Search Results (28 total)

An analysis of inclusive pion production in proton-beryllium collisions at 6.4, 12.3, and 17.5 GeV/c proton beam momentum has been performed. The data were taken by Experiment 910 at the Alternating Gradient Synchrotron at the Brookhaven National Laboratory. The differential π⁺ and π^- production cross sections (d²σ/dpdΩ) were measured up to 400 mrad in θ_π and up to 6 GeV/c in p_π. The measured cross section was fit with a Sanford-Wang parametrization.

Anisotropic states at half-filled higher Landau levels are investigated in the system with a finite electric current. We study the response of the striped Hall state and the anisotropic charge density wave (ACDW) state against the injected current using the effective action. Current distributions and a current dependence of the total energy are determined for both states. With no injected current, the energy of the ACDW state is lower than that of the striped Hall state. We find that the energy of the ACDW state increases faster than that of the striped Hall state as the injected current increases. Hence, the striped Hall state becomes the lower energy state when the current exceeds the critical value. The critical value is estimated at about 0.04–0.07  nA, which is much smaller than the current used in the experiments.

We calculate the polarization function including only intra-Landau-level correlation effects of striped Hall gas. Using the polarization function, the dielectric function, the dispersion of the plasmon, and the correlation energy are computed in a random phase approximation (RPA) and generalized random phase approximation (GRPA). The plasmon becomes anisotropic and gapless owing to the anisotropy of the striped Hall gas and two dimensionality of the quantum Hall system. The plasmon approximately agrees with the phonon derived before by the single mode approximation. The (G)RPA correlation energy is compared with other numerical calculations.

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on a layered transition-metal dichalcogenide (TMDC) 1T-VSe₂ to study the (4×4) charge-density wave (CDW) mechanism. We observed a partial Fermi-surface (FS) nesting on the electronlike FS centered at the M (L) point. The spectral weight near E_F is considerably suppressed below the transition temperature (T_c=110 K) around the nested portion, while a negligible spectral change is observed even across T_c in other portions of FS. This suggests that the CDW transition in 1T-VSe₂ is caused by the three-dimensional FS nesting. Implications are discussed in relation to the physical properties of 1T-VSe₂ as well as the ARPES results of other TMDC’s.

The NA44 experiment has measured single-particle inclusive spectra for charged pions, kaons, and protons as a function of transverse mass near midrapidity in 158A GeV/c Pb+Pb collisions. From the particle mass dependence of the observed m_T distributions, we are able to deduce a value of about 120 MeV for the temperature at thermal freeze-out. From the observed ratios of the rapidity densities, we find values of the chemical potentials for light and strange quarks and a chemical freeze-out temperature of approximately 140 MeV.

We investigate neutral collective excitations in the striped Hall state. In the striped Hall state, the magnetic translation and rotation symmetries are spontaneously broken. Using the commutation relation between charges and currents corresponding to the broken and unbroken symmetry, the existence of the gapless neutral excitation is proved. The spectrum of the neutral collective excitation at the half filled third Landau level is obtained in the single-mode approximation. We find the periodic line nodes in the spectrum. The spectrum is compared with the particle-hole excitation spectrum in the Hartree-Fock approximation.

NA44 uses a 512-channel Si pad array covering 1.5<η<3.3 to study charged hadron production in 158A GeV Pb+Pb collisions at the CERN SPS. We apply a multiresolution analysis, based on a discrete wavelet transformation, to probe the texture of particle distributions event by event, allowing a simultaneous localization of features in space and scale. Scanning a broad range of multiplicities, we search for signals of clustering and of critical behavior in the power spectra of local density fluctuations. The data are compared with detailed simulations of detector response, using heavy-ion event generators, and with a reference sample created via event mixing. An upper limit is set on the probability and magnitude of dynamical fluctuations.

Differential cross sections are presented for the inclusive production of charged pions in the momentum range 0.1–1.2 GeV/c in interactions of 12.3 and 17.5 GeV/c protons with Be, Cu, and Au targets. The measurements were made by Experiment 910 at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory. The cross sections are presented as a function of pion total momentum and production polar angle θ with respect to the beam.

Inclusive and semi-inclusive measurements are presented for antiproton (p̅ ) production in proton-nucleus collisions at the Alternating Gradient Synchrotron (AGS). The inclusive yields per event increase strongly with increasing beam energy and decrease slightly with increasing target mass. The p̅ yield in 17.5 GeV/c p+Au collisions decreases with grey track multiplicity, N_g, for N_g>0, consistent with annihilation within the target nucleus. The relationship between N_g and the number of scatterings of the proton in the nucleus is used to estimate the p̅ annihilation cross section in the nuclear medium. The resulting cross section is at least a factor of 5 smaller than the free p̅ -p annihilation cross section when assuming a small or negligible formation time. Only with a long formation time can the data be described with the free p̅ -p annihilation cross section.

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta 〈p_T〉≈0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p_T data, while a two-dimensional analysis was used for the higher p_T data. We find that the source-size parameters are consistent with the m_T scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.

The first detailed measurements of the centrality dependence of strangeness production in p-A collisions are presented. Λ and K_S dn/dy distributions from 17.5 GeV/ c p-Au collisions are shown as a function of “grey” track multiplicity and the estimated number of collisions, ν, made by the proton. The ν dependence of the Λ yield deviates from a scaling of p-p data by the number of participants, increasing faster than this scaling for ν≤5 and saturating for larger ν. A slower growth in K_S multiplicity with ν is observed, consistent with a weaker ν dependence of KK̅ production than YK production.

Thin (thickness h≈3 nm) films of n-octadecane and n-hexadecane adsorbed on mica surfaces from vapor close to their bulk melting points (T_m) have been studied in a surface force apparatus. Using data on the growth rate of capillary condensates between the mica surfaces in contact and measurements of h, we have identified a transition in the structure of the adsorbed films a few degrees above T_m. As T decreases the alkane layers appear to undergo a transition to a more ordered structure, akin to the postulated “surface freezing” of long-chain liquid n-alkanes.

Multiple-vacancy states for outer-shell electrons created in heavy-ion–atom collision have been studied through high-resolution measurement of the Lγ x rays emitted from ₆₅Tb induced by 15- and 30-MeV carbon ions. For the Lγ₁ x-ray emission, it is found that the satellite structure due to N-shell multiple vacancies can be well resolved from the satellite structures due to M-shell multiple vacancies. The spectral features observed provide experimental evidence for the manifestation of the exchange interaction between the 4d_3/2 hole and the unpaired 4f electrons in the presence of several N vacancies. An interpretation of the observed satellite structure for the complex Lγ_2,3 x-ray emission is presented.

Using the von Neumann lattice formalism, we study compressible anisotropic states around the half-filled Landau levels in the quantum Hall system. In these states the unidirectional charge-density wave (UCDW) state seems to be the most plausible state. The charge-density profile and Hartree-Fock energy of the UCDW are calculated self-consistently. The wavelength dependence of the energy for the UCDW is also obtained numerically. We show that the UCDW is regarded as a collection of the one-dimensional lattice Fermi-gas systems which extend to the uniform direction. The kinetic energy of the gas system is generated dynamically from the Coulomb interaction.

The surface tension of n-octadecane was studied in the vicinity of the bulk melting point using both the maximum bubble pressure and Wilhelmy plate methods. The bubble surfaces were found to be supercooled below the surface freezing point. The onset of surface freezing is indicated by a sharp drop in surface tension at a constant temperature. This transition is accompanied by an increased film stability resulting in longer bubble lifetimes at the liquid surface. Variations in bubble lifetime reflect changes in the interfacial mechanical properties of the film from liquidlike to solidlike.

Experiment E910 has measured slow protons and deuterons from collisions of 18 GeV/c protons with Be, Cu, and Au targets at the BNL AGS. These correspond to the “grey tracks” first observed in emulsion experiments. We report on their momentum and angular distributions and investigate their use in measuring the centrality of a collision, as defined by the mean number of projectile-nucleon interactions. The relation between the measured N_grey and the mean number of interactions ν̅ (N_grey) is studied using several simple models, one newly proposed, as well as the RQMD event generator. RQMD is shown to reproduce the N_grey distribution, and exhibits a dependence of N_grey on centrality that is similar to the behavior of the simple models. We find a strong linear dependence of N_grey̅ on ν, with a constant of proportionality that varies with target. For the Au target, we report a relative systematic error for extracting ν̅ (N_grey) that lies between 10 and 20 % over all N_grey.

The rate of growth of capillary condensates in a model wedgelike pore has been studied using the surface force apparatus. The method described allows the direct measurement of the diffusion-limited growth of liquid condensates with interface radii of curvature as small as 20 nm. The observed rates of condensation of vapors of n-pentane, n-hexane, cyclohexane, and water are close to, but consistently lower than, values calculated from a model based on Langmuir's theory of droplet growth.

We study the compressible states in the quantum Hall system using a mean field theory on the von Neumann lattice. In the lowest Landau level, a kinetic energy is generated dynamically from Coulomb interaction. The compressibility of the state is calculated as a function of the filling factor ν and the width d of the spacer between the charge carrier layer and dopants. The compressibility becomes negative below a critical value of d and the state becomes unstable at ν  =  1/2. Within a finite range around ν  =  1/2, the stable compressible state exists above the critical value of d.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

By studying the quantum Hall effect of stationary states with high values of injected current using a von Neumann lattice representation, we found that broadening of extended state bands due to a Hall electric field occurs and causes the breakdown of the quantum Hall effect. The Hall conductance agrees with a topological invariant that is quantized exactly below a critical field and is not quantized above a critical field. The critical field is proportional to B^3/2 and is enhanced substantially if the extended states occupy a small fraction of the system.

Two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume.

Using a momentum representation of a magnetic von Neumann lattice, we study a two-dimensional electron in a uniform magnetic field and obtain one-particle spectra of various periodic short-range potential problems in the lowest Landau level. We find that the energy spectra satisfy a duality relation between a period of the potential and a magnetic length. The energy spectra consist of the Hofstadter-type bands and flat bands. We also study the connection between a periodic short-range potential problem and a tight-binding model.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.