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Y. Gu

Publications and source records attributed to Y. Gu.

At least 37 records · Page 2Linked to original sources
arXiv2018

A Catalog of Post-starburst Quasars from Sloan Digital Sky Survey Data Release 7

We present a catalog of nearby (z $\leq$ 0.5) quasars with significant features of post-starburst stellar populations in their optical spectra, so-called post-starburst quasars, or PSQs. After carefully decomposing spectra from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) Quasar Catalog into quasar and host-galaxy components, we derive a sample of 208 PSQs. Their host-galaxy components have strong Hδ absorption ($\rm EW \geq 6 Å$) indicating a significant contribution of an intermediate-aged stellar population formed in a burst of star formation within the past 1 Gyr, which makes them potentially useful for studying the co-evolution of supermassive black holes and their host galaxies.

astro-ph.GA↗
arXiv2017

Angular decay coefficients of $J/ψ$ mesons at forward rapidity from $p+p$ collisions at $\sqrt{s}=510$ GeV

We report the first measurement of the full angular distribution for inclusive $J/ψ\rightarrowμ^{+}μ^{-}$ decays in $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV. The measurements are made for $J/ψ$ transverse momentum $2<p_{T}<10$ GeV/$c$ and rapidity $1.2<y<2.2$ in the Helicity, Collins-Soper, and Gottfried-Jackson reference frames. In all frames the polar coefficient $λ_θ$ is strongly negative at low $p_{T}$ and becomes close to zero at high $p_{T}$, while the azimuthal coefficient $λ_ϕ$ is close to zero at low $p_{T}$, and becomes slightly negative at higher $p_{T}$. The frame-independent coefficient $\tildeλ$ is strongly negative at all $p_{T}$ in all frames. The data are compared to the theoretical predictions provided by nonrelativistic quantum chromodynamics models.

hep-ex↗
arXiv2017

Measurements of double-helicity asymmetries in inclusive $J/ψ$ production in longitudinally polarized $p+p$ collisions at $\sqrt{s}=510$ GeV

We report the double helicity asymmetry, $A_{LL}^{J/ψ}$, in inclusive $J/ψ$ production at forward rapidity as a function of transverse momentum $p_T$ and rapidity $|y|$. The data analyzed were taken during $\sqrt{s}=510$ GeV longitudinally polarized $p$$+$$p$ collisions at the Relativistic Heavy Ion Collider (RHIC) in the 2013 run using the PHENIX detector. At this collision energy, $J/ψ$ particles are predominantly produced through gluon-gluon scatterings, thus $A_{LL}^{J/ψ}$ is sensitive to the gluon polarization inside the proton. We measured $A_{LL}^{J/ψ}$ by detecting the decay daughter muon pairs $μ^+ μ^-$ within the PHENIX muon spectrometers in the rapidity range $1.2<|y|<2.2$. In this kinematic range, we measured the $A_{LL}^{J/ψ}$ to be $0.012 \pm 0.010$~(stat)~$\pm$~$0.003$(syst). The $A_{LL}^{J/ψ}$ can be expressed to be proportional to the product of the gluon polarization distributions at two distinct ranges of Bjorken $x$: one at moderate range $x \approx 0.05$ where recent RHIC data of jet and $π^0$ double helicity spin asymmetries have shown evidence for significant gluon polarization, and the other one covering the poorly known small-$x$ region $x \approx 2\times 10^{-3}$. Thus our new results could be used to further constrain the gluon polarization for $x< 0.05$.

hep-ex↗
arXiv2016

Explosion of relativistic electron vortices in laser plasmas

The interaction of high intensity laser radiation with underdense plasma may lead to the formation of electron vortices. Though being quasistationary on an electron timescales, these structures tend to expand on a proton timescale due to Coloumb repulsion of ions. Using a simple analytical model of a stationary vortex as initial condition, 2D PIC simulations are performed. A number of effects are observed such as vortex boundary field intensification, multistream instabilities at the vortex boundary, and bending of the vortex boundary with the subsequent transformation into smaller electron vortices.

physics.plasm-ph↗
arXiv2014

Nuclear matter effects on $J/ψ$ production in asymmetric Cu+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

We report on $J/ψ$ production from asymmetric Cu+Au heavy-ion collisions at $\sqrt{s_{_{NN}}}$=200 GeV at the Relativistic Heavy Ion Collider at both forward (Cu-going direction) and backward (Au-going direction) rapidities. The nuclear modification of $J/ψ$ yields in Cu$+$Au collisions in the Au-going direction is found to be comparable to that in Au$+$Au collisions when plotted as a function of the number of participating nucleons. In the Cu-going direction, $J/ψ$ production shows a stronger suppression. This difference is comparable in magnitude and has the same sign as the difference expected from shadowing effects due to stronger low-$x$ gluon suppression in the larger Au nucleus. The relative suppression is opposite to that expected from hot nuclear matter dissociation, since a higher energy density is expected in the Au-going direction.

nucl-ex↗
arXiv2014

Concept for an Electron Ion Collider (EIC) detector built around the BaBar solenoid

The PHENIX collaboration presents here a concept for a detector at a future Electron Ion Collider (EIC). The EIC detector proposed here, referred to as ePHENIX, will have excellent performance for a broad range of exciting EIC physics measurements, providing powerful investigations not currently available that will dramatically advance our understanding of how quantum chromodynamics binds the proton and forms nuclear matter.

nucl-ex↗
arXiv2011

Tidal Waves -- a non-adiabatic microscopic description of the yrast states in near-spherical nuclei

The yrast states of nuclei that are spherical or weakly deformed in their ground states are described as quadrupole waves running over the nuclear surface, which we call "tidal waves". The energies and E2 transition probabilities of the yrast states in nuclides with $Z$= 44, 46, 48 and $N=56, ~58,..., 66$ are calculated by means of the cranking model in a microscopic way. The nonlinear response of the nucleonic orbitals results in a strong coupling between shape and single particle degrees of freedom.

nucl-th↗
arXiv2011

Measured g factors and the tidal-wave description of transitional nuclei near A = 100

The transient-field technique has been used in both conventional kinematics and inverse kinematics to measure the g factors of the 2+ states in the stable even isotopes of Ru, Pd and Cd. The statistical precision of the g(2+) values has been significantly improved, allowing a critical comparison with the tidal-wave version of the cranking model recently proposed for transitional nuclei in this region.

nucl-ex↗
arXiv2010

Tidal waves as yrast states in transitional nuclei

The yrast states of transitional nuclei are described as quadrupole waves running over the nuclear surface, which we call tidal waves. In contrast to a rotor, which generates angular momentum by increasing the angular velocity at approximately constant deformation, a tidal wave generates angular momentum by increasing the deformation at approximately constant angular velocity. The properties of the tidal waves are calculated by means of the cranking model in a microscopic way. The calculated energies and E2 transition probabilities of the yrast states in the transitional nuclides with $Z$= 44, 46, 48 and $N=56, 58, ..., 66$ reproduce the experiment in detail. The nonlinear response of the nucleonic orbitals results in a strong coupling between shape and single particle degrees of freedom.

nucl-th↗
arXiv2007

Lifetime measurements of Triaxial Strongly Deformed bands in $^{163}$Tm

With the Doppler Shift Attenuation Method, quadrupole transition moments, $Q_t$, were determined for the two recently proposed Triaxial Strongly Deformed (TSD) bands in $^{163}$Tm. The measured $Q_t$ moments indicate that the deformation of these bands is larger than that of the yrast, signature partners. However, the measured values are smaller than those predicted by theory. This observation appears to be valid for TSD bands in several nuclei of the region

nucl-ex↗
arXiv2006

Evidence for particle-hole excitations in the triaxial strongly-deformed well of ^{163}Tm

Two interacting, strongly-deformed triaxial (TSD) bands have been identified in the Z = 69 nucleus ^{163}Tm. This is the first time that interacting TSD bands have been observed in an element other than the Z = 71 Lu nuclei, where wobbling bands have been previously identified. The observed TSD bands in ^{163}Tm appear to be associated with particle-hole excitations, rather than wobbling. Tilted-Axis Cranking (TAC) calculations reproduce all experimental observables of these bands reasonably well and also provide an explanation for the presence of wobbling bands in the Lu nuclei, and their absence in the Tm isotopes.

nucl-ex↗
arXiv2003

RF Surface Resistance of a HIPped MgB2 Sample at 21 GHz

Magnesium diboride (MgB2) is attractive for RF cavity application for particle accelerators because it might not show an increase of RF surface losses at high magnetic surface fields, a phenomenon that has prevented high-Tc superconducting materials such as YBCO from being used for this application. We have measured the RF surface resistance (Rs) at 21 GHz of a MgB2 sample fabricated using Hot Isostatic Press (HIP) at 200 MPa and 1000 C. The results show that polishing with 0.1-micron diamond lapping film followed by a 1500-psi DI water rinse in a clean room reduced the Rs by a factor of 6.2 at 15 K and it is the lowest compared to other published data. The Rs data near the lowest temperature (~13 K) scatter between 0.6 and 1.3 mOhm. The penetration depth and energy gap were estimated to be 263 nm and 1.9-2.7, respectively, for the polished surface.

cond-mat.supr-con↗
arXiv2003

Elastic properties of hot isostatically pressed magnesium diboride

Powder of magnesium diboride was hot isostatically pressed using three qualitatively different cycles: cooling under pressure (DMCUP), "standard" cycle with pressure and temperature reduced simultaneously and isothermal presure release. Elastic properties of dense material were measured at normal conditions using resonant ultrasound specroscopy (RUS) method. The highest value of density (2.66 g/cc) and elastic moduli (Young's modulus 273.5 GPa, bulk modulus 143.9 GPa, Poisson's ratio 0.183) correspond to the sample processed using DMCUP cycle. These data agree very well with some theoretical predictions based on quantum mechanics calculations. Effect of lower density on elastic constants is consistent with theoretical approach based on elasticity theory taking into account effect of low porosity.

cond-mat.supr-con↗
arXiv2003

Microwave performance of high-density bulk MgB2

We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.

cond-mat.supr-con↗
arXiv2003

Hot isostatic pressing of powder in tube MgB2 wires

The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB2 crystallites.

cond-mat.mtrl-sci↗
arXiv2003

Defect structures in MgB2 wires introduced by hot isostatic pressing

The microstructures of MgB2 wires prepared by the powder-in-tube technique and subsequent hot isostatic pressing were investigated using transmission electron microscopy. Large amount of crystalline defects including small angle twisting, tilting, and bending boundaries, in which high densities of dislocations reside, were found forming sub-grains within MgB2 grains. It is believed that these defects resulted from particle deformation during the hot isostatic pressing process and are effective flux pinning centers that contribute to the high critical current densities of the wires at high temperatures and at high fields.

cond-mat.mtrl-sci↗