119285801
Modified entropic gravity revisited
Inspired by Verlinde’s idea, some modified versions of entropic gravity have been suggested. Extending them in a unified formalism, herein we derive the generalized gravitational equations accordingly. From gravitational equations, the energy-momentum conservation law and cosmological equations are investigated. The covariant conservation law of energy-momentum tensor severely constrains viable modifications of entropic gravity. A discrepancy arises when two independent methods are applied to the homogeneous isotropic universe, posing a serious challenge to modified models of entropic gravity.
118139263
Replaying Neutrino Bremsstrahlung with General Dispersion Relations
It is generally held that neutrinos with superluminal velocity will lose their energy spontaneously by radiating electron-positron pairs, similar to bremsstrahlung process. Recently, this process was closely studied for neutrinos whose energy is roughly proportional to their momentum. Confronted with an increasing amount of superluminal neutrino models, it is urgent to calculate the same process for general dispersion relations. The calculation is performed in this paper, without resorting to any nontrivial frame such as the effective "rest frame".
119449490
A Rotating Kaluza-Klein black hole with squashed horizons
We find a rotating Kaluza-Klein black hole solution with squashed S-3 horizons in five dimensions. This is a Kerr counterpart of the charged one found by Ishihara and Matsuno [H. Ishihara, K. Matsuno, Kaluza-Klein black holes with squashed horizons. hep-th/0510094] recently. The space-time is geodesic complete and free of naked singularities. Its asymptotic structure is a twisted S' fiber bundle over a four-dimensional Minkowski space-time. We also study the mass and thermodynamics of this black hole. (c) 2006 Elsevier B.V. All rights reserved.
119186452
Curvature and entropy perturbations in generalized gravity
We investigate the cosmological perturbations in generalized gravity, where the Ricci scalar and a scalar field are nonminimally coupled via an arbitrary function f({phi},R). In the Friedmann-Lemaitre-Robertson-Walker background, by studying the linear perturbation theory, we separate the scalar-type perturbations into the curvature perturbation and the entropy perturbation, whose evolution equations are derived. Then we apply this framework to inflation. We consider the generalized slow-roll conditions and the quantization initial condition. Under these conditions, two special examples are studied analytically. One example is the case with no entropy perturbation. The other example is a model with the entropy perturbation large initially but decaying significantly after crossing the horizon.
119201058
Cosmic steps in modeling dark energy
Past and recent data analyses gave some hints of steps in dark energy. Considering dark energy as a dynamical scalar field, we investigate several models with various steps: a step in the scalar potential, a step in the kinetic term, a step in the energy density, and a step in the equation-of-state parameter $w$. These toy models provide a workable mechanism to generate steps and features of dark energy. Remarkably, a single real scalar can cross $w=\ensuremath{-}1$ dynamically with a step in the kinetic term.
119235960
Note on Non-Gaussianities in Two-field Inflation
Two-field slow-roll inflation is the most conservative modification of a single-field model. The main motivations to study it are its entropic mode and non-Gaussianity. Several years ago, for a two-field model with additive separable potentials, Vemizzi and Wands invented an analytic method to estimate its non-Gaussianities. Later on, Choi et al. applied this method to the model with multiplicative separable potentials. In this note, we design a larger class of models whose non-Gaussianity can be estimated by the same method. Under some simplistic assumptions, roughly these models are unlikely able to generate a large non-Gaussianity. We look over some specific models of this class by scanning the full parameter space, but still no large non-Gaussianity appears in the slow-roll region. These models and scanning techniques would be useful for a future model hunt if observational evidence shows up for two-field inflation.
122861815
Beam test of the CsI(Tl) calorimeter for the BELLE detector at the KEK-B factory
We studied the performance of a prototype electromagnetic calorimeter for the BELLE detector at the KEK proton synchrotron for an energy range of 0.25-3.5 GeV. The prototype consisted of an array of 6 x 5 CsI(Tl) crystals with 30 cm length (16.2 radiation lengths) and about 6 cm x 6 cm cross section. The scintillation light of each CsI(Tl) crystal was read out by two large-area PIN photodiodes and charge-sensitive preamplifiers attached at the rear face of the crystal. We measured the energy and position resolution for electrons and the e/pi separation for two sets of matrix configurations: one corresponded to the center and the other to the edge of the barrel calorimeter. The overall performance measured by the test proves that the prototype calorimeter is satisfactory for the use in the BELLE detector.
11739512
A weak gravity conjecture for scalar field theories
We put forward a conjecture for a class of scalar field theories analogous to the recently proposed weak gravity conjecture [4] for U(1) gauge field theories. Taking gravity into account, we find an upper bound on the gravity interaction strength, expressed in terms of scalar coupling parameters. This conjecture is supported by some two-dimensional models and noncommutative field theories.
118507158
Selection and Mid-infrared Spectroscopy of Ultraluminous Star-forming Galaxies at z ~ 2
Starting from a sample of 24 ?m sources in the Extended Groth Strip, we use 3.6-8 ?m color criteria to select ultraluminous infrared galaxies (ULIRGs) at z ~ 2. Spectroscopy from 20-38 ?m of 14 objects verifies their nature and gives their redshifts. Multi-wavelength data for these objects imply stellar masses >1011 M ? and star formation rates ?410 M ? yr?1. Four objects of this sample observed at 1.6 ?m (rest-frame visible) with Hubble Space Telescope/WFC3 show diverse morphologies, suggesting that multiple formation processes create ULIRGs. Of the 14 objects, 4 show signs of active galactic nuclei, but the luminosity appears to be dominated by star formation in all cases.
118388877
Lepton-number violating four-body decays of heavy mesons
A bstractNeutrinoless hadron Lepton Number Violating (LNV) decays can be induced by virtual Majorana neutrino, which in turn indubitably show the Majorana nature of neutrinos. Many three-body LNV processes and Lepton Flavour Violating (LFV) processes have been studied extensively in theory and by experiment. As a supplement, we here study 75 four-body LNV (LFV) processes from heavy pseudoscalar B and D decays. Most of these processes have not been studied in theory and searched for in experiment, while they may have sizable decay rates. Since the four-body decay modes have the same vertexes and mixing parameters with three-body cases, so their branching fractions are comparable with the corresponding three-body decays. We calculate their decay widths and branching fractions with current bounds on heavy Majorana neutrino mixing parameters, and estimate some channels’ reconstruction events using the current experimental data from Belle.
14636359
Some low dimensional evidence for the weak gravity conjecture
We discuss a few examples in 2+1 demensions and 1+1 dimensions supporting a recent conjecture concerning the relation between the Planck scale and the coupling strength of a non-gravitional interaction. Unlike those examples in 3+1 dimensions, we do not have to resort to exotic physics such as small black holes. However, the result concerning these low dimensional examples is a direct consequence of the 3+1 dimensional conjecture.