We report on measurements of dielectron (\(e^+e^-\)) production in Au\(+\)Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at RHIC. Systematic ...measurements of the dielectron yield as a function of transverse momentum (\(p_{\rm T}\)) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region (\(M_{ee}<\) 1 GeV/\(c^2\)). This enhancement cannot be reproduced by the \(\rho\)-meson vacuum spectral function. In minimum-bias collisions, in the invariant-mass range of 0.30 \(-\) 0.76 GeV/\(c^2\), integrated over the full \(p_{\rm T}\) acceptance, the enhancement factor is 1.76 \(\pm\) 0.06 (stat.) \(\pm\) 0.26 (sys.) \(\pm\) 0.29 (cocktail). The enhancement factor exhibits weak centrality and \(p_{\rm T}\) dependence in STAR's accessible kinematic regions, while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 \(\pm\) 0.10. Models that assume an in-medium broadening of the \(\rho\) meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of \(\omega\) and \(\phi\)-meson production through their \(e^+e^-\) decay channel. These measurements show good agreement with Tsallis Blast-Wave model predictions as well as, in the case of the \(\phi\)-meson, results through its \(K^+K^-\) decay channel. In the intermediate invariant-mass region (1.1\(<M_{ee}<\) 3 GeV/\(c^2\)), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed.
We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in \(p^\uparrow+p\) collisions at RHIC. These ...correlations directly probe quark transversity distributions. We measure signals in excess of five standard deviations at high transverse momenta, at high pseudorapidities eta>0.5, and for pair masses around the mass of the rho-meson. This is the first direct transversity measurement in p+p collisions. Comparing the results to data from lepton-nucleon scattering will test the universality of these spin-dependent quantities.
Dihadron correlations are analyzed in \(\sqrt{s_{_{\rm NN}}} = 200\) GeV \(d\)+Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam ...direction. It is found that the jetlike correlated yield increases with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.
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 on a polarization measurement of inclusive \(J/\psi\) mesons in the di-electron decay channel at mid-rapidity at 2 \(<p_{T}<\) 6 GeV/\(c\) in \(p+p\) collisions at \(\sqrt{s}\) = 200 GeV. ...Data were taken with the STAR detector at RHIC. The \(J/\psi\) polarization measurement should help to distinguish between different models of the \(J/\psi\) production mechanism since they predict different \(p_{T}\) dependences of the \(J/\psi\) polarization. In this analysis, \(J/\psi\) polarization is studied in the helicity frame. The polarization parameter \(\lambda_{\theta}\) measured at RHIC becomes smaller towards high \(p_{T}\), indicating more longitudinal \(J/\psi\) polarization as \(p_{T}\) increases. The result is compared with predictions of presently available models.
Chin. Phys. C 45 (2021) 044002 Dihadron azimuthal correlations containing a high transverse momentum ($p_T$)
trigger particle are sensitive to the properties of the nuclear medium created
at RHIC ...through the strong interactions occurring between the traversing parton
and the medium, i.e. jet-quenching. Previous measurements revealed a strong
modification to dihadron azimuthal correlations in Au+Au collisions with
respect to p+p and d+Au collisions. The modification increases with the
collision centrality, suggesting a path-length or energy density dependence to
the jet-quenching effect. This paper reports STAR measurements of dihadron
azimuthal correlations in mid-central (20-60%) Au+Au collisions at
$\sqrt{s_{_{\rm NN}}}=200$ GeV as a function of the trigger particle's
azimuthal angle relative to the event plane, $\phi_s=|\phi_t-\psi_{\rm EP}|$.
The azimuthal correlation is studied as a function of both the trigger and
associated particle $p_T$. The subtractions of the combinatorial background and
anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The
correlation results are first discussed with subtraction of the even harmonic
(elliptic and quadrangular) flow backgrounds. The away-side correlation is
strongly modified, and the modification varies with $\phi_s$, with a
double-peak structure for out-of-plane trigger particles. The near-side ridge
(long range pseudo-rapidity $\Delta\eta$ correlation) appears to drop with
increasing $\phi_s$ while the jet-like component remains approximately
constant. The correlation functions are further studied with subtraction of odd
harmonic triangular flow background arising from fluctuations. It is found that
the triangular flow, while responsible for the majority of the amplitudes, is
not sufficient to explain the $\phi_s$-dependence of the ridge or the away-side
double-peak structure. ...
The \(\jpsi\) \(\pt\) spectrum and nuclear modification factor (\(\raa\)) are reported for \(\pt < 5 \ \gevc\) and \(|y|<1\) from 0\% to 60\% central Au+Au and Cu+Cu collisions at \(\snn = 200 \ ...\gev\) at STAR. A significant suppression of \(\pt\)-integrated \(\jpsi\) production is observed in central Au+Au events. The Cu+Cu data are consistent with no suppression, although the precision is limited by the available statistics. \(\raa\) in Au+Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with \(\pt\). The data are compared to high-\(\pt\) STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low \(\pt\) are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration.
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.
