952 research outputs found
Characterization of a ballistic supermirror neutron guide
We describe the beam characteristics of the first ballistic supermirror
neutron guide H113 that feeds the neutron user facility for particle physics
PF1B of the Institute Laue-Langevin, Grenoble (ILL). At present, the neutron
capture flux density of H113 at its 20x6cm2 exit window is 1.35x10^10/cm^2/s,
and will soon be raised to above 2x10^10/cm^2/s. Beam divergence is no larger
than beam divergence from a conventional Ni coated guide. A model is developed
that permits rapid calculation of beam profiles and absolute event rates from
such a beam. We propose a procedure that permits inter-comparability of the
main features of beams emitted from ballistic or conventional neutron guides.Comment: 15 pages, 11 figures, to be submitted to Nuclear Instruments and
Methods
Measurement of the Neutrino Asymmetry Parameter B in Neutron Decay
A new measurement of the neutrino asymmetry parameter B in neutron decay, the
angular correlation between neutron spin and anti-neutrino momentum, is
presented. The result, B=0.9802(50), agrees with the Standard Model expectation
and earlier measurements, and permits improved tests on ``new physics'' in
neutron decay.Comment: 4 pages, 2 figures; v2: revised PRL versio
On gravity as an entropic force
We consider E. Verlinde's proposal that gravity is an entropic force -- we
shall call this theory entropic gravity (EG) -- and reanalyze a recent claim
that this theory is in contradiction with the observation of the
gravitationally-bound ground state of neutrons in the GRANIT experiment. We
find that EG does not necessarily contradict the existence of
gravitationally-bound quantum states of neutrons in the Earth's gravitational
field, since EG is equivalent to Newtonian gravity in this case. However,
certain transitions between the gravitationally-bound quantum states of
neutrons, in particular spontaneous decays of excited states, which can
hopefully be observed in future experiments, cannot be explained in the
framework of EG, unless essential ingredients are introduced into it.
Otherwise, a quantized description of gravity will be required.Comment: 6 pages, v2: the possibility that graviton may appear as an emergent
concept in EG is noted, few improvements in arguments and presentation, some
typos and grammar corrected. To appear in Phys. Lett.
GRANIT project: a trap for gravitational quantum states of UCN
Previous studies of gravitationally bound states of ultracold neutrons showed
the quantization of energy levels, and confirmed quantum mechanical predictions
for the average size of the two lowest energy states wave functions.
Improvements in position-like measurements can increase the accuracy by an
order of magnitude only. We therefore develop another approach, consisting in
accurate measurements of the energy levels. The GRANIT experiment is devoted to
the study of resonant transitions between quantum states induced by an
oscillating perturbation.
According to Heisenberg's uncertainty relations, the accuracy of measurement
of the energy levels is limited by the time available to perform the
transitions. Thus, trapping quantum states will be necessary, and each source
of losses has to be controlled in order to maximize the lifetime of the states.
We discuss the general principles of transitions between quantum states, and
consider the main systematical losses of neutrons in a trap.Comment: presented in ISINN 15 seminar, Dubn
Dynamical response of the Hodgkin-Huxley model in the high-input regime
The response of the Hodgkin-Huxley neuronal model subjected to stochastic
uncorrelated spike trains originating from a large number of inhibitory and
excitatory post-synaptic potentials is analyzed in detail. The model is
examined in its three fundamental dynamical regimes: silence, bistability and
repetitive firing. Its response is characterized in terms of statistical
indicators (interspike-interval distributions and their first moments) as well
as of dynamical indicators (autocorrelation functions and conditional
entropies). In the silent regime, the coexistence of two different coherence
resonances is revealed: one occurs at quite low noise and is related to the
stimulation of subthreshold oscillations around the rest state; the second one
(at intermediate noise variance) is associated with the regularization of the
sequence of spikes emitted by the neuron. Bistability in the low noise limit
can be interpreted in terms of jumping processes across barriers activated by
stochastic fluctuations. In the repetitive firing regime a maximization of
incoherence is observed at finite noise variance. Finally, the mechanisms
responsible for spike triggering in the various regimes are clearly identified.Comment: 14 pages, 24 figures in eps, submitted to Physical Review
Measurement of the Proton Asymmetry Parameter C in Neutron Beta Decay
The proton asymmetry parameter C in neutron decay describes the correlation
between neutron spin and proton momentum. In this Letter, the first measurement
of this quantity is presented. The result C=-0.2377(26) agrees with the
Standard Model expectation. The coefficient C provides an additional parameter
for new and improved Standard Model tests. From a differential analysis of the
same data (assuming the Standard Model), we obtain lambda=-1.275(16) as ratio
of axial-vector and vector coupling constant.Comment: 4 pages, 2 figure
On entropic gravity: the entropy postulate, entropy content of screens and relation to quantum mechanics
We consider the controversial hypothesis that gravity is an entropic force
that has its origin in the thermodynamics of holographic screens. Several key
aspects of entropic gravity are discussed. In particular, we revisit and
elaborate on our criticism of the recent claim that entropic gravity fails to
explain observations involving gravitationally-bound quantum states of neutrons
in the GRANIT experiment and gravitationally induced quantum interference. We
argue that the analysis leading to this claim is troubled by a
misinterpretation concerning the relation between the microstates of a
holographic screen and the state of a particle in the emergent space,
engendering inconsistencies. A point of view that could resolve the
inconsistencies is presented. We expound the general idea of the aforementioned
critical analysis of entropic gravity in such a consistent setting. This
enables us to clarify the problem and to identify a premise whose validity will
decide the faith of the criticism against entropic gravity. It is argued that
in order to reach a sensible conclusion we need more detailed knowledge on
entropic gravity. These arguments are relevant to any theory of emergent space,
where the entropy of the microscopic system depends on the distribution of
matter in the emergent space.Comment: 15 pages; v2: presentation and arguments improved, particularly in
section 5; accepted to Phys. Lett.
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