arXiv subjects

S. X. Nakamura

Publications and source records attributed to S. X. Nakamura.

At least 19 recordsLinked to original sources

arXiv2025

Global coupled-channel analysis of $e^+e^-\to c\bar{c}$ processes in $\sqrt{s}=3.75-4.7$ GeV

Recent high-precision $e^+e^-\to c\bar{c}$ data from the BESIII and Belle are highly useful to understand vector charmonium ($ψ$) pole structures and puzzling lineshapes due to the exotic hadron candidates $Y$. We thus perform a global coupled-channel analysis of most of the available data (10 two-body, 9 three-body, and 1 four-body final states) in $\sqrt{s}=3.75-4.7$ GeV. Not only cross sections but also invariant-mass distributions of subsystems are fitted. The $e^+e^-\to μ^+μ^-$ cross sections are also predicted. Our model includes dozens of (quasi) two-body states that nonperturbatively couple with each other through bare $ψ$ excitations, particle-exchange, and short-range mechanisms; approximate three-body unitarity is considered. The amplitudes obtained from the fit are analytically continued to $ψ$ and $Z_c$ poles. We find $ψ$ states similar to those in the Particle Data Group listing and $Y(4320)$. Moreover, several $ψ$ states, including new ones, are found close to open charm thresholds. Trajectories and compositeness of the near-threshold poles suggest dominant hadron-molecule contents in their internal structures. Two $Z_c$ poles are found as virtual states $\sim$40 MeV below the $D^*\bar{D}^{(*)}$ thresholds, being consistent with lattice QCD results. This work presents the first global analysis to determine $ψ$ and $Z_c$ poles, thereby paving the way to extracting detailed properties of the prominent exotic hadron candidates from data.

hep-ph↗

arXiv2024

Three-body unitary coupled-channel approach to radiative $J/ψ$ decays and $η(1405/1475)$

Recent BESIII data on radiative $J/ψ$ decays from $\sim 10^{10}$ $J/ψ$ samples should significantly advance our understanding of the controversial nature of $η(1405/1475)$. This motivates us to develop a three-body unitary coupled-channel model for radiative $J/ψ$ decays to three-meson final states of any partial wave ($J^{PC}$). Basic building blocks of the model are bare resonance states such as $η(1405/1475)$ and $f_1(1420)$, and $πK$, $K\bar{K}$, and $πη$ two-body interactions that generate resonances such as $K^*(892)$, $K^*_0(700)$, and $a_0(980)$. This model reasonably fits $K_SK_Sπ^0$ Dalitz plot pseudo data generated from the BESIII's $J^{PC}=0^{-+}$ amplitude for $J/ψ\toγK_SK_Sπ^0$. The experimental branching ratios of $η(1405/1475)\toηππ$ and $η(1405/1475)\toγρ$ relative to that of $η(1405/1475)\to K\bar{K}π$ are simultaneously fitted. Our $0^{-+}$ amplitude is analytically continued to find three poles, two of which correspond to $η(1405)$ on different Riemann sheets of the $K^*\bar{K}$ channel, and the third one for $η(1475)$. This is the first pole determination of $η(1405/1475)$ and, furthermore, the first-ever pole determination from analyzing experimental Dalitz plot distributions with a manifestly three-body unitary coupled-channel framework. Process-dependent $ηππ$, $γπ^+π^-$, and $πππ$ lineshapes of $J/ψ\toγ(0^{-+})\to γ(ηππ)$, $γ(γρ)$, and $γ(πππ)$ are predicted, and are in reasonable agreement with data. A triangle singularity is shown to play a crucial role to cause the large isospin violation of $J/ψ\toγ(πππ)$.

hep-ph↗

arXiv2023

Threshold-cusp explanation for $X$ and $Z_{cs}$ in $B^+\to J/ψϕK^+$

Several $X$ and $Z_{cs}$ exotic hadrons were claimed in the LHCb's amplitude analysis on $B^+\to J/ψϕK^+$. The data shows that all the peaks and also dips in the spectra are located at thresholds of seemingly relevant meson-meson channels. While the LHCb analysis fitted the peaks with Breit-Wigner resonances, threshold kinematical cusps might be the cause of such structures. We thus analyze the LHCb data considering the threshold cusps. Our model is simultaneously fitted to $J/ψϕ$, $J/ψK^+$, and $K^+ϕ$ invariant mass distributions. The threshold cusps fit well all the $X$, $Z_{cs}$, and dip structures. Our analysis indicates that spin-parity of the $X(4274)$ and $X(4500)$ structures are $0^-$ and $1^-$, respectively. This is different from the LHCb's spin-parity assignments ($1^+$ and $0^+$). The number of fitting parameters can be significantly reduced by considering the relevant threshold cusps. Our analysis shows that $D_s^{(*)}\bar{D}^{*}$ scattering lengths are consistent with zero. This disfavors an explanation of $Z_{cs}(4000)$ and $Z_{cs}(4220)$ as $D_s^{(*)}\bar{D}^{*}$ molecules, which is consistent with lattice QCD via the SU(3) relation.

hep-ph↗

arXiv2023

Pole determination of $P_{ψs}^Λ(4338)$ and possible $P_{ψs}^Λ(4255)$ in $B^-\to J/ψΛ\bar{p}$

First hidden-charm pentaquark candidate with strangeness, $P_{ψs}^Λ(4338)$, was recently discovered in $B^-\to J/ψΛ\bar{p}$ by the LHCb Collaboration. $P_{ψs}^Λ(4338)$ shows up as a bump at the $Ξ_c\bar{D}$ threshold in the $J/ψΛ$ invariant mass ($M_{J/ψΛ}$) distribution. The $M_{J/ψΛ}$ distribution also shows a large fluctuation at the $Λ_c\bar{D}_s$ threshold, hinting the existence of a possible $P_{ψs}^Λ(4255)$. In this work, we determine the $P_{ψs}^Λ(4338)$ and $P_{ψs}^Λ(4255)$ pole positions for the first time. For this purpose, we fit a $B^-\to J/ψΛ\bar{p}$ model to the $M_{J/ψΛ}$, $M_{J/ψ\bar{p}}$, $M_{Λ\bar{p}}$, and $\cosθ_{K^*}$ distributions from the LHCb simultaneously; $χ^2/{\rm ndf}\sim 1.21$. Then we extract $P_{ψs}^Λ$ poles from a unitary $Ξ_c\bar{D}$-$Λ_c\bar{D}_s$ coupled-channel scattering amplitude built in the model. In our default fit, the $P_{ψs}^Λ(4338)$ pole is found at $( 4338.2\pm 1.4)-( 1.9\pm 0.5 )\,i$ MeV while the $P_{ψs}^Λ(4255)$ pole at $4254.7\pm 0.4$ MeV. The $P_{ψs}^Λ(4338)$ and $P_{ψs}^Λ(4255)$ are mostly $Ξ_c \bar{D}$ bound and $Λ_c\bar{D}_s$ virtual states, respectively. Through our analysis, the data disfavors a hypothesis of $P_{ψs}^Λ(4338)$ as merely a kinematical effect. This pole determination, which is important in its own right, sets a primary basis to study the nature of the $P_{ψs}^Λ$ states.

hep-ph↗

arXiv2023

Three-Body Unitary Coupled-Channel Analysis on $η(1405/1475)$

The recent BESIII data on $J/ψ\toγ(K_SK_Sπ^0)$, which is significantly more precise than earlier $η(1405/1475)$-related data, enables quantitative discussions on $η(1405/1475)$ at the previously unreachable level. We conduct a three-body unitary coupled-channel analysis of experimental Monte-Carlo outputs for radiative $J/ψ$ decays via $η(1405/1475)$: $K_SK_Sπ^0$ Dalitz plot distributions from the BESIII, and branching ratios of $γ(ηπ^+π^-)$ and $γ(γπ^+π^-)$ final states relative to that of $γ(K\bar{K}π)$. Our model systematically considers (multi-)loop diagrams and an associated triangle singularity, which is critical in making excellent predictions on $η(1405/1475)\to πππ$ lineshapes and branching ratios. The $η(1405/1475)$ pole locations are revealed for the first time. Two poles for $η(1405)$ are found on different Riemann sheets of the $K^*\bar{K}$ channel, while one pole for $η(1475)$. The $η(1405/1475)$ states are described with two bare states dressed by continuum states. The lower bare state would be an excited $η^\prime$, while the higher one could be an excited $η^{(\prime)}$, hybrid, glueball, or their mixture. This work presents the first-ever pole determination based on a manifestly three-body unitary coupled-channel framework applied to experimental three-body final state distributions (Dalitz plots).

hep-ph↗

arXiv2021

$P_c(4312)^+$ and $P_c(4337)^+$ as interfering $Σ_c\bar{D}$ and $Λ_c\bar{D}^{*}$ threshold cusps

The recent LHCb data on $B_s^0\to J/ψp\bar{p}$ revealed a new pentaquark-like $P_c(4337)^+$ structure, while finding no evidence for $P_c(4312)^+$ discovered earlier in $Λ_b^0\to J/ψp K^-$. Though puzzling, the data actually offer an important hint to understand the nature of the pentaquark candidates. We develop a model to analyze the $B_s^0\to J/ψp\bar{p}$ data. We find that a $Σ_c\bar{D}$ one-loop mechanism causes a threshold cusp that fits well the $P_c(4337)^+$ peak. Also, the $Σ_c\bar{D}$ and $Λ_c\bar{D}^{*}$ threshold cusps interfere with each other to reproduce an oscillating behavior in the proton helicity angle distribution. These results combined with our earlier analysis on $Λ_b^0\to J/ψp K^-$ indicate that $P_c(4312)^+$ and $P_c(4337)^+$ are created by different interference patterns between the $Σ_c\bar{D}$ and $Λ_c\bar{D}^{*}$ (anomalous) threshold cusps. The proposed scenario consistently explains why the $P_c(4312)^+$ and $P_c(4337)^+$ peaks appear in $Λ_b^0\to J/ψp K^-$ and $B_s^0\to J/ψp\bar{p}$, respectively, but not vice versa or both.

hep-ph↗

arXiv2019

The ANL-Osaka Partial-Wave Amplitudes of $πN$ and $γN$ Reactions

The determination of the Argonne National Laboratory-Osaka University (ANL-Osaka) Partial-Wave Amplitudes (PWA) of $πN$ and $γN$ Reactions is reviewed. The predicted PWA are presented on a web page (https://www.phy.anl.gov/theory/research/anl-osaka-pwa). The formulas are given for using the predicted PWA to calculate the cross sections of (1) meson-baryon ($MB$) scattering $MB \rightarrow M'B'$ with $MB, M'B'= πN, ηN, KΛ, KΣ$, (2) two-pion production $πN \rightarrow πΔ, ρN, σN \rightarrow ππN$, (3) Meson photoproduction $γN \rightarrow πN, ηN, KΛ, KΣ$, (4) Pion electroproduction $N(e,e'π)N$, (5) inclusive $N(e,e')X$. We also present sample results from our fits to the data.

nucl-th↗

arXiv2019

$Z_c(4430)$ and $Z_c(4200)$ as triangle singularities

$Z_c(4430)$ discovered by the Belle and confirmed by the LHCb in $\bar{B}^0\toψ(2S)K^-π^+$ is generally considered to be a charged charmonium-like state that includes minimally two quarks and two antiquarks. $Z_c(4200)$ found in $\bar{B}^0\to J/ψK^-π^+$ by the Belle is also a good candidate of a charged charmonium-like state. We demonstrate that kinematical singularities in triangle loop diagrams induce a resonance-like behavior that can consistently explain the properties (mass, width, and Argand plot) of $Z_c(4430)$ and $Z_c(4200)$ from the experimental analyses. The triangle diagrams include only experimentally well-established hadrons. Applying this idea to $Λ_b^0\to J/ψpπ^-$, we also identify triangle singularities that behave like $Z_c(4200)$, but no triangle diagram is available for $Z_c(4430)$. This is consistent with the LHCb's finding that their description of the $Λ_b^0\to J/ψpπ^-$ data is significantly improved by including a $Z_c(4200)$ contribution while $Z_c(4430)$ seems to hardly contribute. Even though the proposed mechanisms have uncertainty in the absolute strengths which are currently difficult to estimate, they are certainly a compelling alternative to tetraquark-based interpretations of $Z_c(4430)$ and $Z_c(4200)$.

hep-ph↗

arXiv2019

Summary of the NuSTEC Workshop on Shallow- and Deep-Inelastic Scattering

The NuSTEC workshop (https://indico.cern.ch/event/727283) held at L'Aquila in October 2018 was devoted to neutrino-nucleus scattering in the kinematic region where hadronic systems with invariant masses above the $Δ(1232)$ resonance are produced: the so-called shallow- and deep-inelastic scattering regime. Not only is the physics in this kinematic region quite intriguing, it is also important for current and future oscillation experiments with accelerator and atmospheric neutrinos. For the benefit of the community, links to the presentations are accompanied by annotations from the speakers.

hep-ph↗

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.

nucl-ex↗

arXiv2019

Impact of final state interactions on neutrino-nucleon pion production cross sections extracted from neutrino-deuteron reaction data

The current and near-future neutrino oscillation experiments require significantly improved neutrino-nucleus reaction models. Neutrino-nucleon pion production data play a crucial role to validate corresponding elementary amplitudes that go into such neutrino-nucleus models. Thus the currently available data extracted from charged-current neutrino-deuteron reaction data ($ν_μd\to μ^-πNN$) must be corrected for nuclear effects such as the Fermi motion and final state interactions (FSI). We study $ν_μd\to μ^-πNN$ with a theoretical model including the impulse mechanism supplemented by FSI from $NN$ and $πN$ rescatterings. An analysis of the spectator momentum distributions reveals that the FSI effects significantly reduce the spectra over the quasi-free peak region, and leads to a useful recipe to extract information of elementary $ν_μN\to μ^-πN$ processes using $ν_μd\to μ^-πNN$ data, with the important FSI corrections taken into account. We provide $ν_μN\to μ^-πN$ total cross sections by correcting the deuterium bubble chamber data for the FSI and Fermi motion. The results will bring a significant improvement on neutrino-nucleus reaction models for the near-future neutrino-oscillation experiments.

hep-ph↗

arXiv2018

$η$ photoproduction off the deuteron and low-energy $η$-nucleon interaction

We study $η$ photoproduction off the deuteron ($γd\toηpn$) at a special kinematics: $\sim 0.94$ GeV of the photon beam energy and $\sim 0^\circ$ of the scattering angle of the proton. This kinematics is ideal to extract the low-energy $η$-nucleon scattering parameters such as $a_{ηN}$ (scattering length) and $r_{ηN}$ (effective range) because the $η$-nucleon elastic scattering is significantly enhanced. We show that if a ratio $R$, the $γd\toηpn$ cross section divided by the $γp\toηp$ cross section convoluted with the proton momentum distribution in the deuteron, is measured with 5% error, ${\rm Re}[a_{ηN}]$ (${\rm Re}[r_{ηN}]$) can be determined at the precision of $\sim\pm$0.1 fm ($\sim\pm$0.5 fm), significantly narrowing down the currently estimated range of the parameters. The measurement is ongoing at the Research Center for Electron Photon Science (ELPH), Tohoku University.

nucl-th↗

arXiv2018

Angular distributions in electroweak pion production off nucleons: odd parity hadron terms, strong relative phases and model dependence

The study of pion production in nuclei is important for signal and background determinations in current and future neutrino oscillation experiments. The first step, however, is to understand the pion production reactions at the free nucleon level. We present an exhaustive study of the charged-current and neutral-current neutrino and antineutrino pion production off nucleons, paying a special attention to the angular distributions of the outgoing pion. We show, using general arguments, that parity violation and time-reversal odd correlations in the weak differential cross sections are generated from the interference between different contributions to the hadronic current that are not relatively real. Next, we present a detailed comparison of three, state of the art, microscopic models for electroweak pion production off nucleons, and we also confront their predictions with polarized electron data, as a test of the vector content of these models. We also illustrate the importance of carrying out a comprehensive test at the level of outgoing pion angular distributions, going beyond comparisons done for partially integrated cross sections, where model differences cancel to a certain extent. Finally, we observe that all charged and neutral current distributions show sizable anisotropies, and identify channels for which parity violating effects are clearly visible. Based on the above results, we conclude that the use of isotropic distributions for the pions in the center of mass of the final pion-nucleon system, as assumed by some of the Monte Carlo event generators, needs to be improved by incorporating the findings of microscopic calculations.

hep-ph↗

arXiv2018

Nuclear applications of ANL-Osaka amplitudes: pion photo-productions on deuteron

The Argonne National Laboratory-Osaka University (ANL-Osaka) amplitudes (http://www.phy.anl.gov/theory/research/anl-osaka-pwa/) are applied to study pion photo-production reactions on the deuteron target. Within the multiple scattering formulation, we predict the cross sections of $d(γ,π)NN$ in the nucleon resonance region. The calculations include the impulse term and the final-state interaction (FSI) terms due to pion-exchange and nucleon-exchange. We show that the off-shell effects, calculated from the meson-exchange mechanisms, on the propagations of the exchanged nucleon and pion are significant in determining the reaction amplitudes. The FSI effects on the predicted cross sections are found to be important at energies near the $Δ$(1232) resonance, and are still significant at higher energies. The results are in good agreement with most of the available data of $d(γ,π^0)np$ and $d(γ,π^-)pp$ reactions.

nucl-th↗

arXiv2017

Low-energy $η$-nucleon interaction studied with $η$ photoproduction off the deuteron

We develop a reaction model for $η$ photoproduction off the deuteron ($γd\toηpn$), and study the reaction at a special kinematics, where the photon beam energy is $\sim 0.94$ GeV and the scattered proton is detected at $\sim 0^\circ$, for the purpose of determining the $η$-nucleon scattering length ($a_{ηN}$) and effective range ($r_{ηN}$). In this kinematics, the $η$-nucleon elastic rescattering is significantly enhanced while other background mechanisms being suppressed. We show that a ratio $R$, the $γd\toηpn$ cross section divided by the $γp\toηp$ cross section convoluted with the proton momentum distribution in the deuteron, has a very good resolving power of $a_{ηN}$ and $r_{ηN}$. We conclude that the $R$ data with 5% error, binned in 1 MeV width of the $η$-neutron invariant mass, can determine ${\rm Re}[a_{ηN}]$ (${\rm Re}[r_{ηN}]$) at the precision of $\sim\pm$0.1 fm ($\sim\pm$0.5 fm), significantly narrowing down the previously estimated ranges of the parameters. To arrive at the conclusion, it is essential to use the $γd\toηpn$ reaction model equipped with elementary amplitudes that are well constrained by $πN$ and $γN$ reaction data through a sophisticated coupled-channels analysis. This result strongly motivates the Research Center for Electron Photon Science (ELPH) at Tohoku University to measure $R$.

nucl-th↗

arXiv2017

Towards a Unified Model of Neutrino-Nucleus Reactions for Neutrino Oscillation Experiments

A precise description of neutrino-nucleus reactions will play a key role in addressing fundamental questions such as the leptonic CP violation and the neutrino mass hierarchy through analyzing data from next-generation neutrino oscillation experiments. The neutrino energy relevant to the neutrino-nucleus reactions spans a broad range and, accordingly, the dominant reaction mechanism varies across the energy region from quasi-elastic scattering through nucleon resonance excitations to deep inelastic scattering. This corresponds to transitions of the effective degree of freedom for theoretical description from nucleons through meson-baryon to quarks. The main purpose of this review is to report our recent efforts towards a unified description of the neutrino-nucleus reactions over the wide energy range; recent overall progress in the field is also sketched. Starting with an overview of the current status of neutrino-nucleus scattering experiments, we formulate the cross section to be commonly used for the reactions over all the energy regions. A description of the neutrino-nucleon reactions follows and, in particular, a dynamical coupled-channels model for meson productions in and beyond the $Δ$(1232) region is discussed in detail. We then discuss the neutrino-nucleus reactions, putting emphasis on our theoretical approaches. We start the discussion with electroweak processes in few-nucleon systems studied with the correlated Gaussian method. Then we describe quasi-elastic scattering with nuclear spectral functions, and meson productions with a $Δ$-hole model. Nuclear modifications of the parton distribution functions determined through a global analysis are also discussed. Finally, we discuss issues to be addressed for future developments.

nucl-th↗

arXiv2016

Isospin decomposition of the $γ^{()} N \to N^$ transitions as input for constructing models of neutrino-induced reactions in the nucleon resonance region

We present our recent efforts to determine the matrix elements associated with the transition between the nucleon and a nucleon resonance induced by the vector current, which are necessary ingredients for models of neutrino-induced reactions in the resonance region. This is accomplished by making the comprehensive analysis of the data for various meson photo- and electro-production reactions off the nucleon within a sophisticated coupled-channels framework, which is known as the ANL-Osaka dynamical coupled-channels model. We also give a brief introduction to our project for constructing a unified neutrino reaction model conducted at the J-PARC Branch of the KEK Theory Center.

nucl-th↗

arXiv2016

Isospin decomposition of $γN \to N^*$ transitions within a dynamical coupled-channels model

By extending the dynamical coupled-channels analysis performed in our previous work [Phys. Rev. C 88, 035209 (2013)] to include the available data of photoproduction of pi meson off the neutron, the transition amplitudes for the photo-excitation of the neutron to nucleon resonances, gamma n --> N*, at the resonance pole positions are determined. The combined fits to the data for both the proton- and neutron-target reactions also revise our results for the resonance pole positions and the gamma p --> N* transition amplitudes. Our results allow an isospin decomposition of the gamma N --> N* transition amplitudes for the isospin I=1/2 N* resonances, which is necessary for testing hadron structure models and gives crucial inputs for constructing models of neutrino-induced reactions in the nucleon resonance region.

nucl-th↗