68,483 research outputs found

### The Algebra of Strand Splitting. II. A Presentation for the Braid Group on One Strand

Presentations are computed for a braided version BV of Thompson's group V and
for V itself showing that there is an Artin group/Coxeter group relation
between them. The presentation for V is obtained from that for BV by declaring
all that all generators are involutions.Comment: 15 page

### Nonparametric maximum likelihood estimation of probability densities by penalty function methods

When it is known a priori exactly to which finite dimensional manifold the probability density function gives rise to a set of samples, the parametric maximum likelihood estimation procedure leads to poor estimates and is unstable; while the nonparametric maximum likelihood procedure is undefined. A very general theory of maximum penalized likelihood estimation which should avoid many of these difficulties is presented. It is demonstrated that each reproducing kernel Hilbert space leads, in a very natural way, to a maximum penalized likelihood estimator and that a well-known class of reproducing kernel Hilbert spaces gives polynomial splines as the nonparametric maximum penalized likelihood estimates

### Constraints on z~10 Galaxies from the Deepest HST NICMOS Fields

We use all available fields with deep NICMOS imaging to search for J dropouts
(H<28) at z~10. Our primary data set for this search were the two J+H NICMOS
parallel fields taken with the ACS HUDF. The 5 sigma limiting mags were 28.6 in
J and 28.5 in H. Several shallower fields were also used: J+H NICMOS frames
available over the HDF North, the HDF South NICMOS parallel, and the ACS HUDF.
The primary selection criterion was (J-H)>1.8. 11 such sources were found in
all search fields using this criterion. 8 of these were clearly ruled out as
credible z~10 sources, either as a result of detections (>2 sigma) blueward of
J or their colors redward of the break (H-K~1.5). The nature of the 3 remaining
sources could not be determined from the data. The number appears consistent
with the expected contamination from low-z interlopers. Analysis of the stacked
images for the 3 candidates also suggests contamination. Regardless of their
true redshifts, the actual number of z~10 sources must be <=3. To assess the
significance of these results, two lower redshift samples (a z~3.8 B-dropout
and z~6 i-dropout sample) were projected to z~8-12 using a (1+z)^{-1} size
scaling. They were added to the image frames, and the selection repeated,
giving 15.6 and 4.8 J-dropouts, respectively. This suggests that to the limit
of this probe (0.3 L*) there has been evolution from z~3.8 and possibly from
z~6. This is consistent with the strong evolution already noted at z~6 and
z~7.5 relative to z~3-4. Even assuming that 3 sources from this probe are at
z~10, the rest-frame continuum UV (~1500 A) luminosity density at z~10
(integrated down to 0.3 L*) is just 0.19_{-0.09}^{+0.13}x that at z~3.8 (or
0.19_{-0.10}^{+0.15}x including cosmic variance). However, if none of our
sources is at z~10, this ratio has a 1 sigma upper limit of 0.07. (abridged)Comment: 13 pages, 3 figures, 2 tables, accepted for publication in the
Astrophysical Journal Letter

### Chemical propulsion research at MSFC

Chemical propulsion research reviews at Marshall Space Flight Cente

Recommended from our members

### Deaf and hearing children's picture naming Impact of age of acquisition and language modality on representational gesture

Stefanini, Bello, Caselli, Iverson, & Volterra (2009) reported that Italian 24-36 month old children use a high proportion of representational gestures to accompany their spoken responses when labelling pictures. The two studies reported here used the same naming task with (1) typically developing 24-46-month-old hearing children acquiring English and (2) 24-63-month-old deaf children of deaf and hearing parents acquiring British Sign Language (BSL) and spoken English. In Study 1 children scored within the range of correct spoken responses previously reported, but produced very few representational gestures. However, when they did gesture, they expressed the same action meanings as reported in previous research. The action bias was also observed in deaf children of hearing parents in Study 2, who labelled pictures with signs, spoken words and gestures. The deaf group with deaf parents used BSL almost exclusively with few additional gestures. The function of representational gestures in spoken and signed vocabulary development is considered in relation to differences between native and non-native sign language acquisition

### How does breakup influence the total fusion of $^{6,7}$Li at the Coulomb barrier?

Total (complete + incomplete) fusion excitation functions of $^{6,7}$Li on
$^{59}$Co and $^{209}$Bi targets around the Coulomb barrier are obtained using
a new continuum discretized coupled channel (CDCC) method of calculating
fusion. The relative importance of breakup and bound-state structure effects on
total fusion is particularly investigated. The effect of breakup on fusion can
be observed in the total fusion excitation function. The breakup enhances the
total fusion at energies just around the barrier, whereas it hardly affects the
total fusion at energies well above the barrier. The difference between the
experimental total fusion cross sections for $^{6,7}$Li on $^{59}$Co is notably
caused by breakup, but this is not the case for the $^{209}$Bi target.Comment: 9 pages, 9 figures, Submitted to Phys. Rev.

### Laser cooling in the Penning trap: an analytical model for cooling rates in the presence of an axializing field

Ions stored in Penning traps may have useful applications in the field of
quantum information processing. There are, however, difficulties associated
with the laser cooling of one of the radial motions of ions in these traps,
namely the magnetron motion. The application of a small radio-frequency
quadrupolar electric potential resonant with the sum of the two radial motional
frequencies has been shown to couple these motions and to lead to more
efficient laser cooling. We present an analytical model that enables us to
determine laser cooling rates in the presence of such an 'axializing' field. It
is found that this field leads to an averaging of the laser cooling rates for
the two motions and hence improves the overall laser cooling efficiency. The
model also predicts shifts in the motional frequencies due to the axializing
field that are in qualitative agreement with those measured in recent
experiments. It is possible to determine laser cooling rates experimentally by
studying the phase response of the cooled ions to a near resonant excitation
field. Using the model developed in this paper, we study the expected phase
response when an axializing field is present.Comment: 22 pages, 7 figure

- …