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Cloaking Core-Shell Nanoparticles from Conducting Electrons in Solids

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Cloaking Core-Shell Nanoparticles from Conducting Electrons in Solids Liao, Bolin; Zebarjadi, Mona; Esfarjani, Keivan; Chen, Gang In this Letter, we aim at making nanoparticles embedded in a host semiconductor with a size comparable to electronic wavelengths “invisible” to the electron transport. Inspired by the recent progress made in optics and working within the framework of the expansion of partial waves, we demonstrate that the opposite effects imposed by potential barriers and wells of a core-shell nanoparticle on the phase shifts associated with the scattered electron wave could make the scattering cross section of the first two partial waves vanish simultaneously. We show that this is sufficient to cloak the nanoparticle from being detected by electrons with specific energy in the sense that a total scattering cross section smaller than 0.01% of the physical cross section can be obtained and a 4 orders of magnitude difference in the total scattering cross section can be presented within an energy range of only 40 meV, indicating possible applications of the “electron cloaks” as novel electronic switches and sensors, and in efficient energy harvesting and conversion technologies.

First-Order Phase Transitions in Optical Lattices with Tunable Three-Body Onsite Interaction

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First-Order Phase Transitions in Optical Lattices with Tunable Three-Body Onsite Interaction Safavi-Naini, Arghavan; Stecher, Javier von; Capogrosso-Sansone, Barbara; Rittenhouse, Seth T. We study the two-dimensional Bose-Hubbard model in the presence of a three-body interaction term, both at a mean-field level and via quantum Monte Carlo simulations. The three-body term is tuned by coupling the triply occupied states to a trapped universal trimer. We find that, for a sufficiently attractive three-body interaction, the n=2 Mott lobe disappears and the system displays first-order phase transitions separating the n=1 from the n=3 lobes and the n=1 and n=3 Mott insulator from the superfluid. We also analyze the effect of finite temperature and find that transitions are still of first order at temperatures T∼J, where J is the hopping matrix element.

The Gag protein of the Drosphila telomeric retrotransposon TAHRE collaborates with HeT-A and TART Gags for nuclear localization

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The Gag protein of the Drosphila telomeric retrotransposon TAHRE collaborates with HeT-A and TART Gags for nuclear localization Fuller, Adelaide M.; Cook, Elizabeth G.; Kelley, Kerry J. TAHRE, the least abundant of the three retrotransposons forming telomeres in Drosophila melanogaster, has high sequence similarity to the gag gene and untranslated regions of HeT-A, the most abundant telomere-specific retrotransposon. Despite TAHRE's apparent evolutionary relationship to HeT-A, we find TAHRE Gag cannot locate to telomere-associated “Het dots” unless collaborating with HeT-A Gag. TAHRE Gag is carried into nuclei by HeT-A or TART Gag, but both TART and TAHRE Gags need HeT-A Gag to localize to Het dots. When coexpressed with the appropriate fragment of HeT-A and/or TART Gags, TAHRE Gag multimerizes with either protein. HeT-A and TART Gags form homo- and heteromultimers using a region containing major homology region (MHR) and zinc knuckle (CCHC) motifs, separated by a pre_C2HC motif (motifs common to other retroelements). This region's sequence is strongly conserved among the three telomeric Gags, with precise spacing of conserved residues. Nontelomeric Gags neither interact with the telomeric Gags nor have this conserved spacing. TAHRE Gag is much less able to enter the nucleus by itself than HeT-A or TART Gags. The overall telomeric localization efficiency for each of the three telomeric Gag proteins correlates with the relative abundance of that element in telomere arrays, suggesting an explanation for the relative rarity of TAHRE elements in telomere arrays and supporting the hypothesis that Gag targeting to telomeres is important for the telomere-specific transposition of these elements.

Philosophers Ponder the Afterlife

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Philosophers Ponder the Afterlife Byrne, Alex Star Trek–style teleportation may one day become a reality. You step into the transporter, which instantly scans your body and brain, vaporizing them in the process. The information is transmitted to Mars, where it is used by the receiving station to reconstitute your body and brain exactly as they were on Earth. You then step out of the receiving station, slightly dizzy, but pleased to arrive on Mars in a few minutes, as opposed to the year it takes by old-fashioned spacecraft.

Breakdown in the Wetting Transparency of Graphene

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Breakdown in the Wetting Transparency of Graphene Shih, Chih-Jen; Wang, Qing Hua; Lin, Shangchao; Park, Kyoo Chul; Jin, Zhong; Strano, Michael S.; Blankschtein, Daniel We develop a theory to model the van der Waals interactions between liquid and graphene, including quantifying the wetting behavior of a graphene-coated surface. Molecular dynamics simulations and contact angle measurements were also carried out to test the theory. We show that graphene is only partially transparent to wetting and that the predicted highest attainable contact angle of water on a graphene-coated surface is 96°. Our findings reveal a more complex picture of wetting on graphene than what has been reported recently as complete “wetting transparency.”

Sloppy Models, Parameter Uncertainty, and the Role of Experimental Design

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Sloppy Models, Parameter Uncertainty, and the Role of Experimental Design Apgar, Joshua F.; Witmer, David K.; White, Forest M.; Tidor, Bruce Computational models are increasingly used to understand and predict complex biological phenomena. These models contain many unknown parameters, at least some of which are difficult to measure directly, and instead are estimated by fitting to time-course data. Previous work has suggested that even with precise data sets, many parameters are unknowable by trajectory measurements. We examined this question in the context of a pathway model of epidermal growth factor (EGF) and neuronal growth factor (NGF) signaling. Computationally, we examined a palette of experimental perturbations that included different doses of EGF and NGF as well as single and multiple gene knockdowns and overexpressions. While no single experiment could accurately estimate all of the parameters, experimental design methodology identified a set of five complementary experiments that could. These results suggest optimism for the prospects for calibrating even large models, that the success of parameter estimation is intimately linked to the experimental perturbations used, and that experimental design methodology is important for parameter fitting of biological models and likely for the accuracy that can be expected from them.

Quantum Time Crystals

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Quantum Time Crystals Wilczek, Frank Some subtleties and apparent difficulties associated with the notion of spontaneous breaking of time-translation symmetry in quantum mechanics are identified and resolved. A model exhibiting that phenomenon is displayed. The possibility and significance of breaking of imaginary time-translation symmetry is discussed.

Classical Time Crystals

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Classical Time Crystals Shapere, Alfred; Wilczek, Frank We consider the possibility that classical dynamical systems display motion in their lowest-energy state, forming a time analogue of crystalline spatial order. Challenges facing that idea are identified and overcome. We display arbitrary orbits of an angular variable as lowest-energy trajectories for nonsingular Lagrangian systems. Dynamics within orbits of broken symmetry provide a natural arena for formation of time crystals. We exhibit models of that kind, including a model with traveling density waves.

A new layout optimization technique for interferometric arrays and the final MWA antenna layout

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A new layout optimization technique for interferometric arrays and the final MWA antenna layout Capallo, R.J.; Lonsdale, Colin John; Corey, Brian E.; Kratzenberg, Eric; McWhirter, Stephen R.; Oberoi, Divya; Rogers, Alan E. E.; Salah, Joseph E.; Whitney, Alan R.; Goeke, Robert F.; Hewitt, Jacqueline N.; Morgan, Edward H.; Remillard, Ronald Alan; Williams, Christopher Leigh Antenna layout is an important design consideration for radio interferometers because it determines the quality of the snapshot point spread function (PSF, or array beam). This is particularly true for experiments targeting the 21-cm Epoch of Reionization signal as the quality of the foreground subtraction depends directly on the spatial dynamic range and thus the smoothness of the baseline distribution. Nearly all sites have constraints on where antennas can be placed – even at the remote Australian location of the Murchison Widefield Array (MWA) there are rock outcrops, flood zones, heritages areas, emergency runways and trees. These exclusion areas can introduce spatial structure into the baseline distribution that enhances the PSF sidelobes and reduces the angular dynamic range. In this paper we present a new method of constrained antenna placement that reduces the spatial structure in the baseline distribution. This method not only outperforms random placement algorithms that avoid exclusion zones, but surprisingly outperforms random placement algorithms without constraints to provide what we believe are the smoothest constrained baseline distributions developed to date. We use our new algorithm to determine antenna placements for the originally planned MWA, and present the antenna locations, baseline distribution and snapshot PSF for this array choice.

Fast holographic deconvolution: a new technique for precision radio interferometry

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Fast holographic deconvolution: a new technique for precision radio interferometry Goeke, Robert F.; Hewitt, Jacqueline N.; Morgan, Edward H.; Remillard, Ronald Alan; Williams, Christopher Leigh We introduce the Fast Holographic Deconvolution method for analyzing interferometric radio data. Our new method is an extension of A-projection/software-holography/forward modeling analysis techniques and shares their precision deconvolution and wide-field polarimetry, while being significantly faster than current implementations that use full direction-dependent antenna gains. Using data from the MWA 32 antenna prototype, we demonstrate the effectiveness and precision of our new algorithm. Fast Holographic Deconvolution may be particularly important for upcoming 21 cm cosmology observations of the Epoch of Reionization and Dark Energy where foreground subtraction is intimately related to the precision of the data reduction.

Chandra Evidence of a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720

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Chandra Evidence of a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720 Jeltema, Tesla E.; Canizares, Claude R.; Garmire, Gordon P.; Buote, David A. We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720, to verify the existence of a dark matter halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources distributed throughout the field, most of which were undetected and therefore unaccounted for in previous X-ray studies. For semimajor axes a [ over ~]150" out to a = 185" (22.4 h[-1 over 70] kpc), which is near the edge of the S3 CCD, ϵ[subscript X] and P.A. diverge from their values at smaller a. Possible origins of this behavior at the largest a are discussed. Overall, the ellipticities and P.A. twist for a [

X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications

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X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications Schulz, Norbert S.; Canizares, Claude R.; Huenemoerder, David P.; Tibbetts, Kevin The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of ~3 × 10[superscript 5] yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 Å of three of its X-ray-brightest members (Θ1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe XVII to Fe XXV ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of Θ[superscript 1] Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day wind period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km [superscript s-1] (all widths are stated as half-width at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s[superscript -1]. The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for Θ[superscript 1] Ori E and 4.8-42.7 MK for Θ[superscript 1] Ori A. The high-temperature emissivity contributes over 70% to the total X-ray flux. None of the lines are resolved for Θ[superscript 1] Ori A and E with a confidence limit of 160 km s[superscript -1]. The influence of the strong UV radiation field on the forbidden line in the He-like triplets allows us to set an upper limit on distance of the line-emitting region from the photosphere. The bulk of the X-ray emission cannot be produced by shock instabilities in a radiation-driven wind and are likely the result of magnetic confinement in all three stars. Although confinement models cannot explain all the results, the resemblance of the unresolved lines and of the DEM with recent observations of active coronae in II Peg and AR Lac during flares is quite obvious. Thus we speculate that the X-ray production mechanism in these stars is similar, with the difference that the Orion stars may be in a state of almost continuous flaring driven by the wind. We clearly rule out major effects due to X-rays from a possible companion. The fact that all three stars appear to be magnetic and are near zero age on the main sequence also raises the issue of whether the Orion stars are simply different or whether young massive stars enter the main sequence carrying significant magnetic fields. The ratio log L[subscript X]/L[subscript bol] using the "wind" component of the spectrum is -7 for the Trapezium stars, consistent with the expectation from O stars. This suggests that massive ZAMS stars generate their X-ray luminosities like normal O stars and magnetic confinement provides an additional source of X-rays.

Etching of Graphene Devices with a Helium Ion Beam

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Etching of Graphene Devices with a Helium Ion Beam Baugher, Britton William Herb; Jarillo-Herrero, Pablo; Lemme, Max C.; Bell, David C.; Williams, James R.; Stern, Lewis A.; Marcus, Charles M. We report on the etching of graphene devices with a helium ion beam, including in situ electrical measurement during lithography. The etching process can be used to nanostructure and electrically isolate different regions in a graphene device, as demonstrated by etching a channel in a suspended graphene device with etched gaps down to about 10 nm. Graphene devices on silicon dioxide (SiO[subscript 2]) substrates etch with lower He ion doses and are found to have a residual conductivity after etching, which we attribute to contamination by hydrocarbons.

Differential Maintenance of DNA Sequences in Telomeric and Centromeric Heterochromatin

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Differential Maintenance of DNA Sequences in Telomeric and Centromeric Heterochromatin DeBaryshe, P. G.; Pardue, Mary-Lou Repeated DNA in heterochromatin presents enormous difficulties for whole-genome sequencing; hence, sequence organization in a significant portion of the genomes of multicellular organisms is relatively unknown. Two sequenced BACs now allow us to compare telomeric retrotransposon arrays from Drosophila melanogaster telomeres with an array of telomeric retrotransposons that transposed into the centromeric region of the Y chromosome >13 MYA, providing a unique opportunity to compare the structural evolution of this retrotransposon in two contexts. We find that these retrotransposon arrays, both heterochromatic, are maintained quite differently, resulting in sequence organizations that apparently reflect different roles in the two chromosomal environments. The telomere array has grown only by transposition of new elements to the chromosome end; the centromeric array instead has grown by repeated amplifications of segments of the original telomere array. Many elements in the telomere have been variably 5′-truncated apparently by gradual erosion and irregular deletions of the chromosome end; however, a significant fraction (4 and possibly 5 or 6 of 15 elements examined) remain complete and capable of further retrotransposition. In contrast, each element in the centromere region has lost ≥40% of its sequence by internal, rather than terminal, deletions, and no element retains a significant part of the original coding region. Thus the centromeric array has been restructured to resemble the highly repetitive satellite sequences typical of centromeres in multicellular organisms, whereas, over a similar or longer time period, the telomere array has maintained its ability to provide retrotransposons competent to extend telomere ends.

Adapting to life at the end of the line: How Drosophila telomeric retrotransposons cope with their job

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Adapting to life at the end of the line: How Drosophila telomeric retrotransposons cope with their job Pardue, Mary-Lou; DeBaryshe, P. G. Drosophila telomeres are remarkable because they are maintained by telomere-specific retrotransposons, rather than the enzyme telomerase that maintains telomeres in almost every other eukaryotic organism. Successive transpositions of the Drosophila retrotransposons onto chromosome ends produce long head-to-tail arrays that are analogous in form and function to the long arrays of short repeats produced by telomerase in other organisms. Nevertheless, Drosophila telomere repeats are retrotransposons, complex entities three orders of magnitude longer than simple telomerase repeats. During the >40–60 My they have been coevolving with their host, these retrotransposons perforce have evolved a complex relationship with Drosophila cells to maintain populations of active elements while carrying out functions analogous to those of telomerase repeats in other organisms. Although they have assumed a vital role in maintaining the Drosophila genome, the three Drosophila telomere-specific elements are non-LTR retrotransposons, closely related to some of the best known non-telomeric elements in the Drosophila genome. Thus, these elements offer an opportunity to study ways in which retrotransposons and their host cells can coevolve cooperatively. The telomere-specific elements display several characteristics that appear important to their roles at the telomere; for example, we have recently reported that they have evolved at least two innovative mechanisms for protecting essential sequence on their 5' ends. Because every element serves as the end of the chromosome immediately after it transposes, its 5'end is subject to chromosomal erosion until it is capped by a new transposition. These two mechanisms make it possible for at least a significant fraction of elements to survive their initial time as the chromosome end without losing sequence necessary to be competent for subsequent transposition. Analysis of sequence from >90 kb of assembled telomere array shows that these mechanisms for small scale sequence protection are part of a unified set which maintains telomere length homeostasis. Here we concentrate on recently elucidated mechanisms that have evolved to provide this small scale 5’ protection.

Drosophila telomeres: A variation on the telomerase theme

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Drosophila telomeres: A variation on the telomerase theme Pardue, Mary-Lou; DeBaryshe, P. G. In Drosophila, the role of telomerase is carried out by three specialized retrotransposable elements, HeT-A, TART, and TAHRE. Telomeres contain long tandem head-to-tail arrays of these elements. Within each array, the three elements occur in random, but polarized, order. Some are truncated at the 5’ end, giving the telomere an enriched content of the large 3’ untranslated regions which distinguish these telomeric elements from other retrotransposons. Thus, Drosophila telomeres resemble other telomeres because they are long arrays of repeated sequences, albeit more irregular arrays than those produced by telomerase. The telomeric retrotransposons are reverse-transcribed directly onto the end of the chromosome, extending the end by successive transpositions. Their transposition uses exactly the same method by which telomerase extends chromosome ends - copying an RNA template. In addition to these similarities in structure and maintenance, Drosophila telomeres have strong functional similarities to other telomeres, and, as variants, provide an important model for understanding general principles of telomere function and evolution.

Onset Mechanism of Strain-Rate-Induced Flow Stress Upturn

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Onset Mechanism of Strain-Rate-Induced Flow Stress Upturn Fan, Yue; Osetsky, Yuri N.; Yip, Sidney; Yildiz, Bilge The strain-rate response of flow stress in a plastically deforming crystal is formulated through a stress-sensitive dislocation mobility model that can be evaluated by atomistic simulation. For the flow stress of a model crystal of bcc Fe containing a [1 over 2] {111} screw dislocation, this approach describes naturally a non-Arrhenius upturn at high strain rate, an experimentally established transitional behavior for which the underlying mechanism has not been clarified. Implications of our findings regarding the previous explanations of strain-rate effects on flow stress are discussed.

Mechanistic studies of Gemcitabine-loaded nanoplatforms in resistant pancreatic cancer cells

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Mechanistic studies of Gemcitabine-loaded nanoplatforms in resistant pancreatic cancer cells Papa, Anne-Laure; Basu, Sudipta; Sengupta, Poulomi; Banerjee, Deboshri; Sengupta, Shiladitya; Harfouche, Rania Background: Pancreatic cancer remains the deadliest of all cancers, with a mortality rate of 91%. Gemcitabine is considered the gold chemotherapeutic standard, but only marginally improves life-span due to its chemical instability and low cell penetrance. A new paradigm to improve Gemcitabine’s therapeutic index is to administer it in nanoparticles, which favour its delivery to cells when under 500 nm in diameter. Although promising, this approach still suffers from major limitations, as the choice of nanovector used as well as its effects on Gemcitabine intracellular trafficking inside pancreatic cancer cells remain unknown. A proper elucidation of these mechanisms would allow for the elaboration of better strategies to engineer more potent Gemcitabine nanotherapeutics against pancreatic cancer. Methods: Gemcitabine was encapsulated in two types of commonly used nanovectors, namely poly(lactic-co-glycolic acid) (PLGA) and cholesterol-based liposomes, and their physico-chemical parameters assessed in vitro. Their mechanisms of action in human pancreatic cells were compared with those of the free drug, and with each others, using cytotoxity, apoptosis and ultrastructural analyses. Results: Physico-chemical analyses of both drugs showed high loading efficiencies and sizes of less than 200 nm, as assessed by dynamic light scattering (DLS) and transmission electron microscopy (TEM), with a drug release profile of at least one week. These profiles translated to significant cytotoxicity and apoptosis, as well as distinct intracellular trafficking mechanisms, which were most pronounced in the case of PLGem showing significant mitochondrial, cytosolic and endoplasmic reticulum stresses. Conclusions: Our study demonstrates how the choice of nanovector affects the mechanisms of drug action and is a crucial determinant of Gemcitabine intracellular trafficking and potency in pancreatic cancer settings.

Reply to Comment on “Sloppy models, parameter uncertainty, and the role of experimental design"

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Reply to Comment on “Sloppy models, parameter uncertainty, and the role of experimental design" Hagen, David Robert; Apgar, Joshua F.; Witmer, David K.; White, Forest M.; Tidor, Bruce We welcome the commentary from Chachra, Transtrum, and Sethna1 regarding our paper“Sloppy models, parameter uncertainty, and the role of experimental design,”2 as their intriguing work shaped our thinking in this area.3 Sethna and colleagues introduced the notion of sloppy models, in which the uncertainty in the values of some combinations of parameters is many orders of magnitude greater than others.4 In our work we explored the extent to which large parameter uncertainties are an intrinsic characteristic of systems biology network models, or whether uncertainties are instead closely related to the collection of experiments used for model estimation. We were gratified to find the latter result –– that parameters are in principle knowable, which is important for the field of systems biology. The work also showed that small parameter uncertainties can be achieved and that the process can be greatly accelerated by using computational experimental design approaches5–9 deployed to select sets of experiments that effectively exercise the system in complementary directions. available in PMC 2012 November 10.

Exclusive measurements of b→sγ transition rate and photon energy spectrum

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Exclusive measurements of b→sγ transition rate and photon energy spectrum Dujmic, Denis; Sciolla, Gabriella We use 429  fb[superscript -1] of e[superscript +]e[superscript -] collision data collected at the Υ(4S) resonance with the BABAR detector to measure the radiative transition rate of b→sγ with a sum of 38 exclusive final states. The inclusive branching fraction with a minimum photon energy of 1.9 GeV is found to be B(B̅ →X[subscript s]γ)=(3.29±0.19±0.48)×10[superscript -4] where the first uncertainty is statistical and the second is systematic. We also measure the first and second moments of the photon energy spectrum and extract the best-fit values for the heavy-quark parameters, m[subscript b] and μ[subscript π][superscript 2], in the kinetic and shape function models.
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