAMI facility. The proposed method for determining $a_{nn}$ and $r_{nn}$ from $γd\to π^+ nn$ has a great experimental advantage over the previous one utilizing $π^- d\toγnn$ for being free from the formidable task of controlling the neutron detection efficiency and its uncertainty.

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arXiv2019

$Z_c(4430)$, $Z_c(4200)$, $Z_1(4050)$, and $Z_2(4250)$ as triangle singularities

$Z_c(4430)$ discovered in $\bar{B}^0\toψ(2S)K^-π^+$, $Z_c(4200)$ found in $\bar{B}^0\to J/ψK^-π^+$, and $Z_1(4050)$ and $Z_2(4250)$ observed in $\bar{B}^0\toχ_{c1}K^-π^+$ are candidates of charged charmonium-like states. All surviving theoretical models interpreted these candidates as four-quark states, until we recently identified a compelling alternative. We discuss that kinematical singularities in triangle loop diagrams induce a resonance-like behavior that can consistently explain the properties (such as spin-parity, mass, width, and Argand plot) of $Z_c(4430)$, $Z_c(4200)$, $Z_1(4050)$ and $Z_2(4250)$ from experiments. In terms of the triangle singularities, we can also naturally understand interesting experimental findings such as the appearance (absence) of $Z_c(4200)$($Z_c(4430)$)-like contribution in $Λ_b^0\to J/ψpπ^-$, and the highly asymmetric shape of the spectrum bump for $Z_1(4050)$; the other theoretical models have not successfully addressed these points. Although Pakhlov et al. proposed another triangle diagram to generate a $Z_c(4430)$-like bump, we argue that this scenario is very unlikely.

hep-ph↗
arXiv2019

Triangle singularities in $\bar{B}^0\to χ_{c1}K^-π^+$ relevant to $Z_1(4050)$ and $Z_2(4250)$

$Z_1(4050)$ and $Z_2(4250)$ observed in $\bar{B}^0\toχ_{c1}K^-π^+$ by the Belle Collaboration are candidates of charged charmonium-like states that minimally includes two quarks and two antiquarks. While $Z_1(4050)$ and $Z_2(4250)$ have been interpreted as tetraquark states previously, we propose a completely different scenario based on a kinematical effect called the triangle singularity. We demonstrate that the triangle singularities cause in the $χ_{c1}π^+$ invariant mass distribution resonance-like bumps that fit very well the Belle data. If these bumps are simulated by the $Z_1(4050)$ and $Z_2(4250)$ resonance excitations, the spin-parity of them are predicted to be $1^-$ for $Z_1(4050)$ and $1^+$ or $1^-$ for $Z_2(4250)$. The bump corresponding to $Z_1(4050)$ has a highly asymmetric shape, which the Belle data exactly indicate. We show that the asymmetric shape originates from an interplay between the triangle singularity and the opening of the $X(3872)π^+$ channel near the triangle-singularity energy. This characteristic lineshape could be used to discriminate different interpretations of $Z_1(4050)$. An interesting prediction from interpretting $Z_1(4050)$ and $Z_2(4250)$ as the triangle singularities is that similar bumps caused by the same mechanisms possibly appear also in $\bar{B}^0\to J/ψK^-π^+$ data; the already observed $Z_c(4200)$ corresponds to $Z_2(4250)$ of $J^P=1^+$.

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arXiv2019

Electroweak pion-production on nuclei within the extended factorization scheme

We have applied the extended factorization scheme to investigate the electroweak pion production on nuclei. The ANL-Osaka model, which was obtained by analyzing the data of $πN$, $γN$, $N(e,e'π)$ and $N(ν,μ\,π) $ reactions up to invariant mass $W=$ 2 GeV, is used to generate the matrix elements of current operators relevant to pion-production off the nucleon. Medium effects on the $Δ$ (1232) component of meson-exchange current are included by using a $Δ$-nucleus potential determined from the previous $Δ$-hole model studies of pion-nucleus reactions. Nuclear correlations in the initial target state and in the spectator system(s) are modeled using realistic hole spectral functions. As a first step, we show that the data of $^{12}$C$(e,e')$ up to the $Δ$ (1232) region can be described reasonably well. The interplay between the pion production and two-body meson-exchange mechanisms is shown to be essential in improving the agreement with the data in the ``dip'' region, between the quasielastic and the $Δ$ (1232) peaks. Predictions for $^{12}$C$(ν,μ\,π)$ have also been made. They can be used to estimate pion-emission rates in neutrino-nucleus cross section, which constitutes an important systematic uncertainty to the reconstructed neutrino energy. With further improvements of the Metropolis Monte-Carlo techniques to account for final states comprised of more than two particles, our approach can be employed up to $W=$ 2 GeV, where two-pion production and higher mass nucleon resonances must be included for analyzing the data from accelerator-based neutrino-oscillation experiments.

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arXiv2019

Dynamical coupled-channels approach to electroweak meson productions on nucleon and deuteron

I overview our recent activity with the Argonne-Osaka dynamical coupled-channels (DCC) approach that provides a unified description of various electroweak meson productions on single nucleon and nucleus. First I discuss the DCC model of a single nucleon. The DCC model has been developed through a comprehensive analysis of $πN, γN\to πN, ηN, KΛ, KΣ$ reaction data. The model has been further extended to finite $Q^2$ region by analyzing pion electroproduction data, and to neutrino-induced reactions using the PCAC relation. Next I discuss applications of the DCC model to electroweak meson productions on the deuteron. We consider impulse mechanism supplemented by final state interactions (FSI) due to $NN$ and meson-nucleon rescatterings. Using this model, I discuss FSI corrections needed to extract $γ$-neutron reaction observables from $γd\to πNN$, and a novel method to extract $ηN$ scattering length from $γd\to ηpn$. I also discuss FSI corrections on the existing neutrino-nucleon pion production data that had been extracted from neutrino-deuteron data.

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arXiv2018

Extraction of $γn\toπN$ observables from deuteron-target data

An examination is conducted on a commonly used procedure for extracting (un)polarized $γn\to π^-p$ and $γn\to π^0n$ observables from $d(γ,π^-)pp$ and $d(γ,π^0)pn$ data, using a model that consists of the impulse term and the final-state interaction (FSI) terms due to nucleon- and pion-exchange. Recent experimental and theoretical analyses used an extraction method that does not impose a cut on the final $πN$ invariant mass $W$. I demonstrate that the use of this method can result in the $γn\to πN$ observables that are seriously distorted by the nucleon Fermi motion, and that one can efficiently avoid this problem by imposing a cut on $W$. It is also shown that the use of kinematical cuts of recent experimental analyses can still leave in the selected samples substantial FSI effects that must be corrected in extracting the $γn \toπN$ cross sections. In terms of the nucleon- and pion-exchange mechanisms, I give the first qualitative explanation of the FSI corrections, obtained in a recent MAMI experiment, for extracting $γn\to π^0 n$ cross sections.

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arXiv2018

Photo- and Electro-excitation of Bound Neutrons and Protons

Data for pion photo-productions off the neutron ($γn\to πN$) have been primarily extracted from deuteron-target data ($γd\to πNN$) by applying kinematical cuts thereby isolating the quasi-free samples. We critically examine if the neutron-target data obtained through this conventional procedure can be contaminated by final state interactions (FSI) and/or the kinematical cuts. The analysis is conducted with a theoretical model for $γd\to πNN$ that takes account of the impulse mechanisms supplemented by the $NN$ and $πN$ rescattering mechanisms. We show that the FSI effects still visibly remain in the extracted $γ$`$n$'$\toπN$ unpolarized cross sections and polarization asymmetries $E$ even after the kinematical cuts are applied. We also find the FSI effects on $γ$`$n$'$\toπ^0n$ can be somewhat different from those on $γ$`$p$'$\toπ^0p$.

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arXiv2016

Coupled-channel Dalitz plot analysis of $D^+\to K^- π^+π^+$ decay

We demonstrate that partial wave amplitudes extracted from $D^+\to K^-π^+π^+$ Dalitz plot with a unitary coupled-channel model are significantly different from those obtained with an isobar model. The unitary coupled-channel model takes account of hadronic rescattering mechanisms involving all three mesons that have been missed in conventioanl isobar model analyses. The rescattering mechanisms contribute largely, and can triplicate the $D^+\to {K}^-π^+π^+$ decay width within our analysis. These findings deliver a warning that analysis results obtained with isobar models should be looked with a caution. The determination of the CKM angle $γ/ϕ_3$ is a highly relevant problem.

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