70,141 research outputs found
Speaker-normalized sound representations in the human auditory cortex
The acoustic dimensions that distinguish speech sounds (like the vowel differences in âbootâ and âboatâ) also differentiate speakersâ voices. Therefore, listeners must normalize across speakers without losing linguistic information. Past behavioral work suggests an important role for auditory contrast enhancement in normalization: preceding context affects listenersâ perception of subsequent speech sounds. Here, using intracranial electrocorticography in humans, we investigate whether and how such context effects arise in auditory cortex. Participants identified speech sounds that were preceded by phrases from two different speakers whose voices differed along the same acoustic dimension as target words (the lowest resonance of the vocal tract). In every participant, target vowels evoke a speaker-dependent neural response that is consistent with the listenerâs perception, and which follows from a contrast enhancement model. Auditory cortex processing thus displays a critical feature of normalization, allowing listeners to extract meaningful content from the voices of diverse speakers
Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic,
short time-scale variability that is difficult to reconcile with the standard
relativistic shock model. We interpret the observed light curves as a
microlensing event superimposed on power-law flux decays typical of afterglows.
In general, a relativistic GRB shock appears on the sky as a thin ring
expanding at a superluminal speed. Initially the ring is small relative to its
angular separation from the lens and so its flux is magnified by a constant
factor. As the ring grows and sweeps across the lens its magnification reaches
a maximum. Subsequently, the flux gradually recovers its unlensed value. This
behavior involves only three free parameters in its simplest formulation and
was predicted theoretically by Loeb & Perna (1998). Fitting the available
R-band photometric data of GRB 000301C to a simple model of the microlensing
event and a broken power-law for the afterglow, we find reasonable values for
all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99
for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8
days after the burst. The entire optical-IR data imply a width of the GRB ring
of order 10% of its radius, similar to theoretical expectations. The angular
resolution provided by microlensing is better than a micro-arcsecond. We infer
a mass of approximately 0.5 M_Sun for a lens located half way to the source at
z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens,
but current data provides only an upper-limit on its surface brightness at the
GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional
figure included); all data used for the fits available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Composite fermion theory of rapidly rotating two-dimensional bosons
Ultracold neutral bosons in a rapidly rotating atomic trap have been
predicted to exhibit fractional quantum Hall-like states. We describe how the
composite fermion theory, used in the description of the fractional quantum
Hall effect for electrons, can be applied to interacting bosons. Numerical
evidence supporting the formation of composite fermions, each being the bound
state of a boson and one flux quantum, is shown for filling fractions of the
type nu=p/(p+1), both by spectral analysis and by direct comparison with trial
wave functions. The rapidly rotating system of two-dimensional bosons thus
constitutes an interesting example of "statistical transmutation," with bosons
behaving like composite fermions. We also describe the difference between the
electronic and the bosonic cases when p approaches infinity. Residual
interactions between composite fermions are attractive in this limit, resulting
in a paired composite-fermion state described by the Moore-Read wave function.Comment: 12 pages, 9 figures. Conference proceeding. BEC 2005 Ital
Pickoff and spin-conversion quenchings of ortho-positronium in oxygen
The quenching processes of the thermalized ortho-positronium(o-Ps) on an
oxygen molecule have been studied by the positron annihilation age-momentum
correlation techinique(AMOC). The Doppler broadening spectrum of the 511 keV
gamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as a
function of the o-Ps age. The rate of the quenching, consisting of the pickoff
and the spin-conversion, is estimated from the positron lifetime spectrum. The
ratio of the pickoff quenching rate to the spin-conversion rate is deduced from
the Doppler broadening of the 511 keV gamma-rays from the annihilation of the
o-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons per
molecule which contribute to the pickoff quenching, for O_2 is determined to be
0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching is
determined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4
Molecular-beam epitaxy of CrSi_2 on Si(111)
Chromium disilicide layers have been grown on Si(111) in a commercial molecularâbeam epitaxy machine. Thin layers (10 nm) exhibit two epitaxial relationships, which have been identified as CrSi_2(0001)//Si(111) with CrSi_2[1010]//Si[101], and CrSi_2(0001)//Si(111) with CrSi_2[1120]//Si[101]. The latter case represents a 30° rotation of the CrSi_2 layer about the Si surface normal relative to the former case. Thick (210 nm) layers were grown by four different techniques, and the bestâquality layer was obtained by codeposition of Cr and Si at an elevated temperature. These layers are not single crystal; the largest grains are observed in a layer grown at 825â°C and are 1â2 ÎŒm across
Vacuum Energy Density and Cosmological Constant in dS Brane World
We discuss the vacuum energy density and the cosmological constant of dS
brane world with a dilaton field. It is shown that a stable AdS brane can
be constructed and gravity localization can be realized. An explicit relation
between the dS bulk cosmological constant and the brane cosmological constant
is obtained. The discrete mass spectrum of the massive scalar field in the
AdS brane is used to acquire the relationship between the brane
cosmological constant and the vacuum energy density. The vacuum energy density
in the brane gotten by this method is in agreement with astronomical
observations.Comment: 16 pages,4 figure
Geometric CP Violation with Extra Dimensions
We discuss how CP symmetry can be broken geometrically through orbifold
projections in hidden extra dimensions in the context of D-brane models for
particle unifications. We present a few toy models to illustrate the idea and
suggest ways to incorporate this technique in the context of realistic models.Comment: 6 pages, one figure; references updated and a new model adde
Perturbative QCD Fragmentation Functions for Production of P-wave Mesons with Charm and Beauty
We calculate the leading order QCD fragmentation functions for the production
of -wave charmed beauty mesons. Long-distance effects are factored into two
nonperturbative parameters: the derivative of the radial wavefunction at the
origin and a second parameter related to the probability for a
heavy quark pair that is produced in a color-octet -wave state to form a
color-singlet -wave bound state. The four states and those states
which lie below the flavor threshold eventually all decay into the
ground state through hadronic cascades or by emitting photons. The total
fragmentation probabilities for production of the ground state from
the cascades of the and states are about and
respectively. Thus the direct production of the -wave
states via fragmentation may account for a significant fraction of the
inclusive production rate of the at large transverse momentum in high
energy colliders. Our analytic results for the -wave fragmentation functions
disagree with those obtained earlier in the literature.Comment: 31 pages, Latex file, 1 figure (postscript file appended at the end
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