Dielectric responses of the layered cobalt oxysulfide Sr_2Cu_2CoO_2S_2 with CoO_2 square-planes

We have studied the dielectric responses of the layered cobalt oxysulfide Sr$_2$Cu$_2$CoO$_2$S$_2$ with the CoO$_2$ square-planes. With decreasing temperature below the N\'eel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the N\'eel temperature.Comment: 3 pages, 4 eps figures, to be published in J. Appl. Phy

Gravitino dark matter from increased thermal relic particles

We investigate the so-called superWIMP scenario with gravitino as the lightest supersymmetric particle (LSP) in the context of non-standard cosmology, in particular, brane world cosmology. As a candidate of the next-to-LSP (NLSP), we examine slepton and sneutrino. Brane world cosmological effects dramatically enhance the relic density of the slepton or sneutrino NLSP, so that the NLSP with mass of order 100 GeV can provide the correct abundance of gravitino dark matter through its decay. We find that with an appropriate five dimensional Planck mass, this scenario can be realized consistently with the constraints from Big Bang Nucleosynthesis (BBN) for both NLSP candidates of slepton and sneutrino. The BBN constraints for slepton NLSP are more stringent than that for sneutrino, as the result, the gravitino must be rather warm in the slepton NLSP case. The energy density of gravitino produced by thermal scattering is highly suppressed and negligible due to the brane world cosmological effects.Comment: 15 pages, discussion and references added, the final versio

Top quark spin correlations in the Randall-Sundrum model at the CERN Large Hadron Collider

In the Randall-Sundrum model, we study top-antitop pair production and top spin correlations at the Large Hadron Collider. In addition to the Standard Model processes, there is a new contribution to the top-antitop pair production process mediated by graviton Kaluza-Klein modes in the s-channel. We calculate the density matrix for the top-antitop pair production including the new contribution. With a reasonable parameter choice in the Randall-Sundrum model, we find a sizable deviation of the top-antitop pair production cross section and the top spin correlations from those in the Standard Model. In particular, resonant productions of the graviton Kaluza-Klein modes give rise to a remarkable enhancement of such a deviation.Comment: 18 pages, 7 figures, discussion added, typos corrected, references added, version accepted by PR

Neutralino dark matter in brane world cosmology

The thermal relic density of the neutralino dark matter in the brane world cosmology is studied. Since the expansion law at a high energy regime in the brane world cosmology is modified from the one in the standard cosmology, the resultant relic density can be altered. It has been found that, if the five dimensional Planck mass $M_5$ is lower than $10^4$ TeV, the brane world cosmological effect is significant at the decoupling time and the resultant relic density is enhanced. We calculate the neutralino relic density in the Constrained Minimal Supersymmetric Standard Model (CMSSM) and show that the allowed region is dramatically modified from the one in the standard cosmology and eventually disappears as $M_5$ is decreasing. We also find a new lower bound on $M_5 \gtrsim 600$ TeV based on the neutralino dark matter hypothesis, namely the lower bound in order for the allowed region of the neutralino dark matter to exist.Comment: 12 pages, 8 figure

Light wino dark matter in brane world cosmology

The thermal relic density of the wino-like neutralino dark matter in the brane world cosmology is studied. The expansion law at a high energy regime in the brane world cosmology is modified from the one in the standard cosmology, and the resultant relic density can be enhanced if the five dimensional Planck mass $M_5$ is low enough. We calculate the wino-like neutralino relic density in the anomaly mediated supersymmetry breaking scenario and show that the allowed region is dramatically modified from the one in the standard cosmology and the wino-like neutralino with mass of order 100 GeV can be a good candidate for the dark matter. Since the allowed region disappears eventually as $M_5$ is decreasing, we can find a lower bound on $M_5 \gtrsim 100$ TeV according to the neutralino dark matter hypothesis, namely the lower bound in order for the allowed region of the neutralino dark matter to exist.Comment: 16 pages, 9 figures, final versio

Gauss-Bonnet braneworld cosmological effect on relic density of dark matter

In Gauss-Bonnet braneworld cosmology, the Friedmann equation of our four-dimensional universe on 3-brane is modified in a high energy regime (Gauss-Bonnet regime), while the standard expansion law is reproduced in low energies (standard regime). We investigate the Gauss-Bonnet braneworld cosmological effect on the thermal relic density of cold dark matter when the freeze-out of the dark matter occurs in the Gaugss-Bonnet regime. We find that the resultant relic density is considerably reduced when the transition temperature, which connects the Gauss-Bonnet regime with the standard regime, is low enough. This result is in sharp contrast with the result previously obtained in the Randall-Sundrum braneworld cosmology, where the relic density is enhanced.Comment: 11 pages, 2 figures, new references were added, some typos were correcte

Statistical mechanics of lossy compression using multilayer perceptrons

Statistical mechanics is applied to lossy compression using multilayer perceptrons for unbiased Boolean messages. We utilize a tree-like committee machine (committee tree) and tree-like parity machine (parity tree) whose transfer functions are monotonic. For compression using committee tree, a lower bound of achievable distortion becomes small as the number of hidden units K increases. However, it cannot reach the Shannon bound even where K -> infty. For a compression using a parity tree with K >= 2 hidden units, the rate distortion function, which is known as the theoretical limit for compression, is derived where the code length becomes infinity.Comment: 12 pages, 5 figure

The X-ray binary population in M33: II. X-ray spectra and variability

In this paper we investigate the X-ray spectra and X-ray spectral variability of compact X-ray sources for 3 Chandra observations of the Local Group galaxy M33. The observations are centered on the nucleus and the star forming region NGC 604. In the observations 261 sources have been detected. For a total of 43 sources the number of net counts is above 100, sufficient for a more detailed spectral fitting. Of these sources, 25 have been observed in more than one observation, allowing the study of spectral variability on ~months timescales. A quarter of the sources are found to be variable between observations. However, except for two foreground sources, no source is variable within any observation above the 99% confidence level. Only six sources show significant spectral variability between observations. A comparison of N_H values with HI observations shows that X-ray absorption values are consistent with Galactic X-ray binaries and most sources in M33 are intrinsically absorbed. The pattern of variability and the spectral parameters of these sources are consistent with the M33 X-ray source population being dominated by X-ray binaries: Two thirds of the 43 bright sources have spectral and timing properties consistent with X-ray binaries; we also find two candidates for super-soft sources and two candidates for quasi-soft sources.Comment: 25 pages, ApJ accepte

Thermal leptogenesis in brane world cosmology

The thermal leptogenesis in brane world cosmology is studied. In brane world cosmology, the expansion law is modified from the four-dimensional standard cosmological one at high temperature regime in the early universe. As a result, the well-known upper bound on the lightest light neutrino mass induced by the condition for the out-of-equilibrium decay of the lightest heavy neutrino, $\tilde{m}_1 \lesssim 10^{-3}$ eV, can be moderated to be $\tilde{m}_1 \lesssim 10^{-3} {eV} \times (M_1/T_t)^2$ in the case of $T_t \leq M_1$ with the lightest heavy neutrino mass ($M_1$) and the ``transition temperature'' ($T_t$), at which the modified expansion law in brane world cosmology is smoothly connecting with the standard one. This implies that the degenerate mass spectrum of the light neutrinos can be consistent with the thermal leptogenesis scenario. Furthermore, as recently pointed out, the gravitino problem in supersymmetric case can be solved if the transition temperature is low enough $T_t \lesssim 10^{6-7}$ GeV. Therefore, even in the supersymmetric case, thermal leptogenesis scenario can be successfully realized in brane world cosmology.Comment: 9 pages, final versio

Resonant Leptogenesis in the Minimal B-L Extended Standard Model at TeV

We investigate the resonant leptogenesis scenario in the minimal B-L extended standard model(SM) with the B-L symmetry breaking at the TeV scale. Through detailed analysis of the Boltzmann equations, we show how much the resultant baryon asymmetry via leptogenesis is enhanced or suppressed, depending on the model parameters, in particular, the neutrino Dirac Yukawa couplings and the TeV-scale Majorana masses of heavy degenerate neutrinos. In order to consider a realistic case, we impose a simple ansatz for the model parameters and analyze the neutrino oscillation parameters and the baryon asymmetry via leptogenesis as a function of only a single CP-phase. We find that for a fixed CP-phase all neutrino oscillation data and the observed baryon asymmetry of the present universe can be simultaneously reproduced.Comment: 25 pages, 15 figures, version to be published in Phys. Rev.