Over 99% of the known mass in the world is caused by the strong interaction between quarks and gluons. The most successful theory of this strong force is Quantum Chromodynamics (QCD) which describes how colored quarks interact by exchanging gluons. Its notable feature is that gluons also carry color charge and hence can interact with other gluons.

QCD has well-developed tools for calculating the rates for interactions between two quarks or gluons. These successful tools are perturbation series in the strong coupling as(q), where q is the momentum transferred during the interaction; this coupling decreases as the momentum q increases and perturbative calculations  can reproduce experimental results very accurately. However, for those interactions that determine the masses of protons, neutrons, and nuclei, the coupling strength as(q) is of order unity. Hence no perturbation series has any chance of convergence. This is the regime of non-perturbative QCD, a broad area of physics that is the focus of my research. My work is guided by the following questions: 

  • What happens when the QCD interaction is non-perturbatively strong? 
  • What new phenomena occur and can we quantify these? 
  • Can we unambiguously test precise calculations of non-perturbative QCD? 
  • What general lessons on strong coupling physics can we apply to other areas of science?

To answer these questions I work in a large experiment (PHENIX) at RHIC. Where we collide two large nuclei togther at close to the speed of light. This produces a small volume of quark-gluon plasma, a hot dense system where quarks and gluons are no longer confined. We have learnt that the plasma is a dense, strongly-coupled, nearly ideal fluid. We however do not know the degrees-of-freedom of this surprising liquid-like plasma, i.e. how do quarks and gluons interact to drive the strong-coupling. The best experimental way forward is to vary the coupling by measuring the production of heavy-flavor (charm and beauty) which requires a major upgrade of PHENIX’s capability: the VTX upgrade. We completed this $9M upgrade in 2010.

Contact me if you are interested in working on these projects either as an undergraduate or graduate student.


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Physics  research

Physics talks

Heavy flavor systematics from PHENIX (ppt 4MB) (pdf 3MB)

1012 in the Shade: the quark-gluon plasma
(ppt 10MB) (pdf (2 MB)