Engineering Papers⌕ Search

Engineering topics

Chen, X.

Publications and source records attributed to Chen, X..

At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Energy dependence of J / ψ production in Au + Au collisions at s N N = 39 , 62.4 and 200 GeV

The inclusive J / ψ transverse momentum spectra and nuclear modification factors are reported at mid-rapidity ( | y | < 1.0 ) in Au + Au collisions at s N N = 39, 62.4 and 200 GeV taken by the STAR experiment. A suppression of J / ψ production, with respect to the production in p + p scaled by the number of binary nucleon–nucleon collisions, is observed in central Au + Au collisions at these three energies. No significant energy dependence of nuclear modification factors is found within uncertainties. The measured nuclear modification factors can be described by model calculations that take into account both suppression of direct J / ψ production due to the color screening effect and J / ψ regeneration from recombination of uncorrelated charm–anticharm quark pairs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7−3946

We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7−3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti)correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H i emission. We present a series of simulated images of RX J1713.7−3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton , whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H i observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Measurement of the cross section and longitudinal double-spin asymmetry for dijet production in polarized p p collisions at s = 200 GeV

We report the first measurement of the longitudinal double-spin asymmetry A LL for mid-rapidity dijet production in polarized pp collisions at a center-of-mass energy of $\sqrt{s}$ = 200 GeV. The dijet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. A LL results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. Lastly, the measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-x > 0.05.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Electrochemical corrosion of a noble metal-bearing alloy-oxide composite

The effects of added Ru and Pd on the microstructure and electrochemical behaviour of a composite material made by melting those metals with AISI 410 stainless steel, Zr, Mo, and lanthanide oxides were assessed using electrochemical and microscopic methods Furthermore, the lanthanide oxides reacted with Zr to form durable lanthanide zirconates and Mo alloyed with steel to form FeMoCr intermetallics. The noble metals alloyed with the steel to provide solid solution strengthening and inhibit carbide/nitride formation. In a passive film formed during electrochemical tests in acidic NaCl solution, but became less effective as corrosion progressed and regions over the intermetallics eventually failed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Direct virtual photon production in Au+Au collisions at s N N = 200 GeV

We report the direct virtual photon invariant yields in the transverse momentum ranges 1 < p T < 3 GeV / c and 5 < p T < 10 GeV / c at mid-rapidity derived from the dielectron invariant mass continuum region 0.10 < M e e < 0.28 GeV / c 2 for 0–80% minimum-bias Au+Au collisions at s N N = 200 GeV . A clear excess in the invariant yield compared to the nuclear overlap function T A A scaled p + p reference is observed in the p T range 1 < p T < 3 GeV / c . For p T > 6 GeV / c the production follows T A A scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Stress distribution during cold compression of a quartz aggregate using synchrotron X-ray diffraction: Observed yielding, damage, and grain crushing

Herein, we report new experimental results that quantify the stress distribution within a quartz aggregate during pore collapse and grain crushing. The samples were probed with synchrotron X-ray diffraction as they were compressed in a multianvil deformation apparatus at room temperature from low pressure (tens of megapascal) to pressures of a few gigapascal. In such a material, stress is likely to concentrate at grain-to-grain contacts and vanish where grains are bounded by open porosity. Therefore, internal stress is likely to vary significantly from point to point in such an aggregate, and hence, it is important to understand both the heterogeneity and anisotropy of such variation with respect to the externally applied stress. In our quartz aggregate (grain size of ~4 μm), the measured diffraction peaks broaden asymmetrically at low pressure (tens of megapascal), suggesting that open pores are still a dominant characteristic of grain boundaries. In contrast, a reference sample of novaculite (a highly dense quartz polycrystal, grain size of ~6–9 μm) showed virtually no peak broadening with increasing pressure. In the quartz aggregate, we observed significant deviation in the pressure-volume curves in the range of P = 400–600 MPa. We suggest that this marks the onset of grain crushing (generally denoted as P* in the rock mechanic literature), which is commonly reported to occur in sandstones at pressures of this order, in general agreement with a Hertzian analysis of fracturing at grain contacts.

58 GEOSCIENCES↗
DOE OSTI2017

Magnetic and magnetocaloric properties of spin-glass material DyNi 0.67 Si 1.34

Structural, magnetic, and magnetocaloric properties of DyNi 0.67 Si 1.34 were investigated using X-ray powder diffraction, magnetic susceptibility, and magnetization measurements. X-ray powder diffraction pattern shows that DyNi 0.67 Si 1.34 crystallizes in the AlB 2 -type hexagonal structure (space group: P 6/ mmm , No. 191, a = b = 3.9873(9) Å, and c = 3.9733(1) Å). The compound is a spin-glass with the freezing temperature TG = 6.2 K. The ac magnetic susceptibility measurements confirm magnetic frustration in DyNi 0.67 Si 1.34 . Furthermore, the maximum value of the magnetic entropy change determined from M(H) data is –16.1 J/kg K at 10.5 K for a field change of 70 kOe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Measurement of higher-order multipole amplitudes in ψ ( 3686 ) → γ χ c 1 , 2 with χ c 1 , 2 → γ J / ψ and search for the transition η c ( 2 S ) → γ J / ψ

Using 106 ×10 6 ψ (3686) events collected with the BESIII detector, we measure multipole amplitudes for the decay ψ (3686) → γχ c1,2 → γγ J/ψ beyond the dominant electric-dipole amplitudes. The normalized magnetic-quadrupole (M2) amplitude for ψ (3686) → γχ c1,2 → γγ J/ψ and the normalized electric-octupole amplitudes for ψ ( 3686 ) → γχ c2 , χ c2 → γJ/ψ are determined. The M2 amplitudes for ψ (3686) → γ χ c1 and χ c1,2 → γ J / ψ are found to differ significantly from zero and are consistent with theoretical predictions. We also obtain the ratios of M2 contributions of ψ ( 3686 ) and J /ψ decays to χ c1,2 , b$1\atop{2}$/ b$2\atop{2}$ = 1.35 ± 0.72 and a$1\atop{2}$/a$2\atop{2}$ = 0.617 ± 0.083 , which agree well with theoretical expectations. By considering the multipole contributions of χ c 1 , 2 , we measure the product branching fractions for the cascade decays ψ ( 3686 ) → γχ c0,1,2 → γγJ/ψ and search for the process η c (2S) → γJ/ψ through ψ ( 3686 ) → γη c (2S). The product branching fraction for ψ (3686) → γχ c0 → γγJ/ψ is 3σ larger than published measurements, while those of ψ (3686) → γχ c1,2 → γγJ/ψ are consistent. In conclusion, no significant signal for the decay ψ ( 3686 ) → γη c (2S) → γγJ/ψ is observed, and the upper limit of the product branching fraction at the 90% confidence level is determined.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Elliptic flow of electrons from heavy-flavor hadron decays in Au + Au collisions at s N N = 200 , 62.4, and 39 GeV

Here, we present measurements of elliptic flow (v 2 ) of electrons from the decays of heavy-flavor hadrons (e HF ) by the STAR experiment. For Au+Au collisions at $\sqrt{s}$$_ {NN}$=200 GeV we report v 2 , for transverse momentum (p T ) between 0.2 and 7 GeV/c, using three methods: the event plane method (v 2 {EP}), two-particle correlations (v 2 {2}), and four-particle correlations (v 2 {4}). For Au+Au collisions at $\sqrt{s}$$_ {NN}$=62.4 and 39 GeV we report v 2 {2} for p T <2GeV/c. v 2 {2} and v 2 {4} are nonzero at low and intermediate p T at 200 GeV, and v 2 {2} is consistent with zero at low p T at other energies. The v 2 {2} at the two lower beam energies is systematically lower than at $\sqrt{s}$$_ {NN}$=200 GeV for p T <1GeV/c. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s}$$_ {NN}$=200 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Precise Measurement of the e + e - → π + π - J / ψ Cross Section at Center-of-Mass Energies from 3.77 to 4.60 GeV

The statistical significance of this resonance is estimated to be larger than 7.6 σ . The mass and width of the second resonance agree with the Y(4360) resonance reported by the BABAR and Belle experiments within errors. Finally, the Y ( 4008 ) resonance previously observed by the Belle experiment is not confirmed in the description of the BESIII data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Evidence of Two Resonant Structures in e + e - → π + π - h c

The cross sections of e + e - →π + π - h c at center-of-mass energies from 3.896 to 4.600 GeV are measured using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The cross sections are found to be of the same order of magnitude as those of e + e - →π + π - J/ψ and e + e - →π + π - ψ (2S), but the line shape is inconsistent with the Y states observed in the latter two modes. Two structures are observed in the e + e - → π + π - h c cross sections around 4.22 and 4.39 GeV / c 2 , which we call Y ( 4220 ) and Y ( 4390 ) , respectively. A fit with a coherent sum of two Breit-Wigner functions results in a mass of (4218.4 $+5.5\atop{-4.5 ± 0.9) MeV/c 2 and a width of 66.0$+12.3\atop-8.3$±0.4 MeV for the Y (4220), and a mass of (4391.5 $+6.3\atop-16.8$ ± 1.0) MeV/c 2 and a width of (139.5$+16.2\atop-20.6 ± 0.6) MeV for the Y (4390), where the first uncertainties are statistical and the second ones systematic. The statistical significance of Y ( 4220 ) and Y(4390) is 10σ over one structure assumption.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

A Luminous and Isolated Gamma-Ray Flare from the Blazar B2 1215+30

We report that B2 1215+30 is a BL-Lac-type blazar that was first detected at TeV energies by the MAGIC atmospheric Cherenkov telescopes and subsequently confirmed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) observatory with data collected between 2009 and 2012. In 2014 February 08, VERITAS detected a large-amplitude flare from B2 1215+30 during routine monitoring observations of the blazar 1ES 1218+304, located in the same field of view. The TeV flux reached 2.4 times the Crab Nebula flux with a variability timescale of $\lt 3.6\,\mathrm{hr}$. Multiwavelength observations with Fermi-LAT, Swift, and the Tuorla Observatory revealed a correlated high GeV flux state and no significant optical counterpart to the flare, with a spectral energy distribution where the gamma-ray luminosity exceeds the synchrotron luminosity. Lastly, when interpreted in the framework of a one-zone leptonic model, the observed emission implies a high degree of beaming, with Doppler factor $\delta \gt 10$, and an electron population with spectral index $p\lt 2.3$.

79 ASTRONOMY AND ASTROPHYSICS↗