At extremely high temperature and density, the normal nuclear matter we experience every day "melts'" into the so called Quark-gluon plasma (QGP) phase consisting of "quasi-free'" quarks and gluons, a phase believed to have existed during the first 10 microseconds after our universe was born in the Big Bang. This QGP can be created by heating matter above a temperature of 170 millon electron volts. Experimentally, this is done by colliding two large nuclei (typically Gold or Lead) at very high energy. The resulting hot and dense fireball is expected to expand under its own pressure, and cool down. Properties of the QGP such as its temperature, pressure, chemical potential, viscosity, diffusion coefficient, speed of sound, etc. can be deduced from thousands of particles emitted from the fireball and detected with large scale particle detectors surrounding the interaction region. Such studies are being carried out in the PHENIX experiment at the Relativistic Heavy-ion Collider (RHIC) and planned for the ATLAS experiment at the Large Hadron Collider (LHC), respectively. This project is focused on a set of exclusive measurements that can establish the properties of the high-density matter created at RHIC via the PHENIX detector, as well as applying some of these measurement techniques at LHC using the ATLAS detector. Specifically, we will probe the properties of this matter via measurements of high-energy single jets and back-to-back jet pairs (di-jets). The research will improve our present understanding of the fundamental nature of the matter under extreme conditions, and therefore has a broad impact on other fields of endeavor such as astrophysics and super-string theories.
Participation by graduate students and undergraduate students is expected throughout the program. The PHENIX experiment provide a diverse array of research topics at all levels of education, as well as an excellent working environment involving interactions between hundreds of scientists in the field, and unique opportunities for career development in programming, hardware, leadership and academics.
We have performed a set of collective flow measurements, which improved our understanding of the properties of the high energy density matter created in relativistic heavy ion collisions both at the RHIC and at the LHC. Specifically, we have made 1) the most comprehensive result of the first six flow harmonics v_1-v_6 over broad ranges of centrality, pseudorapidity and p_T for charged particles; 2) first distribution measurements of the event-by-event v_2, v_3 and v_4 over broad centrality ranges in Pb+Pb collisions at LHC; 3) first measurement of the angular correlations among event planes associated with v_2-v_6 in Pb+Pb collisions at LHC; 4) the discovery and detailed characterization of the ``double-ridge'' in p+Pb collisions at LHC; The Pb+Pb measurements have provided unprecedented insights on the nature of the initial density fluctuations and dynamics of the collective evolution. A coherent picture of initial condition and collective flow based on linear and non-linear hydrodynamic responses is derived, which qualitatively describe most experimental results. Specifically, the data show that the first three flow harmonics are driven mainly by a linear response to the corresponding eccentricity of the initial state, $v_nproptoepsilon_n$ for n<=3. For the higher-order flow harmonics v_4, v_5 and v_6, non-linear contributions from lower-order harmonics are very important, especially in mid-central and peripheral collisions. The relative contributions of linear and non-linear effects are sensitive to the expansion dynamics and dictate the experimentally measured correlations between event-plane angles and flow magnitudes. For a small collision system such as p+Pb collisions, it was assumed that the transverse size of the produced system is too small for the hydrodynamic flow description to be applicable. Our discovery of large v_2 and v_3, with a magnitude comparable to those in Pb+Pb system, come as a suprise. It fuels an ongoing intense debate on the mechanistic origin for the collectiviy in small system.
