arXiv subjects

K. Nakayama

Publications and source records attributed to K. Nakayama.

At least 73 records · Page 4Linked to original sources

arXiv2011

CANGAROO-III observation of TeV gamma rays from the unidentified gamma-ray source HESS J1614-518

We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov telescope array, of a very high energy gamma-ray signal from the unidentified gamma-ray source HESS J1614-518, which was discovered in the H.E.S.S. Galactic plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9 sigma level during an effective exposure of 54 hr from 2008 May to August. The spectrum can be represented by a power-law: 8.2+-2.2_{stat}+-2.5_{sys}x10^{-12}x (E/1TeV)^{-Gamma} cm^{-2} s^{-1} TeV^{-1} with a photon index Gamma of 2.4+-0.3_{stat}+-0.2_{sys}, which is compatible with that of the H.E.S.S. observations. By combining our result with multi-wavelength data, we discuss the possible counterparts for HESS J1614-518 and consider radiation mechanisms based on hadronic and leptonic processes for a supernova remnant, stellar winds from massive stars, and a pulsar wind nebula. Although a leptonic origin from a pulsar wind nebula driven by an unknown pulsar remains possible, hadronic-origin emission from an unknown supernova remnant is preferred.

astro-ph.HE↗

arXiv2011

Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides

High-temperature superconductivity in iron-arsenic materials (pnictides) near an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of the underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the SC gap develops on underdoping. This pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic wavevector, where the superconducting pairing is stronger as well, suggesting that antiferromagnetic fluctuations drive both the pseudogap and superconductivity. Interestingly, we found that the pseudogap and the spectral lineshape vary with the Fermi surface quasi-nesting conditions in a fashion that shares similarities with the nodal-antinodal dichotomous behaviour observed in underdoped copper oxide superconductors.

cond-mat.supr-con↗

arXiv2011

Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe0.55Se0.45

The momentum distribution of the energy gap opening at the Fermi level of superconductors is a direct fingerprint of the pairing mechanism. While the phase diagram of the iron-based superconductors promotes antiferromagnetic fluctuations as a natural candidate for electron pairing, the precise origin of the interaction is highly debated. We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all iron-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, strongly suggesting local antiferromagnetic exchange interactions as the pairing origin.

cond-mat.supr-con↗

arXiv2011

Two pseudogaps with different energy scales at the antinode of the high-temperature Bi$_2$Sr$_2$CuO$_6$ superconductor using angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on single-layered cuprate Bi$_2$Sr$_2$CuO$_6$ to clarify the origin of the pseudogap. By using various photon energies, we have succeeded in directly observing two different pseudogaps with two different energy scales which coexist in the antinodal region: one reflects the $d_{x^2-y^2}$-wave pairing strength while the other has a larger energy scale suggesting an origin distinct from superconductivity. The observed two-pseudogap behavior provides a key to fully understand the pseudogap phenomena in cuprates.

cond-mat.supr-con↗

arXiv2011

Measurement and analysis of the pp -> pp gamma reaction at 310 MeV

The pp -> pp gamma reaction has been studied at a beam energy of 310 MeV by detecting both final protons at the PROMICE-WASA facility and identifying the photon through the resulting missing-mass peak. The photon angular distribution in the center-of-mass system and those of the proton-proton relative momentum with respect to the beam direction and to that of the recoil photon were determined reliably up to a final pp excitation energy of Epp ~ 30 MeV. Except for very small Epp values, the behavior of these distributions with excitation energy is well reproduced by a new refined model of the hard bremsstrahlung process. The model reproduces absolutely the total cross section and its energy dependence to within the experimental and theoretical uncertainties.

nucl-ex↗

arXiv2011

Dressing the electromagnetic nucleon current

A field-theory-based approach to pion photoproduction off the nucleon is used to derive a microscopically consistent formulation of the fully dressed electromagnetic nucleon current in an effective Lagrangian formalism. It is shown how the rigorous implementation of local gauge invariance at all levels of the reaction dynamics provides equations that lend themselves to practically manageable truncations of the underlying nonlinearities of the problem. The requirement of consistency also suggests a novel way of treating the pion photoproduction problem. Guided by a phenomenological implementation of gauge invariance for the truncated equations that has proved successful for pion photoproduction, an expression for the fully dressed nucleon current is given that satisfies the Ward-Takahashi identity for a fully dressed nucleon propagator as a matter of course. Possible applications include meson photo- and electroproduction processes, bremsstrahlung, Compton scattering, and $ee'$ processes off nucleons.

nucl-th↗

arXiv2011

Role of high-spin hyperon resonances in the reaction of $γp \to K^+ K^+ Ξ^-$

The recent data taken by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility for the reaction of $γp \to K^+ K^+ Ξ^-$ are reanalyzed within a relativistic meson-exchange model of hadronic interactions. The present model is an extension of the one developed in an earlier work by Nakayama, Oh, and Haberzettl [Phys. Rev. C 74, 035205 (2006)]. In particular, the role of the spin-5/2 and -7/2 hyperon resonances, which were not included in the previous model, is investigated in the present study. It is shown that the contribution of the $Σ(2030)$ hyperon having spin-7/2 and positive parity has a key role to bring the model predictions into a fair agreement with the measured data for the $K^+Ξ^-$ invariant mass distribution.

nucl-th↗

arXiv2011

Universality of superconducting gaps in overdoped Ba0.3K0.7Fe2As2 observed by angle-resolved photoemission spectroscopy

We have performed angle-resolved photoemission spectroscopy on the overdoped Ba$_{0.3}$K$_{0.7}$Fe$_2$As$_2$ superconductor ($T_c$ = 22 K). We demonstrate that the superconducting (SC) gap on each Fermi surface (FS) is nearly isotropic whereas the gap value varies from 4.4 to 7.9 meV on different FSs. By comparing with under- and optimally-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, we find that the gap value on each FS nearly scales with $T_c$ over a wide doping range (0.25 $\textyen leq$ $x$ $\textyen leq$ 0.7). Although the FS volume and the SC gap magnitude are strongly doping dependent, the multiple nodeless gaps can be commonly fitted by a single gap function assuming pairing up to the second-nearest-neighbor, suggesting the universality of the short-range pairing states with the $s_{\yenpm}$-wave symmetry.

cond-mat.supr-con↗

arXiv2010

Photoproduction of $η'$-mesons off the deuteron

Quasi-free photoproduction of $η'$ mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5 GeV. The $η'$-mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the $γn \rightarrow nη'$ reaction. In addition, also first estimates for coherent $η'$-production off the deuteron have been obtained. In agreement with model predictions, the total cross section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and $t$-channel exchanges.

nucl-ex↗

arXiv2010

A model-independent analysis of p + p --> pp(1S0) + gamma

The pp hard bremsstrahlung reaction, p + p --> pp(1S0) + gamma, in which the two protons in the final state are in the 1S0 state, is investigated theoretically. Here, the most general spin structure of the NN hard bremsstrahlung reaction, consistent with symmetry principles, is derived from a partial-wave expansion of this amplitude. Based on this spin structure, it is shown that there are only four independent spin matrix elements in this reaction, which is a direct consequence of reflection symmetry in the reaction plane. It is also shown that it requires at least eight independent observables to determine them uniquely. The present method provides the coefficients multiplying each spin operator in terms of the partial-wave or, equivalently, multipole amplitudes. Some observables are expressed explicitly in terms of these multipoles and a partial-wave analysis is performed. The results should be useful in the analyses of the experimental data on the p + p --> pp(1S0) + gamma reaction being taken, in particular, at the COSY accelerator facility, as well as in providing some theoretical guidance to the future experiments in this area.

nucl-th↗

arXiv2010

Gauge-invariant formulation of $N N \to N N γ$

A complete, rigorous relativistic field-theory formulation of the nucleon-nucleon (NN) bremsstrahlung reaction is presented. The resulting amplitude is unitary as a matter of course and it is gauge invariant, i.e., it satisfies a generalized Ward-Takahashi identity. The novel feature of this approach is the consistent microscopic implementation of local gauge invariance across all interaction mechanisms of the hadronic systems, thus serving as a constraint for all subprocesses. The formalism is quite readily adapted to approximations and thus can be applied even in cases where the microscopic dynamical structure of the underlying interacting hadronic systems is either not known in detail or too complex to be treated in detail. We point out how the interaction currents resulting from the photon being attached to nucleon-nucleon-meson vertices can be treated by phenomenological four-point contact currents that preserve gauge invariance. In an advance application of the present formalism [K. Nakayama and H. Haberzettl, PRC 80, 051001(R) (2009)], such interaction currents were found to contribute significantly in explaining experimental data. In addition, we provide a scheme that permits the approximate treatment of current contributions resulting from pieces of the NN interaction that cannot be incorporated exactly. In each case, the approximation procedure ensures gauge invariance of the entire bremsstrahlung amplitude. We also discuss the necessary modifications when taking into account baryonic states other than the nucleon N; in detail, we consider the $Delta(1232)$ resonance by incorporating the couplings of the $NN$ to the $NΔ$ and $ΔΔ$ systems, and the $γN \to Δ$ transitions. We apply the formalism to the 280-MeV bremsstrahlung data from TRIUMF [PRD 41, 2689 (1990)] incorporating $γNΔ$ transition currents and find good agreement with the data.

nucl-th↗

arXiv2010

Pion photoproduction in a dynamical coupled-channel model

Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich pi-N model, which presently includes the hadronic pi-N and eta-N stable channels as well as the pi-Delta, sigma-N and rho-N effective channels. This model has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi+ n, gamma p to pi0 p and gamma n to pi- p are in good agreement with the experimental data.

nucl-th↗

arXiv2010

Fermiology of Sr4V2O6Fe2As2: Quasi-Nested Fe vs Mott-Insulating V Orbitals

We have performed an angle-resolved photoemission spectroscopy study of a new iron-based superconductor Sr4V2O6Fe2As2. While V 3d orbitals are found to be in a Mott insulator state and show an incoherent peak at ~ 1 eV below the Fermi level, the dispersive Fe 3d bands form several hole- and electron-like Fermi surfaces (FSs), some of which are quasi-nested by the (pi, 0) wave vector. This differs from the local density approximation (LDA) calculations, which predict non-nested FSs for this material. However, LDA+U with a large effective Hubbard energy U on V 3d electrons can reproduce the experimental observation reasonably well. The observed fermiology in superconducting Sr4V2O6Fe2As2 strongly supports that (pi, 0) interband scattering between quasi-nested FSs is indispensable to superconductivity in pnictides.

cond-mat.supr-con↗

arXiv2010

Electron-Hole Asymmetry in Superconductivity of Pnictides Originated from the Observed Rigid Chemical Potential Shift

We have performed a systematic photoemission study of the chemical potential shift as a function of carrier doping in a pnictide system based on BaFe$_2$As$_2$. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different effective masses for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which matches well with observed asymmetric superconducting domes in the phase diagram, strongly supports FS near-nesting driven superconductivity in the iron pnictides.

cond-mat.supr-con↗

arXiv2010

Measurement of the invariant mass distributions for the pp -> ppeta' reaction at excess energy of Q = 16.4 MeV

The proton-proton and proton-eta' invariant mass distributions have been determined for the pp -> ppeta' reaction at an excess energy of Q = 16.4 MeV. The measurement was carried out using the COSY-11 detector setup and the proton beam of the cooler synchrotron COSY. The shapes of the determined invariant mass distributions are similar to those of the pp -> ppeta reaction and reveal an enhancement for large relative proton-proton momenta. This result, together with the fact that the proton-eta interaction is much stronger that the proton-eta' interaction, excludes the hypothesis that the observed enhancement is caused by the interaction between the proton and the meson.

nucl-ex↗

arXiv2009

On the cross section ratio sigma_n/sigma_p in eta photoproduction

The recently discovered enhancement of eta photoproduction on the quasi-free neutron at energies around sqrt(s)~1.67 GeV is addressed within a SU(3) coupled channel model. The quasi-free cross sections on proton and neutron, sigma_n and sigma_p, can be quantitatively explained. In this study, the main source for the peak in sigma_n/sigma_p is a coupled channel effect in S-wave that explains the dip-bump structure in gamma n --> eta n. In particular, the photon coupling to the intermediate meson-baryon states is important. The stability of the result is extensively tested and consistency with several pion- and photon-induced reactions is ensured.

nucl-th↗

arXiv2009

Interaction current in $p p \to p p γ$

The nucleon-nucleon bremsstrahlung reaction is investigated based on a fully gauge-invariant relativistic meson-exchange model approach. In order to account consistently for the complicated part of the interaction current (which at present is too demanding to be calculated explicitly), a generalized contact current is introduced following the approach of Haberzettl, Nakayama, and Krewald [Phys. Rev. C74, 045202 (2006)]. The contact interaction current is constructed phenomenologically such that the resulting full bremsstrahlung amplitude satisfies the generalized Ward-Takahashi identity. The formalism is applied to describe the high-precision proton-proton bremsstrahlung data at 190 MeV obtained at KVI [H. Huisman et al., Phys. Rev. C65, 031001(R) (2002)]. The present results show good agreement with the data, thus removing the longstanding discrepancy between the theoretical predictions and experimental data. The present investigation, therefore, points to the importance of properly taking into account the interaction current for this reaction.

nucl-th↗

arXiv2009

The phase and pole structure of the N*(1535) in piN-->piN and gammaN-->piN

The nature of some baryonic resonances is still an unresolved issue. The case of the N*(1535) is particularly interesting in this respect due to the nearby etaN threshold and interference with the N*(1650). The N*(1535) has been described as a threshold effect, as a genuine 3-quark resonance, or as dynamically generated from the interaction of the octet of baryons with the octet of mesons. In the scheme of dynamical generation, predictions for the interaction of the N*(1535) with the photon can be made. In this study, we simultaneously analyze the role of the N*(1535) in the piN-->piN and gammaN-->piN reactions and compare to the respective amplitudes from partial wave analyses. This test is very sensitive to the meson-baryon components of the N*(1535).

nucl-th↗