13,407 research outputs found
From Cosmology to Cold Atoms: Observation of Sakharov Oscillations in Quenched Atomic Superfluids
Sakharov oscillations, conventionally discussed in the context of early
universe evolution and the anisotropy of cosmic microwave background radiation,
is the manifestation of interfering acoustic waves synchronously generated in
an ideal fluid. Here we report the laboratory demonstration of Sakharov
oscillations in a quenched atomic superfluid. We quench the sample by Feshbach
tuning and monitor the subsequent density fluctuations at different time and
length scales by in situ imaging. Sakharov oscillations are identified as the
multi-peak structure in the atomic density power spectrum, resembling that of
the cosmic microwave background. We also observe Sakharov oscillations in the
time domain, from which we extract the energy dispersion of the superfluid, and
determine the sonic horizon of the excitations
Mining and Incentive Concession Contracts
This paper studies the design of a mining concession contract as a multi-period autoselection problem where production is the depletion of a non renewable resource. As compared to symmetric information, we show that overproduction (resp. underproduction) is optimal in the initial phase (resp. terminal phase ) of the resource extraction program. Also, asymmetric information lengthens the contract duration but reduces the scarcity rent. Finally, when there are several agents competing for contract bid, we show that optimal auctioning could be used to award the concession, assigning the lowest cost agent to carry out the extraction.ADVERSE SELECTION; EXHAUSTIBILITY; OVERPRODUCTION
Quantum Spin Dynamics with Pairwise-Tunable, Long-Range Interactions
We present a platform for the simulation of quantum magnetism with full
control of interactions between pairs of spins at arbitrary distances in one-
and two-dimensional lattices. In our scheme, two internal atomic states
represent a pseudo-spin for atoms trapped within a photonic crystal waveguide
(PCW). With the atomic transition frequency aligned inside a band gap of the
PCW, virtual photons mediate coherent spin-spin interactions between lattice
sites. To obtain full control of interaction coefficients at arbitrary
atom-atom separations, ground-state energy shifts are introduced as a function
of distance across the PCW. In conjunction with auxiliary pump fields,
spin-exchange versus atom-atom separation can be engineered with arbitrary
magnitude and phase, and arranged to introduce non-trivial Berry phases in the
spin lattice, thus opening new avenues for realizing novel topological spin
models. We illustrate the broad applicability of our scheme by explicit
construction for several well known spin models.Comment: 18 pages, 10 figure
CHARACTERISTIC GROUND REACTION FORCES IN SOFTBALL PITCHING
The purpose of this study was to investigate the ground reaction forces of pushing leg and landing leg during softball pitching with windmill style fastball. Four female elite fastpitch softball pitchers served as subjects. A Peak high-speed camera (120Hz) was synchronized with a force plate (600Hz) to collect the parameters of lower extremity during pitching. Results indicated that the higher impulse of pushing leg and the peak slope of landing leg improve softball pitching performance. In addition, the peak ground reaction forces appeared before the human neuromuscular system responded to modify these forces. The greater ground reaction forces created during the landing phase putting a heavy load on lower extremity. It was suggested that the improvement of lower extremities strength would reduce the injury and improve softball pitching performance
A new mechanism for a naturally small Dirac neutrino mass
A mechanism is proposed in which a right-handed neutrino zero mode and a
right-handed charged lepton zero mode can be localized at the same place along
an extra compact dimension while having markedly different spreads in their
wave functions: a relatively narrow one for the neutrino and a rather broad one
for the charged lepton. In their overlaps with the wave function for the
left-handed zero modes, this mechanism could produce a natural large hierarchy
in the effective Yukawa couplings in four dimensions, and hence a large
disparity in masses.Comment: 6 pages (2 with figures), twocolumn forma
Exotic fermion multiplets as a solution to baryon asymmetry, dark matter and neutrino masses
We propose an extension to the standard model where three exotic fermion
5-plets and one scalar 6-plet are added to the particle content. By demanding
that all interactions are renormalizable and standard model gauge invariant, we
show that the lightest exotic particle in this model can be a dark matter
candidate as long as the new 6-plet scalar does not develop a nonzero vacuum
expectation value. Furthermore, light neutrino masses are generated radiatively
at one-loop while the baryon asymmetry is produced by the CP-violating decays
of the second lightest exotic particle. We have demonstrated using concrete
examples that there is a parameter space where a consistent solution to the
problems of baryon asymmetry, dark matter and neutrino masses can be obtained.Comment: 17 pages, 2 figures (REVTeX4.1), v2: some refs added, v3: typos
corrected, Sec.VI.B, C modified, this version to appear in PR
Near-Infrared MOSFIRE Spectra of Dusty Star-Forming Galaxies at 0.2<z<4
We present near-infrared and optical spectroscopic observations of a sample
of 450m and 850m-selected dusty star-forming galaxies (DSFGs)
identified in a 400 arcmin area in the COSMOS field. Thirty-one sources of
the 102 targets were spectroscopically confirmed at , identified
primarily in the near-infrared with Keck MOSFIRE and some in the optical with
Keck LRIS and DEIMOS. The low rate of confirmation is attributable both to high
rest-frame optical obscuration in our targets and limited sensitivity to
certain redshift ranges. The high-quality photometric redshifts available in
the COSMOS field allow us to test the robustness of photometric redshifts for
DSFGs. We find a subset (11/31%) of DSFGs with inaccurate () or non-existent photometric redshifts; these have very distinct
spectral energy distributions from the remaining DSFGs, suggesting a decoupling
of highly obscured and unobscured components. We present a composite rest-frame
4300--7300\AA\ spectrum for DSFGs, and find evidence of 20030 km s
gas outflows. Nebular line emission for a sub-sample of our detections indicate
that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more
so than typical mass or UV-selected high-z galaxies. We also confirm the
extreme level of dust obscuration in DSFGs, measuring very high Balmer
decrements, and very high ratios of IR to UV and IR to H luminosities.
This work demonstrates the need to broaden the use of wide bandwidth technology
in the millimeter to the spectroscopic confirmations of large samples of high-z
DSFGs, as the difficulty in confirming such sources at optical/near-infrared
wavelengths is exceedingly challenging given their obscuration.Comment: 14 pages, 13 figures, ApJ accepted. Composite DSFG Halpha spectrum
available at www.as.utexas.edu/~cmcasey/downloads.htm
Quantum many-body models with cold atoms coupled to photonic crystals
Using cold atoms to simulate strongly interacting quantum systems represents
an exciting frontier of physics. However, as atoms are nominally neutral point
particles, this limits the types of interactions that can be produced. We
propose to use the powerful new platform of cold atoms trapped near
nanophotonic systems to extend these limits, enabling a novel quantum material
in which atomic spin degrees of freedom, motion, and photons strongly couple
over long distances. In this system, an atom trapped near a photonic crystal
seeds a localized, tunable cavity mode around the atomic position. We find that
this effective cavity facilitates interactions with other atoms within the
cavity length, in a way that can be made robust against realistic
imperfections. Finally, we show that such phenomena should be accessible using
one-dimensional photonic crystal waveguides in which coupling to atoms has
already been experimentally demonstrated
- …