Directed Flow At STAR

It is widely believed that the universe began in an explosion from a small volume with all energy of the universe. During the first a few millionths of a second in the explosion, the universe passed through a phase which is made of Quark Gluon Plasma (QGP)-a de-confined state of quark matter. To understand the origin of the universe as well as the strong interaction itself, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in USA was built to re-create the hot and dense conditions of the universe's first few microseconds. It is currently taking data. At RHIC, two nuclei are smashed together with a nearly light speed, and thousands particles are produced due to the tremendous energy deposited. In order to characterize the collision, the fluid-like dynamics are applied and directed flow is one of them.Directed flow is the first coefficient (v1) of Fourier expansion of the azimuthal distri-bution for emitted particles with respect to the reaction plane. It describes the collective sideward motion of produced particles and nuclear fragments. It carries information from the very early stage of the collision since it generated in the very early stage. The mag-nitude and the shape of directed flow, in particular those for the identified particles, are of the special interest since they are sensitive to the equation of the state (EOS). Recent theoretical work shows that directed flowu1, as a function of rapidity (y), may exhibit a small slope (nearly flatness) at midrapidity due to a strong expansion of the fireball being tilted away from the collision axis. Such tilted expansion gives rise to the new direction of flow which is called anti-flow or a 3rd flow component. The anti-flow is perpendicular to the source surface. It is in the opposite direction to the bouncing-off motion of nucleons (the spectators). If the tilted expansion is strong enough, it can even overcome the bouncing-off motion and results in a negative v1 (y) slope at midrapidity. It potentially produces a wiggle-like structure in v1(y). Although the calculations for both anti-flow and the 3rd flow component are made for collisions at SPS energies, the direct cause of the negative slope is the strong tilted expansion and it is also important at RHIC's top energies. Indeed, the hydrodynamic calculations for Au+Au collisions at (?)=200 GeV with a tilted source as the initial condition can give a similar negative v1(y) slope as that found in experiment. A wiggle structure is also predicted by the Rcl-ativistic Quantum Molecular Dynamics (RQMD) model. In the RQMD model, it is only attributed to baryon stopping together with a positive space-momentum correlation. In this picture, pions and nuclcons'directed flow v1(y) in opposite directions without the phase transition.To distinguish between baryon stopping and anti-flow associated with a phase tran-sition, it is desirable to do the v1(y) for identified particles measurements and compare their slope signs at midrapidity. Furthermore, the observation of a ccntrality dependence of proton v1(y) may reveal some information from the character of a possible first order phase transition. It is expected that in very peripheral collisions, protons'directed flow will be in the same direction as the spectators. In mid-central collisions, if there is a phase transition, the proton v1 (y) slope in the midrapidity may have a sign change, and it becomes negative. Finally, the slope diminishes in central collisions because of the symmetry of collisions.At top RHIC energies, v1 has been studied mostly for charged particles by both the Solcnoidal Tracker At RHIC-STAR and the PHOBOS collaborations. It is found that v1 (PT) in the forward region follows the limiting fragmentation hypothesis, and v1(η) depends only on the incident energy, but not on the size of the colliding system at a given centrality in Au+Au collisions arc measured by the STAR experiment before 2005. The system size independence of v1 can be explained by the hydrodynamie calculation with a tilted initial condition.In this thesis, p, p, KOS, A and A's directed flow in Au+Au collisions at (?) 62 GcV and (?)=200 GeV are measured by the STAR experiment in Run 4. To improve the resolution of the event plane, we determine the event plane from sideward deflection of the spectator neutrons measured by STAR's Shower Maximum Detector at Zero Degree Calorimeters (ZDC-SMDs), together with tracks reconstructed with the Forward Time Projection Chambers (FTPCs). The directed results of these particles are presented as a function of pseudorapidityη, transverse momentum PT and centrality. Within the rapidity range studied, at both energies, the magnitude of p v1 (y) is less than 1%, and its of p is found not more than 2%, v1 for KOS, A and A arc less than 5%. The systematic study of v1 for identified particles at RHIC did not begin until re-cently because it is more challenging work for two reasons as following. Firstly, v1 for some identified particles (such as p, p) is much smaller than that of all charged particles. Thus it has more difficulty to measure. Secondly, more statistics arc needed to measure directed flow as function of y, centrality for identified particles other thanπ+. In the year of 2007 (the Run7 period in STAR), with the high statistics-60M data, the study of the directed flow for the identified particles becomes possible. STAR's measurements of directed flow (v1) at midrapidity forπ±, p, p, K±and KOS in Au+Au collisions at (?)=200 GeV in Run 7 arc presented. A negative v1(y) slope is observed for most produced particles (π±, K±, KOS and p). In 10-70% central collisions, v1(y) slopes ofπ±, K±as well as KOS, p are found to be negative at mid-rapidity. However, p's directed flow exhibits a clearly flatter shape than that for p. A sizable difference is observed between v1 of p and p in 5-30% central collisions. Comparisons with the availblc models (such as:the Relativistic Quantum Molecular Dynamics Model RQMD-RQMD, the Ultra-relativistic Quantum Molecular Dynamics-UrQMD, A Multiphase Transport Model-AMPT, the Quark Gluon String Model with Parton Recombination-QGSM with par-ton recombination, the Hydrodynamic Calculation with Tilted Source-hydrodynamics with a tilted source) arc made. None of models explored can describe v1(y) forπ±and p simultaneously. Furthermore, an additional mechanism besides the anti-flow may need to explain the centrality dependence of the difference between the v1(y) slopes of p and P.The directed flow of charged hadrons and identified particles have been studied in the framework of A Multi-Phase Transport (AMPT) model, for 197 Au+197 Au collisions at (?)=200,130,62.4,39,17.2 and 9.2 GeV. The rapidity (y), centrality (impact parameter-b) and energy (?) dependence of directed flow for charged particles over a wide rapidity range are presented in this thesis. The v1 values calculated from the AMPT model with two versions for different energies are also discussed. It is found that the AMPT model gives the right shape of v1 vs. y although it underestimates the directed flow's magnitude. The reason is possibly due to the lack of mean-field in its hadron cascade. AMPT model can describe the trend of the v1 slope's energy dependence well with missing the magnitude by a fraction of 75%. Hadronic rcscattcring is found to be less important at higher energies because the strong collective motion becomes to be the dominant dynamics at these energy range.Exploring the Quantum Chromo-Dynamics (QCD) phase diagram is one of the most important target of heavy-ion collision experiments. The QCD phase diagram is usually plotted as the temperature (T) us. the baryon chemical potential (μB). Experimentally, we can access this phase diagram and vary these initial conditions by changing the collision beam energy. As an initial step to exam the capabilities of the collider and experiments, a test run is made for 197 Au+197Au collisions at (?) GcV in the year of 2008. The period of data taken lasted for less than 5 hour at the RHIC-STAR experiment. The results on directed flow v1(y) from STAR arc reported in this thesis later. The directed flow results from Au+Au (?)=9.2 GcV arc similar to those obtained from collisions at similar previous energies. The results besides the directed flow from Au+Au at (?)=9.2 GcV demonstrates the capabilities of the STAR detector to pursue the proposal Beam Energy Scan.