arXiv subjects

T. Matsumura

Publications and source records attributed to T. Matsumura.

At least 109 records · Page 6Linked to original sources

arXiv2008

Forward coherent $ϕ$-meson photoproduction from deuterons near threshold

Differential cross sections and decay asymmetries for coherent $ϕ$-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at $E_γ$= 1.5-2.4 GeV. This reaction offers a unique way to directly access natural-parity Pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity $π$-meson exchange is small. Nevertheless the deduced cross sections of $ϕ$-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional Pomeron model.

nucl-ex↗

arXiv2007

Backward-angle photoproduction of $π^0$ mesons on the proton at $E_γ$ = 1.5--2.4 GeV

Differential cross sections and photon beam asymmetries for $π^0$ photoproduction have been measured at $E_γ$ = 1.5--2.4 GeV and at the $π^0$ scattering angles, --1 $<$ cos$Θ_{c.m.} <$ --0.6. The energy-dependent slope of differential cross sections for $u$-channel $π^0$ production has been determined. An enhancement at backward angles is found above $E_γ$ = 2.0 GeV. This is inferred to be due to the $u$-channel contribution and/or resonances. Photon beam asymmetries have been obtained for the first time at backward angles. A strong angular dependence has been found at $E_γ>$ 2.0 GeV, which may be due to the unknown high-mass resonances.

nucl-ex↗

arXiv2006

MAXIPOL: Cosmic Microwave Background Polarimetry Using a Rotating Half-Wave Plate

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization of the cosmic microwave background radiation (CMB). MAXIPOL is the first bolometric CMB experiment to observe the sky using rapid polarization modulation. To build MAXIPOL, the CMB temperature anisotropy experiment MAXIMA was retrofitted with a rotating half-wave plate and a stationary analyzer. We describe the instrument, the observations, the calibration and the reduction of data collected with twelve polarimeters operating at 140 GHz and with a FWHM beam size of 10 arcmin. We present maps of the Q and U Stokes parameters of an 8 deg^2 region of the sky near the star Beta Ursae Minoris. The power spectra computed from these maps give weak evidence for an EE signal. The maximum-likelihood amplitude of l(l+1)C^{EE}_{l}/(2 pi) is 55_{-45}^{+51} uK^2 (68%), and the likelihood function is asymmetric and skewed positive such that with a uniform prior the probability that the amplitude is positive is 96%. This result is consistent with the expected concordance LCDM amplitude of 14 uK^2. The maximum likelihood amplitudes for l(l+1)C^{BB}_{l}/(2 pi) and $\ell(\ell+1)C^{EB}_{\ell}/2π$ are -31_{-19}^{+31} and 18_{-34}^{+27} uK^2 (68%), respectively, which are consistent with zero. All of the results are for one bin in the range 151 < l < 693. Tests revealed no residual systematic errors in the time or map domain. A comprehensive discussion of the analysis of the data is presented in a companion paper.

astro-ph↗

arXiv2006

Differential cross section and photon beam asymmetry for the gamma n -> K+ Sigma- reaction at Egamma=1.5-2.4 GeV

Differential cross sections and photon beam asymmetries have been measured for the gamma n -> K+ Sigma- and gamma p -> K+ Sigma0 reactions separately using liquid deuterium and hydrogen targets with incident linearly polarized photon beams of Egamma=1.5-2.4 GeV at 0.6<cosTheta<1. The cross section ratio of sigma(K+Sigma-)/sigma(K+Sigma0), expected to be 2 on the basis of the isospin 1/2 exchange, is found to be close to 1. For the K+Sigma- reaction, large positive asymmetries are observed indicating the dominance of the K*-exchange. A large difference between the asymmetries for the K+Sigma- and K+Sigma0 reactions can not be explained by simple theoretical considerations.

hep-ex↗

arXiv2005

Diffractive $ϕ$-meson photoproduction on proton near threshold

Photoproduction of $ϕ$-meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to $E_γ$= 2.37 GeV. The differential cross sections at $t = -|t|_{min}$ do not increase smoothly as $E_γ$ increases, but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that $ϕ$-mesons are photo-produced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.

nucl-ex↗

arXiv2005

Mobility gap in intermediate valent TmSe

The infrared optical conductivity of intermediate valence compound TmSe reveals clear signatures for hybridization of light $d$- and heavy f-electronic states with m* ~ 1.6 m_0 and m* ~ 16 m_0, respectively. At moderate and high temperatures, the metal-like character of the heavy carriers dominate the low-frequency response while at low temperatures (T_N < T < 100 K) a gap-like feature is observed in the conductivity spectra below 10 meV which is assigned to be a mobility gap due to localization of electrons on local Kondo singlets, rather than a hybridization gap in the density of states.

cond-mat.str-el↗

arXiv2005

A Millimeter-Wave Achromatic Half Wave Plate

We have constructed an achromatic half wave plate (AHWP) suitable for the millimeter wavelength band. The AHWP was made from a stack of three sapphire a-cut birefringent plates with the optical axes of the middle plate rotated by 50.5 degrees with respect to the aligned axes of the other plates. The measured modulation efficiency of the AHWP at 110 GHz was $96 \pm 1.5$%. In contrast, the modulation efficiency of a single sapphire plate of the same thickness was $43 \pm 4$%. Both results are in close agreement with theoretical predictions. The modulation efficiency of the AHWP was constant as a function of incidence angles between 0 and 15 degrees. We discuss design parameters of an AHWP in the context of astrophysical broad band polarimetry at the millimeter wavelength band.

physics.optics↗

arXiv2005

Orbital Ordering in ferromagnetic Lu2V2O7

We have observed the orbital ordering in the ferromagnetic Mott-insulator Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering pattern determined from the observed magnetic form factors can be explained in terms of a linear combination of wave functions |yz>, |zx> and |xy>; |0> = (1/3)^(1/2) |xy> + (1/3)^(1/2)|yz> + (1/3)^(1/2) |zx> which is proportional to |(x + y + z)^2 - r^2>; where each orbital is extended toward the center-of-mass of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7, using a Hubbard Hamiltonian with these three orbitals.

cond-mat.str-el↗

arXiv2005

The EBEX Experiment

EBEX is a balloon-borne polarimeter designed to measure the intensity and polarization of the cosmic microwave background radiation. The measurements would probe the inflationary epoch that took place shortly after the big bang and would significantly improve constraints on the values of several cosmological parameters. EBEX is unique in its broad frequency coverage and in its ability to provide critical information about the level of polarized Galactic foregrounds which will be necessary for all future CMB polarization experiments. EBEX consists of a 1.5 m Dragone-type telescope that provides a resolution of less than 8 arcminutes over four focal planes each of 4 degree diffraction limited field of view at frequencies up to 450 GHz. The experiment is designed to accommodate 330 transition edge bolometric detectors per focal plane, for a total of up to 1320 detectors. EBEX will operate with frequency bands centered at 150, 250, 350, and 450 GHz. Polarimetry is achieved with a rotating achromatic half-wave plate. EBEX is currently in the design and construction phase, and first light is scheduled for 2008.

astro-ph↗

arXiv2004

phi photo-production from Li, C, Al, and Cu nuclei at Egamma=1.5 - 2.4 GeV

The photo-production of $ϕ$ mesons from Li, C, Al, and Cu at forward angles has been measured at $E_gamma$=1.5--2.4 GeV. The number of events for incoherent phi photo-production is found to have a target mass number dependence of $A^{0.72\pm 0.07}$ in the kinematical region of $|t|\le 0.6$ ${\rm GeV}^2/c^2$. The total cross section of the $ϕ$-nucleon interaction, $σ_{ϕN}$, has been estimated as $35^{+17}_{-11}$ mb using the $A$-dependence of the $ϕ$ photo-production yield and a Glauber-type multiple scattering theory. This value is much larger than $σ_{ϕN}$ in free space, suggesting that the $ϕ$ properties might change in the nuclear medium.

nucl-ex↗

arXiv2004

d-f Coulomb and quadrupole-strain interactions in DyB2C2 observed by resonant x-ray scattering

Experimental evidence of the d-f Coulomb interaction responsible for the resonant x-ray scattering (RXS) from antiferroquadrupolar order in DyB2C2 is presented. The energy dependences of the RXS intensity with polarization analysis are analyzed by considering the interference between the resonances of dipolar (E1) and quadrupolar (E2) transition processes. It is found that the structure factors for the E1 and E2 processes have the same sign for sigma-pi' but the opposite sign for sigma-sigma' channel. This result, when compared with the calculated structure factors, means that the quadrupolar moments of the 4f and 5d electrons have opposite signs to each other. Interference between nonresonant Thomson scattering from the lattice distortion and the resonant scatterings is also studied and a direct coupling between 4f and the lattice is concluded.

cond-mat.str-el↗

arXiv2004

Spin Fluctuation and Crystal Field Excitation of a Heavy Fermion Compound YbAgGe studied by Inelastic Neutron Scattering

Inelastic neutron scattering experiment has been performed on a new heavy fermion compound YbAgGe for a polycrystalline sample at zero magnetic field. A quasielastic scattering and a crystal field excitation at 12 meV was observed. Both of them are broadened with an intrinsic width of 0.9 meV at 1.5 K which increases to about 3 meV at 300 K. The temperature dependence follows Gamma_0+AT^(1/2) typically observed in Kondo systems. Crystal field parameters were also deduced. To explain the result that only one excitation isobserved at 12 meV, it is concluded that the B22 term should be the main component.

cond-mat.str-el↗

arXiv2003

MAXIPOL: A Balloon-borne Experiment for Measuring the Polarization Anisotropy of the Cosmic Microwave Background Radiation

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10 arcmin to 2 degrees. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary data set was collected during a 26 hour turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress.

astro-ph↗

arXiv2003

A Cosmic Microwave Background Radiation Polarimeter Using Superconducting Bearings

Measurements of the polarization of the cosmic microwave background (CMB) radiation are expected to significantly increase our understanding of the early universe. We present a design for a CMB polarimeter in which a cryogenically cooled half wave plate rotates by means of a high-temperature superconducting (HTS) bearing. The design is optimized for implementation in MAXIPOL, a balloon-borne CMB polarimeter. A prototype bearing, consisting of commercially available ring-shaped permanent magnet and an array of YBCO bulk HTS material, has been constructed. We measured the coefficient of friction as a function of several parameters including temperature between 15 and 80 K, rotation frequency between 0.3 and 3.5 Hz, levitation distance between 6 and 10 mm, and ambient pressure between 10^{-7} and 1 torr. The low rotational drag of the HTS bearing allows rotations for long periods of time with minimal input power and negligible wear and tear thus making this technology suitable for a future satellite mission.

astro-ph↗

arXiv2003

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)Sigma0 reactions at Egamma=1.5-2.4 GeV

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)sigma0 reactions are measured for the first time for Egamma=1.5-2.4 GeV and 0.6<cos(theta_cm(K+))<1.0 by using linearly polarized photons at the Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are positive and gradually increase with rising photon energy. The data are not consistent with theoretical predictions based on tree-level effective Lagrangian approaches. Including the new results in the development of the models is, therefore, crucial for understanding the reaction mechanism and to test the presence of baryon resonances which are predicted in quark models but are sofar undiscovered.

nucl-ex↗

arXiv2001

Antiferroquadrupolar Order in the Magnetic Semiconductor TmTe

The physical properties of the antiferroquadrupolar state occurring in TmTe below TQ=1.8 K have been studied using neutron diffraction in applied magnetic fields. A field-induced antiferromagnetic component k = (1/2,1/2,1/2) is observed and, from its magnitude and direction for different orientations of H, an O(2,2) quadrupole order parameter is inferred. Measurements below TN ~= 0.5 K reveal that the magnetic structure is canted, in agreement with theoretical predictions for in-plane antiferromagnetism. Complex domain repopulation effects occur when the field is increased in the ordered phases, with discontinuities in the superstructure peak intensities above 4 T.

cond-mat.str-el↗

arXiv2001

Observation of the antiferroquadrupolar order in DyB2C2 by resonant x-ray scattering

We have investigated the antiferroquadrupolar(AFQ) order in DyB$_2$C$_2$ by resonant x-ray scattering. X-rays with energies near the $L_3$ absorption edge of Dy were employed. Superlattice peaks that correspond to three kinds of propagation vectors of (1 0 0), (1 0 1/2) and (0 0 1/2) were investigated in detail with polarization analyses. The experimental results are analyzed using a formalism on resonant x-ray scattering and a model of the AFQ order. The magnetic and quadrupolar scatterings are explained by the model satisfactorily. A detailed investigation on the critical behavior of the AFQ ordering is also reported. The critical exponent $β$ is deduced to be 0.35, not far from the three-dimensional Heisenberg system. We have also succeeded in detecting the diffuse scattering above $T_{Q}$.

cond-mat.str-el↗

arXiv2001

Orbital excitations in CMR progenitor of LaMnO3 studied by resonant inelastic x-ray scattering

We report resonant inelastic x-ray scattering experiments of the orbital ordered manganite LaMnO3. When the incident photon energy is tuned near the Mn K absorption edge, the spectra reveal three features at 2.5 eV, 8 eV and 11 eV. The 8 eV and 11 eV peaks are considered charge-transfer type excitations. On the other hand, theoretical calculations identify the 2.5 eV peak as an orbital excitation across the Mott gap, i.e. an electron excitation from the lower Hubbard band with the d3x2-r2 and d3y2-r2 orbital characters to the upper Hubbard band with the dy2-z2 and dz2-x2 ones. The observed weak dispersion and characteristic azimuthal angle dependence of this new type of excitations are well reproduced by the theory which includes orbital degeneracy and strong electron correlation.

cond-mat.str-el↗