arXiv subjects

Z. Xu

Publications and source records attributed to Z. Xu.

At least 73 records · Page 4Linked to original sources

arXiv2015

Cosmic Ray Test of Mini-drift Thick Gas Electron Multiplier Chamber for Transition Radiation Detector

A thick gas electron multiplier (THGEM) chamber with an effective readout area of 10$\times$10 cm$^{2}$ and a 11.3 mm ionization gap has been tested along with two regular gas electron multiplier (GEM) chambers in a cosmic ray test system. The thick ionization gap makes the THGEM chamber a mini-drift chamber. This kind mini-drift THGEM chamber is proposed as part of a transition radiation detector (TRD) for identifying electrons at an Electron Ion Collider (EIC) experiment. Through this cosmic ray test, an efficiency larger than 94$\%$ and a spatial resolution $\sim$220 $μ$m are achieved for the THGEM chamber at -3.65 kV. Thanks to its outstanding spatial resolution and thick ionization gap, the THGEM chamber shows excellent track reconstruction capability. The gain uniformity and stability of the THGEM chamber are also presented.

physics.ins-det↗

arXiv2015

$v_2$ of charged hadrons in a`soft + hard' model for PbPb collisions at $\sqrt s$ = 2.76 ATeV

We describe transverse spectra as well as azimuthal anisotropy ($v_2$) of charged hadrons stemming from various centrality PbPb collisions at $\sqrt s$ = 2.76 ATeV analytically in a `soft + hard' model. In this model, we propose that hadron yields produced in heavy-ion collisions are simply the sum of yields stemming from jets (hard yields) and yields stemming from the Quark-Gluon Plasma (soft yields). The hadron spectra in both types of yields are approximated by the Tsallis distribution. It is found that the anisotropy decreases for more central collisions.

hep-ph↗

arXiv2015

A `soft + hard' model for heavy-ion collisions

We describe the spectra of neutral pions stemming from AuAu collisions at $\sqrt s$ = 200 AGeV in a `soft + hard' model. The model is based on the assumptions that hadrons stemming from hard processes are described via perturbative quantum chromodynamics improved parton model calculations, while those stemming from the Quark-Gluon Plasma (we refer to as soft yields) can be described in a super-statistical model induced by multiplicity fluctuations. The obtained dependence of the parameters of the model on the event centrality is compared to what is observed in PbPb collisions at $\sqrt s$ = 2.76 ATeV.

hep-ph↗

arXiv2014

Performance of the gas gain monitoring system of the CMS RPC muon detector

The RPC muon detector of the CMS experiment at the LHC (CERN, Geneva, Switzerland) is equipped with a Gas Gain Monitoring (GGM) system. A report on the stability of the system during the 2011-2012 data taking run is given, as well as the observation of an effect which suggests a novel method for the monitoring of gas mixture composition.

physics.ins-det↗

arXiv2014

Mach cones in viscous heavy-ion collisions

The formation of Mach cones is studied in a full $(3+1)$-dimensional setup of ultrarelativistic heavy-ion collisions, considering a transverse and longitudinal expanding medium at Relativistic Heavy-Ion Collider energies. For smooth initial conditions and central collisions the jet-medium interaction is investigated using high-energy jets and various values of the ratio of shear viscosity over entropy density, $η/s$. For small viscosities, the formation of Mach cones is proven, whereas for larger viscosities the characteristic structures smear out and vanish eventually. The formation of a double-peak structure both in a single- and in a multiple-jet event is discussed.

hep-ph↗

arXiv2014

Calibration and performance of the STAR Muon Telescope Detector using cosmic rays

We report the timing and spatial resolution from the Muon Telescope Detector (MTD) installed in the STAR experiment at RHIC. Cosmic ray muons traversing the STAR detector have an average transverse momentum of 6 GeV/c. Due to their very small multiple scattering, these cosmic muons provide an ideal tool to calibrate the detectors and measure their timing and spatial resolution. The values obtained were ~100 ps and ~1-2 cm, respectively. These values are comparable to those obtained from cosmic-ray bench tests and test beams.

physics.ins-det↗

arXiv2013

Transition From Ideal To Viscous Mach Cones In A Kinetic Transport Approach

Using a microscopic transport model we investigate the evolution of conical structures originating from the supersonic projectile moving through the hot matter of ultrarelativistic particles. Using different scenarios for the interaction between projectile and matter, and different transport properties of the matter, we study the formation and structure of Mach cones. Especially, a dependence of the Mach cone angle on the details and rate of the energy deposition from projectile to the matter is investigated. Furthermore, the two-particle correlations extracted from the numerical calculations are compared to an analytical approximation. We find that the propagation of a high energetic particle through the matter does not lead to the appearance of a double peak structure as observed in the ultrarelativistic heavy-ion collision experiments. The reason is the strongly forward-peaked energy and momentum deposition in the head shock region. In addition, by adjusting the cross section we investigate the influence of the viscosity to the structure of Mach cones. A clear and unavoidable smearing of the profile depending on a finite ratio of shear viscosity to entropy density is clearly visible.

nucl-th↗

arXiv2013

Pair Breaking Caused by Magnetic Impurities in the High-T$_\text{C}$ Superconductor Bi$_{2.1}$Sr$_{1.9}$Ca(Cu$_{1-x}$Fe$_{x}$)$_{2}$O$_{y}$

Conventional superconductivity is robust against the addition of impurities unless the impurities are magnetic in which case superconductivity is quickly suppressed. Here we present a study of the cuprate superconductor Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+δ}$ that is intentionally doped with the magnetic impurity, Fe. Through the use of our Tomographic Density of States (TDoS) technique, we find that while the superconducting gap magnitude is essentially unaffected by the inclusion of iron, the onset of superconductivity, T$_{C}$, and the pair-breaking rate are strongly dependent and correlated. These findings suggest that, in the cuprates, the pair-breaking rate is critical to the determination of T$_{C}$ and that magnetic impurities do not disrupt the strength of pairing but rather the lifetime of the pairs.

cond-mat.supr-con↗

arXiv2013

Investigation of Heat Conductivity in Relativistic Systems using a Partonic Cascade

Motivated by the classical picture of heat flow we construct a stationary temperature gradient in a relativistic microscopic transport model. Employing the relativistic Navier-Stokes ansatz we extract the heat conductivity κ for a massless Boltzmann gas using only binary collisions with isotropic cross sections. We compare the numerical results to analytical expressions from different theories and discuss the final results. The directly extracted value for the heat conductivity can be referred to as a literature reference within the numerical uncertainties.

hep-ph↗

arXiv2012

Coupled Spin-Orbital Texture in a Prototypical Topological Insulator

One of the most important properties of topological insulators (TIs) is the helical spin texture of the Dirac surface states, which has been theoretically and experimentally argued to be left-handed helical above the Dirac point and right handed helical below. However, since the spin is not a good quantum number in these strongly spin-orbit coupled systems, this can not be a complete statement, and we must consider the total angular momentum J = L + S that is a contribution of the spin and orbital terms. Using a combination of orbital and spin-resolved angle-resolved photoemission spectroscopy (ARPES), we show a direct link between the different orbital and spin components, with a "backwards" spin texture directly observed for the in-plane orbital states of Bi2Se3.

cond-mat.str-el↗

arXiv2012

A Trapped Field of >3T in Bulk MgB2 Fabricated by Uniaxial Hot Pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetised stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot-Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.

cond-mat.supr-con↗

arXiv2012

In-Plane Orbital Texture Switch at the Dirac Point in the Topological Insulator Bi2Se3

Topological insulators are novel macroscopic quantum-mechanical phase of matter, which hold promise for realizing some of the most exotic particles in physics as well as application towards spintronics and quantum computation. In all the known topological insulators, strong spin-orbit coupling is critical for the generation of the protected massless surface states. Consequently, a complete description of the Dirac state should include both the spin and orbital (spatial) parts of the wavefunction. For the family of materials with a single Dirac cone, theories and experiments agree qualitatively, showing the topological state has a chiral spin texture that changes handedness across the Dirac point (DP), but they differ quantitatively on how the spin is polarized. Limited existing theoretical ideas predict chiral local orbital angular momentum on the two sides of the DP. However, there have been neither direct measurements nor calculations identifying the global symmetry of the spatial wavefunction. Here we present the first results from angle-resolved photoemission experiment and first-principles calculation that both show, counter to current predictions, the in-plane orbital wavefunctions for the surface states of Bi2Se3 are asymmetric relative to the DP, switching from being tangential to the k-space constant energy surfaces above DP, to being radial to them below the DP. Because the orbital texture switch occurs exactly at the DP this effect should be intrinsic to the topological physics, constituting an essential yet missing aspect in the description of the topological Dirac state. Our results also indicate that the spin texture may be more complex than previously reported, helping to reconcile earlier conflicting spin resolved measurements.

cond-mat.mtrl-sci↗

arXiv2012

Solving the heat-flow problem with transient relativistic fluid dynamics

Israel-Stewart theory is a causal, stable formulation of relativistic dissipative fluid dynamics. This theory has been shown to give a decent description of the dynamical behavior of a relativistic fluid in cases where shear stress becomes important. In principle, it should also be applicable to situations where heat flow becomes important. However, it has been shown that there are cases where Israel-Stewart theory cannot reproduce phenomena associated with heat flow. In this paper, we derive a relativistic dissipative fluid-dynamical theory from kinetic theory which provides a good description of all dissipative phenomena, including heat flow. We explicitly demonstrate this by comparing this theory with numerical solutions of the relativistic Boltzmann equation.

nucl-th↗

arXiv2012

Treatment of Errors in Efficiency Calculations

In this report we discuss the treatment of statistical errors in cut efficiencies. The two commonly used methods for the calculation of the errors, Poissonian and Binomial, are shown to be defective. We derive the form of the underlying probability density function and characterize its mean, mode, and variance. A method for the calculation of errors based on the variance of the distribution is discussed.

physics.data-an↗

arXiv2012

Magnetic fields of an active region filament from full Stokes analysis of Si I 1082.7 nm and He I 1083.0 nm

Vector magnetic fields of an active region filament in the photosphere and upper chromosphere are obtained from spectro-polarimetric observations recorded with the Tenerife Infrared Polarimeter (TIP II) at the German Vacuum Tower Telescope (VTT). We apply Milne-Eddington inversions on full Stokes vectors of the photospheric Si I 1082.7 nm and the upper chromospheric He I triplet at 1083.0 nm to obtain magnetic field vector and velocity maps in two atmosphere layers. We find that: (1)A complete filament was already present in H$α$ at the beginning of the TIP II data acquisition. Only a partially formed one, composed of multiple small threads, was present in He I. (2) The AR filament comprises two sections. One shows strong magnetic field intensities, about 600 - 800 G in the upper chromosphere and 800 - 1000 G in the photosphere. The other exhibits only comparatively weak magnetic field strengths in both layers. (3) The Stokes V signal is indicative of a dip in the magnetic field strength close to the chromospheric PIL. (3) In the chromosphere consistent upflows are found along the PIL flanked by downflows. (4) The transversal magnetic field is nearly parallel to the PIL in the photosphere and inclined by 20 - 30 degree in the chromosphere. (5) The chromospheric magnetic field around the filament is found to be in normal configuration, while the photospheric field presents a concave magnetic topology. The observations are consistent with the emergence of a flux rope with a subsequent formation of a filament.

astro-ph.SR↗

arXiv2011

NMR Studies of pseudogap and electronic inhomogeneity in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We report $^{17}$O NMR measurements in single crystals of overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ with $T_c=82$ K. We measure the full anisotropy of the planar oxygen Knight shift, electric field gradient, and spin lattice relaxation rate tensors, and show that the entire temperature dependence is determined by the suppression of the density of states in the pseudogap below $T^*\sim 94$ K. The linewidth can be explained by a combination of magnetic and quadrupolar broadening as a result of an inhomogeneous distribution of local hole concentrations that is consistent with scanning tunneling microscopy measurements. This distribution is temperature independent, does not break $C_4$ symmetry, and exhibits no change below $T^*$ or $T_c$.

cond-mat.str-el↗

arXiv2011

Calculation of shear viscosity using Green-Kubo relations within a parton cascade

The shear viscosity of a gluon gas is calculated using the Green-Kubo relation. Time correlations of the energy-momentum tensor in thermal equilibrium are extracted from microscopic simulations using a parton cascade solving various Boltzmann collision processes. We find that the pQCD based gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers the shear viscosity by a factor of 3-8 compared to elastic scatterings. The shear viscosity scales with the coupling as 1/(alpha_s^2\log(1/alpha_s)). For a constant coupling constant the shear viscosity to entropy density ratio has no dependence on temperature. Replacing the pQCD-based collision angle distribution of binary scatterings by an isotropic form decreases the shear viscosity by a factor of 3.

hep-ph↗

arXiv2011

Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120dBc/Hz at 1 Hz offset from a 11.55GHz carrier frequency. We furthermore report a detailed investigation into the fundamental noise sources which afflicts the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects we implement an ultra-low noise carrier-suppression measurement system, which circumvents the detection and amplification noise of more conventional techniques. This technique now allows the suppression of these unwanted sources of noise to very low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a fed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented in order to reduce the relative intensity noise which causes unwanted phase noise through power to phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements on the relative intensity noise of the fiber comb.

physics.optics↗