arXiv subjects

T. Sekihara

Publications and source records attributed to T. Sekihara.

At least 19 recordsLinked to original sources

arXiv2024

Mini-Proceedings of the "Fourth International Workshop on the Extension Project for the J-PARC Hadron Experimental Facility (HEF-ex 2024)"

The mini proceedings of the "Fourth International Workshop on the Extension Project for the J-PARC Hadron Experimental Facility (HEF-ex 2024) [https://kds.kek.jp/event/46965]" held at J-PARC, February 19-21, 2024, are presented. The workshop was devoted to discussing the physics case that connects both the present and the future Hadron Experimental Facility at J-PARC, covering a wide range of topics in flavor, hadron, and nuclear physics related to both experimental and theoretical activities being conducted at the facility.

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arXiv2016

Exotic-Hadron Signature by Constituent-Counting Rule in Perturbative QCD

We explain a method to find internal quark configurations of exotic hadron candidates by using the constituent counting rule. The counting rule was theoretically predicted in perturbative QCD for hard exclusive hadron reactions, and it has been tested in experiments for stable hadrons including compound systems of hadrons such as the deuteron, $^3$H, and $^3$He. It indicates that the cross section scales as $dσ/dt \sim 1/s^{n-2}$, where $s$ is the center-of-mass energy squared and $n$ is the total number of constituents. We apply this method for finding internal configurations of exotic hadron candidates, especially $Λ(1405)$. There is a possibility that $Λ(1405)$ could be five-quark state or a $\bar K N$ molecule, and scaling properties should be different between the ordinary three-quark state or five-quark one. We predict such a difference in $π^- + p \to K^0 + Λ(1405)$, and it could be experimentally tested, for example, at J-PARC. On the other hand, there are already measurements for $γ+ p \to K^+ + Λ(1405)$ as well as the ground $Λ$ in photoproduction reactions. Analyzing such data, we found an interesting indication that $Λ(1405)$ looks like a five-quark state at medium energies and a three-quark one at high energies. However, accurate higher-energy measurements are necessary for drawing a solid conclusion, and it should be done at JLab by using the updated 12 GeV electron beam. Furthermore, we discuss studies of exotic hadron candidates, such as $f_0 (980)$ and $a_0 (980)$, in electron-positron annihilation by using generalized distribution amplitudes and the counting rule. These studies should be possible as a KEKB experiment.

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arXiv2015

Constraint on K Kbar compositeness of the a0(980) and f0(980) resonances from their mixing intensity

Structure of the $a_{0}(980)$ and $f_{0}(980)$ resonances is investigated with the $a_{0}(980)$-$f_{0}(980)$ mixing intensity from the viewpoint of compositeness, which corresponds to the amount of two-body states composing resonances as well as bound states. For this purpose we first formulate the $a_{0}(980)$-$f_{0}(980)$ mixing intensity as the ratio of two partial decay widths of a parent particle, in the same manner as the recent analysis in BES experiments. Calculating the $a_{0}(980)$-$f_{0}(980)$ mixing intensity with the existing Flatte parameters from experiments, we find that many combinations of the $a_{0}(980)$ and $f_{0}(980)$ Flatte parameters can reproduce the experimental value of the $a_{0}(980)$-$f_{0}(980)$ mixing intensity by BES. Next, from the same Flatte parameters we also calculate the $K \bar{K}$ compositeness for $a_{0}(980)$ and $f_{0}(980)$. Although the compositeness with the correct normalization becomes complex in general for resonance states, we find that the Flatte parameters for $f_{0}(980)$ imply large absolute value of the $K \bar{K}$ compositeness and the parameters for $a_{0}(980)$ lead to small but nonnegligible absolute value of the $K \bar{K}$ compositeness. Then, connecting the mixing intensity and the $K \bar{K}$ compositeness via the $a_{0}(980)$- and $f_{0}(980)$-$K \bar{K}$ coupling constants, we establish a relation between them. As a result, a small mixing intensity indicates a small value of the product of the $K \bar{K}$ compositeness for the $a_{0}(980)$ and $f_{0}(980)$ resonances. Moreover, the experimental value of the $a_{0}(980)$-$f_{0}(980)$ mixing intensity implies that the $a_{0}(980)$ and $f_{0}(980)$ resonances cannot be simultaneously $K \bar{K}$ molecular states.

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arXiv2015

Testing the molecular nature of $\bm{D_{s 0}^{\ast}(2317)}$ and $\bm{D_{0}^{\ast} (2400)}$ in semileptonic $\bm{B_s}$ and $\bm{B}$ decays

We study the semileptonic $B_s$ and $B$ decays into the $D_{s 0}^{\ast} (2317)$ and $D_{0}^{\ast} (2400)$ resonances, respectively. With the help of a chiral unitarity model in coupled channels we compute the ratio of the decay widths of both processes. Using current values of the width for the $\bar{B}^{0} \to D_{0}^{\ast} (2400)^{+} \barν_{l} l^{-}$ decay we make predictions for the rate of the $\bar{B}_{s}^{0} \to D_{s0}^{\ast} (2317)^{+} \barν_{l} l^{-}$ decay.

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arXiv2014

Internal structure of exotic hadrons by high-energy exclusive reactions

We propose to use high-energy exclusive reactions for probing internal structure of exotic hadron candidates. First, the constituent counting rule of perturbative QCD can be used for finding internal configurations of an exotic hadron candidate. It is because the number of constituents ($n$), which participate in the exclusive reaction, is found by the scaling behavior of the cross section $dσ/dt \propto 1/s^{n-2}$ at large momentum transfer, where $s$ is the center-of-mass energy squared. As an example, we show that the internal structure of $Λ\, (1405)$ should be found, for example, by the reaction $π^- + p \to K^0 + Λ(1405)$. Second, the internal structure of exotic hadron candidates should be investigated by hadron tomography with generalized parton distributions (GPDs) and generalized distribution amplitudes (GDAs) in exclusive reactions. Exotic nature should be reflected in the GPDs which contain two factors, longitudinal parton distributions as indicated by the constituent counting rule and transverse form factors as suggested by the hadron size. The GDAs should be investigated by the two-photon process $γ^* γ\to h\bar h$, for example $h=f_0$ or $a_0$, in electron-positron annihilation. Since the GDAs contain information on a time-like form factor, exotic nature should be found by studying the $h\bar h$ invariant mass dependence of the cross section. The internal structure of exotic hadron candidates should be clarified by the exclusive reactions at facilities such as J-PARC and KEKB.

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arXiv2014

Determining Compositeness of Hadronic Resonances: the $Λ(1405)$ Radiative Decay and the $a_{0} (980)$-$f_{0} (980)$ Mixing

Recently the concept of compositeness has been developed so as to distinguish whether interested hadrons are hadronic molecules or not. Here, in terms of compositeness, we investigate $\bar{K} N$ molecular structure of the $Λ(1405)$ resonance with the $Λ(1405)$ radiative decay and $K \bar{K}$ molecular structure of the $a_{0} (980)$ and $f_{0} (980)$ resonances with the $a_{0} (980)$-$f_{0} (980)$ mixing.

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arXiv2014

Exotic hadron production in hard exclusive reactions

We consider hard exclusive production of exotic hadrons to study their internal structure. Revisiting the constituent-counting rule for the large-angle exclusive scattering, we discuss general features expected for the production cross section of exotic hadrons whose leading Fock states are given by multi-quark states other than the ordinary baryon ($qqq$) or meson ($q\bar{q}$) states.We take the production of $Λ(1405)$ as an example and propose to study its partonic configuration from the asymptotic scaling of the cross section, which is measurable at J-PARC. We also discuss the production of a pair of the light-hadrons such as $f_0(980)$s and $a_0(980)$s in $γ^*γ$ collisions in the framework of QCD factorization, in which the cross section is expressed as a convolution of the perturbative coefficients and the generalized distribution amplitudes (GDAs). We demonstrate how the internal structure of $f_0(980)$ or $a_0(980)$ can be explored by measuring the GDAs at $e^+e^-$ experiments such as the B-factories.

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arXiv2014

Determination of compositeness of the Lambda(1405) resonance from its radiative decay

The radiative decay of $Λ(1405)$ is investigated from the viewpoint of compositeness, which corresponds to the amount of two-body states composing resonances as well as bound states. For a $\bar{K}N (I=0)$ bound state without couplings to other channels, we establish a relation between the radiative decay width and the compositeness. Especially the radiative decay width of the bound state is proportional to the compositeness. Applying the formulation to $Λ(1405)$, we observe that the decay to $Λγ$ is dominated by the $K^{-}p$ component inside $Λ(1405)$, because in this decay $π^{+} Σ^{-}$ and $π^{-} Σ^{+}$ strongly cancel each other and the $πΣ$ component can contribute to the $Λγ$ decay only through the slight isospin breaking. This means that the decay $Λ(1405) \to Λγ$ is suitable for the study of the $\bar{K} N$ component in $Λ(1405)$. Fixing the $Λ(1405)$-$πΣ$ coupling constant from the usual decay of $Λ(1405) \to πΣ$, we show a relation between the absolute value of the $\bar{K} N$ compositeness for $Λ(1405)$ and the radiative decay width of $Λ(1405) \to Λγ$ and $Σ^{0} γ$, and we find that large decay width to $Λγ$ implies large $\bar{K}N$ compositeness for $Λ(1405)$. By using the "experimental" data on the radiative decay widths, which is based on an isobar model fitting of the $K^{-}p$ atom data, we estimate the $\bar{K}N$ compositeness for $Λ(1405)$. We also discuss the pole position dependence of our relation on the $Λ(1405)$ radiative decay width and the effects of the two-pole structure for $Λ(1405)$.

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arXiv2013

Studies of exotic hadrons by high-energy exclusive reactions

We investigate the possibility of clarifying internal structure of exotic hadrons by high-energy exclusive reactions. In particular, the constituent-counting rule could be used for determining the internal configuration in large-angle exclusive scattering. As an example, we show the cross section pi^- + p -> K^0 + Lambda(1405) in comparison with the one for the ground-state Lambda production pi^- + p -> K^0 + Lambda. The counting rule indicates that the cross section scales as s^8 dsigma /dt=constant if Lambda(1405) is an ordinary three-quark baryon, whereas it is s^{10} dsigma /dt=constant if Lambda(1405) is a five-quark baryon. Here, s and t are Mandelstam variables. Such experiments could be possible at J-PARC, LEP, JLab, CERN-COMPASS, and other high-energy facilities.

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arXiv2013

Exotic hadrons and hadron-hadron interactions in heavy ion collisions

We discuss the exotic hadron structure and hadron-hadron interactions in view of heavy ion collisions. First, we demonstrate that a hadronic molecule with a large spatial size would be produced more abundantly in the coalescence model compared with the statistical model result. Secondly, we constrain the Lambda-Lambda interaction by using the recently measured Lambda-Lambda correlation data. We find that the RHIC-STAR data favor the Lambda-Lambda scattering parameters in the range 1/a_0 <= -0.8 fm^{-1} and r_{eff} >= 3 fm.

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arXiv2013

Production of hyperon resonances induced by kaon on a deuteron target

The $K^- d \to πYN$reaction is theoretically studied so as to investigate the $K^{-}$-induced production of the hyperon resonances $Σ(1385)$ and $Λ(1405)$. For this purpose we take into account the p-wave amplitudes for the meson-baryon two-body scatterings as well as the s-wave amplitudes. Due to the fact that the hyperon resonances are selectively produced from the $\bar{K} N$ channel in this reaction, the $Λ(1405)$ peak appears at 1420 MeV, which implies that $Λ(1405)$ and $Σ(1385)$ could be separately seen in the missing mass spectrum of the emitted nucleon in the $K^{-} d \to n X$ reaction. The $πY$ invariant mass spectrum in this study is consistent with experimental data both for $Σ(1385)$ and $Λ(1405)$. The pion exchange contributions are also included and are found to give smooth background without distorting the peak structure of the hyperon resonances.

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arXiv2012

A new perspective on the Faddeev equations and the $\bar{K}NN$ system from chiral dynamics and unitarity in coupled channels

We review recent work concerning the $\bar{K}N$ interaction and Faddeev equations with chiral dynamics which allow us to look at the $\bar{K}NN$ from a different perspective and pay attention to problems that have been posed in previous studies on the subject. We show results which provide extra experimental evidence on the existence of two $Λ(1405)$ states. We then show the findings of a recent approach to Faddeev equations using chiral unitary dynamics, where an explicit cancellation of the two body off shell amplitude with three body forces stemming from the same chiral Lagrangians takes place. This removal of the unphysical off shell part of the amplitudes is most welcome and renders the approach unambiguous, showing that only on shell two body amplitudes need to be used. With this information in mind we use an approximation to the Faddeev equations within the fixed center approximation to study the $\bar{K}NN$ system, providing answers within this approximation to questions that have been brought before and evaluating binding energies and widths of this three body system. As a novelty with respect to recent work on the topic we find a bound state of the system with spin S=1, like a bound state of $\bar{K}$-deuteron, less bound that the one of S=0, where all recent efforts have been devoted. The width is relatively large in this case, suggesting problems in a possible experimental observation.

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arXiv2011

The K d -> Lambda(1405) n reaction with the DAFNE set up and the $\bar{K}NN$ system revisited

The $K^{-}$ induced production of $Λ(1405)$ in the $K^{-} d \to πΣn$ reaction is investigated having in mind the conditions of the DAFNE facility at Frascati. We find that the fastest kaons from the decay of the $ϕ$ at DAFNE are well suited to see this resonance if one selects forward going neutrons in the center of mass, which reduce the contribution of single scattering and stress the contribution of the double scattering where the signal of the resonance appears clearer. We take advantage to report briefly on a recent work in which in addition to the $\bar{K}NN$ system with total spin S=0, we find a less bound state (although with equally large width) with S=1, like in the $K^{-} d$ reported in the first part.

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arXiv2010

Kaon induced Lambda(1405) production on a deuteron target at DAFNE

The K^- induced production of Lambda(1405) in the K^- d to pi Sigma n reaction is investigated having in mind the conditions of the DAFNE facility at Frascati where kaons are obtained from the decay of slow moving phi mesons. We find that the K^- d to Lambda(1405) n process favors the production of Lambda(1405) initiated by the K^- p channel, which gives largest weight to the higher mass Lambda(1405) appearing at 1420 MeV in chiral theories. We find that the fastest kaons from the decay of the phi are well suited to see this resonance, particularly if one selects forward going neutrons in the center of mass, which reduce the contribution of single scattering and make the double scattering dominate where the signal of the resonance appears clearer.

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arXiv2010

The nature of Lambda(1405) hyperon resonance in chiral dynamics

The nature of the Lambda(1405) is discussed based on the unitarised coupled-channels approach with chiral dynamics (chiral unitary model). This approach describes the Kbar N scattering cross sections and the Lambda(1405) spectra phenomenologically very well. With this successful description of Lambda(1405), it is found that the Lambda(1405) is composed by two resonance states having different coupling nature to the meson-baryon states. As a consequence, the resonance position in the pi Sigma invariant mass spectrum depends on the initial channel of the Lambda(1405) production. To observe the Lambda(1405) initiated by the Kbar N channel, K^- d to Lambda(1405) n is one of the most favorable reactions. Hadronic molecule states with kaons are also discussed by emphasizing an important role of Lambda(1405) as a quasibound state of Kbar N.

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arXiv2009

Kaonic production of Lambda(1405) off deuteron target in chiral dynamics

The K^- induced production of Lambda(1405) is investigated in K^- d to pi Sigma n reactions based on coupled-channels chiral dynamics, in order to discuss the resonance position of the Lambda(1405) in the KbarN channel. We find that the K^-d to Lambda(1405)n process favors the production of Lambda(1405) initiated by the KbarN channel. The present approach indicates that the Lambda(1405) resonance position is 1420 MeV rather than 1405 MeV in the pi Sigma invariant mass spectra of K- d to pi Sigma n reactions. This is consistent with an observed spectrum of the K^- d to pi^+ Sigma^- n with 686-844 MeV/c incident K^- by bubble chamber experiments done in the 70's. Our model also reproduces the measured Lambda(1405) production cross section.

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arXiv2009

Possibility of s-wave pion condensates in neutron stars revisited

We examine possibilities of pion condensation with zero momentum (s-wave condensation) in neutron stars by using the pion-nucleus optical potential U and the relativistic mean field (RMF) models. We use low-density phenomenological optical potentials parameterized to fit deeply bound pionic atoms or pion-nucleus elastic scatterings. Proton fraction (Y_p) and electron chemical potential (mu_e) in neutron star matter are evaluated in RMF models. We find that the s-wave pion condensation hardly takes place in neutron stars and especially has no chance if hyperons appear in neutron star matter and/or b_1 parameter in U has density dependence.

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arXiv2009

Dynamically generated resonances

In this talk I report on recent work related to the dynamical generation of baryonic resonances, some made up from pseudoscalar meson-baryon, others from vector meson-baryon and a third type from two meson-one baryon systems. We can establish a correspondence with known baryonic resonances, reinforcing conclusions previously drawn and bringing new light on the nature of some baryonic resonances of higher mass.

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