arXiv subjects

F. Liu

Publications and source records attributed to F. Liu.

89 records · Page 5Linked to original sources

arXiv2009

Energy Dependence of Directed Flow in Au+Au Collisions from a Multi-phase Transport Model

The directed flow of charged hadron and identified particles has been studied in the framework of a multi-phase transport (AMPT) model, for $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}=$200, 130, 62.4, 39, 17.2 and 9.2 GeV. The rapidity, centrality and energy dependence of directed flow for charged particles over a wide rapidity range are presented. AMPT model gives the correct $v_1(y)$ slope, as well as its trend as a function of energy, while it underestimates the magnitude. Within the AMPT model, the proton $v_1$ slope is found to change its sign when the energy increases to 130 GeV - a feature that is consistent with ``anti-flow''. Hadronic re-scattering is found having little effect on $v_1$ at top RHIC energies. These studies can help us to understand the collective dynamics at early times in relativistic heavy-ion collisions, and they can also be served as references for the RHIC Beam Energy Scan program.

nucl-th↗

arXiv2009

Intense high contrast femtosecond K-shell x-ray source from laser-driven Ar clusters

Bright Ar K-shell x-ray with very little background has been generated using an Ar clustering gas jet target irradiated with an 800 mJ, 30 fs ultra-high contrast laser, with the measured flux of 1.1 x 10^4 photons/mrad^2/pulse. This intense x-ray source critically depends on the laser contrast and the laser energy and the optimization of this source with interaction is addressed. Electron driven by laser electric field directly via nonlinear resonant is proved in simulation, resulting in effective electron heating and the enhancement of x-ray emission. The x-ray pulse duration is demonstrated to be only 10 fs, as well as a source size of 20 um, posing great potential application for single-shot ultrafast x-ray imaging.

physics.plasm-ph↗

arXiv2008

Initial orientation effect and selecting desired events in 520AMeV/u U-U collisions

How to select out those collisions with the desired geometry such as tip-tip and/or body-body in experiment is one key point for performing high energy UU collisions. With a relativistic transport model, we performed a simulation for deformed UU collision with vast different orientations at CSR energy area corresponding to the high net-baryon density region in QCD phase diagram. By investigating the centrality and initial collision orientation dependence of the center baryon density, we found that the tip-tip like UU collisions with extended high density phase, which is very important for studying the nuclear EoS of high baryon density matter and the possible end-point of the phase boundary, are those events with small initial orientations ($\leq20^{0}$) for bath projectile and target in reaction plane and small impact parameter ($\leq2.6fm$). We pointed out quantificationally two observations -- multiplicity of forward neutron and nuclear stopping power that both allows us to select out those most interesting events (i.e. tip-tip like), which will be very helpful for the future experiments at performing UU collisions.

nucl-th↗

arXiv2008

Quantum walks on Erdos-Renyi networks

We study the coherent exciton transport of continuous-time quantum walks (CTQWs) on Erdos-Renyi networks. The Erdos-Renyi network of N nodes is constructed by connecting every pair of nodes with probability $p$. We numerically calculate the ensemble averaged transition probability of quantum transport between two nodes of the networks. For finite networks, we find that the limiting transition probability is reached very quickly. For infinite networks whose spectral density follows the semicircle law, the efficiencies of the classical and quantum-mechanical transport are compared on networks of different average degree. In the long time limiting, we consider the distribution of the ensemble averaged transition probabilities, and show that there is a high probability to find the exciton at the initial node. Such high return probability almost do not alter in a wide range of connection probability p but increases rapidly when the network approaches to be fully connected. For networks whose topology is not extremely connected, the return probability is inversely proportional to the network size N. Furthermore, the transport dynamics are compared with that on a random graph model in which the degree of each node equals to the average degree of the Erdos-Renyi networks.

quant-ph↗

arXiv2007

Computer model validation with functional output

A key question in evaluation of computer models is Does the computer model adequately represent reality? A six-step process for computer model validation is set out in Bayarri et al. [Technometrics 49 (2007) 138--154] (and briefly summarized below), based on comparison of computer model runs with field data of the process being modeled. The methodology is particularly suited to treating the major issues associated with the validation process: quantifying multiple sources of error and uncertainty in computer models; combining multiple sources of information; and being able to adapt to different, but related scenarios. Two complications that frequently arise in practice are the need to deal with highly irregular functional data and the need to acknowledge and incorporate uncertainty in the inputs. We develop methodology to deal with both complications. A key part of the approach utilizes a wavelet representation of the functional data, applies a hierarchical version of the scalar validation methodology to the wavelet coefficients, and transforms back, to ultimately compare computer model output with field output. The generality of the methodology is only limited by the capability of a combination of computational tools and the appropriateness of decompositions of the sort (wavelets) employed here. The methods and analyses we present are illustrated with a test bed dynamic stress analysis for a particular engineering system.

stat.ME↗

arXiv2007

Low-lying ud anti-s anti-s configurations in a non-relativistic constituent quark model

The energies of the low-lying isoscalar and isovector ud anti-s anti-s configurations with spin-parity J^P=0^+, 1^+, and 2^+ are calculated in a non-relativistic constituent quark model by use of the variational method. The contributions of various parts of the quark-quark interacting potentials including the s-channel interaction are investigated, and the effect of different forms of confinement potential is examined. The model parameters are determined by the same method as in our previous work, and they still can satisfactorily describe the nucleon-nucleon scattering phase shifts and the hyperon-nucleon cross sections. The parameters of the s-channel interaction are fixed by the masses of K and K^* mesons, for which the size parameter is taken to be two possible values. When it is chosen as the same as baryons', the numerical results show that the masses of all the ud anti-s anti-s configurations are higher than the corresponding meson-meson thresholds. But when the size parameter for the K and K^* mesons is adjusted to be smaller than that for the baryons, the ud anti-s anti-s configuration with I=0 and J^P=1^+ is found to lie lower than the K^*K^* threshold, furthermore, this state has a very small KK^* component and the interaction matrix elements between this state and KK^* is comparatively small, thus its coupling to the KK^* channel will consequently be weak and it might be regarded as a possible tetraquark candidate.

nucl-th↗

arXiv2006

A possible Omega-pi molecular state

The structure of Omega-pi state with isospin I=1 and spin-parity J^p=3/2^- are dynamically studied in both the chiral SU(3) quark model and the extended chiral SU(3) quark model by solving a resonating group method (RGM) equation. The model parameters are taken from our previous work, which gave a satisfactory description of the energies of the baryon ground states, the binding energy of the deuteron, the nucleon-nucleon (NN) scattering phase shifts, and the hyperon-nucleon (YN) cross sections. The calculated results show that the Omega-pi state has an attractive interaction, and in the extended chiral SU(3) quark model such an attraction can make for an Omega-pi quasi-bound state with the binding energy of about several MeV.

nucl-th↗

arXiv2006

Anisotropic flow at RHIC: How unique is the number-of-constituent-quark scaling?

The transverse momentum dependence of the anisotropic flow $v_2$ for $π$, $K$, nucleon, $Λ$, $Ξ$ and $Ω$ is studied for Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured $v_2$, they both predict the particle type dependence of $v_2$, as observed by the RHIC experiments: $v_2$ exhibits a hadron-mass hierarchy (HMH) in the low $p_T$ region and a number-of-constituent-quark (NCQ) dependence in the intermediate $p_T$ region. The failure of the hadronic models to reproduce the absolute magnitude of the observed $v_2$ indicates that transport calculations of heavy ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependencies observed in heavy-ion collisions at intermediate $p_T$ might be related to the hadronic cross sections in vacuum rather than to the hadronization process itself.

nucl-th↗

arXiv2005

Preliminary Design of The Wide-Field Infrared Survey Explorer (WISE)

The Wide-field Infrared Survey Explorer (WISE), a NASA MIDEX mission, will survey the entire sky in four bands from 3.3 to 23 microns with a sensitivity 1000 times greater than the IRAS survey. The WISE survey will extend the Two Micron All Sky Survey into the thermal infrared and will provide an important catalog for the James Webb Space Telescope. Using 1024x1024 HgCdTe and Si:As arrays at 3.3, 4.7, 12 and 23 microns, WISE will find the most luminous galaxies in the universe, the closest stars to the Sun, and it will detect most of the main belt asteroids larger than 3 km. The single WISE instrument consists of a 40 cm diamond-turned aluminum afocal telescope, a two-stage solid hydrogen cryostat, a scan mirror mechanism, and reimaging optics giving 5" resolution (full-width-half-maximum). The use of dichroics and beamsplitters allows four color images of a 47'x47' field of view to be taken every 8.8 seconds, synchronized with the orbital motion to provide total sky coverage with overlap between revolutions. WISE will be placed into a Sun-synchronous polar orbit on a Delta 7320-10 launch vehicle. The WISE survey approach is simple and efficient. The three-axis-stabilized spacecraft rotates at a constant rate while the scan mirror freezes the telescope line of sight during each exposure. WISE is currently in its Preliminary Design Phase, with the mission Preliminary Design Review scheduled for July, 2005. WISE is scheduled to launch in mid 2009; the project web site can be found at www.wise.ssl.berkeley.edu.

astro-ph↗

arXiv2004

Precision Measurements of the Semileptonic Charm Decays $D^0 \to π^- \ell^+ ν$ and $D^0 \to K^- \ell^+ ν$

We investigate the decays $D^0\toπ^-\ell^+ν$ and $D^0\to K^-\ell^+ ν$, where $\ell$ is $e$ or $μ$, using approximately 7 ${\rm fb}^{-1}$ of data collected with the CLEO III detector. We find $R_0\equiv {\cal B}(D^0\to π^-e^+ν)/{\cal B}(D^0\to K^-e^+ν)= 0.082 \pm 0.006 \pm 0.005$. Fits to the kinematic distributions of the data provide parameters describing the form factor of each mode. Combining the form factor results and $R_0$ gives $|f^π_{+}(0)|^2 |V_{cd}|^2/|f^K_{+}(0)|^2 |V_{cs}|^2 = 0.038^{+0.006+0.005}_{-0.007-0.003}$.

hep-ex↗

arXiv2004

Measurements of Absolute Branching Fractions for Exclusive $D^0$ Semileptonic Decays

Based on a data sample of 60 pb$^{-1}$ collected at the $ψ(3770)$ resonance with the CLEO-c detector at CESR, we present improved measurements of absolute branching fractions for exclusive $D^0$ semileptonic decays into $K^-e^+ν$, $π^-e^+ν$ and $K^{*-}e^+ν$, and the first observation and measurement of $D^0 \to ρ^-e^+ν$. The determinations of the CKM matrix elements $V_{cs}$ and $V_{cd}$ are reviewed.

hep-ex↗

arXiv2004

Speed-Up of DNA Melting Algorithm with Complete Nearest Neighbor Properties

We describe a faster and more accurate algorithm for computing the statistical mechanics of DNA denaturation according to the Poland-Scheraga type. Nearest neighbor thermodynamics is included in a complete and general way. The algorithm represents an optimization with respect to algorithmic complexity of the partition function algorithm of Yeramian et al.: We reduce the computation time for a base-pairing probability profile from O(N2) to O(N). This speed-up comes in addition to the speed-up due to a multiexponential approximation of the loop entropy factor as introduced by Fixman and Freire. The speed-up, however, is independent of the multiexponential approximation and reduces time from O(N3) to O(N2) in the exact case. In addition to calculating the standard base-pairing probability profiles, we propose to use the algorithm to calculate various other probabilities (loops, helices, tails) for a more direct view of the melting regions and their positions and sizes.

q-bio.BM↗

arXiv2003

The TOFp/pVPD Time of Flight System for STAR

A time-of-flight system was constructed for the STAR Experiment for the direct identification of hadrons produced in 197Au+197Au collisions at RHIC. The system consists of two separate detector subsystems, one called the pVPD (the "start" detector) and the other called the TOFp tray (the "stop" detector). Each detector is based on conventional scintillator/phototube technology and includes custom high-performance front-end electronics and a common CAMAC-based digitization and read-out. The design of the system and its performance during the 2001 RHIC run will be described. The start resolution attained by the pVPD was 24 ps, implying a pVPD single-detector resolution of 58 ps. The total time resolution of the system averaged over all detector channels was 87 ps, allowing direct pi/K/p discrimination for momenta up to 1.8 GeV/c, and direct (pi+K)/p discrimination up to 3 GeV/c.

nucl-ex↗

arXiv2003

Insulator-metal transition shift related to magnetic polarons in La0.67-xYxCa0.33MnO3

The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3 (0 <= x <= 0.14) system. It was found that the transition temperature Tp almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above Tp, there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of Tp vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7~15.4 angstrom which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than Tp conform to the variable-range hopping mechanism.

cond-mat.str-el↗

arXiv2002

New results for hadronic collisions in the framework of the Parton-Based Gribov-Regge Theory

We recently proposed a new approach to high energy nuclear scattering, which treats hadronic collisions in a sophisticated way. Demanding theoretical consistency as a minimal requirement for a realistic model, we provide a solution for the energy conservation, screening problems and identical elementary interactions, the so-called "Parton-Based Gribov-Regge Theory" including enhanced diagrams. We can now present some of our results for SPS and RHIC energies.

hep-ph↗

arXiv2001

A Spin-Resolved Photoemission Study of Photohole Lifetimes in Ferromagnetic Gadolinium

High resolution spin-resolved photoemission is used to probe the properties of a Gd(0001) surface state. The state shows both a spin-mixing behavior reflecting the exchange of magnons with the local moments and a reduction of the exchange splitting with increasing temperature. The surface state polarization at low T suggests that the surface layer has an enhanced Tc of 365K or greater. Measurements of the photoemission linewidths show that at low temperatures, the lifetime of a majority spin photohole is predominantly limited by electron-phonon scattering and that of a minority spin photohole by electron-magnon scattering. Since similar behavior may be expected for bulk states close to the Fermi level, the transport properties of this material will also be determined by different decay mechanisms in the two channels.

cond-mat.str-el↗

arXiv2000

Recent Psi(2S) Results at BES

Based on $(3.79\pm0.31)\times10^6 ~ψ'$ data sets collected with the BES detector at BEPC, the recent $ψ'$, $χ_{cJ}$ and $η_c$ results from BES are presented. Some results are compared with NRQCD.

hep-ex↗