Engineering topics
Cheng, J.
Publications and source records attributed to Cheng, J..
Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au + Au collisions at s N N = 200 GeV
Here, the STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at √ s NN = 200 GeV. Charged jets are reconstructed with the anti-k T algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents (p T > 0.2 GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at midrapidity, for charged-jet transverse momentum p ch T,jet < 30 GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at √s = 2.76 TeV, to calculations for p+p collisions at √s = 200 GeV based on the pythia Monte Carlo generator and on a next-to-leading order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of 2.8 ± 0.2(stat) ± 1.5(sys) GeV/c, for 10 < p ch T,jet < 20 GeV/c and R = 0.5. No medium-induced change in jet shape is observed for R < 0.5. The azimuthal distribution of low-p ch T,jet recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasiparticles in the hot QCD medium. As a result, measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au + Au collisions of 50 ± 30(sys)% of the large-angle yield in p+p collisions predicted by pythia.
Dijet imbalance measurements in Au + Au and p p collisions at s N N = 200 GeV at STAR
In this paper, we report the first dijet transverse momentum asymmetry measurements from Au + Au and pp collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above 2 GeV/c display a significantly higher momentum imbalance in heavy-ion collisions than in the pp reference. Finally, when reexamined with correlated soft particles included, we observe that these dijets then exhibit a unique new feature—momentum balance is restored to that observed in pp for a jet resolution parameter of R = 0.4, while rebalancing is not attained with a smaller value of R = 0.2.
Global Λ hyperon polarization in nuclear collisions
The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. But, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin–orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark–gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. Furthermore, these data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.
Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay
Here, the Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GW th reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective 239 Pu fission fractions F 239 from 0.25 to 0.35, Daya Bay measures an average IBD yield ¯σf of (5.90±0.13)×10 –43 cm 2 /fission and a fuel-dependent variation in the IBD yield, dσ f /dF 239 , of (–1.86±0.18)×10 –43 cm 2 /fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the 239 Pu fission fraction at 10 standard deviations. The variation in IBD yield is found to be energy dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1σ. This discrepancy indicates that an overall deficit in the measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes 235 U, 239 Pu, 238 U, and 241 Pu. Based on measured IBD yield variations, yields of (6.17±0.17) and (4.27±0.26)×10 –43 cm 2 /fission have been determined for the two dominant fission parent isotopes 235 U and 239 Pu. A 7.8% discrepancy between the observed and predicted 235 U yields suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
Campus Capability Plan
Lawrence Livermore National Laboratory Campus Capability Plan for 2018-2028. Lawrence Livermore National Laboratory (LLNL) is one of three national laboratories that are part of the National Nuclear Security Administration. LLNL provides critical expertise to strengthen U.S. security through development and application of world-class science and technology that: Ensures the safety, reliability, and performance of the U.S. nuclear weapons stockpile; Promotes international nuclear safety and nonproliferation; Reduces global danger from weapons of mass destruction; Supports U.S. leadership in science and technology. Essential to the execution and continued advancement of these mission areas are responsive infrastructure capabilities. This report showcases each LLNL capability area and describes the mission, science, and technology efforts enabled by LLNL infrastructure, as well as future infrastructure plans.
Measurement of D 0 Azimuthal Anisotropy at Midrapidity in Au + Au Collisions at s N N = 200 GeV
In this paper, we report the first measurement of the elliptic anisotropy (v2) of the charm meson D 0 at midrapidity (|y| < 1) in Au + Au collisions at $\sqrt{s}$$_ {NN}$ = 200 GeV. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured D 0 v 2 in 0%–80% centrality Au + Au collisions can be described by a viscous hydrodynamic calculation for a transverse momentum (p T ) of less than 4 GeV/c . The D 0 v 2 as a function of transverse kinetic energy (m T - m 0 , where m T = $\sqrt{p}$$2\atop{T}$ + m$2\atop{0}$) is consistent with that of light mesons in 10%–40% centrality Au + Au collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. In conclusion, several theoretical models, with the temperature-dependent, dimensionless charm spatial diffusion coefficient (2πTD s ) in the range of ~2–12 , are able to simultaneously reproduce our D 0 v 2 result and our previously published results for the D 0 nuclear modification factor.
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.
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.
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.
Measurement of electron antineutrino oscillation based on 1230 days of operation of the Daya Bay experiment
Here, a measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of ¯ν e ’s. Comparison of the ¯ν e rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors (~1500–1950 m) relative to detectors near the reactors (~350–600 m) allowed a precise measurement of ¯ν e disappearance. More than 2.5 million ¯ν e inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012–July, 2015). The ¯ν e rate observed at the far detectors relative to the near detectors showed a significant deficit, R = 0.949 ± 0.002(stat) ± 0.002(syst). The energy dependence of ¯ν e disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle sin 2 2θ 13 = 0.0841 ± 0.0027(stat) ± 0.0019(syst) and the effective neutrino mass-squared difference of |Δm 2 ee | = (2.50 ± 0.06(stat) ± 0.06(syst)) × 10 –3 eV 2 . Analysis using the exact three-flavor probability found Δm 2 32 = (2.45 ± 0.06(stat) ± 0.06(syst)) × 10 –3 eV 2 assuming the normal neutrino mass hierarchy and Δm 2 32 = (–2.56 ± 0.06(stat) ± 0.06(syst)) × 10 –3 eV 2 for the inverted hierarchy.
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.
Charge-Dependent Directed Flow in Cu + Au Collisions at s N N = 200 GeV
Here we present the first measurement of charge-dependent directed flow in Cu + Au collisions at $\sqrt{s}$$_ {NN}$ = 200 GeV . The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics model, which suggests that most of the electric charges, i.e., quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, 1fm / c .
Improved measurement of the reactor antineutrino flux and spectrum at Daya Bay
Here, a new measurement of the reactor antineutrino flux and energy spectrum by the Daya Bay reactor neutrino experiment is reported. The antineutrinos were generated by six 2.9 GW th nuclear reactors and detected by eight antineutrino detectors deployed in two near (560 m and 600 m flux-weighted baselines) and one far (1640 m flux-weighted baseline) underground experimental halls. With 621 days of data, more than 1.2 million inverse beta decay (IBD) candidates were detected. The IBD yield in the eight detectors was measured, and the ratio of measured to predicted flux was found to be 0.946 ± 0.020 (0.992 ± 0.021) for the Huber+Mueller (ILL+Vogel) model. A 2.9σ deviation was found in the measured IBD positron energy spectrum compared to the predictions. In particular, an excess of events in the region of 4$-$6 MeV was found in the measured spectrum, with a local significance of 4.4σ. Finally, a reactor antineutrino spectrum weighted by the IBD cross section is extracted for model-independent predictions.
Limits on Active to Sterile Neutrino Oscillations from Disappearance Searches in the MINOS, Daya Bay, and Bugey-3 Experiments
Searches for a light sterile neutrino have been performed independently by the MINOS and the Daya Bay experiments using the muon (anti)neutrino and electron antineutrino disappearance channels, respectively. In this Letter, results from both experiments are combined with those from the Bugey-3 reactor neutrino experiment to constrain oscillations into light sterile neutrinos. The three experiments are sensitive to complementary regions of parameter space, enabling the combined analysis to probe regions allowed by the Liquid Scintillator Neutrino Detector (LSND) and MiniBooNE experiments in a minimally extended four-neutrino flavor framework. Stringent limits on sin22θμe are set over 6 orders of magnitude in the sterile mass-squared splitting Δm412. The sterile-neutrino mixing phase space allowed by the LSND and MiniBooNE experiments is excluded for Δm412<0.8 eV2 at 95% CLs.
Improved Search for a Light Sterile Neutrino with the Full Configuration of the Daya Bay Experiment
Here this Letter reports an improved search for light sterile neutrino mixing in the electron antineutrino disappearance channel with the full configuration of the Daya Bay Reactor Neutrino Experiment. With an additional 404 days of data collected in eight antineutrino detectors, this search benefits from 3.6 times the statistics available to the previous publication, as well as from improvements in energy calibration and background reduction. A relative comparison of the rate and energy spectrum of reactor antineutrinos in the three experimental halls yields no evidence of sterile neutrino mixing in the 2 x 10 -4 $\lesssim$| Δ$2\atop{41}$| $\lesssim$ 0.3 eV 2 . The resulting limits on sin 2 $2\theta$ 14 are improved by approximately a factor of two over previous results and constitute the most stringent constraints to date in the |Δm$2\atop{41}$| $\lesssim$ 0.2 eV 2 region.
Synchronization of Geodesic Acoustic Modes and Magnetic Fluctuations in Toroidal Plasmas
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