Global polarizations (P) of Λ (Λ¯) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split ...the Λ and Λ¯ global polarizations (ΔP=PΛ–PΛ¯<0). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance (Δn = NL–NR/< NL+NR > ≠0) between left- and right-handed Λ (Λ¯) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator (Δγ) and parity-odd azimuthal harmonic observable (Δa1). Measurements of ΔP, Δγ, and Δa1 have not led to definitive conclusions concerning the CME or the magnetic field, and Δn has not been measured previously. Correlations among these observables may reveal new insights. Furthermore, this paper reports measurements of correlation between Δn and Δa1, which is sensitive to chirality fluctuations, and correlation between ΔP and Δγ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
Angular distributions of charged particles relative to jet axes are studied in $\sqrt{^SNN}$ = 200 GeV Au+Au collisions as a function of the jet orientation with respect to the event plane. This ...differential study tests the expected path-length dependence of energy loss experienced by a hard-scattered parton as it traverses the hot and dense medium formed in heavy-ion collisions. A second-order event plane is used in the analysis as an experimental estimate of the reaction plane formed by the collision impact parameter and the beam direction. Charged-particle jets with 15 < $p_{\text {T,jet}}$ < 20 and 20 < $p_{\text {T,jet}}$ < 40 GeV / $c$ were reconstructed with the anti-$k_{\text {T}}$ algorithm with radius parameter setting of R = 0.4 in the 20-50% centrality bin to maximize the initial-state eccentricity of the interaction region. The reaction plane fit method is implemented to remove the flow-modulated background with better precision than prior methods. Yields and widths of jet-associated charged-hadron distributions are extracted in three angular bins between the jet axis and the event plane. The event-plane (EP) dependence is further quantified by ratios of the associated yields in different EP bins. No dependence on orientation of the jet axis with respect to the event plane is seen within the uncertainties in the kinematic regime studied. Finally, this finding is consistent with a similar experimental observation by ALICE in $\sqrt{^SNN}$ = 2.76 TeV Pb-Pb collision data.
Azimuthal anisotropy of produced particles is one of the most important observables used to access the collective properties of the expanding medium created in relativistic heavy-ion collisions. ...Here, in this paper, we present second (v2) and third (v3) order azimuthal anisotropies of $K_{S}^{0}$, Φ, Λ, Ξ, and Ω at midrapidity (|y| < 1) in Au+Au collisions at $\sqrt{s_{NN}}$= 54.4 GeV measured by the STAR detector. The v2 and v3 are measured as a function of transverse momentum and centrality. Their energy dependence is also studied. v3 is found to be more sensitive to the change in the center-of-mass energy than v2. Scaling by constituent quark number is found to hold for v2 within 10%. This observation could be evidence for the development of partonic collectivity in 54.4 GeV Au+Au collisions. Differences in v2 and v3 between baryons and antibaryons are presented, and ratios of v3/v$^{3/2}_{2}$ are studied and motivated by hydrodynamical calculations. The ratio of v2 of Φ mesons to that of antiprotons v2(Φ)/v2($\overline{p}$) shows centrality dependence at low transverse momentum, presumably resulting from the larger effects from hadronic interactions on antiproton v2.
Density fluctuations near the QCD critical point can be probed via an intermittency analysis in relativistic heavy-ion collisions. We report the first measurement of intermittency in Au+Au collisions ...at $\sqrt{s{NN}}$ = 7.7-200 GeV measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The scaled factorial moments of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space. We observe a power-law behavior of scaled factorial moments in Au+Au collisions and a decrease in the extracted scaling exponent (ν) from peripheral to central collisions. The ν is consistent with a constant for different collisions energies in the mid-central (10-40%) collisions. Moreover, the ν in the 0-5% most central Au+Au collisions exhibits a non-monotonic energy dependence that reaches a minimum around $\sqrt{s{NN}}$ = 27 GeV. The physics implications on the QCD phase structure are discussed.
A
bstract
We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (|
y
|
<
0.7) in Au+Au collisions at
s
NN
= 200 GeV. Invariant yields ...of HFEs are measured for the transverse momentum range of 3
.
5
< p
T
<
9 GeV/
c
in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in
p
+
p
collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.
The ALICE (A Large Ion Collider Experiment) detector yields a huge sample of data from different sub-detectors. On-line data processing is applied to select and reduce the volume of the stored data. ...ALICE applies a multi-level hardware trigger scheme where fast detectors are used to feed a three-level (L0, L1, and L2) deep chain. The High-Level Trigger (HLT) is a fourth filtering stage sitting logically between the L2 trigger and the data acquisition event building. The EMCal detector comprises a large area electromagnetic calorimeter that extends the momentum measurement of photons and neutral mesons up to pT = 250 GeV/c, which improves the ALICE capability to perform jet reconstruction with measurement of the neutral energy component of jets. An online reconstruction and trigger chain has been developed within the HLT framework to sharpen the EMCal hardware triggers, by combining the central barrel tracking information with the shower reconstruction (clusters) in the calorimeter. In the present report the status and the functionality of the software components developed for the EMCal HLT online reconstruction and trigger chain will be discussed, as well as preliminary results from their commissioning performed during the 2011 LHC running period.
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