Beam-energy dependence of charge balance functions from Au plus Au collisions at energies available at the BNL Relativistic Heavy Ion Collider

Balance functions have been measured in terms of relative pseudorapidity (Delta(eta)) for charged particle pairs at the BNL Relativistic Heavy Ion Collider from Au + Au collisions at root s(NN) = 7.7 GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the CERN Large Hadron Collider from Pb + Pb collisions at root s(NN) = 2.76 TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at root s(NN) = 7.7 GeV implies that a QGP is still being created at this relatively low energy

Jet-Hadron Correlations in root s(NN)=200 GeV p plus p and Central Au plus Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au + Au and p + p collisions at root 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 (p(T)(assoc)) and enhanced at low p(T)(assoc) 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

Beam-Energy Dependence of the Directed Flow of Protons, Antiprotons, and Pions in Au plus Au Collisions

Rapidity-odd directed flow (upsilon 1) measurements for charged pions, protons, and antiprotons near midrapidity (y = 0) are reported in root(S)(NN) = 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeVAu+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider. At intermediate impact parameters, the proton and net-proton slope parameter d upsilon(1) = d upsilon(1)vertical bar (y=0) shows a minimum between 11.5 and 19.6 GeV. In addition, the net-proton d upsilon(1) = d upsilon(1)vertical bar (y=0) changes sign twice between 7.7 and 39 GeV. The proton and net-proton results qualitatively resemble predictions of a hydrodynamic model with a first-order phase transition from hadronic matter to deconfined matter, and differ from hadronic transport calculations

Beam-Energy Dependence of Charge Separation along the Magnetic Field in Au plus Au Collisions at RHIC

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in Au + Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7 GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies

Charged-to-neutral correlation at forward rapidity in Au plus Au collisions at root s(NN)=200 GeV

Event-by-event fluctuations of the multiplicities of inclusive charged particles and photons at forward rapidity in Au + Au collisions at root s(NN) = 200 GeV have been studied. The dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidence 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 base lines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A nonzero 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 owing to particle decay. Results are compared to the expectations based on the generic production mechanism of pions owing to isospin symmetry, for which no significant (<1%) deviation is observed

Observation of Charge Asymmetry Dependence of Pion Elliptic Flow and the Possible Chiral Magnetic Wave in Heavy-Ion Collisions

We present measurements of pi(-) and pi(+) elliptic flow, v(2), at midrapidity in Au + Au collisions at root s(NN) = 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, A(ch), based on data from the STAR experiment at RHIC. We find that pi(-) (pi(+)) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at root s(NN) = 27 GeV and higher. At root s(NN) = 200 GeV, the slope of the difference of v(2) between pi(-) and pi(+) as a function of A(ch) exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies

Azimuthal Anisotropy in U plus U and Au plus Au Collisions at RHIC

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, v(2){2} and v(2){4}, for charged hadrons from U + U collisions at root s(NN) = 193 GeV and Au + Au collisions at root s(NN) = 200 GeV. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in zero degree calorimeters (ZDCs). Within this sample, the observed dependence of v(2){2} on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U + U collisions. We also show that v(2) vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions