Dihadron angular correlations in \(d\)+Au collisions at \(\sqrt{s_{\rm NN}}=200\) GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron ...multiplicity in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (\(\Delta\eta\)) on the near side (i.e. relative azimuth \(\Delta\phi\sim0\)). This correlated yield as a function of \(\Delta\eta\) appears to scale with the dominant, primarily jet-related, away-side (\(\Delta\phi\sim\pi\)) yield. The Fourier coefficients of the \(\Delta\phi\) correlation, \(V_{n}=\langle\cos n\Delta\phi\rangle\), have a strong \(\Delta\eta\) dependence. In addition, it is found that \(V_{1}\) is approximately inversely proportional to the mid-rapidity event multiplicity, while \(V_{2}\) is independent of it with similar magnitude in the forward (\(d\)-going) and backward (Au-going) directions.
We report the first measurement of charmed-hadron (\(D^0\)) production via the hadronic decay channel (\(D^0\rightarrow K^- + \pi^+\)) in Au+Au collisions at \(\sqrt{s_{_{\mathrm{NN}}}\) = 200\,GeV ...with the STAR experiment. The charm production cross-section per nucleon-nucleon collision at mid-rapidity 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.
A data-driven method was applied to measurements of Au+Au collisions at \(\sqrt{s_{_{\rm NN}}} =\) 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance ...\(\Delta\eta\)-dependent and \(\Delta\eta\)-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a component of the correlation that is \(\Delta\eta\)-independent, which is likely dominated by anisotropic flow and flow fluctuations. It was also found to be independent of \(\eta\) within the measured range of pseudorapidity \(|\eta|<1\). The relative flow fluctuation was found to be \(34\% \pm 2\% (stat.) \pm 3\% (sys.)\) for particles of transverse momentum \(p_{T}\) less than \(2\) GeV/\(c\). The \(\Delta\eta\)-dependent part may be attributed to nonflow correlations, and is found to be \(5\% \pm 2\% (sys.)\) relative to the flow of the measured second harmonic cumulant at \(|\Delta\eta| > 0.7\).
Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at \(\sqrt{s_{NN}}\)=200 GeV have been studied. Dominant contribution to ...such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.
We report the STAR measurements of dielectron (\(e^+e^-\)) production at midrapidity (\(|y_{ee}|<\)1) in Au+Au collisions at \(\sqrt{s_{\rm NN}}\) = 200\,GeV. The measurements are evaluated in ...different invariant mass regions with a focus on 0.30-0.76 (\(\rho\)-like), 0.76-0.80 (\(\omega\)-like), and 0.98-1.05 (\(\phi\)-like) GeV/\(c^{2}\). The spectrum in the \(\omega\)-like and \(\phi\)-like regions can be well described by the hadronic cocktail simulation. In the \(\rho\)-like region, however, the vacuum \(\rho\) spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77\(\pm\)0.11(stat.)\(\pm\)0.24(sys.)\(\pm\)0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the \(\rho\) meson. The excess yield in the \(\rho\)-like region increases with the number of collision participants faster than the \(\omega\) and \(\phi\) yields. Theoretical models with broadened \(\rho\) contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.
We report a new high-precision measurement of the mid-rapidity inclusive jet longitudinal double-spin asymmetry, \(A_{LL}\), in polarized \(pp\) collisions at center-of-mass energy \(\sqrt{s}=200\) ...GeV. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep inelastic scattering (DIS), semi-inclusive DIS, and RHIC \(pp\) data. The measured asymmetries provide evidence for positive gluon polarization in the Bjorken-\(x\) region \(x>0.05\).
