18,799 research outputs found
Predicting the synaptic information efficacy in cortical layer 5 pyramidal neurons using a minimal integrate-and-fire model
Synaptic information efficacy (SIE) is a statistical measure to quantify the efficacy of a synapse. It measures how much information is gained, on the average, about the output spike train of a postsynaptic neuron if the input spike train is known. It is a particularly appropriate measure for assessing the inputâoutput relationship of neurons receiving dynamic stimuli. Here, we compare the SIE of simulated synaptic inputs measured experimentally in layer 5 cortical pyramidal neurons in vitro with the SIE computed from a minimal model constructed to fit the recorded data. We show that even with a simple model that is far from perfect in predicting the precise timing of the output spikes of the real neuron, the SIE can still be accurately predicted. This arises from the ability of the model to predict output spikes influenced by the input more accurately than those driven by the background current. This indicates that in this context, some spikes may be more important than others. Lastly we demonstrate another aspect where using mutual information could be beneficial in evaluating the quality of a model, by measuring the mutual information between the modelâs output and the neuronâs output. The SIE, thus, could be a useful tool for assessing the quality of models of single neurons in preserving inputâoutput relationship, a property that becomes crucial when we start connecting these reduced models to construct complex realistic neuronal networks
Electrodynamics of superconductors
An alternate set of equations to describe the electrodynamics of
superconductors at a macroscopic level is proposed. These equations resemble
equations originally proposed by the London brothers but later discarded by
them. Unlike the conventional London equations the alternate equations are
relativistically covariant, and they can be understood as arising from the
'rigidity' of the superfluid wave function in a relativistically covariant
microscopic theory. They predict that an internal 'spontaneous' electric field
exists in superconductors, and that externally applied electric fields, both
longitudinal and transverse, are screened over a London penetration length, as
magnetic fields are. The associated longitudinal dielectric function predicts a
much steeper plasmon dispersion relation than the conventional theory, and a
blue shift of the minimum plasmon frequency for small samples. It is argued
that the conventional London equations lead to difficulties that are removed in
the present theory, and that the proposed equations do not contradict any known
experimental facts. Experimental tests are discussed.Comment: Small changes following referee's and editor's comments; to be
published in Phys.Rev.
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New Physics Signals through CP Violation in B -> rho,pi
We describe here a method for detecting physics beyond the standard model via
CP violation in B->rho,pi decays. Using a Dalitz-plot analysis to obtain alpha,
along with an analytical extraction of the various tree (T) and penguin (P)
amplitudes, we obtain a criterion for the absence of new physics (NP). This
criterion involves the comparison of the measured |P/T| ratio with its value as
predicted by QCD factorization. We show that the detection of NP via this
method has a good efficiency when compared with the corresponding technique
using B->pi,pi decays.Comment: 8 pages, 4 figures, talk given at MRST 2004: From Quarks to
Cosmology, Concordia University, Montreal, May 200
All-optical reconstruction of atomic ground-state population
The population distribution within the ground-state of an atomic ensemble is
of large significance in a variety of quantum optics processes. We present a
method to reconstruct the detailed population distribution from a set of
absorption measurements with various frequencies and polarizations, by
utilizing the differences between the dipole matrix elements of the probed
transitions. The technique is experimentally implemented on a thermal rubidium
vapor, demonstrating a population-based analysis in two optical pumping
examples. The results are used to verify and calibrate an elaborated numerical
model, and the limitations of the reconstruction scheme which result from the
symmetry properties of the dipole matrix elements are discussed.Comment: 6 pages, 4 figure
Can One Measure the Weak Phase of a Penguin Diagram?
The b -> d penguin amplitude receives contributions from internal u, c and
t-quarks. We show that it is impossible to measure the weak phase of any of
these penguin contributions without theoretical input. However, it is possible
to obtain the weak phase if one makes a single assumption involving the
hadronic parameters. With such an assumption, one can test for the presence of
new physics in the b -> d flavour-changing neutral current by comparing the
weak phase of B_d^0-{\bar B}_d^0 mixing with that of the t-quark contribution
to the b -> d penguin.Comment: 20 pages, no figure
Diagrammatic Analysis of Charmless Three-Body B Decays
We express the amplitudes for charmless three-body B decays in terms of
diagrams. In addition, we show how to use Dalitz-plot analyses to obtain decay
amplitudes which are symmetric or antisymmetric under the exchange of two of
the final-state particles. When annihilation-type diagrams are neglected, as in
two-body decays, many of the exact, purely isospin-based results are modified,
leading to new tests of the standard model (SM). Some of the tests can be
performed now, and we find that present data agree with the predictions of the
SM. Furthermore, contrary to what was thought previously, it is possible to
cleanly extract weak-phase information from three-body decays, and we discuss
methods for B -> K pi pi, K K Kbar, K Kbar pi and pi pi pi.Comment: 30 pages, 10 figures, reference updated, sentences added regarding
indirect CP violation and CP of the final state. Significant text added
describing how to obtain symmetric/antisymmetric decay amplitudes, results of
the comparison of the predictions of the SM with present data for several
decays, and the momentum dependence of the diagram
Exploring CP Violation with B_d -> D K_s Decays
We (re)examine CP violation in the decays B_d -> D K_s, where D represents
D^0, D(bar), or one of their excited states. The quantity can be extracted from the time-dependent rates for and , where the decays to
. If one considers a non-CP-eigenstate hadronic final state to
which both D(bar) and D^0 can decay (e.g. ), then one can obtain two
of the angles of the unitarity triangle from measurements of the time-dependent
rates for and .
There are no penguin contributions to these decays, so all measurements are
theoretically clean.Comment: 15 pages, LaTeX, no figure
Symmetries and collective excitations in large superconducting circuits
The intriguing appeal of circuits lies in their modularity and ease of
fabrication. Based on a toolbox of simple building blocks, circuits present a
powerful framework for achieving new functionality by combining circuit
elements into larger networks. It is an open question to what degree modularity
also holds for quantum circuits -- circuits made of superconducting material,
in which electric voltages and currents are governed by the laws of quantum
physics. If realizable, quantum coherence in larger circuit networks has great
potential for advances in quantum information processing including topological
protection from decoherence. Here, we present theory suitable for quantitative
modeling of such large circuits and discuss its application to the fluxonium
device. Our approach makes use of approximate symmetries exhibited by the
circuit, and enables us to obtain new predictions for the energy spectrum of
the fluxonium device which can be tested with current experimental technology
Hadronic B Decays: A General Approach
In this paper, we propose a general approach for describing hadronic B
decays. Using this method, all amplitudes for such decays can be expressed in
terms of contractions, though the matrix elements are not evaluated. Many years
ago, Buras and Silvestrini proposed a similar approach. However, our technique
goes beyond theirs in several ways. First, we include recent theoretical and
experimental developments which indicate which contractions are negligible, and
which are expected to be smaller than others. Second, we show that all B-decay
diagrams can be simply expressed in terms of contractions. This constitutes a
formal proof that the diagrammatic method is rigourous. Third, we show that one
reproduces the relations between tree and electroweak-penguin diagrams
described by Neubert and Rosner, and by Gronau, Pirjol and Yan. Fourth,
although the previous results hold to all orders in alpha_s, we show that it is
also possible to work order-by-order in this approach. In this way it is
possible to make a connection with the matrix-element evaluation methods of QCD
factorization (QCDfac) and perturbative QCD (pQCD). Finally, using the
contractions approach, we re-evaluate the question of whether there is a ``B ->
pi K puzzle.'' At O(alpha_s^0), we find that the diagram ratio |C'/T| is about
0.17, a factor of 10 too small to explain all the B -> pi K data. Both QCDfac
and pQCD find that, at O(\alpha_s^1), the value of |C'/T'| may be raised to
only about 2-3 times its lowest-order value. We therefore conclude that,
assuming the effect is not a statistical fluctuation, it is likely that the
value of |C'/T'| is similar to its O(\alpha_s^0) result, and that there really
is a B -> pi K puzzle.Comment: 33 pages, plain latex, 10 figures (included
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