Identified baryon and meson distributions at large transverse momenta from Au + Au collisions at square root sNN=200 GeV

Transverse momentum spectra of pi+/-, p, and p up to 12 GeV/c at midrapidity in centrality selected Au + Au collisions at square root sNN=200 GeV are presented. In central Au + Au collisions, both pi +/- and p(p) show significant suppression with respect to binary scaling at pT approximately >4 GeV/c. Protons and antiprotons are less suppressed than pi+/-, in the range 1.5 approximately < pT approximately < 6 GeV/c. The pi-/pi+ and p/p ratios show at most a weak pT dependence and no significant centrality dependence. The p/pi ratios in central Au + Au collisions approach the values in p + p and d + Au collisions at pT approximately >5 GeV/c. The results at high pT indicate that the partonic sources of pi+/-, p, and p have similar energy loss when traversing the nuclear medium.     

Forward neutral pion production in p + p and d + Au collisions at square root sNN=200 GeV

Measurements of the production of forward pi0 mesons from p + p and d + Au collisions at square root sNN=200 GeV are reported. The p + p yield generally agrees with next-to-leading order perturbative QCD calculations. The d + Au yield per binary collision is suppressed as eta increases, decreasing to approximately 30% of the p + p yield at eta =4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward pi0 with charged hadrons at eta approximately 0 show a recoil peak in p + p that is suppressed in d + Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.     

Direct observation of dijets in central Au+Au collisions at sqrt[sNN]=200 GeV

The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum (pT) charged hadrons in Au+Au collisions at higher pT than reported previously. As (pT) is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter.     

Influence of hydrocarbon surfactant on the aggregation behavior of silicone surfactant: observation of intermediate structures in the vesicle-micelle transition

Intermediate structures of the aggregates in the aqueous solution of an ABA-type silicone surfactant and in the process of an SDS-induced vesicle-micelle transition are reported. In single ABA silicone surfactant aqueous solutions, large multilamellar vesicles (MLV), small single lamellar vesicles (SLV), threadlike micelles (TLM), and spheroidal micelles were observed. Interestingly, a large amount of TLMs were found entrapped into the large MLVs, but not in SLVs. Disintegration of the small vesicles inside the MLVs indicates that the entrapped TLM are from the disintegrated membrane of the entrapped small vesicles. Addition of SDS induced a transition from vesicles or threadlike micelles to spheroidal micelles. The intermediate structures, such as the appearance of small holes in the vesicle membrane, the budding of threadlike micelles from the membrane fracture, and the clusters of spheroidal micelles, were observed with increase of the SDS concentration. The electrical conductivity measurements indicated that complex micelles of SDS and silicone surfactant were formed in the solution due to the interaction between the SDS and PEO part of the silicone surfactant.     

Thermosensitive core-shell particles as carrier systems for metallic nanoparticles

We present a new system that allows us to modulate the catalytic activity of metal nanoparticles (Ag) by a thermodynamic transition that takes place within the carrier system. Thermosensitive core-shell particles have been used as the carrier system in which the core consists of poly(styrene) (PS), whereas the shell consists of a poly(N-isopropylacrylamide) (PNIPA) network cross-linked by N,N'-methylenebisacrylamide (BIS). Immersed in water, the shell of these particles is swollen. Heating the suspension above 32 degrees C leads to a volume transition within the shell that is followed by a marked shrinking of the network of the shell. The maximum degree of swelling can be adjusted by the degree of cross-linking. Silver nanoparticles with diameters ranging from 6.5 to 8.5 nm have been embedded into thermosensitive PNIPA networks with different cross-linking densities. The Ag nanoparticles do not influence the swelling and the shrinking of the network in the shell. The surface plasmon absorption band of the nanoparticles is shifted to higher wavelengths with temperature. This is traced back to the varying distance of the nanoparticles caused by the swelling and the shrinking of the shell. The catalytic activity is investigated by monitoring photometrically the reduction of 4-nitrophenol by an excess of NaBH4 in the presence of the silver nanocomposite particles. The rate constant kapp was found to be strictly proportional to the total surface of the nanoparticles in the system. Moreover, kapp is first decreasing with increasing temperature when approaching the volume transition. This is due to the strong shrinking of the network. Only at temperatures above the volume transition is the normal Arrhenius-type dependence of kapp found again. In this way, catalytic activity of the metal nanoparticles enclosed in a "nanoreactor" can be modulated by volume transition over a wide range.     

The effect of melt processing conditions on the hydrogen-bonded sheet orientation and polarization of nylon 11 films

It is known that melt-quenched, cold-drawn, and then annealed nylon 11 films possess a particular doubly oriented hydrogen-bonded sheet structure: the hydrogen-bonded sheets being in the plane of the film and the molecular chain direction being in the direction of draw. These films have been shown to be ferroelectric. In order to clarify the role of this special double orientation on polarization mechanisms and, in addition, provide insight into the reasons for this unique orientation, a systematic study was undertaken. Nylon 11 was melted at 210°C in a hot press for different melting times (ranging from 30 s to 20 min) prior to quenching into an ice-water bath. The resulting orientation of the hydrogen-bonded sheet structure in these films was examined using wide-angle X-ray diffraction and FTIR spectroscopy. For undrawn, melt-quenched films with short times in the melt, a degree of preferred orientation of the hydrogen-bonded sheets in the plane of the film was observed. As time in the melt increased, this preferred orientation in the plane decreased. This was also observed for films which were cold drawn before annealing. However, following cold drawing and several cycles of polarization using a maximum field of 150 MV/m at room temperature, the uniaxially drawn films with different times in the melt possessed the same remanent polarization and the same final orientation of the hydrogen-bonded sheets in the film thickness direction. © 1996 John Wiley & Sons, Inc.     

Neutron texture investigations of hard magnetic Mn?Al bars

In the present work connection between magnetic properties and texture of two samples of the tetragonal MnAl phase are investigated. For texture studies classical angle dispersive neutron diffraction and neutron time-of-flight method have been applied. the process of specimen preparation is shortly described. The mathematical treatment of experimental data for complete texture analysis has been outlined. The texture of investigated samples is weak. Some main components in inverse pole figures are inclined with respect to the direction of easy magnetization. In this way the non-optimal magnetic properties of the investigated material can be explained.