27,138 research outputs found
The electromagnetic form factors of the proton in the timelike region
The reactions ppbar -> e+e- and e+e- -> ppbar are analyzed in the
near-threshold region. Specific emphasis is put on the role played by the
interaction in the initial- or final antinucleon-nucleon state which is taken
into account rigorously. For that purpose a recently published NNbar potential
derived within chiral effective field theory and fitted to results of a new
partial-wave analysis of ppbar scattering data is employed. Our results provide
strong support for the conjecture that the pronounced energy dependence of the
e+e- ppbar cross section, seen in pertinent experiments, is primarily due
to the ppbar interaction. Predictions for the proton electromagnetic form
factors G_E and G_M in the timelike region, close to the NNbar threshold, and
for spin-dependent observables are presented. The steep rise of the effective
form factor for energies close to the ppbar threshold is explained solely in
terms of the ppbar interaction. The corresponding experimental information is
quantitatively described by our calculation.Comment: 14 pages, 11 figure
Top-N Recommendation on Graphs
Recommender systems play an increasingly important role in online
applications to help users find what they need or prefer. Collaborative
filtering algorithms that generate predictions by analyzing the user-item
rating matrix perform poorly when the matrix is sparse. To alleviate this
problem, this paper proposes a simple recommendation algorithm that fully
exploits the similarity information among users and items and intrinsic
structural information of the user-item matrix. The proposed method constructs
a new representation which preserves affinity and structure information in the
user-item rating matrix and then performs recommendation task. To capture
proximity information about users and items, two graphs are constructed.
Manifold learning idea is used to constrain the new representation to be smooth
on these graphs, so as to enforce users and item proximities. Our model is
formulated as a convex optimization problem, for which we need to solve the
well-known Sylvester equation only. We carry out extensive empirical
evaluations on six benchmark datasets to show the effectiveness of this
approach.Comment: CIKM 201
Multipole Gravitational Lensing and High-order Perturbations on the Quadrupole Lens
An arbitrary surface mass density of gravitational lens can be decomposed
into multipole components. We simulate the ray-tracing for the multipolar mass
distribution of generalized SIS (Singular Isothermal Sphere) model, based on
the deflection angles which are analytically calculated. The magnification
patterns in the source plane are then derived from inverse shooting technique.
As have been found, the caustics of odd mode lenses are composed of two
overlapping layers for some lens models. When a point source traverses such
kind of overlapping caustics, the image numbers change by \pm 4, rather than
\pm 2. There are two kinds of images for the caustics. One is the critical
curve and the other is the transition locus. It is found that the image number
of the fold is exactly the average value of image numbers on two sides of the
fold, while the image number of the cusp is equal to the smaller one. We also
focus on the magnification patterns of the quadrupole (m = 2) lenses under the
perturbations of m = 3, 4 and 5 mode components, and found that one, two, and
three butterfly or swallowtail singularities can be produced respectively. With
the increasing intensity of the high-order perturbations, the singularities
grow up to bring sixfold image regions. If these perturbations are large enough
to let two or three of the butterflies or swallowtails contact, eightfold or
tenfold image regions can be produced as well. The possible astronomical
applications are discussed.Comment: 24 pages, 6 figure
Massive and Red Objects predicted by a semianalytical model of galaxy formation
We study whether hierarchical galaxy formation in a concordance CDM
universe can produce enough massive and red galaxies compared to the
observations. We implement a semi-analytical model in which the central black
holes gain their mass during major mergers of galaxies and the energy feedback
from active galaxy nuclei (AGN) suppresses the gas cooling in their host halos.
The energy feedback from AGN acts effectively only in massive galaxies when
supermassive black holes have been formed in the central bulges. Compared with
previous models without black hole formation, our model predicts more massive
and luminous galaxies at high redshift, agreeing with the observations of K20
up to . Also the predicted stellar mass density from massive galaxies
agrees with the observations of GDDS. Because of the energy feedback from AGN,
the formation of new stars is stopped in massive galaxies with the termination
of gas cooling and these galaxies soon become red with color 5 (Vega
magnitude), comparable to the Extremely Red Objects (EROs) observed at redshift
1-2. Still the predicted number density of very EROs is lower than
observed at , and it may be related to inadequate descriptions of dust
extinction, star formation history and AGN feedback in those luminous galaxies.Comment: Accepted for Publication in ApJ, added reference
Is the Number of Giant Arcs in LCDM Consistent With Observations?
We use high-resolution N-body simulations to study the galaxy-cluster
cross-sections and the abundance of giant arcs in the CDM model.
Clusters are selected from the simulations using the friends-of-friends method,
and their cross-sections for forming giant arcs are analyzed. The background
sources are assumed to follow a uniform ellipticity distribution from 0 to 0.5
and to have an area identical to a circular source with diameter 1\arcsec. We
find that the optical depth scales as the source redshift approximately as
\tau_{1''} = 2.25 \times 10^{-6}/[1+(\zs/3.14)^{-3.42}] (0.6<\zs<7). The
amplitude is about 50% higher for an effective source diameter of 0.5\arcsec.
The optimal lens redshift for giant arcs with the length-to-width ratio ()
larger than 10 increases from 0.3 for \zs=1, to 0.5 for \zs=2, and to
0.7-0.8 for \zs>3. The optical depth is sensitive to the source redshift, in
qualitative agreement with Wambsganss et al. (2004). However, our overall
optical depth appears to be only 10% to 70% of those from previous
studies. The differences can be mostly explained by different power spectrum
normalizations () used and different ways of determining the
ratio. Finite source size and ellipticity have modest effects on the optical
depth. We also found that the number of highly magnified (with magnification
) and ``undistorted'' images (with ) is comparable to the
number of giant arcs with and . We conclude that our
predicted rate of giant arcs may be lower than the observed rate, although the
precise `discrepancy' is still unclear due to uncertainties both in theory and
observations.Comment: Revised version after the referee's reports (32 pages,13figures). The
paper has been significantly revised with many additions. The new version
includes more detailed comparisons with previous studies, including the
effects of source size and ellipticity. New discussions about the redshift
distribution of lensing clusters and the width of giant arcs have been adde
Extracting CP violation and strong phase in D decays by using quantum correlations in psi(3770)-> D0\bar{D}0 -> (V1V2)(V3V4) and psi(3770)->D0\bar{D}0 -> (V1V2)(K pi)
The charm quark offers interesting opportunities to cross-check the mechanism
of CP violation precisely tested in the strange and beauty sectors. In this
paper, we exploit the angular and quantum correlations in the D\bar{D} pairs
produced through the decay of the psi(3770) resonance in a charm factory to
investigate CP-violation in two different ways. We build CP-violating
observables in psi(3770) -> D\bar{D} -> (V_1V_2)(V_3 V_4) to isolate specific
New Physics effects in the charm sector. We also consider the case of psi(3770)
-> D\bar{D} -> (V_1V_2)(K\pi) decays, which provide a new way to measure the
strong phase difference delta between Cabibbo-favored and doubly-Cabibbo
suppressed D decays required in the determination of the CKM angle gamma.
Neglecting the systematics, we give a first rough estimate of the sensitivities
of these measurements at BES-III with an integrated luminosity of 20 fb^-1 at
psi(3770) peak and at a future Super tau-charm factory with a luminosity of
10^35 cm^-2.s^-1.Comment: 13 pages
Full-scale metamaterial window for building application
The research on acoustic metamaterials (AMMs) has progressed rapidly over the last decades. One of the applications is for noise control and airflow in duct-like systems. These are useful features for natural ventilation window design; however, the visual impact between indoor and outdoor environment, as another key factor of windows, makes the existing AMMs not directly useable for this application due to their geometrical complexity and size limitations. In this research, an AMM previously developed by the authors is exploited for full-scale window design. The AMM is packed only in the window frame so that the window transparency is not compromised. A broadband attenuation performance is obtained by the resonant unit cells constituting the AMM. The effect of the geometric variation on the window performance in terms of both acoustics and the airflow is analysed numerically through Finite Element Method (FEM) models. The performances of different AMM windows are evaluated and compared with those of conventional window designs. The simulation results show that this new AMM-based window design can overcome the limitations of the conventional windows, with great potential in real applications
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