We report the triton ($t$) production in mid-rapidity ($|y| <$ 0.5) Au+Au collisions at $\sqrt{s_\mathrm{NN}}$= 7.7--200 GeV measured by the STAR experiment from the first phase of the beam energy scan at the Relativistic Heavy Ion Collider (RHIC). The nuclear compound yield ratio ($\mathrm{N}_t \times \mathrm{N}_p/\mathrm{N}_d^2$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/d\eta$) and follows a scaling behavior. The $dN_{ch}/d\eta$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3$\sigma$ and 3.4$\sigma$, respectively, giving a combined significance of 4.1$\sigma$. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_{T}$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
Invariant yields of tritons at 7.7 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
Invariant yields of tritons at 11.5 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
Invariant yields of tritons at 14.5 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
A decisive experimental test of the Chiral Magnetic Effect (CME) is considered one of the major scientific goals at the Relativistic Heavy-Ion Collider (RHIC) towards understanding the nontrivial topological fluctuations of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is expected to result in a charge separation phenomenon across the reaction plane, whose strength could be strongly energy dependent. The previous CME searches have been focused on top RHIC energy collisions. In this Letter, we present a low energy search for the CME in Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}}=27$ GeV. We measure elliptic flow scaled charge-dependent correlators relative to the event planes that are defined at both mid-rapidity $|\eta|<1.0$ and at forward rapidity $2.1 < |\eta|<5.1$. We compare the results based on the directed flow plane ($\Psi_1$) at forward rapidity and the elliptic flow plane ($\Psi_2$) at both central and forward rapidity. The CME scenario is expected to result in a larger correlation relative to $\Psi_1$ than to $\Psi_2$, while a flow driven background scenario would lead to a consistent result for both event planes. In 10-50% centrality, results using three different event planes are found to be consistent within experimental uncertainties, suggesting a flow driven background scenario dominating the measurement. We obtain an upper limit on the deviation from a flow driven background scenario at the 95% confidence level. This work opens up a possible road map towards future CME search with the high statistics data from the RHIC Beam Energy Scan Phase-II.
This dataset corresponds to Figure 2, the v2 value estimated by tpc (\Psi_2) in the paper
This dataset corresponds to Figure 2, the v2 value estimated by epd (\Psi_2) in the paper
This dataset corresponds to Figure 2, the v2 value estimated by epd (\Psi_1) in the paper
We report on measurements of sequential $\Upsilon$ suppression in Au+Au collisions at $\sqrt{s_{_\mathrm{NN}}}$ = 200 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) through both the dielectron and dimuon decay channels. In the 0-60% centrality class, the nuclear modification factors ($R_{\mathrm{AA}}$), which quantify the level of yield suppression in heavy-ion collisions compared to $p$+$p$ collisions, for $\Upsilon$(1S) and $\Upsilon$(2S) are $0.40 \pm 0.03~\textrm{(stat.)} \pm 0.03~\textrm{(sys.)} \pm 0.09~\textrm{(norm.)}$ and $0.26 \pm 0.08~\textrm{(stat.)} \pm 0.02~\textrm{(sys.)} \pm 0.06~\textrm{(norm.)}$, respectively, while the upper limit of the $\Upsilon$(3S) $R_{\mathrm{AA}}$ is 0.17 at a 95% confidence level. This provides experimental evidence that the $\Upsilon$(3S) is significantly more suppressed than the $\Upsilon$(1S) at RHIC. The level of suppression for $\Upsilon$(1S) is comparable to that observed at the much higher collision energy at the Large Hadron Collider. These results point to the creation of a medium at RHIC whose temperature is sufficiently high to strongly suppress excited $\Upsilon$ states.
Inclusive Y(1S) $R_{AA}$ as a function of centrality in Au+Au collisions at 200 GeV. The bin corresponding to $N_{part}$ = 162 is for 0-60% centrality. Global uncertainty of 20.0% not shown.
Inclusive Y(1S) $R_{AA}$ as a function of centrality in Au+Au collisions at 200 GeV. The bin corresponding to $N_{part}$ = 162 is for 0-60% centrality. Global uncertainty of 20.0% not shown.
Inclusive Y(2S) $R_{AA}$ as a function of centrality in Au+Au collisions at 200 GeV. The bin corresponding to $N_{part}$ = 162 is for 0-60% centrality. Global uncertainty of 20.5% not shown.
In high-energy heavy-ion collisions, the correlations between the emitted particles can be used as a probe to gain insight into the charge creation mechanisms. In this Letter, we report the first results of such studies using the electric charge balance function in the relative pseudorapidity ($\Delta\eta$) and azimuthal angle ($\Delta\varphi$) in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV with the ALICE detector at the Large Hadron Collider. The width of the balance function decreases with growing centrality (i.e. for more central collisions) in both projections. This centrality dependence is not reproduced by HIJING, while AMPT, a model which incorporates strings and parton rescattering, exhibits qualitative agreement with the measured correlations in $\Delta\varphi$ but fails to describe the correlations in $\Delta\eta$. A thermal blast-wave model incorporating local charge conservation and tuned to describe the $p_{\rm T}$ spectra and v$_2$ measurements reported by ALICE, is used to fit the centrality dependence of the width of the balance function and to extract the average separation of balancing charges at freeze-out. The comparison of our results with measurements at lower energies reveals an ordering with $\sqrt{s_{\rm NN}}$: the balance functions become narrower with increasing energy for all centralities. This is consistent with the effect of larger radial flow at the LHC energies but also with the late stage creation scenario of balancing charges. However, the relative decrease of the balance function widths in $\Delta\eta$ and $\Delta\varphi$ with centrality from the highest SPS to the LHC energy exhibits only small differences. This observation cannot be interpreted solely within the framework where the majority of the charge is produced at a later stage in the evolution of the heavy--ion collision.
The Balance Function as a function of the relative pseudorapidity of two charged particles for the centrality class 0-5%. Also shown in the second column is the result from the mixed data set.
The Balance Function as a function of the relative pseudorapidity of two charged particles for the centrality class 30-40%.
The Balance Function as a function of the relative pseudorapidity of two charged particles for the centrality class 70-80%.
We report the first measurement of the net-charge fluctuations in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions. An additional reduction in the amount of fluctuations is seen in comparison to the results from lower energies. We examine the dependence of fluctuations on the pseudorapidity interval, which may account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations at LHC are smaller compared to the measurements at the Relativistic heavy Ion Collider (RHIC), and as such, closer to what has been theoretically predicted for the formation of Quark-Gluon Plasma (QGP).
The measured NU(+-DYN) as a function of the centrality of the collisions, expressed as the number of participating nucleons, for two values of midrapidity range.
NU(+-DYN), corrected for charge conservation and finite acceptance effects, as a function of the centrality of the collisions, expressed as the number of participating nucleons, for two values of midrapidity range.
The measured and corrected NU(+-DYN) in P P collisions for two values of midrapidity range.
The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|\eta|<0.8$ at different collision centralities and as a function of transverse momentum, $p_{\rm T}$, out to $p_{\rm T}=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for $p_{\rm T}>8$ GeV/$c$. The small $p_{\rm T}$ dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to $p_{\rm T}=8$ GeV/$c$. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least $p_{\rm T}=8$ GeV/$c$ indicating that the particle type dependence persists out to high $p_{\rm T}$.
Elliptic flow (v2) estimated with Event Plane method (with eta gap of 2.0) measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
Elliptic flow (v2) estimated with four-particle cumulants measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
Triangular flow (v3) estimated with Event Plane method (with eta gap of 2.0) measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
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