Search for Compensated Isocurvature Perturbations with Planck Power Spectra

In the standard inflationary scenario, primordial perturbations are adiabatic. The amplitudes of most types of isocurvature perturbations are generally constrained by current data to be small. If, however, there is a baryon-density perturbation that is compensated by a dark-matter perturbation in such a way that the total matter density is unperturbed, then this compensated isocurvature perturbation (CIP) has no observable consequence in the cosmic microwave background (CMB) at linear order in the CIP amplitude. Here we search for the effects of CIPs on CMB power spectra to quadratic order in the CIP amplitude. An analysis of the Planck temperature data leads to an upper bound $\Delta_{\rm rms}^2 \leq 7.1\times 10^{-3}$, at the 68\% confidence level, to the variance $\Delta_{\rm rms}^2$ of the CIP amplitude. This is then strengthened to $\Delta_{\rm rms}^2\leq 5.0\times 10^{-3}$ if Planck small-angle polarization data are included. A cosmic-variance-limited CMB experiment could improve the $1\sigma$ sensitivity to CIPs to $\Delta^2_{\rm rms} \lesssim 9\times 10^{-4}$. It is also found that adding CIPs to the standard $\Lambda$CDM model can improve the fit of the observed smoothing of CMB acoustic peaks just as much as adding a non-standard lensing amplitude.Comment: 9 Pages, 3 Tables, 6 Figures. Accepted in PR

Semileptonic B decays into excited charmed mesons from QCD sum rules

Exclusive semileptonic $B$ decays into excited charmed mesons are studied with QCD sum rules in the leading order of heavy quark effective theory. Two universal Isgur-Wise functions \tau and \zeta for semileptonic B decays into four lowest lying excited $D$ mesons ($D_1$, $D_2^*$, $D'_0$, and $D'_1$) are determined. The decay rates and branching ratios for these processes are calculated.Comment: RevTeX, 17 pages including 2 figure

Neutralino dark matter stars can not exist

Motivated by the recent "Cosmos Project" observation of dark-matter concentrations with no ordinary matter in the same place, we study the question of the existence of compact objects made of pure dark matter. We assume that the dark matter is neutralino, and compare its elastic and annihilation cross sections. We find that the two cross sections are of the same order of magnitude. This result has a straightforward and important consequence that neutralinos comprising a compact object can not achieve thermal equilibrium. To substantiate our arguments, by solving Oppenheimer-Volkoff equation we constructed a model of the star made of pure neutralinos. We explicitly showed that the condition for the thermal equilibrium supported by the Fermi pressure is never fulfilled inside the star. This neutralino state can not be described by the Fermi-Dirac distribution. Thus, a stable neutralino star, which is supported by the Fermi pressure, can not exist. We also estimated that a stable star can not contain more than a few percents of neutralinos, most of the mass must be in the form of the standard model particles.Comment: published in JHE

Hyperaccretion Disks around Neutron Stars

(Abridged) We here study the structure of a hyperaccretion disk around a neutron star. We consider a steady-state hyperaccretion disk around a neutron star, and as a reasonable approximation, divide the disk into two regions, which are called inner and outer disks. The outer disk is similar to that of a black hole and the inner disk has a self-similar structure. In order to study physical properties of the entire disk clearly, we first adopt a simple model, in which some microphysical processes in the disk are simplified, following Popham et al. and Narayan et al. Based on these simplifications, we analytically and numerically investigate the size of the inner disk, the efficiency of neutrino cooling, and the radial distributions of the disk density, temperature and pressure. We see that, compared with the black-hole disk, the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity. Finally, we consider an elaborate model with more physical considerations about the thermodynamics and microphysics in the neutron star disk (as recently developed in studying the neutrino-cooled disk of a black hole), and compare this elaborate model with our simple model. We find that most of the results from these two models are basically consistent with each other.Comment: 44 pages, 10 figures, improved version following the referees' comments, main conclusions unchanged, accepted for publication in Ap

{CoSSL}: {C}o-Learning of Representation and Classifier for Imbalanced Semi-Supervised Learning

In this paper, we propose a novel co-learning framework (CoSSL) with decoupled representation learning and classifier learning for imbalanced SSL. To handle the data imbalance, we devise Tail-class Feature Enhancement (TFE) for classifier learning. Furthermore, the current evaluation protocol for imbalanced SSL focuses only on balanced test sets, which has limited practicality in real-world scenarios. Therefore, we further conduct a comprehensive evaluation under various shifted test distributions. In experiments, we show that our approach outperforms other methods over a large range of shifted distributions, achieving state-of-the-art performance on benchmark datasets ranging from CIFAR-10, CIFAR-100, ImageNet, to Food-101. Our code will be made publicly available

Calculation of the Chiral Lagrangian Coefficients from the Underlying Theory of QCD: A Simple Approach

We calculate the coefficients in the chiral Lagrangian approximately from QCD based on a previous study of deriving the chiral Lagrangian from the first principles of QCD in which the chiral Lagrangian coefficients are defined in terms of certain Green's functions in QCD. We first show that, in the large N(c)-limit, the anomaly part contributions to the coefficients are exactly cancelled by certain terms in the normal part contributions, and the final results of the coefficients only concern the remaining normal part contributions depending on QCD interactions. We then do the calculation in a simple approach with the approximations of taking the large-N(c) limit, the leading order in dynamical perturbation theory, and the improved ladder approximation, thereby the relevant Green's functions are expressed in terms of the quark self energy. By solving the Schwinger-Dyson equation for the quark self energy, we obtain the approximate QCD predicted coefficients and the quark condensate which are consistent with the experimental values.Comment: Further typos corrected, to appear in Phys. Rev.

An analysis of the Isgur-Wise Function and its derivatives within a Heavy-Light QCD Quark Model

In determining the mesonic wave function from QCD inspired potential model, if the linear confinement term is taken as parent (with columbic term as perturbation), Airy's function appears in the resultant wave function - which is an infinite series. In the study of Isgur-Wise function (IWF) and its derivatives with such a wave function, the infinite upper limit of integration gives rise to divergence. In this paper, we have proposed some reasonable cut-off values for the upper limit of such integrations and studied the subsequent effect on the results. We also study the sensitivity of the order of polynomial approximation of the infinite Airy series in calculating the derivatives of IWF.Comment: 14 pages,6 tables 8 figure

Phase Separation, Competition, and Volume Fraction Control in NaFe1−x_{1-x}Cox_xAs

We report a detailed nuclear magnetic resonance (NMR) study by combined $^{23}$Na and $^{75}$As measurements over a broad range of doping to map the phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$ 0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order, which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction $V \simeq 8$\% is approximately constant. Near optimal doping, at $x = 0.0175$, we observe a phase separation between static antiferromagnetism related to the {\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The volume fraction of AFM phase increases upon cooling, but both the N{\'e}el temperature and the volume fraction can be suppressed systematically by applying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity occupies the PM region and its volume fraction grows at the expense of the AFM phase, demonstrating a phase separation of the two types of order based on volume exclusion. At higher dopings, static antiferromagnetism and even critical AFM fluctuations are completely suppressed by superconductivity. Thus the phase diagram we establish contains two distinct types of phase separation and reflects a strong competition between AFM and superconducting phases both in real space and in momentum space. We suggest that both this strict mutual exclusion and the robustness of superconductivity against magnetism are consequences of the extreme two-dimensionality of NaFeAs.Comment: 12 pages, 6 figure