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Guo, Y.

Publications and source records attributed to Guo, Y..

At least 55 records · Page 3
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DOE OSTI2018

Systematic Studies of Jet-medium Interactions in STAR

In these proceedings, we report recent jet results from STAR in heavy-ion collisions. The first measurement of centrality dependence of di-jet momentum imbalance AJ at RHIC is presented. An apparent evolution of the AJ distributions from central to peripheral collisions is observed. A new di-hadron correlation measurement with a method to subtract all orders of flow background using data themselves is also presented to study how the lost energy of jets is redistributed to particles with low-to-modest transverse momentum.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Observation of the helicity-selection-rule suppressed decay of the χ c 2 charmonium state

The decays of χ c 2 → K + K - π 0 , K S K ± π ∓ , and π + π - π 0 are studied with the ψ ( 3686 ) data samples collected with the Beijing Spectrometer (BESIII). For the first time, the branching fractions of χ c 2 → K*$\bar{K}$ , χ c 2 → $a_2^±$(1320)π ∓ /$a_2^0$(1320)π 0 , and χ c 2 → ρ( 770 ) ± π ∓ are measured. Here, K*$\bar{K}$ denotes both K* ± K ∓ and its isospin-conjugated process K* 0 $\bar{K}$ 0 +c.c., and K* denotes the resonances K*(892), $K_2^*$(1430), and $K_3^*$(1780). The observations indicate a strong violation of the helicity selection rule in χ c 2 decays into vector and pseudoscalar meson pairs. The measured branching fractions of χ c 2 → K*(892)$\bar{K}$ are more than ten times larger than the upper limit of χ c 2 → ρ ( 770 ) ± π ∓ , which is so far the first direct observation of a significant U -spin symmetry breaking effect in charmonium decays.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Search for the rare decay D + → D 0 e + ν e

Using a data set with an integrated luminosity of $2.93\text{ }\text{ }{\mathrm{fb}}^{{-}1}$ collected at $\sqrt{s}=3.773\text{ }\mathrm{GeV}$ with the BESIII detector operating at the BEPCII storage rings, we search for the rare decay ${D}^{+}{\rightarrow}{D}^{0}{e}^{+}{{\nu}}_{e}$. No signal events are observed. We set the upper limit on the branching fraction for ${D}^{+}{\rightarrow}{D}^{0}{e}^{+}{{\nu}}_{e}$ to be $1.0\times{}1{0}^{{-}4}$ at the 90% confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Coherent diffractive photoproduction of ρ 0 mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider

Here, the STAR Collaboration reports on the photoproduction of π + π - pairs in goldgold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the π + π - invariant mass spectrum with a combination of 0 and ! resonances and a direct π + π - continuum. This is the first observation of the ! in ultra-peripheral collisions, and the first measurement of ρ - ω interference at energies where photoproduction is dominated by Pomeron exchange.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Bulk properties of the medium produced in relativistic heavy-ion collisions from the beam energy scan program

We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s}$$_ {NN}$= 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons (π ± , K ± , p, and $\bar{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity (| y | < 0.1) results for multiplicity densities dN / dy, average transverse momenta $\langle$pT$\rangle$, and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Determination of the Spin and Parity of the Z c ( 3900 )

In this paper, the spin and parity of the Z c (3900) ± state are determined to be J P = 1 + with a statistical significance larger than 7σ over other quantum numbers in a partial wave analysis of the process e + e - &rarr; &pi; + &pi; - J/Ψ. We use a data sample of 1.92 fb -1 accumulated at $ \sqrt{s}=4.23 $ and 4.26 GeV with the BESIII experiment. When parametrizing the Z c (3900) ± with a Flatté-like formula, we determine its pole mass M pole = (3881.2±4.2 stat ±52.7 syst ) MeV/c 2 and pole width Γ pole = (51.8± 4.6 stat ± 36.0 syst ) MeV. Finally, we also measure cross sections for the process e + e - &rarr; Z c (3900) + &pi; - + c.c. &rarr; J/Ψ&pi; + &pi; - and determine an upper limit at the 90% confidence level for the process e + e - &rarr; Z c (4020) + &pi; - + c.c. &rarr; J/Ψ &pi; + &pi; - .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗