Phenomenological Implications of the Topflavor Model

We explore phenomenologies of the topflavour model for the LEP experiment at $m_{_Z}$ scale and the atomic parity violation (APV) experiment in the $C_s$ atoms at low energies. Implications of the model on the $Z$ peak data are studied in terms of the precision variables $\epsilon_i$'s. We find that the LEP data give more stringent constraints on the model parameters than the APV data.Comment: 23 pages (including 5 .eps figs), ReVTeX, the 1st revised version, to appear in Phys. Lett.

Heating-compensated constant-temperature tunneling measurements on stacks of Bi2_2Sr2_2CaCu2_2O8+x_{8+x} intrinsic junctions

In highly anisotropic layered cuprates such as Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ tunneling measurements on a stack of intrinsic junctions in a high-bias range are often susceptible to self-heating. In this study we monitored the temperature variation of a stack ("sample stack") of intrinsic junctions by measuring the resistance change of a nearby stack ("thermometer stack") of intrinsic junctions, which was strongly thermal-coupled to the sample stack through a common Au electrode. We then adopted a proportional-integral-derivative scheme incorporated with a substrate-holder heater to compensate the temperature variation. This in-situ temperature monitoring and controlling technique allows one to get rid of spurious tunneling effects arising from the self-heating in a high bias range.Comment: 3 pages, 3 figure

Collective Josephson vortex dynamics in a finite number of intrinsic Josephson junctions

We report the experimental confirmation of the collective transverse plasma modes excited by the Josephson vortex lattice in stacks of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ single crystals. The excitation was confirmed by analyzing the temperature ($T$) and magnetic field ($H$) dependencies of the multiple sub-branches in the Josephson-vortex-flow region of the current-voltage characteristics of the system. In the near-static Josephson vortex state for a low tunneling bias current, pronounced magnetoresistance oscillations were observed, which represented a triangular-lattice vortex configuration along the c axis. In the dynamic vortex state in a sufficiently high magnetic field and for a high bias current, splitting of a single Josephson vortex-flow branch into multiple sub-branches was observed. Detailed examination of the sub-branches for varying $H$ field reveals that sub-branches represent the different modes of the Josephson-vortex lattice along the c axis, with varied configuration from a triangular to a rectangular lattices. These multiple sub-branches merge to a single curve at a characteristic temperature, above which no dynamical structural transitions of the Josephson vortex lattice is expected

k-Space Deep Learning for Reference-free EPI Ghost Correction

Nyquist ghost artifacts in EPI are originated from phase mismatch between the even and odd echoes. However, conventional correction methods using reference scans often produce erroneous results especially in high-field MRI due to the non-linear and time-varying local magnetic field changes. Recently, it was shown that the problem of ghost correction can be reformulated as k-space interpolation problem that can be solved using structured low-rank Hankel matrix approaches. Another recent work showed that data driven Hankel matrix decomposition can be reformulated to exhibit similar structures as deep convolutional neural network. By synergistically combining these findings, we propose a k-space deep learning approach that immediately corrects the phase mismatch without a reference scan in both accelerated and non-accelerated EPI acquisitions. To take advantage of the even and odd-phase directional redundancy, the k-space data is divided into two channels configured with even and odd phase encodings. The redundancies between coils are also exploited by stacking the multi-coil k-space data into additional input channels. Then, our k-space ghost correction network is trained to learn the interpolation kernel to estimate the missing virtual k-space data. For the accelerated EPI data, the same neural network is trained to directly estimate the interpolation kernels for missing k-space data from both ghost and subsampling. Reconstruction results using 3T and 7T in-vivo data showed that the proposed method outperformed the image quality compared to the existing methods, and the computing time is much faster.The proposed k-space deep learning for EPI ghost correction is highly robust and fast, and can be combined with acceleration, so that it can be used as a promising correction tool for high-field MRI without changing the current acquisition protocol.Comment: To appear in Magnetic Resonance in Medicin

On the Economics of Information: Three Essays

In this dissertation, we study whether individuals with differing interests are able to achieve a socially efficient outcome in the presence of incomplete information about the others. Unlike the case of complete information, an individual's decision may reveal his private information, thereby impinging on the others' decisions. This signaling aspect of one's decision would force a decision maker to take account of what others would come to know about his private information. Studying this feature leads us to a rigorous examination, first of all, of how the notion of information ought to be understood and thus to be mathematically formulated; and secondly, of how this signaling aspect reduces the range of achievable efficient decision rules relative to the case of complete information. In the first chapter titled ``Formalization of Information: Knowledge and Belief'', we engage in the first task by studying the issue Billingsley (1995) and Dubra and Echenique (2004) raise about the use of sigma-algebra to model information. They provide an example to show that the formalization of information by sigma-algebras and by partitions need not be equivalent. Although Herves-Beloso and Monteiro (2012) provide a method to generate a sigma-algebra from a partition and another method for going in the opposite direction, we show that their two methods are in fact based on two different notions of information: (i) information as belief, (ii) information as knowledge. If information is conceived to allow for falsehood, case (i) above, the equivalence between $\sigma$-algebras and partitions holds after applying the notion of posterior-completion suggested by Brandenburger and Dekel (1987). If information is conceived not to allow for falsehood, case (ii) above, the equivalence holds only for measurable partitions and countably-generated sigma-algebras. In the second chapter titled ``Common Knowledge and Efficiency with Incomplete Information", we engage in the second task. Holmstrom an Myerson (1983) show that we need only check for efficiency on common knowledge events to determine that an incentive compatible decision rule is efficient. By a sharper notion of common knowledge, based on the notion of posterior-completion described in the first chapter, we show that we need only check for efficiency in a strict subset of common knowledge events known as self-evident events and furthermore, that this is the minimal class of events that one needs to check. In the third chapter titled ``Mediator Selection in International Conflict: Bias, Effectiveness, and Incidence", we adapt the question of achieving efficiency to the context of international conflicts and mediation. As war incurs a cost, an efficient outcome is thus a peaceful one in this context. We allow for disputants to make a joint decision whether to accept a potentially biased mediator who would communicate with them and propose a decision rule on their behalf. This extends the mechanism design problem of Horner et al (2015) to allow for mediator bias and its endogenous determination. Our main finding is that both disputants would accept a biased mediator if war is highly likely to occur in a conflict and the mediator's bias is moderate. More importantly, once a mediator has been accepted, the probability of attaining peace is independent of the intensity of her bias: because war is inefficient, the interest of the mediator's favored disputant is best served by promoting peace