397 research outputs found
Population receptive field estimates of human auditory cortex.
Here we describe a method for measuring tonotopic maps and estimating bandwidth for voxels in human primary auditory cortex (PAC) using a modification of the population Receptive Field (pRF) model, developed for retinotopic mapping in visual cortex by Dumoulin and Wandell (2008). The pRF method reliably estimates tonotopic maps in the presence of acoustic scanner noise, and has two advantages over phase-encoding techniques. First, the stimulus design is flexible and need not be a frequency progression, thereby reducing biases due to habituation, expectation, and estimation artifacts, as well as reducing the effects of spatio-temporal BOLD nonlinearities. Second, the pRF method can provide estimates of bandwidth as a function of frequency. We find that bandwidth estimates are narrower for voxels within the PAC than in surrounding auditory responsive regions (non-PAC)
Gravitational waves from self-ordering scalar fields
Gravitational waves were copiously produced in the early Universe whenever
the processes taking place were sufficiently violent. The spectra of several of
these gravitational wave backgrounds on subhorizon scales have been extensively
studied in the literature. In this paper we analyze the shape and amplitude of
the gravitational wave spectrum on scales which are superhorizon at the time of
production. Such gravitational waves are expected from the self ordering of
randomly oriented scalar fields which can be present during a thermal phase
transition or during preheating after hybrid inflation. We find that, if the
gravitational wave source acts only during a small fraction of the Hubble time,
the gravitational wave spectrum at frequencies lower than the expansion rate at
the time of production behaves as with an
amplitude much too small to be observable by gravitational wave observatories
like LIGO, LISA or BBO. On the other hand, if the source is active for a much
longer time, until a given mode which is initially superhorizon (), enters the horizon, for , we find that the gravitational
wave energy density is frequency independent, i.e. scale invariant. Moreover,
its amplitude for a GUT scale scenario turns out to be within the range and
sensitivity of BBO and marginally detectable by LIGO and LISA. This new
gravitational wave background can compete with the one generated during
inflation, and distinguishing both may require extra information.Comment: 21 pages, 2 figures, added discussion about numerical integration and
a new figure to illustrate the scale-invariance of the GW power spectrum,
conclusions unchange
-mass Modification in - a Signal of Restoration of Chiral Symmetry or Test for Nuclear Matter Models ?
Two recent experiments have demonstrated that the effective -mass in
nuclear medium, as extracted from the reaction, is
substantially reduced. This has been advocated as an indication of partial
restoration of chiral symmetry in nuclear matter. We show that even in the
absence of chiral symmetry, effective mean field nuclear matter models can
explain these findings quantitatively.Comment: ReVTeX file with 2 postscript figures include
Study of Two-Step Mechanisms in Pion Absorption on 6Li, 12C via Deuteron Emission
The (pi+,pd), and (pi+,dd) reactions were investigated with pions of 100 and
165 MeV kinetic energy on 6Li and 12C targets. In comparison with previously
published (pi+,pp) data on the same targets and at the same beam energies,
kinematic regions were identified in which the neutron pickup process n+p->d
dominated the observed deuteron yield. The importance of this mechanism
increases with energy, contributing half of the observed cross section at 165
MeV. The contribution of direct quasi-triton absorption is significant only at
100 MeV.Comment: 23 pages, 12 figure
Study of Dipole Resonance Strength in 12-C via the Reactions 12-C(pol.p,p'c)
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
A Measurement of the Longitudinal Acceptance of the K600 Magnetic Spectrometer
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Study of High-Spin States and Three-Quasiparticle (p,π) Transitions on Light Targets
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Isospin Response of the 4-He Continuum
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
The Muonium Atom as a Probe of Physics beyond the Standard Model
The observed interactions between particles are not fully explained in the
successful theoretical description of the standard model to date. Due to the
close confinement of the bound state muonium () can be used as
an ideal probe of quantum electrodynamics and weak interaction and also for a
search for additional interactions between leptons. Of special interest is the
lepton number violating process of sponteanous conversion of muonium to
antimuonium.Comment: 15 pages,6 figure
The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition
We analytically derive the spectrum of gravitational waves due to
magneto-hydrodynamical turbulence generated by bubble collisions in a
first-order phase transition. In contrast to previous studies, we take into
account the fact that turbulence and magnetic fields act as sources of
gravitational waves for many Hubble times after the phase transition is
completed. This modifies the gravitational wave spectrum at large scales. We
also model the initial stirring phase preceding the Kolmogorov cascade, while
earlier works assume that the Kolmogorov spectrum sets in instantaneously. The
continuity in time of the source is relevant for a correct determination of the
peak position of the gravitational wave spectrum. We discuss how the results
depend on assumptions about the unequal-time correlation of the source and
motivate a realistic choice for it. Our treatment gives a similar peak
frequency as previous analyses but the amplitude of the signal is reduced due
to the use of a more realistic power spectrum for the magneto-hydrodynamical
turbulence. For a strongly first-order electroweak phase transition, the signal
is observable with the space interferometer LISA.Comment: 46 pages, 17 figures. Replaced with revised version accepted for
publication in JCA
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