658 research outputs found
Feshbach resonances in a quasi-2D atomic gas
Strongly confining an ultracold atomic gas in one direction to create a
quasi-2D system alters the scattering properties of this gas. We investigate
the effects of confinement on Feshbach scattering resonances and show that
strong confinement results in a shift in the position of the Feshbach resonance
as a function of the magnetic field. This shift, as well as the change of the
width of the resonance, are computed. We find that the resonance is strongly
damped in the thermal gas, but in the condensate the resonance remains sharp
due to many-body effects. We introduce a 2D model system, suited for the study
of resonant superfluidity, and having the same scattering properties as the
tightly confined real system near a Feshbach resonance. Exact relations are
derived between measurable quantities and the model parameters.Comment: 8 pages, 2 figure
Three-body problem in Fermi gases with short-range interparticle interaction
We discuss 3-body processes in ultracold two-component Fermi gases with
short-range intercomponent interaction characterized by a large and positive
scattering length . It is found that in most cases the probability of 3-body
recombination is a universal function of the mass ratio and , and is
independent of short-range physics. We also calculate the scattering length
corresponding to the atom-dimer interaction.Comment: 4 pages, 2 figure
Trapped two-component Fermi gases with up to six particles: Energetics, structural properties, and molecular condensate fraction
We investigate small equal-mass two-component Fermi gases under external
spherically symmetric confinement in which atoms with opposite spins interact
through a short-range two-body model potential. We employ a non-perturbative
microscopic framework, the stochastic variational approach, and determine the
system properties as functions of the interspecies s-wave scattering length a,
the orbital angular momentum L of the system, and the numbers N1 and N2 of
spin-up and spin-down atoms (with N1-N2 =0 or 1 and N < 7, where N=N1+N2). At
unitarity, we determine the energies of the five- and six-particle systems for
various ranges r0 of the underlying two-body model potential and extrapolate to
the zero-range limit. These energies serve as benchmark results that can be
used to validate and assess other numerical approaches. We also present
structural properties such as the pair distribution function and the radial
density. Furthermore, we analyze the one-body and two-body density matrices. A
measure for the molecular condensate fraction is proposed and applied. Our
calculations show explicitly that the natural orbitals and the momentum
distributions of atomic Fermi gases approach those characteristic for a
molecular Bose gas if the s-wave scattering length a, a>0, is sufficiently
small.Comment: 21 pages, 15 figures; accepted for publication in special issue of
CRA
Superfluid transition in quasi2D Fermi gases
We show that atomic Fermi gases in quasi2D geometries are promising for
achieving superfluidity. In the regime of BCS pairing for weak attraction, we
calculate the critical temperature T_c and analyze possibilities of increasing
the ratio of T_c to the Fermi energy. In the opposite limit, where a strong
coupling leads to the formation of weakly bound quasi2D dimers, we find that
their Bose-Einstein condensate will be stable on a long time scale.Comment: 4 pages, 1 figur
Hydrodynamic behavior in expanding thermal clouds of Rb-87
We study hydrodynamic behavior in expanding thermal clouds of Rb-87 released
from an elongated trap. At our highest densities the mean free path is smaller
than the radial size of the cloud. After release the clouds expand
anisotropically. The cloud temperature drops by as much as 30%. This is
attributed to isentropic cooling during the early stages of the expansion. We
present an analytical model to describe the expansion and to estimate the
cooling. Important consequences for time-of-flight thermometry are discussed.Comment: 7 pages with 2 figure
Crossovers in Unitary Fermi Systems
Universality and crossover is described for attractive and repulsive
interactions where, respectively, the BCS-BEC crossover takes place and a
ferromagnetic phase transition is claimed. Crossovers are also described for
optical lattices and multicomponent systems. The crossovers, universal
parameters and phase transitions are described within the Leggett and NSR
models and calculated in detail within the Jastrow-Slater approximation. The
physics of ultracold Fermi atoms is applied to neutron, nuclear and quark
matter, nuclei and electrons in solids whenever possible. Specifically, the
differences between optical lattices and cuprates is discussed w.r.t.
antiferromagnetic, d-wave superfluid phases and phase separation.Comment: 50 pages, 15 figures. Contribution to Lecture Notes in Physics
"BCS-BEC crossover and the Unitary Fermi Gas" edited by W. Zwerge
Perception of Relative Depth Interval: Systematic Biases in Perceived Depth
Given an estimate of the binocular disparity between a pair of points and an estimate of the viewing distance, or knowledge of eye position, it should be possible to obtain an estimate of their depth separation. Here we show that, when points are arranged in different vertical geometric configurations across two intervals, many observers find this task difficult. Those who can do the task tend to perceive the depth interval in one configuration as very different from depth in the other configuration. We explore two plausible explanations for this effect. The first is the tilt of the empirical vertical horopter: Points perceived along an apparently vertical line correspond to a physical line of points tilted backwards in space. Second, the eyes can rotate in response to a particular stimulus. Without compensation for this rotation, biases in depth perception would result. We measured cyclovergence indirectly, using a standard psychophysical task, while observers viewed our depth configuration. Biases predicted from error due either to cyclovergence or to the tilted vertical horopter were not consistent with the depth configuration results. Our data suggest that, even for the simplest scenes, we do not have ready access to metric depth from binocular disparity.</jats:p
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
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