Search

Dark matter which is bound in the Galactic halo might self-annihilate and produce a flux of stable final state particles, e.g. high energy neutrinos. These neutrinos can be detected with IceCube, a cubic-kilometer sized Cherenkov detector. Given IceCube’s large field of view, a characteristic anisotropy of the additional neutrino flux is expected. In this paper we describe a multipole method to search for such a large-scale anisotropy in IceCube data. This method uses the expansion coefficients of a multipole expansion of neutrino arrival directions and incorporates signal-specific weights for each expansion coefficient. We apply the technique to a high-purity muon neutrino sample from the Northern Hemisphere. The final result is compatible with the null-hypothesis. As no signal was observed, we present limits on the self-annihilation cross-section averaged over the relative velocity distribution (Formula Presented.) down to ([Formula Presented.) for a dark matter particle mass of 700–1,000 GeV and direct annihilation into (Formula Presented.). The resulting exclusion limits come close to exclusion limits from γ-ray experiments, that focus on the outer Galactic halo, for high dark matter masses of a few TeV and hard annihilation channels., 0, SCOPUS: ar.j, info:eu-repo/semantics/published

Hyperuniformity characterizes a state of matter for which density fluctuations diminish towards zero at the largest length scales. However, the task of determining whether or not an experimental system is hyperuniform is experimentally challenging due to finite-resolution, noise and sample-size effects that influence characterization measurements. Here we explore these issues, employing video optical microscopy to study hyperuniformity phenomena in disordered two-dimensional jammed packings of soft spheres. Using a combination of experiment and simulation we characterize the detrimental effects of particle polydispersity, image noise, and finite-size effects on the assignment of hyperuniformity, and we develop a methodology that permits improved diagnosis of hyperuniformity from real-space measurements. The key to this improvement is a simple packing reconstruction algorithm that incorporates particle polydispersity to minimize free volume. In addition, simulations show that hyperuniformity can be ascertained more accurately in direct space than in reciprocal space as a result of finite sample-size. Finally, experimental colloidal packings of soft polymeric spheres are shown to be hyperuniform., Comment: 12 pages, 9 figures

We have measured the bulk optical properties of healthy female breast tissues in vivo in the parallel plate, transmission geometry. Fifty-two volunteers were measured. Blood volume and blood oxygen saturation were derived from the optical property data using a novel method that employed a priori spectral information to overcome limitations associated with simple homogeneous tissue models. The measurements provide an estimate of the variation of normal breast tissue optical properties in a fairly large population. The mean blood volume was 34 +/- 9 microM and the mean blood oxygen saturation was 68 +/- 8%. We also investigated the correlation of these optical properties with demographic factors such as body mass index (BMI) and age. We observed a weak correlation of blood volume and reduced scattering coefficient with BMI: correlation with age, however, was not evident within the statistical error of these experiments. The new information on healthy breast tissue provides insight about the potential contrasts available for diffuse optical tomography of breast tumours.

We have incorporated nematic liquid crystal into periodic, polymer host structures templated from self-assembled colloids. Using these composite materials, we demonstrate the first electrically switchable three-dimensional Bragg diffraction. The switchable beam deflection is potentially useful for non-mechanical beam steering and optical beam splitting devices. We compare the electro-optic response of our templated liquid-crystal/polymer composites with conventional polymer-dispersed liquid crystals (PDLCs). Our data reveal a qualitatively different and faster response for liquid crystal distributed within a connected cavity network, as compared to isolated liquid-crystal droplets within a polymer matrix.

We present the clinical implementation of a novel hybrid system that combines magnetic resonance imaging (MRI) and near-infrared (NIR) optical measurements for the noninvasive study of breast cancer in vivo. Fourteen patients were studied with a MR-NIR prototype imager and spectrometer. A diffuse optical tomographic scheme employed the MR images as a priori information to implement an image-guided NIR localized spectroscopic scheme. All patients who entered the study also underwent gadolinium-enhanced MRI and biopsy so that the optical findings were cross-validated with MR readings and histopathology. The technique quantified the oxy- and deoxyhemoglobin of five malignant and nine benign breast lesions in vivo. Breast cancers were found with decreased oxygen saturation and higher blood concentration than most benign lesions. The average hemoglobin concentration ([H]) of cancers was 0.130+/-0.100 mM, and the average hemoglobin saturation (Y) was 60+/-9% compared to [H]=0.018+/-0.005 mM and Y=69+/-6% of background tissue. Fibroadenomas exhibited high hemoglobin concentration [H]=0.060+/-0.010 mM and mild decrease in oxygen saturation Y=67+/-2%. Cysts and other normal lesions were easily differentiated based on intrinsic contrast information. This novel optical technology can be a significant add-on in MR examinations and can be used to characterize functional parameters of cancers with diagnostic and treatment prognosis potential. It is foreseen that the technique can play a major role in functional activation studies of brain and muscle as well.

We investigate the vibrational modes of quasi-two-dimensional disordered colloidal packings of hard colloidal spheres with short-range attractions as a function of packing fraction. Certain properties of the vibrational density of states (vDOS) are shown to correlate with the density and structure of the samples (i.e., in sparsely versus densely packed samples). Specifically, a crossover from dense glassy to sparse gel-like states is suggested by an excess of phonon modes at low frequency and by a variation in the slope of the vDOS with frequency at low frequency. This change in phonon mode distribution is demonstrated to arise largely from localized vibrations that involve individual and/or small clusters of particles with few local bonds. Conventional order parameters and void statistics did not exhibit obvious gel-glass signatures as a function of volume fraction. These mode behaviors and accompanying structural insights offer a potentially new set of indicators for identification of glass-gel transitions and for assignment of gel-like versus glass-like character to a disordered solid material.

We thank Aries et al1 for their interest in on our study.2 We agree that it is too soon to use diffusion correlation spectroscopy (DCS) to guide clinical management, but our experience with this technology suggests that it has strong potential for clinical monitoring of brain perfusion. The key findings of our study were that DCS detected significant head-of-bed (HOB)-induced changes in cerebral perfusion, whereas middle cerebral artery transcranial Doppler (TCD) velocity changes were not significant. Moreover, considerable heterogeneity in HOB response was found. Aries et al1 questioned the lack of observed TCD changes in our data, noting that our study monitored patients for only 5 minutes in each position, which might not be sufficient to capture a TCD changes. One prior study monitored for 15 minutes at each HOB angle,3 but …

Microvascular blood flow contrast is an important hemodynamic and metabolic parameter with potential to enhance in vivo breast cancer detection and therapy monitoring. Here we report on non-invasive line-scan measurements of malignant breast tumors with a hand-held optical probe in the remission geometry. The probe employs diffuse correlation spectroscopy (DCS), a near-infrared optical method that quantifies deep tissue microvascular blood flow. Tumor-to-normal perfusion ratios are derived from thirty-two human subjects. Mean (95% confidence interval) tumor-to-normal ratio using surrounding normal tissue was 2.25 (1.92-2.63); tumor-to-normal ratio using normal tissues at the corresponding tumor location in the contralateral breast was 2.27 (1.94-2.66), and using normal tissue in the contralateral breast was 2.27 (1.90-2.70). Thus, the mean tumor-to-normal ratios were significantly different from unity irrespective of the normal tissue chosen, implying that tumors have significantly higher blood flow than normal tissues. Therefore, the study demonstrates existence of breast cancer contrast in blood flow measured by DCS. The new, optically accessible cancer contrast holds potential for cancer detection and therapy monitoring applications, and it is likely to be especially useful when combined with diffuse optical spectroscopy/tomography.

This review collects and describes experiments that employ colloidal suspensions to probe physics in ordered and disordered solids and related complex fluids. The unifying feature of this body of work is its clever usage of poly(N-isopropylacrylamide) (PNIPAM) microgel particles. These temperature-sensitive colloidal particles provide experimenters with a 'knob' for in situ control of particle size, particle interaction and particle packing fraction that, in turn, influence the structural and dynamical behavior of the complex fluids and solids. A brief summary of PNIPAM particle synthesis and properties is given, followed by a synopsis of current activity in the field. The latter discussion describes a variety of soft matter investigations including those that explore formation and melting of crystals and clusters, and those that probe structure, rearrangement and rheology of disordered (jammed/glassy) and partially ordered matter. The review, therefore, provides a snapshot of a broad range of physics phenomenology which benefits from the unique properties of responsive microgel particles.

We report on the search for neutrino-induced particle-showers, so-called cascades, in the IceCube-40 detector. The data for this search was collected between April 2008 and May 2009 when the first 40 IceCube strings were deployed and operational. Three complementary searches were performed, each optimized for different energy regimes. The analysis with the lowest energy threshold (2 TeV) targeted atmospheric neutrinos. A total of 67 events were found, consistent with the expectation of 41 atmospheric muons and 30 atmospheric neutrino events. The two other analyses targeted a harder, astrophysical neutrino flux. The analysis with an intermediate threshold of 25 TeV lead to the observation of 14 cascade-like events, again consistent with the prediction of 3.0 atmospheric neutrino and 7.7 atmospheric muon events. We hence set an upper limit of $E^2 \Phi_{lim} \leq 7.46\times10^{-8}\,\mathrm{GeV sr^{-1} s^{-1} cm^{-2}}$ (90% C.L.) on the diffuse flux from astrophysical neutrinos of all neutrino flavors, applicable to the energy range 25 TeV to 5 PeV, assuming an $E_{\nu}^{-2}$ spectrum and a neutrino flavor ratio of 1:1:1 at the Earth. The third analysis utilized a larger and optimized sample of atmospheric muon background simulation, leading to a higher energy threshold of 100 TeV. Three events were found over a background prediction of 0.04 atmospheric muon events and 0.21 events from the flux of conventional and prompt atmospheric neutrinos. Including systematic errors this corresponds to a $2.7\sigma$ excess with respect to the background-only hypothesis. Our observation of neutrino event candidates above 100 TeV complements IceCube's recently observed evidence for high-energy astrophysical neutrinos., Comment: 19 pages, 13 figures, submitted to Phys. Rev. D

We explore micropatterned director structures of aqueous lyotropic chromonic liquid crystal (LCLC) films created on square-lattice cylindrical-micropost substrates. The structures are manipulated by modulating the LCLC mesophases and their elastic properties via concentration through drying. Nematic LCLC films exhibit preferred bistable alignment along the diagonals of the micropost lattice. Columnar LCLC films, dried from nematics, form two distinct director and defect configurations: a diagonally aligned director pattern with local squares of defects, and an off-diagonal configuration with zig-zag defects. The formation of these states appears to be tied to the relative splay and bend free energy costs of the initial nematic films. The observed nematic and columnar configurations are understood numerically using a Landau-de Gennes free energy model. Among other attributes, the work provide first examples of quasi-2D micropatterning of LC films in the columnar phase and lyotropic LC films in general, and it demonstrates alignment and configuration switching of typically difficult-to-align LCLC films via bulk elastic properties.

Background and Purpose— A primary goal of acute ischemic stroke (AIS) management is to maximize perfusion in the affected region and surrounding ischemic penumbra. However, interventions to maximize perfusion, such as flat head-of-bed (HOB) positioning, are currently prescribed empirically. Bedside monitoring of cerebral blood flow (CBF) allows the effects of interventions such as flat HOB to be monitored and may ultimately be used to guide clinical management. Methods— Cerebral perfusion was measured during HOB manipulations in 17 patients with unilateral AIS affecting large cortical territories in the anterior circulation. Simultaneous measurements of frontal CBF and arterial flow velocity were performed with diffuse correlation spectroscopy and transcranial Doppler ultrasound, respectively. Results were analyzed in the context of available clinical data and a previous study. Results— Frontal CBF, averaged over the patient cohort, decreased by 17% ( P =0.034) and 15% ( P =0.011) in the ipsilesional and contralesional hemispheres, respectively, when HOB was changed from flat to 30°. Significant (cohort-averaged) changes in blood velocity were not observed. Individually, varying responses to HOB manipulation were observed, including paradoxical increases in CBF with increasing HOB angle. Clinical features, stroke volume, and distance to the optical probe could not explain this paradoxical response. Conclusions— A lower HOB angle results in an increase in cortical CBF without a significant change in arterial flow velocity in AIS, but there is variability across patients in this response. Bedside CBF monitoring with diffuse correlation spectroscopy provides a potential means to individualize interventions designed to optimize CBF in AIS.

Objective Hypoxic-ischemic white mater brain injury commonly occurs in neonates with hypoplastic left heart syndrome (HLHS). Approximately one half of HLHS survivors will exhibit neurobehavioral symptoms believed to be associated with this injury, although the exact timing of the injury is unknown. Methods Neonates with HLHS were recruited for pre- and postoperative monitoring of cerebral oxygen saturation, cerebral oxygen extraction fraction, and cerebral blood flow using 2 noninvasive optical-based techniques: diffuse optical spectroscopy and diffuse correlation spectroscopy. Anatomic magnetic resonance imaging was performed before and approximately 1 week after surgery to quantify the extent and timing of the acquired white matter injury. The risk factors for developing new or worsened white matter injury were assessed using uni- and multivariate logistic regression. Results A total of 37 neonates with HLHS were studied. On univariate analysis, neonates who developed a large volume of new, or worsened, postoperative white matter injury had a significantly longer time to surgery ( P = .0003). In a multivariate model, a longer time between birth and surgery, delayed sternal closure, and greater preoperative cerebral blood flow were predictors of postoperative white matter injury. Additionally, a longer time to surgery and greater preoperative cerebral blood flow on the morning of surgery correlated with lower cerebral oxygen saturation ( P = .03 and P = .05, respectively) and greater oxygen extraction fraction ( P = .05 for both). Conclusions A longer time to surgery was associated with new postoperative white matter injury in otherwise healthy neonates with HLHS. The results suggest that earlier Norwood palliation might decrease the likelihood of acquiring postoperative white matter injury.

Cerebral oxygen extraction, defined as the difference between arterial and venous oxygen saturations (SaO2 and SvO2), is a critical parameter for managing intensive care patients at risk for neurological collapse. Although quantification of SaO2 is easily performed with pulse oximetry or moderately invasive arterial blood draws in peripheral vessels, cerebral SvO2 is frequently not monitored because of the invasiveness and risk associated with obtaining jugular bulb or super vena cava (SVC) blood samples.In this study, near-infrared spectroscopy (NIRS) was used to noninvasively measure cerebral SvO2 in anesthetized and mechanically ventilated pediatric patients (n = 10). To quantify SvO2, the NIRS signal component that fluctuates at the respiration frequency is isolated. This respiratory component is dominated by the venous portion of the interrogated vasculature. The NIRS measurements of SvO2 were validated against the clinical gold standard: invasively measured oxygen saturations from SVC blood samples. This technique was also applied in healthy volunteers (n = 5) without mechanical ventilation to illustrate its potential for use in healthy populations with natural airways.Ten pediatric patients with pulmonary hypertension were studied. In these patients, SvO2 in the SVC exhibited good agreement with NIRS-measured SvO2 (R(2) = 0.80, P = .001, slope = 1.16 ± 0.48). Furthermore, in the healthy adult volunteers, mean (standard deviation) NIRS-measured SvO2 was 79.4 (6.8)%. This value is in good agreement with the expected average central venous saturation reported in literature.Respiration frequency-selected NIRS can noninvasively quantify cerebral SvO2. This bedside technique can be used to help assess brain health in neurologically unstable patients.

Concatameric 5 S rDNA templates reconstituted in vitro into nucleosomal arrays provide very popular chromatin models for many kinds of studies. Here, atomic force microscopy is used to determine the population distributions for one such nucleosomal array, the 172-12, reconstituted to various subsaturated levels with nonacetylated or hyperacetylated HeLa histones. This array is a model for short linker length genomes and transcriptionally active and newly replicated chromatins. The analysis shows that as input histone levels increase, template occupation increases progressively as discrete population distributions. The distributions are random at low (n(av)4) and high (n(av)8) loadings but display specific nonrandom features, such as a deficit of molecules with one nucleosome more or less than the peak species in the distribution and enhanced distribution breadths, in the mid-range (n(av) = 4-8). Thus, the mid-range of occupation on polynucleosomal arrays may be a special range for chromatin structure and/or assembly. The mid-range nonrandom features are enhanced in distributions from short repeat (172-12) arrays, particularly for unacetylated chromatin, and in distributions from hyperacetylated chromatin, particularly for long repeat (208-12) arrays. Thus, short repeat length and acetylation can affect basic chromatin properties, like population tendencies, in very similar ways and therefore may cause similar changes in chromatin structure. Some possible effects are suggested. The data also indicate that it is thermodynamically more difficult for hyperacetylated nucleosomes to assemble onto the 172-12 templates, a result having implications for in vivo chromatin assembly.

In this paper we present fundamental studies elucidating the role of entropy in parti- cle suspensions. We focus on systems composed of large colloidal particles along with a second, usually smaller species such as a particle or polymer. We describe direct measurements of these interactions in suspension, and we systematically show how these forces can be used to control the self-assembly of colloidal particles. The paper provides a unified review of the experiments from our laboratory, and in a few cases touches on very recent results. Entropic forces between macromolecules in suspension are sometimes produced by the addition of other constituents to the background solvent. These added con- stituents are often other (usually smaller) particles, rods or polymers. The resulting entropic forces influence suspension stability, and are of considerable importance in a wide variety of practical materials ranging from frozen desserts to paints to motor oils to living cells. During the last few years we have been studying entropic effects in suspension. We have quantified these forces by direct measurement in a variety of complex fluids, we have identified some novel manifestations of these entropic phe- nomena, and we have used these effects to control the self-assembly of particles. In this paper we provide a unified review of the experiments from our laboratory, and in a few cases touch on very recent results. The starting point of our exposition naturally begins with mixtures of large- and small-diameter hard spheres. Hard-sphere colloids lack attractive and long-range interactions, which typically compete with entropic effects to produce ordered phases. Nonetheless, as Asakura & Oosawa (1958) first noted, in mixtures of different-sized spherical particles, an ordered arrangement of large spheres can increase the total entropy of the system by increasing the entropy of the small spheres. This phe- nomenon is depicted in figure 1. Because the centre of mass of the small sphere

We present quantitative optical images of human breastin vivo. The images were obtained by using near-infrared diffuse optical tomography (DOT) after the administration of indocyanine green (ICG) for contrast enhancement. The optical examination was performed concurrently with a magnetic resonance imaging (MRI) exam on patients scheduled for excisional biopsy or surgery so that accurate image coregistration and histopathological information of the suspicious lesions was available. The ICG-enhanced optical images coregistered accurately with Gadolinium-enhanced magnetic resonance images validating the ability of DOT to image breast tissue. In contrast to simple transillumination, we found that DOT provides for localization and quantification of exogenous tissue chromophore concentrations. Additionally our use of ICG, an albumin bound absorbing dye in plasma, demonstrates the potential to differentiate disease based on the quantified enhancement of suspicious lesions.

Genomic processes like transcription initiation typically involve the alteration of nucleosome structure, to expose DNA elements for regulatory factor binding. Nucleosome altering/modifying complexes have been identified, but precisely how these complexes find their specific targets remains unclear. We have shown that nucleosomes can exhibit significant DNA sequence-dependent stability and dynamics variations and have suggested that these inherent variations could facilitate nucleosome recognition and thus aid in specific targeting. Here, we confirm an important prediction of the model, namely, that stability and DNA dynamics features can correlate with the transcriptional involvement of specific promoter nucleosomes. A transcriptionally inert Mouse Mammary Tumor Virus promoter-region nucleosome (MMTV-D) has greater inherent stability than and reduced dynamics compared to a nearby nucleosome (MMTV-B) that is the initial target of transcription activation-associated processes on this promoter. MMTV-D stability could help direct activation-associated events to the less stable and more dynamic target, MMTV-B.

We explore the depletion attractions that arise between hard colloidal spheres immersed in a nonadsorbing polymer solution of DNA. We spatially confine two 1.25 μm silica spheres in a scanning optical tweezer and quantitatively examine their interaction potential in double-stranded DNA solutions of different concentrations. The potentials obtained display variations in depth and range that are consistent with scaling behavior expected for semiflexible polymers near the θ-point. In particular, we clearly observe the crossover from a dilute solution of Gaussian coils to the weakly fluctuating semidilute regime dominated by two-point collisions (Schaefer, D. W.; Joanny, J. F.; Pincus, P. Macromolecules 1980, 13, 1280−1289). We also quantitatively test the Asakura−Oosawa model for these systems and show that it may be used in both the semidilute as well as the dilute regime. At fixed DNA concentration, we find that the range and depth of the interparticle potentials do not change significantly for ionic conce...

The long-range entropic forces that arise between two micrometer-sized colloidal spheres in a fluid of much smaller colloidal spheres were directly measured using a line-scanned optical tweezer. This new technique allowed us to measure the functional form of the potential with sub- ${k}_{B}T$ energy and $15\mathrm{nm}$ spatial resolution. At the lowest small sphere concentrations, the potential was monotonically attractive, while at higher concentrations an oscillatory potential was observed, due to the liquid structure of the small spheres. Surprisingly, the large spheres came together only rarely at the higher concentrations, suggesting a new means for stabilizing suspensions using entropy alone.

We have performed ultrafast second-harmonic generation spectroscopy of ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}.$ A formalism was developed to calculate the nonlinear response of thin nonlinear films excited by an ultrashort laser source $({\mathrm{T}\mathrm{i}:\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}),$ and then used to extract ${\ensuremath{\chi}}_{\mathrm{zxx}}^{(2)}(\ensuremath{\omega}=2{\ensuremath{\omega}}_{o})$ and ${\ensuremath{\chi}}_{\mathrm{xzx}}^{(2)}(\ensuremath{\omega}=2{\ensuremath{\omega}}_{o})$ from our SHG measurements over a two-photon energy range of 2.6--3.4 eV. The spectra are compared to theory [J. L. P. Hughes, Y. Wang, and J. E. Sipe, Phys. Rev. B 55, 13 630 (1997)]. A weak sub-band-gap enhancement of ${\ensuremath{\chi}}_{\mathrm{zxx}}^{(2)}(\ensuremath{\omega}=2{\ensuremath{\omega}}_{o})$ was observed at a two-photon energy of 2.80 eV; it was not present in ${\ensuremath{\chi}}_{\mathrm{xzx}}^{(2)}(\ensuremath{\omega}=2{\ensuremath{\omega}}_{o}).$ The enhancement, which may result from a three-photon process involving a midgap defect state, was independent of the carrier concentration, intentional doping, and the presence of the ``yellow luminescence band'' defects. In addition, we determined sample miscuts by rotational SHG; the miscuts did not generate observable strain induced nonlinearities. The linear optical properties of GaN from 1.38 to 3.35 eV were also determined.

We have developed a multichannel photon counting instrument for near infrared spatially resolved spectroscopy. The instrument uses two laser diodes at 780 and 830 nm. The two wavelengths are time multiplexed at a rate of 5 MHz for virtually simultaneous measurements at the two wavelengths. The photon pulses are then multiplexed with an optical switch so that the incident optical signals are directed to different source positions. Eight time-correlated channels have been employed for simultaneous acquisition of spectroscopic data at multiple locations. Photon detection is performed by multichannel plate photomultiplier tubes. Robust and accurate data analysis tools to perform deconvolution, data fitting, and absorption change quantification are presented. The instrument has been tested with phantoms simulating tissue optical properties. Absolute optical properties, namely absorption and reduced scattering coefficient, have been determined with an accuracy of ±5%. Quantification of absorption changes can be...

We explore the entropic interactions between a pair of micron-sized colloidal spheres in DNA solutions. By confining the particles in a line-scanned optical tweezer, we directly measured the functional form of the interaction potential with sub- kBT resolution in samples where the spheres and the polymer coils were of comparable size. The potential is well described by the Asakura-Oosawa depletion model even in the semidilute regime where DNA coils overlap strongly. Its range and depth increase with increasing concentration in a manner consistent with a crossover from a dilute solution of Gaussian coils to the weakly fluctuating semidilute regime dominated by two-point collisions which is unique to semiflexible polymers. [S0031-9007(98)07491-2] Colloidal suspensions and polymer solutions are two classic soft materials that have been the focus of decades of study, and yet their properties when mixed together are still only poorly understood. These colloid-polymer mixtures exhibit a rich phase behavior which is not only of fundamental interest [1 ‐ 10] but also of relevance to systems as diverse as motor oils and frozen desserts. The structural and dynamical properties of these complex fluids ultimately depend on the microscopic interactions between the suspension constituents. We present the first direct measurement of the functional form of the depletion interaction between two colloidal spheres in a nonadsorbing polymer background. Our measurements probe the variations in depth and range of the potential as a function of polymer concentration, spanning the dilute region where individual coils are noninteracting to the semidilute regime where they become entangled. Unlike other force measurements [3,5], our experiments probe a previously unexplored regime where colloid and polymer sizes are comparable. We find that the traditional hard-sphere depletion potential [8,9] for dilute solutions still applies and that it can be extended into the entangled region by rescaling the effective size of the polymer coils. We obtain quantitative measurements of the polymers’ osmotic pressure and correlation length above and below the overlap concentration. A dilute polymer solution can be modeled as an ideal gas of hard spheres [8] with a mean size given by the radius of gyration of the individual polymer coils. Unlike hard spheres, polymers in solution can interpenetrate, significantly reducing any effects due to liquid structure. The experimental situation is depicted in Fig. 1(a). The centers of the polymer “spheres” are excluded from a region of thickness Rg surrounding the colloidal particles. When the “depletion zones” surrounding the two spheres overlap, the total volume accessible to the polymer increases, leading to a gain in the system entropy. This produces an attractive interaction between the two spheres.

With an effective telescope area of order $10^4$ m$^2$ for TeV neutrinos, a threshold near $\sim$50 GeV and a pointing accuracy of 2.5 degrees per muon track, the AMANDA detector represents the first of a new generation of high energy neutrino telescopes, reaching a scale envisaged over 25 years ago. We describe early results on the calibration of natural deep ice as a particle detector as well as on AMANDA's performance as a neutrino telescope., Comment: 12 pages, Latex2.09, uses espcrc2.sty and epsf.sty, 13 postscript files included. Talk presented at the 18th International Conference on Neutrino Physics and Astrophysics (Neutrino 98), Takayama, Japan, June 1998

Recently, there has been much progress in the self-assembly of colloidal crystals. Major advances include the growth of colloidal crystals on lithographically templated substrates, the synthesis of ordered macroporous materials starting with crystalline emulsions, the investigation of the role of entropy in hard sphere colloids in space and on earth, and the use of electrohydrodynamic forces to create ordered structures.

We have developed a near-infrared spectroscopy system for low-resolution regional imaging of the brain. Our regional imager employs two intensity-modulated (frequency-domain) diode lasers operating at 779 and 834 nm, respectively, in order to produce macroscopic waves of diffusing photons. The interaction of these diffusive waves with tissue depends on laser modulation frequency, laser wavelength and the optical properties of the sample tissue volume. The lasers can be modulated over a range of frequencies from 50 to 400 MHz. Light is coupled to and from the head using a pad that has 12 source and 4 detector positions within an area of approximately 40 cm2. The pad can be moved to different positions on the head. Measurements from different source-detector combinations enable reconstruction of low-resolution images of the tissue volume beneath the pad. For example, we have made two-dimensional back-projection images of model systems in order to demonstrate the capabilities of the regional imager. We also present preliminary results from initial clinical studies at the Children's Hospital of Philadelphia.

We report on the analysis of air showers observed in coincidence by the Antarctic Muon and Neutrino detector array (AMANDA-B10) and the South Pole Air Shower Experiment (SPASE-1 and SPASE-2). We discuss the use of coincident events for calibration and survey of the deep AMANDA detector as well as the response of AMANDA to muon bundles. This analysis uses data taken during 1997 when both SPASE-1 and SPASE-2 were in operation to provide a stereo view of AMANDA.

We are developing a novel nonlinear image reconstruction algorithm for diffuse optical tomography (DOT). We will present the relevant theory and an initial numerical validation.

We used Diffuse Optical Spectroscopy to measure the effect of PDT on hemoglobin concentration changes during intra-operative PDT for mesothelioma. Those with less PDT-created hypoxia had better prognosis than those with severe hypoxia.

Diffuse Optical Tomography measured physiological parameters were compared to pathological properties of breast cancer including Ki67 expression level. The Warburg effect was observed, which indicates a metabolic pathway taken by the cancer.

We demonstrate use of an improved Diffuse Correlation Spectroscopy instrument to monitor cerebral blood flow (CBF) at the bedside during intravenous administration of normal saline, a standard stroke treatment paradigm employed to maximize CBF

We quantified pre-operative cerebral hemodynamics with non-invasive optical spectroscopies in infants with congenital heart disease. We find that cerebral hemodynamics change from birth until surgery and depend on the type of heart defect.

We report on the observation of a significant deficit of cosmic rays from the direction of the Moon with the IceCube detector. The study of this "Moon shadow" is used to characterize the angular resolution and absolute pointing capabilities of the detector. The detection is based on data taken in two periods before the completion of the detector: between April 2008 and May 2009, when IceCube operated in a partial configuration with 40 detector strings deployed in the South Pole ice, and between May 2009 and May 2010 when the detector operated with 59 strings. Using two independent analysis methods, the Moon shadow has been observed to high significance (> 6 sigma) in both detector configurations. The observed location of the shadow center is within 0.2 degrees of its expected position when geomagnetic deflection effects are taken into account. This measurement validates the directional reconstruction capabilities of IceCube., Submitted to Physical Review D

We propose a novel algorithm to reduce extracerebral contamination in diffuse correlation spectroscopy measurements of cerebral blood flow in real time by varying probe pressure against the scalp.

We present measurements and theory of a fluid-solid phase transition at a hard, flat wall in a two-sized hard-sphere mixture. With particle concentrations well below the level for bulk phase separation, a fluid monolayer (enriched in the larger spheres) forms at the wall. At slightly higher concentrations, the wall-fluid freezes into a wall-solid phase which coexists with the bulk fluid. Measurements of large-sphere area fraction vs. small-sphere bulk concentration in the fluid and solid phases are in reasonable agreement with the predictions of a model that accounts for entropic excluded-volume effects at a flat wall.

We have searched for extremely high energy neutrinos using data taken with the IceCube detector between May 2010 and May 2012. Two neutrino induced particle shower events with energies around 1 PeV were observed, as reported previously. In this work, we investigate whether these events could originate from cosmogenic neutrinos produced in the interactions of ultra-high energy cosmic-rays with ambient photons while propagating through intergalactic space. Exploiting IceCube's large exposure for extremely high energy neutrinos and the lack of observed events above 100 PeV, we can rule out the corresponding models at more than 90% confidence level. The model independent quasi-differential 90% CL upper limit, which amounts to $E^2 \phi_{\nu_e + \nu_\mu + \nu_\tau} = 1.2 \times 10^{-7}$ GeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ at 1 EeV, provides the most stringent constraint in the energy range from 10 PeV to 10 EeV. Our observation disfavors strong cosmological evolution of the highest energy cosmic ray sources such as the Fanaroff-Riley type II class of radio galaxies., Comment: A follow-up analysis to the report published in PRL 111 (2013) 021103. 15 pages, 9 figures, 5 tables, the version accepted for publication in Physical Review D

The IceCube project has transformed one cubic kilometer of deep natural Antarctic ice into a Cherenkov detector. Muon neutrinos are detected and their direction inferred by mapping the light produced by the secondary muon track inside the volume instrumented with photomultipliers. Reconstructing the muon track from the observed light is challenging due to noise, light scattering in the ice medium, and the possibility of simultaneously having multiple muons inside the detector, resulting from the large flux of cosmic ray muons. This manuscript describes work on two problems: (1) the track reconstruction problem, in which, given a set of observations, the goal is to recover the track of a muon; and (2) the coincident event problem, which is to determine how many muons are active in the detector during a time window. Rather than solving these problems by developing more complex physical models that are applied at later stages of the analysis, our approach is to augment the detectors early reconstruction with data filters and robust statistical techniques. These can be implemented at the level of on-line reconstruction and, therefore, improve all subsequent reconstructions. Using the metric of median angular resolution, a standard metric for track reconstruction, we improve the accuracy in the initial reconstruction direction by 13%. We also present improvements in measuring the number of muons in coincident events: we can accurately determine the number of muons 98% of the time., Comment: 14 pages, 4 figures, submitted to Nuclear Instruments and Methods in Physics Research Section A

The vibrational modes of pristine and polycrystalline monolayer colloidal crystals composed of thermosensitive microgel particles are measured using video microscopy and covariance matrix analysis. At low frequencies, the Debye relation for two dimensional harmonic crystals is observed in both crystal types; at higher frequencies, evidence for van Hove singularities in the phonon density of states is significantly smeared out by experimental noise and measurement statistics. The effects of these errors are analyzed using numerical simulations. We introduce methods to correct for these limitations, which can be applied to disordered systems as well as crystalline ones, and we show that application of the error correction procedure to the experimental data leads to more pronounced van Hove singularities in the pristine crystal. Finally, quasi-localized low-frequency modes in polycrystalline two-dimensional colloidal crystals are identified and demonstrated to correlate with structural defects such as dislocations, suggesting that quasi-localized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids., 8 pages, 6 figures 09/11/2013 major revisions

Neonatal congenital heart disease (CHD) is associated with altered cerebral hemodynamics and increased risk of brain injury. Two novel noninvasive techniques, magnetic resonance imaging (MRI) and diffuse optical and correlation spectroscopies (diffuse optical spectroscopy (DOS), diffuse correlation spectroscopy (DCS)), were employed to quantify cerebral blood flow ( CBF) and oxygen metabolism ( CMRO2) of 32 anesthetized CHD neonates at rest and during hypercapnia. Cerebral venous oxygen saturation ( SvO2) and CBF were measured simultaneously with MRI in the superior sagittal sinus, yielding global oxygen extraction fraction ( OEF) and global CMRO2in physiologic units. In addition, microvascular tissue oxygenation ( StO2) and indices of microvascular CBF (BFI) and CMRO2( CMRO2i) in the frontal cortex were determined by DOS/DCS. Median resting-state MRI-measured OEF, CBF, and CMRO2were 0.38, 9.7 mL/minute per 100 g and 0.52 mL O2/minute per 100 g, respectively. These CBF and CMRO2values are lower than literature reports for healthy term neonates (which are sparse and quantified using different methods) and resemble values reported for premature infants. Comparison of MRI measurements of global SvO2, CBF, and CMRO2with corresponding local DOS/DCS measurements demonstrated strong linear correlations ( R2=0.69, 0.67, 0.67; P

We study phonon modes in two-dimensional colloidal crystals composed of soft microgel particles with hard polystyrene particle dopants distributed randomly on the triangular lattice. This experimental approach produces close-packed lattices of spheres with random bond strength disorder, i.e., the effective springs coupling nearest neighbors are very stiff, very soft, or of intermediate stiffness. Particle tracking video microscopy and covariance matrix techniques are then employed to derive the phonon modes of the corresponding "shadow" crystals with bond strength disorder as a function of increasing dopant concentration. At low frequencies, hard and soft particles participate equally in the phonon modes, and the samples exhibit Debye-like density of states behavior characteristic of crystals. For mid- and high-frequency phonons, the relative participation of hard versus soft particles in each mode is found to vary systematically with dopant concentration. Additionally, a few localized modes, primarily associated with hard particle motions, are found at the highest frequencies.

Diffuse correlation spectroscopy (DCS) uses the temporal fluctuations of near-infrared (NIR) light to measure cerebral blood flow (CBF) non-invasively. Here, we provide a brief history of DCS applications in the brain with an emphasis on the underlying physical ideas, common instrumentation and validation. Then we describe recent clinical research that employs DCS-measured CBF as a biomarker of patient well-being, and as an indicator of hemodynamic and metabolic responses to functional stimuli.

Sodium bicarbonate (NaHCO3) is a common treatment for metabolic acidemia; however, little definitive information exists regarding its treatment efficacy and cerebral hemodynamic effects. This pilot observational study quantifies relative changes in cerebral blood flow (ΔrCBF) and oxy- and deoxyhemoglobin concentrations (ΔHbO2 and ΔHb) due to bolus administration of NaHCO3 in patients with mild base deficits.Infants and children with hypoplastic left heart syndrome (HLHS) were enrolled before cardiac surgery. NaHCO3 was given as needed for treatment of base deficit. Diffuse optical spectroscopies were used for 15 min postinjection to noninvasively monitor ΔHb, ΔHbO2, and ΔrCBF relative to baseline before NaHCO3 administration.Twenty-two anesthetized and mechanically ventilated patients with HLHS (aged 1 d to 4 y) received a median (interquartile range) dose of 1.1 (0.8, 1.8) mEq/kg NaHCO3 administered intravenously over 10-20 s to treat a median (interquartile range) base deficit of -4 (-6, -3) mEq/l. NaHCO3 caused significant dose-dependent increases in ΔrCBF; however, population-averaged ΔHb and ΔHbO2 as compared with those of controls were not significant.Dose-dependent increases in cerebral blood flow (CBF) caused by bolus administration of NaHCO3 are an important consideration in vulnerable populations wherein risk of rapid CBF fluctuations does not outweigh the benefit of treating a base deficit.

Diffuse optical tomography (DOT) has been employed to derive spatial maps of physiologically important chromophores in the human breast, but the fidelity of these images is often compromised by boundary effects such as those due to the chest wall. We explore the image quality in fast, data-intensive analytic and algebraic linear DOT reconstructions of phantoms with subcentimeter target features and large absorptive regions mimicking the chest wall. Experiments demonstrate that the chest wall phantom can introduce severe image artifacts. We then show how these artifacts can be mitigated by exclusion of data affected by the chest wall. We also introduce and demonstrate a linear algebraic reconstruction method well suited for very large data sets in the presence of a chest wall.

Ultrafast time-resolved studies of the vibrational motions of CO on Cu(111) are described. In particular, the energy transfer rates between substrate electrons and phonons and the frustrated translation of CO are measured. An explicit relaxation model including the temperature dependence of these rates is developed and used in the analysis.

IN a colloidal suspension containing particles of two different sizes, there is an attractive force between the larger particles. This attraction is due to the extra volume that becomes available to the smaller particles when the larger particles approach one another, thus increasing the entropy of the system. Entropic 'excluded-volume' effects of this type have been studied previously in colloids and emulsions, in the context of phase-separation phenomena in the bulk1–15 and at flat surfaces2,16. Here we show how similar effects can be used to position the larger particles of a binary mixture on a substrate, or to move them in a predetermined way. Our experiments demonstrate the entropically driven repulsion of a colloidal sphere (in a suspension of smaller spheres) from the edge of a step; the magnitude of the entropic barrier felt by the sphere is approximately twice its mean thermal energy. These results indicate that passive structures etched into the walls of a container create localized entropic force fields which can trap, repel or induce the controlled drift of particles. Manipulation techniques based on these effects should be useful for making the highly ordered particle arrays required for structures with photonic band gaps17,18, microelectronic mask materials19, and materials for clinical assays20

We present detailed second-order nonlinear optical spectroscopic studies of the interface electronic structure in metal:GaAs thin films and interfaces. The interface spectra from Au:GaAs, Ga-rich n-type systems exhibit two resonance features in the near infrared at 0.715 eV and 0.731 eV. A single resonance feature at 0.715 eV is observed in Au:GaAs, As-rich n-type interfaces. Similar single resonance features at 0.715 eV were observed in As:GaAs n-type samples. No resonances however were observed in oxide:GaAs and metal:GaAs p-type systems. All of the spectral features are sharp one-photon resonances. They provide compelling evidence for the existence and symmetry of atomic displacement-induced defect states just below the buried interface.