We report the first measurement of the elliptic anisotropy (v_{2}) of the charm meson D^{0} at midrapidity (|y|<1) in Au+Au collisions at sqrts_{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}=sqrtp_{T}^{2}+m_{0}^{2}) 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. 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.
We present the first measurement of charge-dependent directed flow in Cu+Au collisions at sqrts_{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, 1 fm/c.
The spatial extents of particle emission source in high-energy collisions can be measured using two-particle femtoscopic correlations. In collisions with small multipli cities, such as proton-proton ...collisions, correlation functions can be distorted by non-femtos copic effects, for example due to the correlations that caused by energy-momentum conservatio n laws, jets and mini-jets. To estimate these effects, a simulation of p+p collisions at \(\sqrt{s}\)=200 and \(\sqrt{s}\)=510 GeV using PYTHIA 6.4.28 and HIJING 1.383, and p+Au collisions at \(\sqrt{s_{NN}}\)=200 GeV using HIJING were performed. Charged pion and kaon correlation functions obtained from the Monte Carlo generators and their comparison to the experimental data are p resented.
We present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at sqrts=500 GeV by the STAR experiment at RHIC. The ...measured observable is sensitive to the Sivers function, one of the transverse-momentum-dependent parton distribution functions, which is predicted to have the opposite sign in proton-proton collisions from that observed in deep inelastic lepton-proton scattering. These data provide the first experimental investigation of the nonuniversality of the Sivers function, fundamental to our understanding of QCD.
We present high precision measurements of elliptic flow near midrapidity (|y|<1.0) for multistrange hadrons and ϕ meson as a function of centrality and transverse momentum in Au+Au collisions at ...center of mass energy √sNN=200 GeV. We observe that the transverse momentum dependence of ϕ and Ω v2 is similar to that of π and p, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0%-30% and 30%-80% collision centrality. There is an indication of the breakdown of previously observed mass ordering between ϕ and proton v2 at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v2.
We present ΛΛ correlation measurements in heavy-ion collisions for Au+Au collisions at sqrts_{NN}=200 GeV using the STAR experiment at the Relativistic Heavy-Ion Collider. The Lednický-Lyuboshitz ...analytical model has been used to fit the data to obtain a source size, a scattering length and an effective range. Implications of the measurement of the ΛΛ correlation function and interaction parameters for dihyperon searches are discussed.
We report the first measurement of charmed-hadron (D(0)) production via the hadronic decay channel (D(0) → K(-) + π(+)) in Au+Au collisions at sqrts(NN) = 200 GeV with the STAR experiment. The charm ...production cross section per nucleon-nucleon collision at midrapidity scales with the number of binary collisions, N(bin), from p+p to central Au+Au collisions. The D(0) meson yields in central Au + Au collisions are strongly suppressed compared to those in p+p scaled by N(bin), for transverse momenta p(T) > 3 GeV/c, demonstrating significant energy loss of charm quarks in the hot and dense medium. An enhancement at intermediate p(T) is also observed. Model calculations including strong charm-medium interactions and coalescence hadronization describe our measurements.
Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at √s(NN)=200 GeV in STAR are presented. The trigger jet ...population in Au+Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (pTassoc) and enhanced at low pTassoc in 0%-20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions.
Phys. Rev. C 110, 014905 (2024) For the search of the chiral magnetic effect (CME), STAR previously presented
the results from isobar collisions
(${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$,
...${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blind
analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the
CME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$),
normalized by elliptic anisotropy ($v_{2}$), was observed to be close to but
systematically larger than the inverse multiplicity ratio. The background
baseline for the isobar ratio, $Y =
\frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, is
naively expected to be $\frac{(1/N)^{\text{Ru}}}{(1/N)^{\text{Zr}}}$; however,
genuine two- and three-particle correlations are expected to alter it. We
estimate the contributions to $Y$ from those correlations, utilizing both the
isobar data and HIJING simulations. After including those contributions, we
arrive at a final background baseline for $Y$, which is consistent with the
isobar data. We extract an upper limit for the CME fraction in the
$\Delta\gamma$ measurement of approximately $10\%$ at a $95\%$ confidence level
on in isobar collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV, with an expected
$15\%$ difference in their squared magnetic fields.
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