Relativistic Heavy Ion Physics

My research activities are centered in the area of experimental relativistic heavy ion physics. Its primary goal is the study of the properties of nuclear matter under extreme conditions of several trillion kelvin temperature, in which quarks and gluons become locally deconfined. This so-called "Quark-Gluon Plasma" (QGP) is predicted to have existed about one microsecond after the Big Bang.

The observation of a strong collective flow behavior at the Relativistic Heavy Ion Collider (RHIC) suggested the formation of a strongly coupled Quantum Chromodynamics (QCD) state of matter that exhibits close-to-zero shear viscosity to entropy density ratio, consistent with an almost ''perfect liquid''. The onset of the LHC is bringing a "COBE-to-WMAP" paradigm shift to the field of heavy ion physics. With a variety of new probes and tools, the next goal is to quantitatively characterize the properties of QGP and pin down the origin of this strongly coupled QCD liquid. This research will shed light on the fundamental dynamics of confinement phenomena emerging in strong force, which is still an unresolved problem from first principle QCD.

High-Density Proton-Proton Physics

Quantum Chromodynamics (QCD) is the theory that describes the strong interactions between quarks and gluons making up hadrons. The advent of the Large Hadron Collider (LHC) brings QCD to a new testing ground, accessing the energy scale beyond TeV level, probing proton structure down to extremely small x and at extreme particle density.

I have a strong interest in the very high-particle-density (high multiplicity) proton-proton interactions. In 2010, the first unexpected phenomena in proton-proton collisions at the LHC was reported by the CMS collaboration during a special seminar at  CERN and a paper entitled "Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions". It was thought that QGP could only be created in large nucleus-nucleus collisions, but this observed characteristic ``ridge-like'' structure in multiparticle correlations for high-particle-density pp collisions provided the first clear evidence of collective effects in pp collisions, and also has the promise of uncovering a deeper internal structure of the proton and studying the quantum fluctuations that occur there. It opens up a new avenue in the exploration of the nature of the strong force in a nonperturbative regime using a tiny but high density quark and gluon system. With future upgrade of the LHC and CMS detector, these high multiplicity pp events will be investigated in great detail and shed more light on the QCD physics in the very high-density regime.

Wei Li, "Unexplained long-range correlations observed in pPb collisions", CMS Times, November, 2012.

Wei Li, "QCD at the LHC", Newsletter for the APS Topical Group on Hadron Physics, February issue, 2011.

Wei Li, "Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions", inside RHIC, October 05 (2010).

Wei Li, "Observation of a long-range, ridge-like correlation structure in high multiplicity proton-proton interactions", Modern Physics Letter A27, 1230018 (2012)

CMS Collaboration, "Observation of long-range near-side angular correlations in proton-lead collisions at the LHC", Physics Letters B 718 (2013) 795.

CMS Collaboration, "Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s_NN) = 2.76 TeV", Physical Review Letter 109, 022301 (2012).

CMS Collaboration, "Measurement of jet fragmentation into charged particles in pp and PbPb collisions at sqrt{s_NN}=2.76 TeV", JHEP 10 (2012) 087.

CMS Collaboration, "Measurement of the Pseudorapidity and Centrality Dependence of the Transverse Energy Density in Pb-Pb Collisions at sqrt{s_NN}=2.76 TeV", Physical Review Letter 109, 152303 (2012).

CMS Collaboration, "Measurement of the azimuthal anisotropy of neutral pions in PbPb collisions at sqrt{s_NN} = 2.76 TeV", accepted by Physical Review Letter (2012).

CMS Collaboration, "Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions at sqrt{s_NN}=2.76 TeV", Physical Review C 87, 014902 (2013).

CMS Collaboration, "Observation of Sequential Υ Suppression in PbPb Collisions", Physical Review Letter 109, 222301 (2012)

CMS Collaboration, "Studies of jet quenching using isolated-photon + jet correlations in PbPb and pp collisions at sqrt{s_NN}=2.76 TeV", Physics Letters B 718, 773 (2013).

CMS Collaboration, "Centrality dependence of dihadron correlations and azimuthal anisotropy harmonics in PbPb collisions at a nucleon-nucleon center of mass energy of 2.76 TeV", The European Physical Journal C72 (2012) 2012.

CMS Collaboration, "Long-range and short-range dihadron angular correlations in central PbPb collisions at sqrt(s_NN)=2.76TeV'', JHEP 07 (2011) 076.

CMS Collaboration, "Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC", JHEP 09 (2010) 091.

PHOBOS Collaboration, "System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at sqrt(s_NN)=200GeV", Phys. Rev. C81, 024904 (2010).

PHOBOS Collaboration, "Cluster properties from two-particle angular correlations in p+p collisions at sqrt(s)=200GeV and 410GeV", Phys. Rev. C75, 054913 (2007).

“Flow and flow fluctuations from the CMS experiment at the LHC”, Workshop on flow fluctuations, ECT, Trento, Italy, July 2012.

“Jet measurements at the LHC”, invited talk, APS DNP 2012, Newport Beach, CA, October 2012.

“Azimuthal flow anisotropy in ultra-central PbPb collisions”, Poster at Quark Matter 2012, Washington D.C., August 2012.

"Observation of the `Ridge' in Proton-Lead Collisions and its Physics Implications", Triangle Nuclear Theory Colloquium, North Carolina State University, Raleigh, NC, December 2012.

"Trillion Degree Matter", Brown Bag seminar, Rice Physics and Astronomy Department, October, 2012