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Y. Zhang

Publications and source records attributed to Y. Zhang.

At least 685 records · Page 38Linked to original sources
arXiv2003

An RFQ injection system for the HRIBF

At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL), molecular ions extracted from a positive ion source and subsequently broken up in a charge exchange cell produce Radioactive Ion Beams (RIBs) with several hundred eV energy spread, preventing effective magnetic isobar separation. In order to perform magnetic isobar separation prior to charge exchange, a multi-harmonic buncher and a 12 MHz RFQ (Radio-Frequency Quadrupole) is proposed to supplement the present 300 kV injection system for the 25 MV tandem electrostatic accelerator. The RFQ will be mounted on a variable high voltage platform to accelerate ions with masses from 10 to 150 amu.

physics.acc-ph↗
arXiv2003

Optical spectrum of the planetary nebula M 2-24

We have obtained medium-resolution, deep optical long-slit spectra of the bulge planetary nebula (PN) M 2-24. The spectrum covers the wavelength range from 3610--7330 A. Over two hundred emission lines have been detected. The spectra show a variety of optical recombination lines (ORLs) from C, N, O and Ne ions. The diagnostic diagram shows significant density and temperature variations across the nebula. Our analysis suggests that the nebula has a dense central emission core. The nebula was thus studied by dividing it into two regions: 1) an high ionization region characterized by an electron temperature of T_e=16,300K and a density of logN_e(cm^{-3})=6.3; and 2) a low ionization region represented by T_e=11,400K and logN_e(cm^{-3})=3.7. A large number of ORLs from C, N, O and Ne ions have been used to determine the abundances of these elements relative to hydrogen. In general, the resultant abundances are found to be higher than the corresponding values deduced from collisionally excited lines (CELs). This bulge PN is found to have large enhancements in two {alpha}-elements, magnesium and neon.

astro-ph↗
arXiv2003

High-p_T Pion Production in Heavy-Ion Collisions at RHIC energies

Perturbative QCD results on pion production are presented in proton-proton, proton-nucleus and nucleus-nucleus collisions from CERN SPS up to RHIC energy. A K_{jet}(s, p_T, Q) factor obtained from jet production is applied to perform next-to-leading order calculations. Using the intrinsic transverse momentum (k_T) we determined transverse momentum spectra for pions in wide energy region. We have investigated nuclear multiscattering and the Cronin effect at RHIC energies.

nucl-th↗
arXiv2002

Insulating behavior of lambda-DNA on the micron scale

We have investigated the electrical conductivity of lambda-DNA using DNA covalently bonded to Au electrodes. Thiol-modified dTTP was incorporated into the `sticky' ends of bacteriophage lambda-DNA using DNA polymerase. Two-probe measurements on such molecules provide a hard lower bound for the resistivity ρ>10^6 (Ohm cm) at bias potentials up to 20 volts, in conflict with recent claims of moderate to high conductivity. By direct imaging, we show that the molecules are present after the measurements. We stress the importance of eliminating salt residues in these measurements.

physics.bio-ph↗
arXiv2002

Perturbative QCD Results on Pion Production in pp, pA and AA Collisions

We summarize new pQCD results on pion production in proton-proton (pp), proton-nucleus (pA) and nucleus-nucleus (AA) collisions. Our calculation introduces intrinsic parton transverse momentum (k_T) and is performed effectively at next-to-leading order (NLO), applying a K factor extracted for jet events. Two different factorization scales, Q=p_{T,jet}/2 and p_{T,jet} are used. Experimental data in pA collisions imply a preference for the latter choice at NLO level. We display our results at CERN SPS for AA collisions.

nucl-th↗
arXiv2002

Charge and stability of strange quark matter at finite temperature

By means of the quark mass density- and temperature- dependent model, it is found that the negative charge and the higher strangeness fraction are in favor of the stability of strange quark matter at finite temperature. A critical baryon number density n_Bc has been found. The strange quark matter is unstable when n_B < n_Bc. The charge and strangeness fraction dependence of stability for strange quark matter is addressed.

nucl-th↗
arXiv2002

Quark mass density- and temperature- dependent model for strange quark matter

It is found that the radius of a stable strangelet decreases as the temperature increases in a quark mass density-dependent model. To overcome this difficulty, we extend this model to a quark mass density- and temperature- dependent model in which the vacuum energy density at zero baryon density limit B depends on temperature. An ansatz is introduced and the regions for the best choice of the parameters are studied.

nucl-th↗
arXiv2001

Jet quenching as a probe of gluon plasma formation

We study, in a pQCD calculation augmented by nuclear effects, the jet energy loss needed to reproduce the pi^0 spectra in Au+Au collisions at large p_T, measured by PHENIX at RHIC. The transverse width of the parton momentum distributions (intrinsic-k_T) is used phenomenologically to obtain a reliable baseline pp result. Jet quenching is applied to the nuclear spectra (including shadowing and multiscattering) to fit the data.

hep-ph↗
arXiv2001

High-p_T pion and kaon production in relativistic nuclear collisions

High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.

hep-ph↗
arXiv2001

Inclusive Soft Pion Production from 12.3 and 17.5 GeV/c Protons on Be, Cu and Au

Differential cross-sections are presented for the inclusive production of charged pions in the momentum range 0.1 to 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 in 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.

nucl-ex↗
arXiv2001

MgB2 tunnel junctions and 19 K low-noise dc superconducting quantum interference devices

Point contact junctions made from two pieces of MgB2 can be adjusted to exhibit either superconductor-insulator-superconductor (SIS) or superconductor-normal metal-superconductor (SNS) current-voltage characteristics. The SIS characteristics are in good agreement with the standard tunneling model for s-wave superconductors, and yield an energy gap of (2.02 +/- 0.08) meV. The SNS characteristics are in good agreement with the predictions of the resistively-shunted junction model. DC Superconducting QUantum Interference Devices made from two SNS junctions yield magnetic field noise as low as 35 fT/Hz^{1/2} at 19 K.

cond-mat.supr-con↗
arXiv2001

Antiproton Production in $p+A$ Collisions at AGS Energies

Inclusive and semi-inclusive measurements are presented for antiproton ($\bar{p}$) production in proton-nucleus collisions at the AGS. The inclusive yields per event increase strongly with increasing beam energy and decrease slightly with increasing target mass. The $\bar{p}$ 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 $\bar{p}$ annihilation cross section in the nuclear medium. The resulting cross section is at least a factor of five smaller than the free $\bar{p}-p$ 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 $\bar{p}-p$ annihilation cross section.

nucl-ex↗
arXiv2001

MHD instability in differentially rotating cylindric flows

The possibility that the magnetic shear-flow instability (MRI, Balbus-Hawley instability) might give rise to turbulence in a cylindric Couette flow is investigated through numerical simulations. The study is linear and the fluid flow is supposed to be incompressible and differentially rotating with a simple velocity profile with $Ω= a+b/R^2$. The simplicity of the model is counterbalanced by the fact that the study is fully global in all three spatial directions with boundaries on each side; finite diffusivities are also allowed. The investigation is also carried out for several values of the azimuthal wavenumber of the perturbations in order to analyse whether non-axisymmetric modes might be preferred, which might produce, in a non-linear extension of the study, a self-sustained magnetic field. We find that with magnetic field the instability is easier to be excited than without magnetic field. The critical Reynolds number for Pm$=1$ is of order 50 independent of whether the nonmagnetic flow is stable or not. Also we find that i) the magnetic field strongly reduces the number of Taylor vortices, ii) the angular momentum is transported outwards and iii) for finite c ylinders a netto dynamo-alpha effect results which is negative for the upper part of the cylinder and which is positive for the lower part of the cylinder. For magnetic Prandtl number smaller than unity the critical Reynolds number scales with Pm$^{-0.65}$. If this was true even for very small magnetic Prandtl number (e.g. liquid sodium) the critical Reynolds number should reach the value 10$^5$ which is, however, also characteristic for nonlinear fi nite-amplitude hydrodynamic Taylor-Couette turbulence -- so that we easily have to expect the simultaneous existence of both sorts of instabi lities in cold accretion disks.

astro-ph↗
arXiv2001

Multiple scattering and $p_t$-broadening at RHIC energies

In ultrarelativistic heavy-ion collisions, in the 2 GeV$<p_\perp<$ 6 GeV transverse momentum region, the soft and semi-hard multiple scattering of the incoming nucleons in the nuclear medium results in the broadening of the expected hadronic (e.g. pion) $p_\perp$ spectra relative to proton-proton ($pp$) collisions. Thus, higher transverse-momentum regions are populated than in $pp$ collisions. In a perturbative QCD based calculation we include the intrinsic transverse momentum ($k_\perp$) of the partons in the nucleon (determined from $pp$ collisions), augmented by the extra broadening obtained via a systematic analysis of proton-nucleus ($pA$) collisions in the energy range 17$<\sqrt{s}<$ 39 AGeV. The original polynomial spectra are modified, and a nearly exponential spectrum appears in the region 2$\lesssim p_\perp\lesssim 3.5$ GeV. At present RHIC energies ($\sqrt{s}=$130 AGeV), the slope of the calculated spectra is reminiscent of that of fluid-dynamical descriptions, but lacks any thermal origin. We determine and discuss the size of the modifications originating in multiple scattering, which lead to this state of affairs.

nucl-th↗
arXiv2001

Discovery of Jet Quenching at RHIC and the Opacity of the Produced Gluon Plasma

The predicted quenching of jets in A+A at RHIC energies has been discovered by STAR and PHENIX in preliminary data reported at this conference. We apply the GLV theory of QCD radiative energy loss to estimate the opacity, L/λ_g, of the gluon plasma produced in Au+Au collisions at 130 AGeV. We show that (in contrast to the factor of two Cronin enhancement of π^0 found at the SPS by WA98) the factor of 5-8 suppression of p_T \sim 2-4 GeV π^0 reported by PHENIX can be accounted for with an effective static plasma opacity L/λ_g \approx 3-4.

nucl-th↗
arXiv2000

Semi-Inclusive Lambda and Kshort Production in p-Au Collisions at 17.5 GeV/c

The first detailed measurements of the centrality dependence of strangeness production in p-A collisions are presented. Lambda and Kshort 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, nu, made by the proton. The nu dependence of the Lambda yield deviates from a scaling of p-p data by the number of participants, increasing faster than this scaling for nu<=5 and saturating for larger nu. A slower growth in Kshort multiplicity with nu is observed, consistent with a weaker nu dependence of K-Kbar production than Y-K production.

nucl-ex↗
arXiv2000

Giant enhancement of the thermal Hall conductivity κ_{xy} in the superconductor YBa_2Cu_3O_7

In high-purity crystals of YBa_2Cu_3O_7, the quasiparticle (qp) lifetime τand the (weak-field) thermal Hall conductivity κ_{xy} undergo dramatic increases below 90K. We present a detailed picture of the behavior of κ_{xy} at low temperature, in particular its scaling properties, which are directly relevant to the issue of whether Landau quantization of the qp states occurs.

cond-mat.supr-con↗