arXiv subjects

M. Oka

Publications and source records attributed to M. Oka.

At least 37 records · Page 2Linked to original sources

arXiv2014

Electromagnetic structure of charmed baryons in Lattice QCD

As a continuation of our recent work on the electromagnetic properties of the doubly charmed $Ξ_{cc}$ baryon, we compute the charge radii and the magnetic moments of the singly charmed $Σ_c$, $Ω_c$ and the doubly charmed $Ω_{cc}$ baryons in 2+1 flavor Lattice QCD. In general, the charmed baryons are found to be compact as compared to the proton. The charm quark acts to decrease the size of the baryons to smaller values. We discuss the mechanism behind the dependence of the charge radii on the light valence- and sea-quark masses. The magnetic moments are found to be almost stable with respect to changing quark mass. We investigate the individual quark sector contributions to the charge radii and the magnetic moments. The magnetic moments of the singly charmed baryons are found to be dominantly determined by the light quark and the role of the charm quark is significantly enhanced for the doubly charmed baryons.

hep-lat↗

arXiv2013

Electromagnetic properties of doubly charmed baryons in Lattice QCD

We compute the electromagnetic properties of Ξ_cc baryons in 2+1 flavor Lattice QCD. By measuring the electric charge and magnetic form factors of Ξ_cc baryons, we extract the magnetic moments, charge and magnetic radii as well as the Ξ_cc Ξ_cc ρcoupling constant, which provide important information to understand the size, shape and couplings of the doubly charmed baryons. We find that the two heavy charm quarks drive the charge radii and the magnetic moment of Ξ_cc to smaller values as compared to those of, e.g., the proton.

hep-lat↗

arXiv2013

Development of High Intensity Laser-Electron Photon Beams up to 2.9 GeV at the SPring-8 LEPS Beamline

A laser-Compton backscattering beam, which we call a `Laser-Electron Photon' beam, was upgraded at the LEPS beamline of SPring-8. We accomplished the gains in backscattered photon beam intensities by factors of 1.5--1.8 with the injection of two adjacent laser beams or a higher power laser beam into the storage ring. The maximum energy of the photon beam was also extended from 2.4 GeV to 2.9 GeV with deep-ultraviolet lasers. The upgraded beams have been utilized for hadron photoproduction experiments at the LEPS beamline. Based on the developed methods, we plan the simultaneous injection of four high power laser beams at the LEPS2 beamline, which has been newly constructed at SPring-8. As a simulation result, we expect an order of magnitude higher intensities close to 10$^7$ sec$^{-1}$ and 10$^6$ sec$^{-1}$ for tagged photons up to 2.4 GeV and 2.9 GeV, respectively.

physics.acc-ph↗

arXiv2013

Backward-angle photoproduction of $ω$ and $η'$ mesons from protons at $E_γ=1.5-3.0$ GeV

We report the measurement of differential cross sections for $ω$ and $η'$ photoproduction from protons at backward angles ($-1.0<\cosΘ_{C.M}^{X}<-0.8$) using linearly polarized photons at $E_γ=$$1.5-3.0$ GeV. Differential cross sections for $ω$ mesons are larger than the predicted $u$-channel contribution in the energy range $2.0\leq\sqrt{s}\leq2.4$ GeV. The differential cross sections for $ω$ and $η'$ mesons become closer to the predicted $u$-channel contribution at $\sqrt{s}>2.4$ GeV. A bump structure in the $\sqrt{s}$ dependence of the differential cross sections for $η'$ mesons was observed at $\sqrt{s}\sim$2.35 GeV.

nucl-ex↗

arXiv2013

Vector and axial-vector couplings of D and D* mesons in 2+1 flavor Lattice QCD

Using the axial-vector coupling and the electromagnetic form factors of the D and D* mesons in 2+1 flavor Lattice QCD, we compute the D*Dπ, DDρand D*D*ρcoupling constants, which play an important role in describing the charm hadron interactions in terms of meson-exchange models. We also extract the charge radii of D and D* mesons and determine the contributions of the light and charm quarks separately.

hep-lat↗

arXiv2012

Kappa Distribution Model for Hard X-Ray Coronal Sources of Solar Flares

Solar flares produce hard X-ray emission of which the photon spectrum is often represented by a combination of thermal and power-law distributions. However, the estimates of the number and total energy of non-thermal electrons are sensitive to the determination of the power-law cutoff energy. Here we revisit an `above-the-loop' coronal source observed by RHESSI on 2007 December 31 and show that a kappa distribution model can also be used to fit its spectrum. Because the kappa distribution has a Maxwellian-like core in addition to the high-energy power-law tail, the emission measure and temperature of the instantaneous electrons can be derived without assuming the cutoff energy. Moreover, the non-thermal fractions of electron number/energy densities can be uniquely estimated because they are functions of the power-law index only. With the kappa distribution model, we estimated that the total electron density of the coronal source region was ~2.4x10^10 cm^-3. We also estimated without assuming the source volume that a moderate fraction (~20%) of electrons in the source region was non-thermal and carried ~52% of the total electron energy. The temperature was 28 MK, and the power-law index d of the electron density distribution was -4.3. These results are compared to the conventional power-law models with and without a thermal core component.

astro-ph.SR↗

arXiv2012

Narrow bound states of the DNN system

We report on a recent calculation of the properties of the $DNN$ system, a charmed meson with two nucleons. The system is analogous to the $\bar K NN$ system substituting a strange quark by a charm quark. Two different methods are used to evaluate the binding and width, the Fixed Center approximation to the Faddeev equations and a variational calculation. In both methods we find that the system is bound by about 200 MeV and the width is smaller than 40 MeV, a situation opposite to the one of the $\bar K NN$ system and which makes this state well suited for experimental observation.

nucl-th↗

arXiv2012

A narrow $DNN$ quasi-bound state

The energies and widths of $DNN$ quasi-bound states with isospin I=1/2 are evaluated in two methods, the fixed center approximation to the Faddeev equation and the variational method approach to the effective one-channel Hamiltonian. The $DN$ interactions are constructed so that they dynamically generate the $Λ_c(2595)$ (I=0, $J^π =1/2^-$) resonance state. We find that the system is bound by about 250 MeV from the $DNN$ threshold, $\sqrt{s} \sim 3500$ MeV. Its width including both the mesonic decay and the $D$ absorption, is estimated to be about 20-40 MeV. The I=0 $DN$ pair in the $DNN$ system is found to form a cluster that is similar to the $Λ_c(2595)$.

hep-ph↗

arXiv2011

Radiation from relativistic shocks with turbulent magnetic fields

Using our new 3-D relativistic electromagnetic particle (REMP) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron-positron jet propagating in an unmagnetized ambient electron-positron plasma. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. We calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We also used the new technique to calculate emission from electrons based on simulations with a small system. We obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability. A fully developed shock within a larger system generates a jitter/synchrotron spectrum.

astro-ph.HE↗

arXiv2010

`Island Surfing' Mechanism of Electron Acceleration During Magnetic Reconnection

One of the key unresolved problems in the study of space plasmas is to explain the production of energetic electrons as magnetic field lines `reconnect' and release energy in a exposive manner. Recent observations suggest possible roles played by small scale magnetic islands in the reconnection region, but their precise roles and the exact mechanism of electron energization have remained unclear. Here we show that secondary islands generated in the reconnection region are indeed efficient electron accelerators. We found that, when electrons are trapped inside the islands, they are energized continuously by the reconnection electric field prevalent in the reconnection diffusion region. The size and the propagation speed of the secondary islands are similar to those of islands observed in the magnetotail containing energertic electrons.

astro-ph.SR↗

arXiv2010

Electron Acceleration by Multi-Island Coalescence

Energetic electrons of up to tens of MeV are created during explosive phenomena in the solar corona. While many theoretical models consider magnetic reconnection as a possible way of generating energetic electrons, the precise roles of magnetic reconnection during acceleration and heating of electrons still remain unclear. Here we show from 2D particle-in-cell simulations that coalescence of magnetic islands that naturally form as a consequence of tearing mode instability and associated magnetic reconnection leads to efficient energization of electrons. The key process is the secondary magnetic reconnection at the merging points, or the `anti-reconnection', which is, in a sense, driven by the converging outflows from the initial magnetic reconnection regions. By following the trajectories of the most energetic electrons, we found a variety of different acceleration mechanisms but the energization at the anti-reconnection is found to be the most important process. We discuss possible applications to the energetic electrons observed in the solar flares. We anticipate our results to be a starting point for more sophisticated models of particle acceleration during the explosive energy release phenomena.

astro-ph.SR↗

arXiv2010

Axial Charges of Octet Baryons in Two-flavor Lattice QCD

We evaluate the strangeness-conserving $N N$, $ΣΣ$, $ΞΞ$, $ΛΣ$ and the strangeness-changing $ΛN$, $ΣN$, $ΛΞ$, $ΣΞ$ axial charges in lattice QCD with two flavors of dynamical quarks and extend our previous work on pseudoscalar-meson-octet-baryon coupling constants so as to include $πΞΞ$, $KΛΞ$ and $KΣΞ$ coupling constants. We find that the axial charges have rather weak quark-mass dependence and the breaking in SU(3)-flavor symmetry is small at each quark-mass point we consider.

hep-lat↗

arXiv2010

Possible quantum numbers of the pentaquark Θ^+(1540) in QCD sum rules

The QCD sum rule technique is employed to investigate pentaquark states with strangeness S = +1 and IJ^P = 0,1/2^\pm, 1,1/2^\pm, 0,3/2^\pm, 1,3/2^\pm. Throughout the calculation, emphasis is laid on the establishment of a valid Borel window, which corresponds to a region of the Borel mass, where the operator product expansion converges and the presumed ground state pole dominates the sum rules. Such a Borel window is achieved by constructing the sum rules from the difference of two independent correlators and by calculating the operator product expansion up to dimension 14. Furthermore, we discuss the possibility of the contamination of the sum rules by possible KN scattering states. As a result, we conclude that the 0,3/2^+ state seems to be the most probable candidate for the experimentally observed Θ^+(1540), while we also obtain states with 0,1/2^-, 1,1/2^-, 1,3/2^+ at somewhat higher mass regions.

hep-ph↗

arXiv2009

Spin Content of Lambda in QCD Sum Rules

We calculate the isoscalar axial-vector coupling constants of the Lambda hyperon using the method of QCD sum rules. A determination of these coupling constants reveals the individual contributions of the u, d and the s quarks to the spin content of Lambda. Our results for the light-quark contributions are in agreement with those from experiment assuming flavor SU(3). We also find that the flavor-SU(3)-breaking effects are small and the contributions from the u and the d quarks to the Lambda polarization are negatively polarized as in the flavor-SU(3) limit.

hep-ph↗

arXiv2009

Spin-3/2 Pentaquark in QCD Sum Rules

The QCD sum rule method is formulated for the strangeness +1 pentaquark baryon with isospin I=0 and spin-parity J^P = 3/2^\pm. The spin-3/2 states are considered to be narrower than the spin-1/2 ones, and thus may provide a natural explanation for the experimentally observed narrow width of Θ^+. In order to obtain reliable results in QCD sum rule calculations, we stress the importance of establishing a wide Borel window, where convergence of the operator product expansion and sufficient low-mass strength of the spectral function are guaranteed. To this end, we employ the difference of two independent correlators so that the high-energy continuum contribution is suppressed. The stability of the physical quantities against the Borel mass is confirmed within the Borel window. It is found that the sum rule gives positive evidence for the (I, J^P) = (0, 3/2^+) state with a mass of about 1.4 \pm 0.2 GeV, while we cannot extract any evidence for the (0, 3/2^-) state.

hep-ph↗

arXiv2009

Pseudoscalar-meson--octet-baryon coupling constants in two-flavor lattice QCD

We evaluate the $πN!N$, $πΣΣ$, $πΛΣ$, $KΛN$ and $K ΣN $ coupling constants and the corresponding monopole masses in lattice QCD with two flavors of dynamical quarks. The parameters representing the SU(3)-flavor symmetry are computed at the point where the three quark flavors are degenerate at the physical $s$-quark mass. In particular, we obtain $α\equiv F/(F+D)=0.395(6)$. The quark-mass dependences of the coupling constants are obtained by changing the $u$- and the $d$-quark masses. We find that the SU(3)-flavor parameters have weak quark-mass dependence and thus the SU(3)-flavor symmetry is broken by only a few percent at each quark-mass point we consider.

hep-lat↗

arXiv2009

Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock

Plasma instabilities (e.g., Buneman, Weibel and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new 3-D relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic (HD) like shock structure. In the leading shock, electron density increases by a factor of 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the AGN and GRB context.

astro-ph.HE↗

arXiv2008

Magnetic Reconnection by a Self-Retreating X-Line

Particle-in-cell (PIC) simulations of collisionless magnetic reconnection are performed to study asymmetric reconnection in which an outflow is blocked by a hard wall while leaving sufficiently large room for the outflow of the opposite direction. This condition leads to a slow, roughly constant motion of the diffusion region away from the wall, the so-called `X-line retreat'. The typical retreat speed is ~0.1 times the Alfven speed. At the diffusion region, ion flow pattern shows strong asymmetry and the ion stagnation point and the X-line are not collocated. A surprise, however, is that the reconnection rate remains the same unaffected by the retreat motion.

astro-ph↗