576 research outputs found
Aspects of Neutrino Interactions (Scattering at small - Region)
The article begins with a description of chiral symmetry and its application
to neutrino induced reactions. For small (forward direction) the process
is dominated by the amplitute with helicity zero where the pion pole disappears
when multiplied with the polarization vector. The remaining part of the
amplitude is determined by PCAC. For GeV the computed cross
sections are in good agreement with data. In coherent pion production we expect
equal yields for neutrinos and antineutrinos a relation which for
GeV is fulfilled. We discuss specific features of the data and suggest methods
for improving them by presenting new estimates for the incoherent background.Comment: Presented at the CETUP-Workshop on Neutrino Interactions, July 22-31,
2014 at Lead/Dead Wood, South Dakota, USA. The resubmission contains minor
correction
Dynamic spin response of a strongly interacting Fermi gas
We present an experimental investigation of the dynamic spin response of a
strongly interacting Fermi gas using Bragg spectroscopy. By varying the
detuning of the Bragg lasers, we show that it is possible to measure the
response in the spin and density channels separately. At low Bragg energies,
the spin response is suppressed due to pairing, whereas the density response is
enhanced. These experiments provide the first independent measurements of the
spin-parallel and spin-antiparallel dynamic and static structure factors and
open the way to a complete study of the structure factors at any momentum. At
high momentum the spin-antiparallel dynamic structure factor displays a
universal high frequency tail, proportional to , where is the probe energy.Comment: Replaced with final versio
Precise determination of the structure factor and contact in a unitary Fermi gas
We present a high-precision determination of the universal contact parameter
in a strongly interacting Fermi gas. In a trapped gas at unitarity we find the
contact to be at a temperature of 0.08 of the Fermi temperature
in a harmonic trap. The contact governs the high-momentum (short-range)
properties of these systems and this low temperature measurement provides a new
benchmark for the zero temperature homogeneous contact. The experimental
measurement utilises Bragg spectroscopy to obtain the dynamic and static
structure factors of ultracold Fermi gases at high momentum in the unitarity
and molecular Bose-Einstein condensate (BEC) regimes. We have also performed
quantum Monte Carlo calculations of the static properties, extending from the
weakly coupled Bardeen-Cooper-Schrieffer (BCS) regime to the strongly coupled
BEC case, which show agreement with experiment at the level of a few percent.Comment: Replaced with accepted versio
Thermodynamics of an attractive 2D Fermi gas
Thermodynamic properties of matter are conveniently expressed as functional
relations between variables known as equations of state. Here we experimentally
determine the compressibility, density and pressure equations of state for an
attractive 2D Fermi gas in the normal phase as a function of temperature and
interaction strength. In 2D, interacting gases exhibit qualitatively different
features to those found in 3D. This is evident in the normalized density
equation of state, which peaks at intermediate densities corresponding to the
crossover from classical to quantum behaviour.Comment: Contains minor revision
Crossover from 2D to 3D in a weakly interacting Fermi gas
We have studied the transition from two to three dimensions in a low
temperature weakly interacting Li Fermi gas. Below a critical atom number,
, only the lowest transverse vibrational state of a highly anisotropic
oblate trapping potential is occupied and the gas is two-dimensional. Above
the Fermi gas enters the quasi-2D regime where shell structure
associated with the filling of individual transverse oscillator states is
apparent. This dimensional crossover is demonstrated through measurements of
the cloud size and aspect ratio versus atom number.Comment: Replaced with published manuscrip
Quantum anomaly and 2D-3D crossover in strongly interacting Fermi gases
We present an experimental investigation of collective oscillations in
harmonically trapped Fermi gases through the crossover from two to three
dimensions. Specifically, we measure the frequency of the radial monopole or
breathing mode as a function of dimensionality in Fermi gases with tunable
interactions. The frequency of this mode is set by the adiabatic
compressibility and probes the thermodynamic equation of state. In 2D, a
dynamical scaling symmetry for atoms interacting via a {\delta}-potential
predicts the breathing mode to occur at exactly twice the harmonic confinement
frequency. However, a renormalized quantum treatment introduces a new length
scale which breaks this classical scale invariance resulting in a so-called
quantum anomaly. Our measurements deep in the 2D regime lie above the
scale-invariant prediction for a range of interaction strengths indicating the
breakdown of a {\delta}-potential model for atomic interactions. As the
dimensionality is tuned from 2D to 3D we see the breathing oscillation
frequency evolve smoothly towards the 3D limit.Comment: 5 pages, 3 figure
Piecewise Potential Vorticity Inversion without Far-Field Response?
Given a flow domain D with subdomains D1 and D2, piecewise potential vorticity inversion (PPVI) inverts a
potential vorticity (PV) anomaly in D2 and assumes vanishing PV in D1 where boundary conditions must be taken into account. It is a widely held view that the PV anomaly exerts a far-field influence on D1, which is revealed by PPVI. Tests of this assertion are conducted using a simple quasi geostrophic model where an upper layer D2 contains a PV anomaly and D1 is the layer underneath. This anomaly is inverted. Any downward physical impact of PV in D2 must also be represented in
the results of a downward piecewise density inversion (PDI) based on the hydrostatic relation and the density in D2 as following from PPVI. There is no doubt about the impact of the mass in D2 on the flow in the lower layer D1. Thus results of PPVI and PDI have to agree closely
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