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K. Nakayama

Publications and source records attributed to K. Nakayama.

At least 109 records · Page 6Linked to original sources
arXiv2006

Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

cond-mat.supr-con↗
arXiv2005

Gauge-Invariant Approach to Meson Photoproduction Including the Final-State Interaction

A gauge-invariant formalism is presented for the practical treatment of photo- and electroproduction of pseudoscalar mesons off nucleons that allows an explicit incorporation of hadronic final-state interactions. The semi-phenomenological approach is based on a field theory developed by one of the authors. It generalizes an earlier approach by allowing for systematic improvement of approximations in a controlled manner. The practical feasibility is illustrated by applying the lowest-order result to the photoproduction of both neutral and charged pions.

nucl-th↗
arXiv2005

Analyzing eta' photoproduction data on the proton at energies of 1.5--2.3 GeV

The recent high-precision data for the reaction $γp\to pη'$ at photon energies in the range 1.5--2.3 GeV obtained by the CLAS collaboration at the Jefferson Laboratory have been analyzed within an extended version of the photoproduction model developed previously by the authors based on a relativistic meson-exchange model of hadronic interactions [Phys. Rev. C \textbf{69}, 065212 (2004)]. The $η'$ photoproduction can be described quite well over the entire energy range of available data by considering $S_{11}$, $P_{11}$, $P_{13}$, and $D_{13}$ resonances, in addition to the $t$-channel mesonic currents. The observed angular distribution is due to the interference between the $t$-channel and the nucleon $s$- and $u$-channel resonance contributions. The $j=3/2$ resonances are required to reproduce some of the details of the measured angular distribution. For the resonances considered, our analysis yields mass values compatible with those advocated by the Particle Data Group. We emphasize, however, that cross-section data alone are unable to pin down the resonance parameters and it is shown that the beam and/or target asymmetries impose more stringent constraints on these parameter values. It is found that the nucleonic current is relatively small and that the $NNη^\prime$ coupling constant is not expected to be much larger than 2.

nucl-th↗
arXiv2005

Photo-excitation of hyperons and exotic baryon in gamma N --> K Anti-K N

We investigate the reaction of $γN \to K \bar{K} N$ focusing on the photoproduction of $Λ(1520)$ and putative exotic $Θ(1540)$. We consider various background production mechanisms including the production of vector mesons, tensor mesons, and other $Λ$ and $Σ$ hyperons. We discuss the angular distribution of $Λ(1520)$ photoproduction cross section and the radiative decays of $Λ(1520)$. We also discuss what we expect for the invariant mass distributions if the $Θ(1540)$ is formed in the reaction, with the parameters studied so far. We find that the peak in the $KN$ invariant mass distribution, if confirmed, can hardly come from the kinematic reflections, especially, due to the tensor meson backgrounds.

hep-ph↗
arXiv2005

Pentaquark Theta^+(1540) production in gamma N --> K Anti-K N

The recent developments in the search of exotic pentaquark hadrons are briefly reviewed. We then focus on investigating how the exotic pentaquark $Θ(1540)$ baryon production can be identified in the $γN \to K \bar{K} N$ reactions, focusing on the influence of the background (non-$Θ$ production) mechanisms. By imposing the SU(3) symmetry and using various quark model predictions, we are able to fix the coupling constants for evaluating the kaon backgrounds, the $K\bar{K}$ production through the intermediate vector meson and tensor meson photoproduction, and the mechanisms involving intermediate $Λ(1116)$, $Λ(1405)$, $Λ(1520)$, $Σ(1193)$, $Σ(1385)$, and $Δ(1232)$ states. The total cross section data of $γp \to K^+K^- p$ is then used to fix the form factors which regularize the background amplitudes so that the signal of $Θ(1540)$ in $γn \to K^+K^-n$ and $γp \to K^0\bar{K}^0p$ cross sections can be predicted. We find that the predicted $K^+K^-$ and $K^+ n$ invariant mass distributions of the $γn \to K^+ K^- n$ reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted $Θ(1540)$ peak can not be identified unambiguously with the data. High statistics experiments are needed to resolve the problem. We also find that an even-parity $Θ$ is more likely to be detected, while it will be difficult to identify an odd-parity $Θ$, even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions.

hep-ph↗
arXiv2005

Spin structure of spin-1/2 baryon and spinless meson production amplitudes in photo and hadronic reactions

The most general spin structures of the spin-1/2 baryon and spinless meson production operator for both photon and nucleon induced reactions are derived from the partial-wave expansions of these reaction amplitudes. The present method provides the coefficients multiplying each spin operator in terms of the partial-wave matrix elements. The result should be useful in studies of these reactions based on partial-wave analyses, especially, when spin observables are considered.

nucl-th↗
arXiv2004

Some aspects of $Θ^+$ parity determination in the reaction $γN\to Θ^+ \bar{K}\to N K \bar{K}$

We analyze the problem of how to determine the parity of the $Θ^+$ pentaquark in the reaction $γN\to KΘ\to NK\bar{K}$, where $N=n,p$. Our model calculations indicate that the contribution of the non-resonant background of the reaction $γN\to NK\bar{K}$ cannot be neglected, and that suggestions to determine the parity based solely on the initial-stage process $γN\to KΘ$ cannot be implemented cleanly. We discuss the various mechanisms that contribute to the background, and we identify some spin observables which are sensitive.

nucl-th↗
arXiv2004

Remarks on the parity determination of narrow resonances

Recently several proposals were put forward to determine the parity of narrow baryonic resonances, in particular the $Θ^+$. In these proceedings we will briefly comment on the general problems in this task and then discuss in detail the potential of reactions of the type $\vec N\vec N\toΘ^+Y$, where $Y$ denotes a hyperon (either $Σ$ or $Λ$) and the arrows indicate that a polarized initial state is required. Besides reiterating the model--independent properties of this class of reactions we discuss the physics content of some model calculations.

hep-ph↗
arXiv2004

Spin observables and the determination of the parity of $Θ^+$ in photoproduction reactions

Spin observables in the photoproduction of the $Θ^+$ are explored for the purpose of determining the parity of the $Θ^+$. Based on reflection symmetry in the scattering plane, we show that certain spin observables in the photoproduction of the $Θ^+$ can be related directly to its parity. We also show that measurements of both the target nucleon asymmetry and the $% Θ^+$ polarization may be useful in determining the parity of $Θ^+$ in a model-independent way. Furthermore, we show that no combination of spin observables involving only the polarization of the photon and/or nucleon in the initial state can determine the parity of $Θ^+$ unambiguously.

hep-ph↗
arXiv2004

On the determination of the parity of the $Θ^+$

We critically examine the possibility of determining the parity of the $Θ^+(1540)$ from the reactions $NN\to Θ^+Y$ ($Y$ = $Λ$, $Σ$) recently discussed in the literature. Specifically, we study the energy dependence of those observables that were suggested to be the most promising ones to unravel the parity of the $Θ^+$, namely the spin correlation coefficient $A_{xx}$, and the spin transfer coefficient $D_{xx}$. We show that the energy dependence of $σ_0(1+A_{xx})$, corresponding to the spin-triplet production cross section, guarantees unambiguous information on the parity of the $Θ^+$. Here, $σ_0$ denotes the unpolarized cross section. Also, the possibility to determine the parity of the $Θ^+$ through the energy dependence of $σ_0D_{xx}$ is discussed.

hep-ph↗
arXiv2004

Consistent analysis of the reaction $γp \to p η^\prime$ and $pp \to ppη^\prime$

The production of $η'$ mesons in the reactions $γp\to pη'$ and $pp\to ppη'$ is described consistently within a relativistic meson exchange model of hadronic interactions. The photoproduction can be described quite well over the entire energy range of available data by considering an $S_{11}$ and a $P_{11}$ resonance, in addition to the $t$-channel mesonic current. The observed angular distribution is due to the interference between the $t$-channel and the nucleon resonance $s$- and $u$-channel contributions. Our analysis yields positions close to 1650 MeV and 1870 MeV for the $S_{11}$ and $P_{11}$ resonances, respectively. We argue that, at present, identifying these states with the known $S_{11}(1650)$ resonance and the missing $P_{11}$ resonance predicted at 1880 MeV, respectively, would be premature. It is found that the nucleonic current is relatively small and that the $NNη^\prime$ coupling constant cannot be much larger than $g_{NNη^\prime}=3$. As for the $p p \to p p η^\prime$ reaction, different current contributions are constrained by a combined analysis of this and the photoproduction reaction. Difficulties to simultaneously account for the 47-MeV and 144-MeV angular distributions measured by the COSY-11 and DISTO collaborations, respectively, are addressed.

nucl-th↗
arXiv2003

Determination of the $Θ^+$ parity from $γn \to K^- K^+ n$

It is demonstrated that measurements of photon asymmetry in the $γn \to K^- K^+ n$ reaction, can most likely determine the parity of the newly discovered $Θ^+$ pentaquark. We predict that if the parity of $Θ^+$ is positive, the photon asymmetry is significantly positive; if the parity is negative, the photon asymmetry is significantly negative. If the background contribution is large, the photon asymmetry may become very small in magnitude, thereby making it difficult to distinguish between the positive and negative parity results. However, even in this case, a combined analysis of the $(K^+ n)$ invariant mass distribution and photon asymmetry should allow a determination of the parity of $Θ^+$.

hep-ph↗
arXiv2003

An analysis of the reaction pp --> ppηnear threshold

It is shown that most of the available data on the $pp\to ppη$ reaction, including the invariant mass distributions in the $pp\to ppη$ reaction recently measured at COSY, can be understood in terms of the partial-wave amplitudes involving final $pp$ $S$ and $P$ states and the $η$ meson s-wave. This finding, together with the fact that results within a meson--exchange model are especially sensitive to the details of the excitation mechanism of the $S_{11}(1535)$ resonance, demonstrates the possibility of investigating the properties of this resonance in $NN$ collisions. The spin correlation function $C_{xx}$ is shown to disentangle the $S$- and $P$-wave contributions. It is also argued that spin correlations may be used to help constrain the contributions of the amplitudes corresponding to the final $pp$ $^3P_0$

nucl-th↗
arXiv2003

Experimental study of pp eta dynamics in the pp --> ppeta reaction

A high statistics measurement of the pp --> ppeta reaction at an excess energy of Q = 15.5 MeV has been performed at the internal beam facility COSY-11. The stochastically cooled proton beam and the used detection system allowed to determine the momenta of the outgoing protons with a precision of 4 MeV/c (sigma) in the center-of-mass frame. The determination of the four-momentum vectors of both outgoing protons allowed to derive the complete kinematical information of the ppeta-system. An unexpectedly large enhancement of the occupation density in the kinematical regions of low proton-eta relative momenta is observed. A description taking the proton-proton and the eta-proton interaction into account and assuming an on-shell incoherent pairwise interaction among the produced particles fails to explain this strong effect. Its understanding will require a rigorous three-body approach to the ppeta system and the precise determination of contributions from higher partial waves. We also present an invariant mass spectrum of the proton-proton system determined at Q = 4.5 MeV. Interestingly, the enhancement at large relative momenta between protons is visible also at such a small excess energy. In contrast to all other determined angular distributions, the orientation of the emission plane with respect to the beam direction is extracted to be anisotropic.

nucl-ex↗
arXiv2003

Near-threshold $ω$ and $ϕ$ meson productions in $pp$ collisions

Using a relativistic effective Lagrangian at the hadronic level, near-threshold $ω$ and $ϕ$ meson productions in proton proton ($pp$) collisions, $p p \to p p ω/ϕ$, are studied within the distorted wave Born approximation. Both initial and final state $pp$ interactions are included. In addition to total cross section data, both $ω$ and $ϕ$ angular distribution data are used to constrain further the model parameters. For the $p p \to p p ω$ reaction we consider two different possibilities: with and without the inclusion of nucleon resonances. The nucleon resonances are included in a way to be consistent with the $π^- p \to ωn$ reaction. It is shown that the inclusion of nucleon resonances can describe the data better overall than without their inclusion. However, the SATURNE data in the range of excess energies $Q < 31$ MeV are still underestimated by about a factor of two. As for the $p p \to p p ϕ$ reaction it is found that the presently limited available data from DISTO can be reproduced by four sets of values for the vector and tensor $ϕNN$ coupling constants. Further measurements of the energy dependence of the total cross section near threshold energies should help to constrain better the $ϕNN$ coupling constant.

nucl-th↗
arXiv2002

Near-threshold $ω$-meson production in proton-proton collisions: With or without resonance excitations ?

We present results for the $p p \to p p ω$ reaction studied by considering two different scenarios: with and without the inclusion of nucleon resonance excitations. The recently measured angular distribution by the COSY-TOF Collaboration at an excess energy of $Q = 173$ MeV and the energy dependence of the total cross section data for $π^- p \to ωn$ are used to calibrate the model parameters. The inclusion of nucleon resonances improves the theoretical prediction for the energy dependence of the total cross section in $pp \to ppω$ at excess energies $Q < 31$ MeV. However, it still underestimates the data by about a factor of two, and remains a problem in understanding the reaction mechanism.

nucl-th↗
arXiv2002

Eta Meson Production in NN Collisions

Eta meson production in both proton-proton and proton-neutron collisions is investigated within a relativistic meson exchange model of hadronic interactions. It is found that the available cross section data can be described equally well by either the vector or pseudoscalar meson exchange mechanism for exciting the S_{11}(1535) resonance. It is shown that the analyzing power data can potentially be very useful in distinguishing these two scenarios for the excitaion of the S_{11}(1535) resonance.

nucl-th↗