1,291 research outputs found
Neural system identification for large populations separating "what" and "where"
Neuroscientists classify neurons into different types that perform similar
computations at different locations in the visual field. Traditional methods
for neural system identification do not capitalize on this separation of 'what'
and 'where'. Learning deep convolutional feature spaces that are shared among
many neurons provides an exciting path forward, but the architectural design
needs to account for data limitations: While new experimental techniques enable
recordings from thousands of neurons, experimental time is limited so that one
can sample only a small fraction of each neuron's response space. Here, we show
that a major bottleneck for fitting convolutional neural networks (CNNs) to
neural data is the estimation of the individual receptive field locations, a
problem that has been scratched only at the surface thus far. We propose a CNN
architecture with a sparse readout layer factorizing the spatial (where) and
feature (what) dimensions. Our network scales well to thousands of neurons and
short recordings and can be trained end-to-end. We evaluate this architecture
on ground-truth data to explore the challenges and limitations of CNN-based
system identification. Moreover, we show that our network model outperforms
current state-of-the art system identification models of mouse primary visual
cortex.Comment: NIPS 201
Quark mass dependence of the nucleon axial-vector coupling constant
We study the quark mass expansion of the axial-vector coupling constant g_A
of the nucleon. The aim is to explore the feasibility of chiral effective field
theory methods for extrapolation of lattice QCD results - so far determined at
relatively large quark masses corresponding to pion masses larger than 0.6 GeV
- down to the physical value of the pion mass. We compare two versions of
non-relativistic chiral effective field theory: One scheme restricted to pion
and nucleon degrees of freedom only, and an alternative approach which
incorporates explicit Delta(1230) resonance degrees of freedom. It turns out
that, in order to approach the physical value of g_A in a leading-one-loop
calculation, the inclusion of the explicit Delta(1230) degrees of freedom is
crucial. With information on important higher order couplings constrained from
analyses of inelastic pion production processes, a chiral extrapolation
function for g_A is obtained, which works well from the chiral limit across the
physical point into the region of present lattice data. The resulting
enhancement of our extrapolation function near the physical pion mass is found
to arise from an interplay between long- and short- distance physics.Comment: 21 pages, LaTeX, 7 figure
Radiative pion capture by a nucleon
The differential cross sections for and are computed up to in heavy baryon chiral perturbation
theory (HBChPT). The expressions at and have no free
parameters. There are three unknown parameters at , low energy
constants of the HBChPT Lagrangian, which are determined by fitting to
experimental data. Two acceptable fits are obtained, which can be separated by
comparing with earlier dispersion relation calculations of the inverse process.
Expressions for the multipoles, with emphasis on the p-wave multipoles, are
obtained and evaluated at threshold. Generally the results obtained from the
best of the two fits are in good agreement with the dispersion relation
predictions.Comment: 24 pages, Latex, using RevTe
A renormalisation group approach to two-body scattering in the presence of long-range forces
We apply renormalisation-group methods to two-body scattering by a
combination of known long-range and unknown short-range potentials. We impose a
cut-off in the basis of distorted waves of the long-range potential and
identify possible fixed points of the short-range potential as this cut-off is
lowered to zero. The expansions around these fixed points define the power
countings for the corresponding effective field theories. Expansions around
nontrivial fixed points are shown to correspond to distorted-wave versions of
the effective-range expansion. These methods are applied to scattering in the
presence of Coulomb, Yukawa and repulsive inverse-square potentials.Comment: 22 pages (RevTeX), 4 figure
Compton Scattering and the Spin Structure of the Nucleon at Low Energies
We analyze polarized Compton scattering which provides information on the
spin-structure of the nucleon. For scattering processes with photon energies up
to 100 MeV the spin-structure dependence can be encoded into four independent
parameters-the so-called spin-polarizabilities of the
nucleon, which we calculate within the framework of the "small scale expansion"
in SU(2) baryon chiral perturbation theory. Specific application is made to
"forward" and "backward" spin- polarizabilities.Comment: 8 pages revtex file, separation between pion-pole and regular
contributions detailed + minor wording changes, results and conclusions
unchange
Chiral Transparency
Color transparency is the vanishing of initial and final state interactions,
predicted by QCD to occur in high momentum transfer quasielastic nuclear
reactions. For specific reactions involving nucleons, the initial and final
state interactions are expected to be dominated by exchanges of pions. We argue
that these interactions are also suppressed in high momentum transfer nuclear
quasielastic reactions; this is ``chiral transparency". We show that studies of
the reaction could reveal the influence of chiral
transparency.Comment: 20 pages, three figures available by fax from
[email protected]; submitted to Phys. Rev.
Chiral unitary theory: application to nuclear problems
In this talk we briefly describe some basic elements of chiral perturbation
theory, , and how the implementation of unitarity and other novel
elements lead to a better expansion of the matrix for meson meson and meson
baryon interactions. Applications are then done to the interaction
in nuclear matter in the scalar and vector channels, antikaons in nuclei and
atoms, and how the meson properties are changed in a nuclear
medium.Comment: 15 pages, 9 figures, Invited talk in the International Symposium on
Nuclear Physics, Bombay, december 200
Virtual Compton Scattering off the Nucleon in Chiral Perturbation Theory
We investigate the spin-independent part of the virtual Compton scattering
(VCS) amplitude off the nucleon within the framework of chiral perturbation
theory. We perform a consistent calculation to third order in external momenta
according to Weinberg's power counting. With this calculation we can determine
the second- and fourth-order structure-dependent coefficients of the general
low-energy expansion of the spin-averaged VCS amplitude based on gauge
invariance, crossing symmetry and the discrete symmetries. We discuss the
kinematical regime to which our calculation can be applied and compare our
expansion with the multipole expansion by Guichon, Liu and Thomas. We establish
the connection of our calculation with the generalized polarizabilities of the
nucleon where it is possible.Comment: 26 pages, 2 Postscript figures, RevTex using epsfi
A new measurement of the properties of the rare decay K -> pi+ e+ e-
A large low-background sample of events (10300) has been collected for the
rare decay of kaons in flight K+ -> pi+ e+ e- by experiment E865 at the
Brookhaven AGS. The decay products were accepted by a broad band
high-resolution charged particle spectrometer with particle identification. The
branching ratio (2.94 +- 0.05(stat.) +- 0.13(syst.) +- 0.05(model))*10**{-7}
was determined normalizing to events from the decay chain K+ -> pi+ pi0; pi0 ->
e+ e- gamma. From the analysis of the decay distributions the vector nature of
this decay is firmly established now, and limits on scalar and tensor
contributions are deduced. From the (e+ e-) invariant mass distribution the
decay form factor f(z)=f0(1+ delta*z) (z=M(ee)**2/m(K)**2) is determined with
delta=2.14 +- 0.13 +- 0.15. Chiral QCD perturbation theory predictions for the
form factor are also tested, and terms beyond leading order O(p**4) are found
to be important.Comment: 4 pages, 5 figure
Chiral dynamics of baryon resonances and hadrons in a nuclear medium
In these lectures I make an introduction to chiral unitary theory applied to
the meson baryon interaction and show how several well known resonances are
dynamically generated, and others are predicted. Two very recent experiments
are analyzed, one of them showing the existence of two
states and the other one providing support for the
resonance as a quasibound state of . The
use of chiral Lagrangians to account for the hadronic interaction at the
elementary level introduces a new approach to deal with the modification of
meson and baryon properties in a nuclear medium. Examples of it for ,
and modification in the nuclear medium are presented.Comment: Lectures given in the Workshop on Hadron Physics, Puri (India), march
200
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