arXiv subjects

J. H. Chen

Publications and source records attributed to J. H. Chen.

At least 217 records · Page 12Linked to original sources

arXiv2014

Thickness and growth-condition dependence of \emph{in-situ} mobility and carrier density of epitaxial thin-film Bi$_2$Se$_3$

Bismuth selenide Bi$_2$Se$_3$ was grown by molecular beam epitaxy while carrier density and mobility were measured directly \emph{in situ} as a function of film thickness. Carrier density shows high interface n-doping (1.5 x 10$^{13}$ cm$^{-2}$) at the onset of film conduction, and bulk dopant density of $\sim$5 x 10$^{18}$ cm$^{-3}$, roughly independent of growth temperature profile. Mobility depends more strongly on the growth temperature and is related to the crystalline quality of the samples quantified by \emph{ex-situ} AFM measurements. These results indicate that Bi$_2$Se$_3$ as prepared by widely employed parameters is \emph{n}-doped before exposure to atmosphere, the doping is largely interfacial in origin, and dopants are not the limiting disorder in present Bi$_2$Se$_3$ films.

cond-mat.mtrl-sci↗

arXiv2014

Precision Measurement of the Longitudinal Double-spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at $\sqrt{s}=200$ GeV

We report a new high-precision measurement of the mid-rapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $\sqrt{s}=200$ GeV. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep inelastic scattering (DIS), semi-inclusive DIS, and RHIC $pp$ data. The measured asymmetries provide evidence for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.

hep-ex↗

arXiv2014

Jet-Hadron Correlations in sqrt{s_{NN}} = 200 GeV p+p and Central Au+Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at $\sqrt{s_{\text{NN}}} = 200 \text{GeV}$ in STAR are presented. The trigger jet population in Au+Au collisions is biased towards jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum ($p_{\text{T}}^{\text{assoc}}$) and enhanced at low $p_{\text{T}}^{\text{assoc}}$ in 0-20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions.

nucl-ex↗

arXiv2014

Measurement of Charge Multiplicity Asymmetry Correlations in High Energy Nucleus-Nucleus Collisions at 200 GeV

A study is reported of the same- and opposite-sign charge-dependent azimuthal correlations with respect to the event plane in Au+Au collisions at 200 GeV. The charge multiplicity asymmetries between the up/down and left/right hemispheres relative to the event plane are utilized. The contributions from statistical fluctuations and detector effects were subtracted from the (co-)variance of the observed charge multiplicity asymmetries. In the mid- to most-central collisions, the same- (opposite-) sign pairs are preferentially emitted in back-to-back (aligned on the same-side) directions. The charge separation across the event plane, measured by the difference, $Δ$, between the like- and unlike-sign up/down $-$ left/right correlations, is largest near the event plane. The difference is found to be proportional to the event-by-event final-state particle ellipticity (via the observed second-order harmonic $v^{\rm obs}_{2}$), where $Δ=(1.3\pm1.4({\rm stat})^{+4.0}_{-1.0}({\rm syst}))\times10^{-5}+(3.2\pm0.2({\rm stat})^{+0.4}_{-0.3}({\rm syst}))\times10^{-3}v^{\rm obs}_{2}$ for 20-40% Au+Au collisions. The implications for the proposed chiral magnetic effect are discussed.

nucl-ex↗

arXiv2014

Nuclear modification factor in intermediate-energy heavy-ion collisions

The transverse momentum dependent nuclear modification factors (NMF), namely $R_{CP}$, is investigated for protons produced in Au + Au at 1$A$ GeV within the framework of the isospin-dependent quantum molecular dynamics (IQMD) model. It is found that the radial collective motion during the expansion stage affects the NMF at low transverse momentum a lot. By fitting the transverse mass spectra of protons with the distribution function from the Blast-Wave model, the magnitude of radial flow can be extracted. After removing the contribution from radial flow, the $R_{CP}$ can be regarded as a thermal one and is found to keep unitary at transverse momentum lower than 0.6 GeV/c and enhance at higher transverse momentum, which can be attributed to Cronin effect.

nucl-th↗

arXiv2014

Beam-Energy Dependence of Directed Flow of Protons, Antiprotons and Pions in Au+Au Collisions

Rapidity-odd directed flow($v_1$) measurements for charged pions, protons and antiprotons near mid-rapidity ($y=0$) are reported in $\sqrt{s_{NN}} =$ 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV Au + Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC). At intermediate impact parameters, the proton and net-proton slope parameter $dv_1/dy|_{y=0}$ shows a minimum between 11.5 and 19.6 GeV. In addition, the net-proton $dv_1/dy|_{y=0}$ changes sign twice between 7.7 and 39 GeV. The proton and net-proton results qualitatively resemble predictions of a hydrodynamic model with a first-order phase transition from hadronic matter to deconfined matter, and differ from hadronic transport calculations.

nucl-ex↗

arXiv2014

Beam energy dependent two-pion interferometry and the freeze-out eccentricity of pions in heavy ion collisions at STAR

We present results of analyses of two-pion interferometry in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV measured in the STAR detector as part of the RHIC Beam Energy Scan program. The extracted correlation lengths (HBT radii) are studied as a function of beam energy, azimuthal angle relative to the reaction plane, centrality, and transverse mass ($m_{T}$) of the particles. The azimuthal analysis allows extraction of the eccentricity of the entire fireball at kinetic freeze-out. The energy dependence of this observable is expected to be sensitive to changes in the equation of state. A new global fit method is studied as an alternate method to directly measure the parameters in the azimuthal analysis. The eccentricity shows a monotonic decrease with beam energy that is qualitatively consistent with the trend from all model predictions and quantitatively consistent with a hadronic transport model.

nucl-ex↗

arXiv2014

Wavelike fracture pattern in metallic glasses: a Kelvin-Helmholtz flow instability

We report a wavelike fracture pattern in a Zr-based bulk metallic glass that has been deformed under quasi-static uniaxial tensions at temperatures between room temperature (300 K) and liquid nitrogen temperature (77 K). We attribute this wavelike pattern to a Kelvin-Helmholtz flow instability that occurred at certain interfaces between local cracking/softening regions. The instability criterion for the pattern formation is achieved via a hydrodynamic perturbation analysis, and furthermore an instability map is built which demonstrates that the shear velocity difference on both sides of the interface is the main destabilizing factor. Finally, the characteristic instability time (the inverse of the instability growth rate) is explored by seeking the dispersion relation in the dominant (fastest) instability mode. The results increase the understanding of the flow and fracture of metallic glasses as well as the nature of their liquid structures.

cond-mat.dis-nn↗

arXiv2014

Production and ratio of $π$, K, p and $Λ$ in Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV

The particle production and their ratios for $π$, $K$, $p$, and $Λ$ are studied in Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV based on a blast-wave model with thermal equilibrium mechanism. The transverse momentum spectra of the above mentioned particles at the kinetic freeze-out stage are discussed. The modification of the inverse slope of pion transverse momentum spectrum due to resonance decay has also been investigated. In addition, we found that the anti-particles to particles ratio as well as kaons to pions ratio agree with the data by the LHC-ALICE Collaboration reasonably well, while the $p/π$ ratio is overestimated by a factor of 1.5, similar to those from other thermal model calculations. It is found that the ratios of $p/π$ and $K/π$ are dominated by the radial flow but slightly affected by the baryon chemical potential. Our study thus constrains the parameters at the chemical and kinetic freeze-out stages within the framework of thermal model in Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, and will help better understand the properties of the dense and hot matter created in high-energy heavy-ion collisions at freeze-out stage.

nucl-th↗

arXiv2013

Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions at sqrt{s} = 200 GeV

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 < η < 2.0 in polarized proton-proton collisions at sqrt{s} = 200 GeV are presented. Neutral pions were detected using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 < p_T < 16 GeV/c and is found to be within the scale uncertainty of a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, A_LL, is measured in the same pseudorapidity range. This quantity is sensitive to the gluonic contribution to the proton spin, Δg(x), at low Bjorken-x (down to x approx 0.01), where it is less constrained by measurements at central pseudorapidity. The measured A_LL is consistent with model predictions. The parity-violating asymmetry, A_L, is also measured and found to be consistent with zero. The transverse single-spin asymmetry, A_N, is measured within a previously unexplored kinematic range in Feynman-x and p_T. Such measurements may aid our understanding of the on-set and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (η approx 3) and their underlying mechanisms. The A_N results presented are consistent with a twist-3 model prediction of a small asymmetry within the present kinematic range.

nucl-ex↗

arXiv2013

Fluctuations of charge separation perpendicular to the event plane and local parity violation in sqrt(sNN)=200 GeV Au+Au collisions at RHIC

Previous experimental results based on data (15 million events) collected by the STAR detector at RHIC suggest event-by-event charge separation fluctuations perpendicular to the event plane in non-central heavy-ion collisions. Here we present the correlator previously used split into its two component parts to reveal correlations parallel and perpendicular to the event plane. The results are from a high statistics 200 GeV Au+Au collisions data set (57 million events) collected by the STAR experiment. We explicitly count units of charge separation from which we find clear evidence for more charge separation fluctuations perpendicular than parallel to the event plane. We also employ a modified correlator to study the possible P-even background in same and opposite charge correlations, and find that the P-even background may largely be explained by momentum conservation and collective motion.

nucl-ex↗

arXiv2013

Extraction of dihadron-jet correlations with rigorous flow-background subtraction in a multiphase transport model

Dihadron azimuthal correlations in Au+Au collisions at $\sqrt{S_{NN}}$=200 GeV have been explored by using a multi-phase transport (AMPT) model. In order to obtain the contributions from jet-medium interactions, the combined harmonic flow background is subtracted from the raw dihadron correlation functions. The signals are compared in three associated transverse momentum ($p_{T}^{assoc}$) bins: 0.2-0.8 GeV/c, 0.8-1.4 GeV/c and 1.4-2.0 GeV/c from central to semi-peripheral geometries. The medium modifications are observed from changes in the signal shape and the relative jet contribution has been obtained within the change in the centrality from peripheral to central one. A strong $p_{T}^{assoc}$ dependence of the RMS width of jet correlation function is observed within the central geometry bin, i.e. 0-10$%$.

nucl-th↗

arXiv2013

Freeze-out Dynamics via Charged Kaon Femtoscopy in sqrt(sNN)=200 GeV Central Au+Au Collisions

We present measurements of three-dimensional correlation functions of like-sign low transverse momentum kaon pairs from sqrt(sNN)=200 GeV Au+Au collisions. A Cartesian surface-spherical harmonic decomposition technique was used to extract the kaon source function. The latter was found to have a three-dimensional Gaussian shape and can be adequately reproduced by Therminator event generator simulations with resonance contributions taken into account. Compared to the pion one, the kaon source function is generally narrower and does not have the long tail along the pair transverse momentum direction. The kaon Gaussian radii display a monotonic decrease with increasing transverse mass m_T over the interval of 0.55<=m_T<=1.15 GeV/c^2. While the kaon radii are adequately described by the m_T-scaling in the outward and sideward directions, in the longitudinal direction the lowest m_T value exceeds the expectations from a pure hydrodynamical model prediction.

nucl-ex↗

arXiv2013

Energy Dependence of Moments of Net-proton Multiplicity Distributions at RHIC

We report the beam energy (\sqrt s_{NN} = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (σ), skewness (S), and kurtosis (κ) of the net-proton multiplicity distributions in Au+Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| < 0.5) and within the transverse momentum range 0.4 < pT < 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the Quantum Chromodynamic (QCD) phase diagram. The products of the moments, Sσand κσ^{2}, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and anti-proton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation, and also to a hadron resonance gas model.

nucl-ex↗

arXiv2013

System size dependence of transverse momentum correlations at $\sqrt{s_{NN}}=$ 62.4 and 200 GeV at the BNL Relativistic Heavy Ion Collider

We present a study of the average transverse momentum ($p_t$) fluctuations and $p_t$ correlations for charged particles produced in Cu+Cu collisions at midrapidity for $\sqrt{s_{NN}} =$ 62.4 and 200 GeV. These results are compared with those published for Au+Au collisions at the same energies, to explore the system size dependence. In addition to the collision energy and system size dependence, the $p_t$ correlation results have been studied as functions of the collision centralities, the ranges in $p_t$, the pseudorapidity $η$, and the azimuthal angle $ϕ$. The square root of the measured $p_t$ correlations when scaled by mean $p_t$ is found to be independent of both colliding beam energy and system size studied. Transport-based model calculations are found to have a better quantitative agreement with the measurements compared to models which incorporate only jetlike correlations.

nucl-ex↗

arXiv2013

Third Harmonic Flow of Charged Particles in Au+Au Collisions at sqrtsNN = 200 GeV

We report measurements of the third harmonic coefficient of the azimuthal anisotropy, v_3, known as triangular flow. The analysis is for charged particles in Au+Au collisions at sqrtsNN = 200 GeV, based on data from the STAR experiment at the Relativistic Heavy Ion Collider. Two-particle correlations as a function of their pseudorapidity separation are fit with narrow and wide Gaussians. Measurements of triangular flow are extracted from the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and also from event plane analysis methods with a large pseudorapidity gap between the particles and the event plane. These results are reported as a function of transverse momentum and centrality. A large dependence on the pseudorapidity gap is found. Results are compared with other experiments and model calculations.

nucl-ex↗

arXiv2013

Single Spin Asymmetry $A_N$ in Polarized Proton-Proton Elastic Scattering at $\sqrt{s}=200$ GeV

We report a high precision measurement of the transverse single spin asymmetry $A_N$ at the center of mass energy $\sqrt{s}=200$ GeV in elastic proton-proton scattering by the STAR experiment at RHIC. The $A_N$ was measured in the four-momentum transfer squared $t$ range $0.003 \leqslant |t| \leqslant 0.035$ $\GeVcSq$, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of $A_N$ and its $t$-dependence are consistent with a vanishing hadronic spin-flip amplitude, thus providing strong constraints on the ratio of the single spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated by the Pomeron amplitude at this $\sqrt{s}$, we conclude that this measurement addresses the question about the presence of a hadronic spin flip due to the Pomeron exchange in polarized proton-proton elastic scattering.

nucl-ex↗

arXiv2013

A brief review of antimatter production

In this article, we present a brief review of the discoveries of kinds of antimatter particles, including positron, antiproton, antideuteron and antihelium-3. Special emphasis is put on the discovery of the antihypertriton and antihelium-4 nucleus which were reported by the RHIC-STAR experiment very recently. In addition, brief discussions about the effort to search for antinuclei in cosmic rays and study of the longtime confinement of the simplest antimatter atom, antihydrogen are also given. Moreover, the production mechanism of anti-light nuclei is introduced.

nucl-ex↗