Growth of long range forward-backward multiplicity correlations with centrality in Au + Au collisions at square root of sNN = 200 GeV

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au + Au and p + p collisions at square root of s(NN) = 200 GeV. Strong short- and long-range correlations (LRC) are seen in central Au + Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au + Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

K/pi Fluctuations at relativistic energies

We report K/pi fluctuations from Au + Au collisions at sqrt[s(NN)]= 19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/pi fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb + Pb collisions at sqrt[s(NN)]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/pi fluctuations and results for charge-separated fluctuations. We observe that the K/pi fluctuations scale with the charged particle multiplicity density.

Observation of two-source interference in the photoproduction reaction AuAu --> AuAurho0

In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a rho{0}. The rho{0} production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing rho{0} production. Since the rho{0} decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is 87%+/-5%(stat.)+/-8%(syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.

Indications of conical emission of charged hadrons at the BNL relativistic heavy ion collider

Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at sqrt[s_{NN}]=200 GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and peripheral Au+Au collisions. An additional structure is observed in central Au+Au data, signaling conical emission of correlated charged hadrons. The conical emission angle is found to be theta=1.37+/-0.02(stat)-0.07+0.06(syst), independent of p_{ perpendicular}.

System-size independence of directed flow measured at the BNL relativistic heavy-ion collider

We measure directed flow (v_{1}) for charged particles in Au+Au and Cu+Cu collisions at sqrt[s_{NN}]=200 and 62.4 GeV, as a function of pseudorapidity (eta), transverse momentum (p_{t}), and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all available model implementations, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to v_{1} in different collision systems, and investigate possible explanations for the observed sign change in v_{1}(p_{t}).

Forward neutral-pion transverse single-spin asymmetries in p + p collisions at sqrt[s] = 200 GeV

We report precision measurements of the Feynman x (xF) dependence, and first measurements of the transverse momentum (pT) dependence, of transverse single-spin asymmetries for the production of pi0 mesons from polarized proton collisions at sqrt[s] = 200 GeV. The xF dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the pT dependence at fixed xF are not consistent with these same perturbative QCD-based calculations.

Lambda spectra in 11.6A GeV/c Au-Au collisions

E896 has measured Lambda production in 11.6A GeV/c Au-Au collisions over virtually the whole rapidity phase space. The midrapidity p(t) distributions have been measured for the first time at this energy and appear to indicate that the Lambda hyperons have different freeze-out conditions than protons. A comparison with the relativistic quantum molecular dynamics model shows that while there is good shape agreement at high rapidity the model predicts significantly different slopes of the m(t) spectra at midrapidity. The data, where overlap occurs, are consistent with previously reported measurements.