Dilepton Polarization as a Signature of Plasma Anisotropy.

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta.

Principal Component Analysis of Event-by-Event Fluctuations.

We apply principal component analysis to the study of event-by-event fluctuations in relativistic heavy-ion collisions. This method brings out all the information contained in two-particle correlations in a physically transparent way. We present a guide to the method, and apply it to multiplicity fluctuations and anisotropic flow, using ALICE data and simulated events.

Anisotropic flow in event-by-event ideal hydrodynamic simulations of √[s(NN)]=200 GeV Au+Au collisions.

We simulate top-energy Au+Au collisions using ideal hydrodynamics in order to make the first comparison to the complete set of midrapidity flow measurements made by the PHENIX Collaboration. A simultaneous calculation of v(2), v(3), v(4), and the first event-by-event calculation of quadrangular flow defined with respect to the v(2) event plane (v(4){Ψ(2)}) gives good agreement with measured values, including the dependence on both transverse momentum and centrality. This provides confirmation that the collision system is indeed well described as a quark-gluon plasma with an extremely small viscosity and that correlations are dominantly generated from collective effects.

Directed flow at midrapidity in sqrt[S(NN)] = 2.76 TeV Pb + Pb collisions.

We analyze published data from the ALICE Collaboration in order to obtain the first extraction of the recently proposed rapidity-even directed flow observable v(1). An accounting of the correlation due to the conservation of transverse momentum restores the factorization seen by ALICE in all other Fourier harmonics and thus indicates that the remaining correlation gives a reliable measurement of directed flow. We then carry out the first viscous hydrodynamic calculation of directed flow, and show that it is less sensitive to viscosity than higher harmonics.

Directed flow at midrapidity in heavy-ion collisions.

It was recently shown that fluctuations in the initial geometry of a heavy-ion collision generally result in a dipole asymmetry of the distribution of outgoing particles. This asymmetry, unlike the usual directed flow, is expected to be present at a wide range of rapidity--including midrapidity. The first evidence of this phenomenon can be seen in recent two-particle correlation data by the STAR Collaboration, providing the last element necessary to quantitatively describe long-range dihadron correlations.

Transverse energy fluctuations and the pattern of J/psi suppression in Pb-Pb collisions.

The NA50 Collaboration has recently observed that the J/psi production rate in Pb-Pb collisions decreases more rapidly as a function of the transverse energy for the most central collisions than for less central ones. We show that this phenomenon can be understood as an effect of transverse energy fluctuations in central collisions. A good fit of the data is obtained using a model which relates J/psi suppression to the local energy density.