3,964 research outputs found
Energy Loss from Reconnection with a Vortex Mesh
Experiments in superfluid 4He show that at low temperatures, energy
dissipation from moving vortices is many orders of magnitude larger than
expected from mutual friction. Here we investigate other mechanisms for energy
loss by a computational study of a vortex that moves through and reconnects
with a mesh of small vortices pinned to the container wall. We find that such
reconnections enhance energy loss from the moving vortex by a factor of up to
100 beyond that with no mesh. The enhancement occurs through two different
mechanisms, both involving the Kelvin oscillations generated along the vortex
by the reconnections. At relatively high temperatures the Kelvin waves increase
the vortex motion, leading to more energy loss through mutual friction. As the
temperature decreases, the vortex oscillations generate additional reconnection
events between the moving vortex and the wall, which decrease the energy of the
moving vortex by transfering portions of its length to the pinned mesh on the
wall.Comment: 9 pages, 10 figure
Thermodynamic inequalities in superfluid
We investigate general thermodynamic stability conditions for the superfluid.
This analysis is performed in an extended space of thermodynamic variables
containing (along with the usual thermodynamic coordinates such as pressure and
temperature) superfluid velocity and momentum density. The stability conditions
lead to thermodynamic inequalities which replace the Landau superfluidity
criterion at finite temperatures.Comment: 7 pages, 1 figur
Relativistic effects in two-particle emission for electron and neutrino reactions
Two-particle two-hole contributions to electroweak response functions are
computed in a fully relativistic Fermi gas, assuming that the electroweak
current matrix elements are independent of the kinematics. We analyze the
genuine kinematical and relativistic effects before including a realistic
meson-exchange current (MEC) operator. This allows one to study the
mathematical properties of the non-trivial seven-dimensional integrals
appearing in the calculation and to design an optimal numerical procedure to
reduce the computation time. This is required for practical applications to CC
neutrino scattering experiments, where an additional integral over the neutrino
flux is performed. Finally we examine the viability of this model to compute
the electroweak 2p-2h response functions.Comment: Major revision (shortened). 22 pages, 18 figure
The Exotic Statistics of Leapfrogging Smoke Rings
The leapfrogging motion of smoke rings is a three dimensional version of the
motion that in two dimensions leads to exotic exchange statistics. The
statistical phase factor can be computed using the hydrodynamical Euler
equation, which is a universal law for describing the properties of a large
class of fluids. This suggests that three dimensional exotic exchange
statistics is a common property of closed vortex loops in a variety of quantum
liquids and gases, from helium superfluids to Bose-Einstein condensed alkali
gases, metallic hydrogen in its liquid phases and maybe even nuclear matter in
extreme conditions.Comment: 9 pages 1 figur
2p-2h excitations in neutrino scattering: angular distribution and frozen approximation
We study the phase-space dependence of 2p-2h excitations in neutrino
scattering using the relativistic Fermi gas model. We follow a similar approach
to other authors, but focusing in the phase-space properties, comparing with
the non-relativistic model. A careful mathematical analysis of the angular
distribution function for the outgoing nucleons is performed. Our goals are to
optimize the CPU time of the 7D integral to compute the hadron tensor in
neutrino scattering, and to conciliate the different relativistic and non
relativistic models by describing general properties independently of the
two-body current. For some emission angles the angular distribution becomes
infinite in the Lab system, and we derive a method to integrate analytically
around the divergence. Our results show that the frozen approximation, obtained
by neglecting the momenta of the two initial nucleons inside the integral of
the hadron tensor, reproduces fairly the exact response functions for constant
current matrix elements.Comment: 8 pages, 4 figures. Contribution to 16th International Workshop on
Neutrino Factories and Future Neutrino Beam Facilities, 25-30 August, 2014.
Held at University of Glasgow, United Kingdo
The frozen nucleon approximation in two-particle two-hole response functions
We present a fast and efficient method to compute the inclusive two-particle
two-hole (2p-2h) electroweak responses in the neutrino and electron
quasielastic inclusive cross sections. The method is based on two
approximations. The first neglects the motion of the two initial nucleons below
the Fermi momentum, which are considered to be at rest. This approximation,
which is reasonable for high values of the momentum transfer, turns out also to
be quite good for moderate values of the momentum transfer . The
second approximation involves using in the "frozen" meson-exchange currents
(MEC) an effective -propagator averaged over the Fermi sea. Within the
resulting "frozen nucleon approximation", the inclusive 2p-2h responses are
accurately calculated with only a one-dimensional integral over the emission
angle of one of the final nucleons, thus drastically simplifying the
calculation and reducing the computational time. The latter makes this method
especially well-suited for implementation in Monte Carlo neutrino event
generators.Comment: 8 pages, 5 figures and 1 tabl
Two-nucleon emission in neutrino and electron scattering from nuclei: the modified convolution approximation
The theoretical formalism of inclusive lepton-nucleus scattering in the
two-nucleon emission channel is discussed in the context of a simplified
approach, the modified convolution approximation. This allows one to write the
2p2h responses of the relativistic Fermi gas as a folding integral of two 1p1h
responses with the energies and momenta transferred to each nucleon. The idea
behind this method is to introduce different average momenta for the two
initial nucleons in the matrix elements of the two-body current, with the
innovation that they depend on the transferred energies and momenta. This
method treats exactly the two-body phase space kinematics, and reduces the
formulae of the response functions from seven-dimensional integrals over
momenta to much simpler three-dimensional ones. The applicability of the method
is checked by comparing with the full results within a model of electroweak
meson-exchange currents. The predictions are accurate enough, especially in the
low-energy threshold region where the average momentum approximation works the
best.Comment: 35 pages, 13 figure
Comparison of cardiovascular and metabolic outcomes in people with type 2 diabetes on insulin versus non-insulin glucose-lowering therapies (GLTs): A systematic review and meta-analysis of clinical trials.
OBJECTIVES: To compare the cardiovascular and metabolic outcomes of Insulin versus non-insulin glucose lowering therapy (GLT). METHODS: We included randomised control trials (RCTs) which randomised patients aged >18years with Type 2 Diabetes (T2D) to insulin vs non-insulin GLT. We used risk ratios (RR), risk difference (RD) and odds ratios (OR) with 95% confidence interval (95%CI) to analyse the treatment effects of dichotomous outcomes and mean differences (with 95% CI) for continuous outcomes. RESULTS: We included 18 RCTs with 19,300 participants. There was no significant difference in the risk of all-cause mortality and CV events between the groups (RR=1.01; 95%CI: 0.96-1.06; p=0.69). In 16 trials, insulin showed greater efficacy in glycaemic control (mean diff=-0.20; 95%CI: -0.28 to -0.11) but the proportion achieving HbA1c level of either ⩽7.0% or 7.4% (53 or 57mmol/mol) was similar in both (OR=1.55; 95%CI=0.92-2.62). The non-insulin group had a significant reduction in weight (mean diff=-3.41; 95%CI: -4.50 to -2.32) and an increase in the proportion of adverse events (54.7% vs 45.3%, p=0.044), but the insulin group showed an (RR=1.90; 95%CI: 1.44-2.51) increased risk of hypoglycaemia. CONCLUSION: There was no difference in the risk of all-cause mortality and adverse cardiovascular (CV) events between Insulin and non-insulin GLTs. Insulin was associated with superior reduction in HbA1c; least reduction in weight and higher risk of hypoglycaemia. Both showed similar proportion of patients achieving HbA1c target. Non-insulin GLTs were associated with a higher risk in reported adverse drug events
Using Electron Scattering Superscaling to predict Charge-changing Neutrino Cross Sections in Nuclei
Superscaling analyses of few-GeV inclusive electron scattering from nuclei
are extended to include not only quasielastic processes, but now also into the
region where -excitation dominates. It is shown that, with reasonable
assumptions about the basic nuclear scaling function extracted from data and
information from other studies of the relative roles played by correlation and
MEC effects, the residual strength in the resonance region can be accounted for
through an extended scaling analysis. One observes scaling upon assuming that
the elementary cross section by which one divides the residual to obtain a new
scaling function is dominated by the transition and employing a
new scaling variable which is suited to the resonance region. This yields a
good representation of the electromagnetic response in both the quasielastic
and regions. The scaling approach is then inverted and predictions are
made for charge-changing neutrino reactions at energies of a few GeV, with
focus placed on nuclei which are relevant for neutrino oscillation
measurements. For this a relativistic treatment of the required weak
interaction vector and axial-vector currents for both quasielastic and
-excitation processes is presented.Comment: 42 pages, 9 figures, accepted for publication in Physical Review
Interaction of Kelvin waves and nonlocality of energy transfer in superfluids
We argue that the physics of interacting Kelvin Waves (KWs) is highly nontrivial and cannot be understood on the basis of pure dimensional reasoning. A consistent theory of KW turbulence in superfluids should be based upon explicit knowledge of their interactions. To achieve this, we present a detailed calculation and comprehensive analysis of the interaction coefficients for KW turbuelence, thereby, resolving previous mistakes stemming from unaccounted contributions. As a first application of this analysis, we derive a local nonlinear (partial differential) equation. This equation is much simpler for analysis and numerical simulations of KWs than the Biot-Savart equation, and in contrast to the completely integrable local induction approximation (in which the energy exchange between KWs is absent), describes the nonlinear dynamics of KWs. Second, we show that the previously suggested Kozik-Svistunov energy spectrum for KWs, which has often been used in the analysis of experimental and numerical data in superfluid turbulence, is irrelevant, because it is based upon an erroneous assumption of the locality of the energy transfer through scales. Moreover, we demonstrate the weak nonlocality of the inverse cascade spectrum with a constant particle-number flux and find resulting logarithmic corrections to this spectrum
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