1,431 research outputs found
Probing Variant Axion Models at LHC
We study collider implications of variant axion models which naturally avoid
the cosmological domain wall problem. We find that in such models the branching
ratio of can be enhanced by a factor of 5 up to 30 as
compared with the standard model prediction. The process
is therefore a promising channel to discover a light Higgs boson at the LHC and
to probe the Peccei-Quinn charge assignment of the standard model fields from
Yukawa interactions.Comment: 16 pages, 4 figure
Quantum-mechanical generation of gravitational waves in braneworld
We study the quantum-mechanical generation of gravitational waves during
inflation on a brane embedded in a five-dimensional anti-de Sitter bulk. To
make the problem well-posed, we consider the setup in which both initial and
final phases are given by a de Sitter brane with different values of the Hubble
expansion rate. Assuming that the quantum state is in a de Sitter invariant
vacuum in the initial de Sitter phase, we numerically evaluate the amplitude of
quantum fluctuations of the growing solution of the zero mode in the final de
Sitter phase. We find that the vacuum fluctuations of the initial Kaluza-Klein
gravitons as well as of the zero mode gravitons contribute to the final
amplitude of the zero mode on small scales, and the power spectrum is quite
well approximated by what we call the rescaled spectrum, which is obtained by
rescaling the standard four-dimensional calculation following a simple mapping
rule. Our results confirm the speculation raised in Ref.
\cite{Kobayashi:2003cn} before.Comment: 11 pages, 11 figure
THREE TECHNIQUES OF SKI JUMP TAKE-OFF MODELED BY CHANGES OF JOINT ANGLE
INTRODUCTION: In ski jumping, take-off action is the most important factor for ascent force. Jumpers should aim for optimum movements of the joints, because reaction force is the result of the integrated kinetic parameters of each joint or segment. In our recent studies, three techniques of take-off action were classified by manner of joint power generation. However, such kinetic parameters are difficult to explain in coaching situations. It would be more useful for coaching to represent the three techniques of take-off action by joint angle rather than joint power. The purpose of this study is to establish visual models of ski jump take-off action of world class jumpers based on changes of joint angle which would be especially useful in coaching.
METHOD: The take-off actions were analyzed from videos taken at Hakuba Intercontinental Cup summer competition at 1997. Camera speed was 240 frames per second. The data from the videos were collected by computer. An inverse kinematics solution was applied to analysis. Jump performance of four jumpers, who received first prize in the team competition of Nagano Olympic games, was analyzed.
RESULTS: The maximum value of angular velocity in the thigh was observed at close to the take-off platform edge in all jumpers. Peak angular velocity in the thigh was larger than in the trunk segment. The action at the hip joint represented the characteristics of jumping techniques rather than the action at the knee joint.
The technique was classified by three manners of angular velocity. The three types of jump action were represented simply as visual models. In the technique of Type-A, motion was observed with regular order in each joint. The angular velocity of the trunk stayed at 2 rad/sec until the thigh’s angular velocity’s appearance. Type-A can be defined simply as an action moving from hip to knee joint. In technique Type-B, the angular velocity in the thigh was 4 rad/sec, higher than in the trunk at the initial jump action, and after some delay, the value in the trunk raised up from a negative value. Action Type-B can be defined simply as moving from knee to hip joint. In the last technique, Type-C, both the trunk and thigh angular velocities increased synchronously. Both hip and knee joints were extended at the same time. Type-C can be defined simply as the technique of synchronous movement at the knee and hip joint. These motions classified by changes of joint angle were shown by three visual models from Type-A to C.
CONCLUSION: Three types of jump action could be represented simply by joint angle as visual models. There are advantages and risks involving jump hieght and the amount of body area subjected to aerodynamic drag force associated with each technique
New Cosmological Solutions in Massive Gravity
We find new, simple cosmological solutions with flat, open, and closed
spatial geometries, contrary to the previous wisdom that only the open model is
allowed. The metric and the St\"{u}ckelberg fields are given explicitly,
showing nontrivial configurations of the St\"{u}ckelberg in the usual
Friedmann-Lema\^{i}tre-Robertson-Walker coordinates. The solutions exhibit
self-acceleration, while being free from ghost instabilities. Our solutions can
accommodate inhomogeneous dust collapse represented by the
Lema\^{i}tre-Tolman-Bondi metric as well. Thus, our results can be used not
only to describe homogeneous and isotropic cosmology but also to study
gravitational collapse in massive gravity.Comment: 5 page
First Detection of Near-Infrared Intraday Variations in the Seyfert 1 Nucleus NGC4395
We carried out a one-night optical V and near-infrared JHK monitoring
observation of the least luminous Seyfert 1 galaxy, NGC4395, on 2004 May 1, and
detected for the first time the intraday flux variations in the J and H bands,
while such variation was not clearly seen for the K band. The detected J and H
variations are synchronized with the flux variation in the V band, which
indicates that the intraday-variable component of near-infrared continuum
emission of the NGC4395 nucleus is an extension of power-law continuum emission
to the near-infrared and originates in an outer region of the central accretion
disk. On the other hand, from our regular program of long-term optical BVI and
near-infrared JHK monitoring observation of NGC4395 from 2004 February 12 until
2005 January 22, we found large flux variations in all the bands on time scales
of days to months. The optical BVI variations are almost synchronized with each
other, but not completely with the near-infrared JHK variations. The color
temperature of the near-infrared variable component is estimated to be
T=1320-1710 K, in agreement with thermal emission from hot dust tori in active
galactic nuclei (AGNs). We therefore conclude that the near-infrared variation
consists of two components having different time scales, so that a small K-flux
variation on a time scale of a few hours would possibly be veiled by large
variation of thermal dust emission on a time scale of days.Comment: 4 pages including figures, accepted for publication in ApJ
Dilute-Bose-Gas Approach to ground state phases of 3D quantum helimagnets under high magnetic field
We study high-field phase diagram and low-energy excitations of
three-dimensional quantum helimagnets. Slightly below the saturation field, the
emergence of magnetic order may be mathematically viewed as Bose-Einstein
condensation (BEC) of magnons. The method of dilute Bose gas enables an
unbiased quantitative analysis of quantum effects in three-dimensional
helimagnets and thereby three phases are found: cone, coplanar fan and an
attraction-dominant one. To investigate the last phase, we extend the usual BEC
approach so that we can handle 2-magnon bound states. In the case of 2-magnon
BEC, the transverse magnetization vanishes and long-range order occurs in the
quadrupolar channel (spin-nematic phase). As an application, we map out the
phase diagram of a 3D helimagnet which consists of frustrated J1-J2 chains
coupled by an interchain interaction J3.Comment: 4pages, 3figures, International Conference on Magnetism (ICM) 2009
(Karlsruhe, Germany, July 26-31, 2009)
Magnetization plateaus as insulator-superfluid transitions in quantum spin systems
We study the magnetization process in two-dimensional S=1/2 spin systems, to
discuss the appearance of a plateau structure. The following three cases are
considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange model
on the triangular lattice, (2) Shastry-Sutherland type lattice, [which is a
possible model for SrCu2(BO3)2,] (3) 1/5-depleted lattice (for CaV4O9). We find
in these systems that magnetization plateaus can appear owing to a transition
from superfluid to a Mott insulator of magnetic excitations. The plateau states
have CDW order of the excitations. The magnetizations of the plateaus depend on
components of the magnetic excitations, range of the repulsive interaction, and
the geometry of the lattice.Comment: 5 pages, RevTeX, 7 figures, note and reference adde
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
- …