arXiv subjects

Takayasu Sekihara

Publications and source records attributed to Takayasu Sekihara.

At least 37 records · Page 2Linked to original sources

arXiv2016

Compositeness of baryonic resonances: Applications to the Delta(1232), N(1535), and N(1650) resonances

We present a formulation of the compositeness for baryonic resonances in order to discuss the meson-baryon molecular structure inside the resonances. For this purpose, we derive a relation between the residue of the scattering amplitude at the resonance pole position and the two-body wave function of the resonance in a sophisticated way, and we define the compositeness as the norm of the two-body wave functions. As applications, we investigate the compositeness of the $Δ(1232)$, $N (1535)$, and $N (1650)$ resonances from precise $πN$ scattering amplitudes in a unitarized chiral framework with the interaction up to the next-to-leading order in chiral perturbation theory. The $πN$ compositeness for the $Δ(1232)$ resonance is evaluated in the $πN$ single-channel scattering, and we find that the $πN$ component inside $Δ(1232)$ in the present framework is nonnegligible, which supports the previous work. On the other hand, the compositeness for the $N (1535)$ and $N (1650)$ resonances is evaluated in a coupled-channels approach, resulting that the $πN$, $ηN$, $K Λ$ and $K Σ$ components are negligible for these resonances.

hep-ph↗

arXiv2016

Weak decays of heavy hadrons into dynamically generated resonances

In this review we give a perspective of the theoretical work done recently on the interpretation of results from $B$, $D$, $Λ_b$, $Λ_c$ weak decays into final states that contain interacting hadrons, and how it is possible to obtain additional valuable information that is increasing our understanding of hadron interactions and the nature of many hadronic resonances. The novelty of these processes is that one begins with a clean picture at the quark level which allows one to select the basic mechanisms by means of which the process proceeds. Finally, one has a final state described in terms of quarks. To make contact with the experiments, where mesons and baryons are observed, one must hadronize, creating pairs of $q \bar q$ and writing the new states in terms of mesons and baryons. This concludes the primary hadron production in these processes. After that, the interaction of these hadrons takes place, offering a rich spectrum of resonances and special features from where it is possible to learn much about the interaction of these hadrons and the nature of many resonances in terms of the components of their wave functions.

hep-ph↗

arXiv2016

Constituent-counting rule in photoproduction of hyperon resonances

We analyze the CLAS data on the photoproduction of hyperon resonances, as well as the available data for the ground state $Λ$ and $Σ^{0}$ of the CLAS and SLAC-E84 collaborations, by considering constituent-counting rule suggested by perturbative QCD. The counting rule emerges as a scaling behavior of cross sections in hard exclusive reactions with large scattering angles, and it enables us to determine the number of elementary constituents inside hadrons. Therefore, it could be used as a new method for identifying internal constituents of exotic-hadron candidates. From the analyses of the $γ\, p \to K^{+} Λ$ and $K^{+} Σ^{0}$ reactions, we find that the number of the elementary constituents is consistent with $n_γ = 1$, $n_{p} = 3$, $n_{K^{+}} = 2$, and $n_Λ = n_{Σ^{0}} = 3$. Then, the analysis is made for the photoproductions of the hyperon resonances $Λ(1405)$, $Σ(1385)^{0}$, and $Λ(1520)$, where $Λ(1405)$ is considered to be a $\bar K N$ molecule and hence its constituent number could be five. However, we find that the current data are not enough to conclude the numbers of their constituent. It is necessary to investigate the higher-energy region at $\sqrt{s} > 2.8$ GeV experimentally beyond the energy of the available CLAS data for counting the number of constituents clearly. We also mention that our results indicate energy dependence in the constituent number, especially for $Λ(1405)$. If an excited hyperon is a mixture of three-quark and five-quark states, the energy dependence of the scaling behavior could be valuable for finding its composition and mixture.

hep-ph↗

arXiv2015

Compositeness of the Delta(1232) resonance in pi N scattering

We evaluate the $πN$ compositeness of the $Δ(1232)$ resonance so as to clarify the internal structure of $Δ(1232)$ in terms of the $πN$ component. Here the compositeness is defined as contributions from two-body wave functions to the normalization of the total wave function and is extracted from the $πN$ scattering amplitude. In this study we employ the chiral unitary approach with the interaction up to the next-to-leading order plus a bare $Δ$ term in chiral perturbation theory and describe $Δ(1232)$ in an elastic $πN$ scattering. Fitting the $πN$ scattering amplitude to the solution of the partial wave analysis, we obtain a large real part of the $πN$ compositeness for $Δ(1232)$ comparable to unity and non-negligible imaginary part as well, with which we reconfirm the result in the previous study on the $πN$ compositeness for $Δ(1232)$.

hep-ph↗

arXiv2015

Investigating the nature of light scalar mesons with semileptonic decays of D mesons

We study the semileptonic decays of $D_{s}^{+}$, $D^{+}$, and $D^{0}$ mesons into the light scalar mesons [$f_{0} (500)$, $K_{0}^{\ast} (800)$, $f_{0} (980)$, and $a_{0}(980)$] and the light vector mesons [$ρ(770)$, $ω(782)$, $K^{\ast} (892)$, and $ϕ(1020)$]. With the help of a chiral unitarity approach in coupled channels, we compute the branching fractions for scalar meson processes of the semileptonic $D$ decays in a simple way. Using current known values of the branching fractions, we make predictions for the branching fractions of the semileptonic decay modes with other scalar and vector mesons. Furthermore, we calculate the $π^{+} π^{-}$, $πη$, $πK$, and $K^{+} K^{-}$ invariant mass distributions in the semileptonic decays of $D$ mesons, which will help us clarify the nature of the light scalar mesons.

hep-ph↗

arXiv2015

$Ξ(1690)$ as a $\bar{K} Σ$ molecular state

We show that a $Ξ^{\ast}$ pole can be dynamically generated near the $\bar{K} Σ$ threshold as an $s$-wave $\bar{K} Σ$ molecular state in a coupled-channels unitary approach with the leading-order chiral interaction. This $Ξ^{\ast}$ state can be identified with the $Ξ(1690)$ resonance with $J^{P} = 1/2^{-}$. We find that the experimental $\bar{K}^{0} Λ$ and $K^{-} Σ^{+}$ mass spectra are qualitatively reproduced with the $Ξ^{\ast}$ state. Moreover we theoretically investigate properties of the dynamically generated $Ξ^{\ast}$ state.

hep-ph↗

arXiv2015

Comprehensive analysis of the wave function of a hadronic resonance and its compositeness

We develop a theoretical framework to investigate the two-body composite structure of a resonance as well as a bound state from its wave function. For this purpose, we introduce both one-body bare states and two-body scattering states, and define the compositeness as a fraction of the contribution of the two-body wave function to the normalization of the total wave function. Writing down explicitly the wave function for a resonance state obtained with a general separable interaction, we formulate the compositeness in terms of the position of the resonance pole, the residue of the scattering amplitude at the pole and the derivative of the Green function of the free two-body scattering system. At the same time, our formulation provides the elementariness expressed with the resonance properties and the two-body effective interaction, and confirms the sum rule showing that the summation of the compositeness and elementariness gives unity. In this formulation the Weinberg's relation for the scattering length and effective range can be derived in the weak binding limit. The extension to the resonance states is performed with the Gamow vector, and a relativistic formulation is also established. As its applications, we study the compositeness of the $Λ(1405)$ resonance and the light scalar and vector mesons described with refined amplitudes in coupled-channel models with interactions up to the next to leading order in chiral perturbation theory. We find that $Λ(1405)$ and $f_{0}(980)$ are dominated by the $\bar{K} N$ and $K \bar{K}$ composite states, respectively, while the vector mesons $ρ(770)$ and $K^{\ast} (892)$ are elementary. We also briefly discuss the compositeness of $N (1535)$ and $Λ(1670)$ obtained in a leading-order chiral unitary approach.

hep-ph↗

arXiv2014

Size measurement of dynamically generated hadronic resonances with finite volume effect

The structures of the hyperon resonance $Λ(1405)$ and the scalar mesons $σ$, $f_{0}(980)$, and $a_{0}(980)$ are investigated based on the coupled-channels chiral dynamics with finite volume effect. The finite volume effect is utilized to extract the coupling constant, compositeness, and mean squared distance between two constituents of a Feshbach resonance state as well as a stable bound state. In this framework, the real-valued size of the resonance can be defined from the downward shift of the resonance pole according to the decreasing finite box size $L$ on a given closed channel. As a result, we observe that, when putting the $\bar{K}N$ and $K\bar{K}$ channels into a finite box while other channels being unchanged, the poles of the higher $Λ(1405)$ and $f_{0}(980)$ move to lower energies while other poles do not show downward mass shift, which implies large $\bar{K}N$ and $K\bar{K}$ components inside higher $Λ(1405)$ and $f_{0}(980)$, respectively. Extracting structures of $Λ(1405)$ and $f_{0}(980)$ in our method, we find that the compositeness of $\bar{K}N$ ($K\bar{K}$) inside $Λ(1405)$ [$f_{0}(980)$] is 0.82-1.03 (0.73-0.97) and the mean distance between two constituents is evaluated as 1.7-1.9 fm (2.6-3.0 fm).

nucl-th↗

arXiv2013

Quantum Fluctuations of Particles and Fields in Smooth Path Integrals

An approach to evaluation of the smooth Feynman path integrals is developed for the study of quantum fluctuations of particles and fields in Euclidean time-space. The paths are described by sum of Gauss functions and are weighted with exp(-S) by appropriate methods. The weighted smooth paths reproduce properties of the ground state of the harmonic oscillator in one dimension with high accuracy. Quantum fluctuations of U(1) and SU(2) gauge fields in four dimensions are also evaluated in our approach.

quant-ph↗

arXiv2013

Determination of exotic hadron structure by constituent-counting rule for hard exclusive processes

We propose to use hard exclusive production of an exotic hadron for finding its internal quark-gluon configuration by the constituent-counting rule in perturbative QCD. In particular, the cross section for the exclusive process $π^- + p \to K^0 + Λ(1405)$ is estimated at the scattering angle $θ=90^\circ$ in the center-of-mass frame by using current experimental data. In comparison, the cross section for the ground-state $Λ$ production $π^- + p \to K^0 + Λ$ is also shown. We suggest that the internal quark configuration of $Λ(1405)$ should be determined by the asymptotic scaling behavior of the cross section. If it is an ordinary three-quark baryon, the scaling of the cross section is $s^{8} dσ/dt=$constant, whereas it is $s^{10} dσ/dt=$constant if $Λ(1405)$ is a five-quark hadron, where $s$ and $t$ are Mandelstam variables. Such a measurement will be possible, for example, by using the high-momentum beamline at J-PARC. In addition, another exclusive process $γ+ p \to K^+ + Λ(1405)$ could be investigated at LEPS and JLab for finding the nature of $Λ(1405)$. We indicate that the constituent-counting rule could be used as a valuable quantity in determining internal structure of exotic hadrons by high-energy exclusive processes, where quark-gluon degrees of freedom explicitly appear. Furthermore, it is interesting to investigate the transition from hadron degrees of freedom to quark-gluon ones for exclusive exotic-hadron production processes.

hep-ph↗

arXiv2013

Size measurement of dynamically generated resonances with finite boxes

The structure of dynamically generated states is studied from a viewpoint of the finite volume effect. We establish the relation between the spatial size of a stable bound state and the finite volume mass shift. In a single-channel scattering model, this relation is shown to be valid for a bound state dominated by the two-body molecule component. We generalize this method to the case of a quasi-bound state with finite width in coupled-channel scattering. We define the real-valued size of the resonance in a given closed channel using the response to the finite volume effect on the channel. Applying this method to physical resonances we find that Lambda(1405) and f_{0}(980) are dominated by the Kbar N and K Kbar scattering states, respectively, and that the distance between Kbar N (K Kbar) inside Lambda(1405) [f_{0}(980)] is 1.7-1.9 fm (2.6-3.0 fm). The root mean squared radii of Lambda(1405) and f_0(980) are also estimated from the mean distance between constituents.

nucl-th↗

arXiv2013

Mesonic and non-mesonic branching ratios of K^- absorption in the nuclear medium

The branching ratios of K^- absorption at rest in nuclear matter are evaluated from the K^- self-energy by using the chiral unitary approach for the s-wave \bar{K} N amplitude. We find that both the mesonic and non-mesonic absorption potentials are dominated by the Λ(1405) contributions. We also observe that the mesonic absorption ratio [π^{-} Σ^{+}] / [π^{+} Σ^{-}] increases as a function of nuclear density due to the interference between Λ(1405) and the I=1 non-resonant background, which is consistent with experimental results. The fraction of the non-mesonic absorption is evaluated to be about 30 % at the saturation density. The branching ratios of the K^- absorption at rest into deuteron and 4He are also calculated.

nucl-th↗

arXiv2012

Branching ratios of mesonic and nonmesonic antikaon absorptions in nuclear medium

The branching ratios of K^- absorption at rest in nuclear matter are theoretically investigated in order to understand the mechanism of K^- absorption into nuclei. For this purpose mesonic and nonmesonic absorption potentials are evaluated as functions of nuclear density, the kaon momentum and energy from one- and two-body K^- self-energy, respectively. By using a chiral unitary approach for the s-wave Kbar N amplitude we find that both the mesonic and nonmesonic absorption potentials are dominated by the Lambda(1405) contributions. The fraction of the mesonic and nonmesonic absorptions are evaluated to be respectively about 70% and 30% at the saturation density almost independently on the kaon momentum. We also observe different behavior of the branching ratios to pi ^+ Sigma^- and pi^- Sigma^+ channels in mesonic absorption due to the interference between Lambda(1405) and the I=1 nonresonant background, which is consistent with experimental results. The nonmesonic absorption ratios [Lambda p]/[Sigma^0 p] and [Lambda n]/[Sigma^0 n] are about unity while [Sigma^+ n]/[Sigma^0 p] and [Sigma^- p]/[Sigma^0 n] are about two due to the Lambda(1405) dominance in absorption. Taking into account the kaon momenta and energies, the absorption potentials become weaker due to the downward shift of the initial K^-N two-body energy, but this does not drastirally change the nonmesonic fraction. The Sigma(1385) contribution in the p-wave Kbar N amplitude is examined and found to be very small compared to the Lambda(1405) contribution in slow K^- absorption.

nucl-th↗

arXiv2012

The $K^- d \to πΣn$ reaction revisited

The appearance of some papers dealing with the $K^- d \to πΣn$ reaction, with some discrepancies in the results and a proposal to measure the reaction at forward $n$ angles at J-PARC justifies to retake the theoretical study with high precision to make accurate predictions for the experiment and extract from there the relevant physical information. We do this in the present paper showing results using the Watson approach and the truncated Faddeev approach. We argue that the Watson approach is more suitable to study the reaction because it takes into account the potential energy of the nucleons forming the deuteron, which is neglected in the truncated Faddeev approach. Predictions for the experiment are done as well as spectra with the integrated neutron angle.

nucl-th↗

arXiv2012

New Approach to Continuum Path Integrals for Particles and Fields

An approach to approximate evaluation of the continuum Feynman path integrals is developed for the study of quantum fluctuations of particles and fields in Euclidean time-space. The paths are described by sum of Gauss functions and are weighted with exp(-S) by the Metropolis method. The weighted smooth paths reproduce properties of the ground state of the harmonic oscillator in one dimension with more than about 90 % accuracy, and the accuracy gets higher by using smaller width of the Gauss functions. Our approach is applied to quantum field theories and quantum fluctuations of U(1) and SU(2) gauge fields in four dimensions respectively provide the Coulomb force and confining linear potential at qualitative levels via the Wilson loops. Distributions of large values of gauge fields are found to be suppressed at least exponentially.

quant-ph↗

arXiv2012

Theoretical study of incoherent phi photoproduction on a deuteron target

We study the photoproduction of phi mesons in deuteron, paying attention to the modification of the cross section from bound protons to the free ones with the aim of comparing with recent results at LEPS. For this purpose we take into account Fermi motion in single scattering and rescattering of the phi to account for phi absorption on a second nucleon as well as the rescattering of the proton. We find that the contribution of the double scattering is much smaller than the typical cross section of gamma p to phi p in free space, which implies a very small screening of the phi production in deuteron. The contribution from the proton rescattering, on the other hand, is found to be not negligible compared to the cross section of gamma p to phi p in free space, and leads to a moderate reduction of the phi photoproduction cross section on a deuteron at forward angles if LEPS set up is taken into account. The Fermi motion allows contribution of the single scattering in regions forbidden by phase space in the free case. In particular, we find that for momentum transferred squared close to the maximum value, the Fermi motion changes drastically the shape of d sigma / dt, to the point that the ratio of this cross section to the free one becomes very sensitive to the precise value of t chosen, or the size of the bin used in an experimental analysis. Hence, this particular region of t does not seem the most indicated to find effects of a possible phi absorption in the deuteron. This reaction is studied theoretically as a function of t and the effect of the experimental angular cuts at LEPS is also discussed, providing guidelines for future experimental analyses of the reaction.

nucl-th↗

arXiv2011

Internal structure of the Lambda(1405) resonance probed in chiral unitary amplitude

The internal structure of the resonant Lambda(1405) state is investigated based on meson-baryon coupled-channels chiral dynamics. We evaluate Lambda(1405) form factors which are extracted from current-coupled scattering amplitudes in meson-baryon degrees of freedom. Using several probe currents and channel decomposition, we find that the resonant Lambda(1405) state is dominantly composed of widely spread Kbar around N, with escaping pi Sigma component.

nucl-th↗

arXiv2011

Studying Exotic Hadrons in Heavy Ion Collisions

We investigate the possibilities of using measurements in present and future experiments on heavy ion collisions to answer some longstanding problems in hadronic physics, namely identifying hadronic molecular states and exotic hadrons with multiquark components. The yields of a selected set of exotic hadron candidates in relativistic heavy ion collisions are discussed in the coalescence model in comparison with the statistical model. We find that the yield of a hadron is typically an order of magnitude smaller when it is a compact multiquark state, compared to that of an excited hadronic state with normal quark numbers. We also find that some loosely bound hadronic molecules are formed more abundantly than the statistical model prediction by a factor of two or more. Moreover, due to the significant numbers of charm and bottom quarks produced at RHIC and even larger numbers expected at LHC, some of the proposed heavy exotic hadrons could be produced with sufficient abundance for detection, making it possible to study these new exotic hadrons in heavy ion collisions.

nucl-th↗