Our research interests, in broad terms, focus on the experimental investigation of the behavior of nuclear matter under extreme conditions of high temperature and/or energy density.

Nuclei are the cores of atoms that make up about 99.99% of the atomic mass. A nucleus is made up of protons and neutrons (called nucleons), which determine its element and isotope. However, nucleons are not the most basic constituents of nuclei. Each nucleon consists of quarks and gluons, which are bounded together by the strong nuclear force, one of the four fundamental interactions of nature. The strong force is described by the theory of Quantum Chromodynamics (QCD) and has the peculiar feature that quarks and gluons interact strongly when they are far removed, called confinement, but appear to be almost free when approaching each other, known as asymptotic freedom. This leads to a rich structure of the phase diagram for matter governed by QCD processes. Lattice QCD calculations predict that a crossover to a new phase of QCD matter will occur at high temperatures around 170 MeV (~ 2 x 1012 kelvins), in which quarks and gluons appear to be deconfined (see picture below). This so-called Quark-Gluon Plasma (QGP) is believed to have existed in the first few microseconds after the Big Bang.



To recreate a "little Big Bang" in the laboratory, accelerator technology was applied to circulate two heavy-element nuclei at nearly the speed of light in the opposite directions so that the nuclei collide head-on. As illustrated below, fast-moving Pb nuclei are initially compressed into a pancake shape due to the Lorentz contraction. Shortly after the reaction (~ 1 fm/c), an enormous amount of energy is released into a tiny volume (~ 10 fm in diameter) wherein a QGP medium may possibly form. The QGP will expand and cool down rapidly. Once its temperature falls below Tc, bound states of quarks and gluons will form, a process called hadronization. The lifetime of the tiny QGP last for about 10 fm/c. Finally, experimental detectors placed around the reaction point will capture the hadronized particles from QGP and reconstruct their trajectories to trace back relevant information during the course of collision.



In the past 15 years, the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL), NY, has made major breakthroughs in discovering a new state of nuclear matter with a temperature of 300-400 MeV in collisions of two gold nuclei, which is postulated to be the QGP. In 2010, with the onset of the Large Hadron Collider (LHC) at CERN, the field of relativistic heavy ion physics has undergone a "COBE-to-WMAP" transition, toward a more precise understanding of the properties of QGP. Our group is participating in the Compact Muon Solenoid (CMS) experiment, one of the four experiments built at the LHC. Featured by a very broad acceptance coverage, and precise particle tracking and calorimeter, the CMS detector is ideally suited to explore the physics of QGP. A beautiful picture of the LHC at CERN, and a schematic of the CMS detector is shown below.



Most strikingly, the hot QGP matter was found to be strongly interacting, behaving like a liquid, instead of a weakly interacting gas, as was originally proposed. This QGP liquid flows better than any known form of matter with almost no frictional resistance or viscosity. The main evidence leading to the discovery of this perfect QGP liquid was acquired in the study of particle azimuthal angular distribution (φ). Because heavy nuclei are compound objects, the size and shape of the collision region depend on the distance between the centers of the nuclei at impact (i.e., the impact parameter). The created medium in the overlap region has an asymmetry in transverse x-y coordinate space. For a liquid-like medium, strong rescattering of quarks and gluons may lead to local thermal equilibrium and the build-up of anisotropic pressure gradients, which drive a collective anisotropic expansion. The expansion is fastest along the largest pressure gradient, i.e., along the short axis of the almond-shaped volume. As a result, constituents emitted from the medium will naturally reflect this azimuthal asymmetry in their momentum space. This anisotropy is typically characterized by the second Fourier coefficient (v2), known as the "Elliptic Flow", in particle azimuthal distribution, dN/d(φ- Ψ2) ~ 1+2 v2 cos[2(φ-Ψ2)], where Ψ2 is the impact parameter direction. An example of φ distribution can be seen in the figure below which exhibits a clear cos(2φ) pattern. A flat distribution would have been seen if QGP was a weakly interacting gas.



It was previously thought that the formation of the QGP liquid is only possible in heavy ion (AA) collisions where the sufficiently large system size and long lifetime allow thermalization to occur. In systems like proton-proton (pp) and proton-ion (pA) collisions, the initial collision zone is about an order of magnitude smaller in size than in AA, and it was, therefore, not anticipated that a QGP liquid state could be formed there. However, shortly after the LHC startup in 2010, a novel long-range, near-side ( Δφ ~ 0) two-particle correlation was observed, led by Prof. Li, in very high-multiplicity (number of final-state particles) pp collisions with the CMS experiment at the LHC. The unexpected finding consists of a novel correlation in which particles coming out of the collision are aligned in their azimuthal angles (Δφ) over a wide pseudorapidity (η) region, a "ridge"-like structure, as shown below (left). This correlation is absent in typical pp events but emerges as particle multiplicities reach very high values. Subsequently, our group led the analysis of first proton-lead (pPb) collision data at the LHC in 2012, which again revealed a surprisingly strong long-range ridge correlation for high-multiplicity pPb events (middle). This phenomenon is reminiscent of the long-range correlation first seen in AA collisions (right).



To quantify the properties of hot QGP medium and elucidate its mysterious nature of perfect fluidity using the technique of particle correlations are the central themes of our current and future research efforts. In particular, to understand the origin of novel ridge correlations in high multiplicity pp and pPb collisions is currently a focal interest in the community, where our group is playing a leading role. With new opportunities emerging from future LHC runs and upgraded CMS detector, we are entering an exciting era in the study of hot QGP matter.