QCD: Quantum Chromodynamics

Understanding the structure of hadrons is one of the great unsolved problems in physics, and as such is the subject of both theoretical and experimental effort at MIT. Robert Jaffe and his collaborators developed the MIT bag model of confinement, still one of the favorite models of quark dynamics, and applied it to the spectrum and structure of hadrons. Professor Jaffe is also one of the leaders in the quest to use new experiments -- including those done by Bernd Surrow here at MIT -- to elucidate the spin structure of the nucleon.

John Negele uses lattice field theory to solve QCD ab initio and thereby understand from first principles how QCD gives rise to the observed quark and gluon structure of protons, neutrons, and other hadrons. The combination of numerical computation and analytic techniques enables one to make fundamental progress in solving complex problems in QCD that are not amenable to either technique alone. Current lattice studies range from calculating the contributions of quarks and gluons to the spatial, momentum, and spin structure of nucleons measured by MIT experimentalists Stanley Kowalski, Richard Milner and Bernd Surrow to understanding the role of diquarks and instantons in hadron structure. Professor Negele is one of the founders of a national initiative to develop Terascale computers optimized for lattice QCD and is leading a collaboration to exploit them to understand hadron structure. As part of this initiative, a dedicated 5.7 Teraflops Blue Gene supercomputer at MIT provides essential resources for lattice research.

Understanding QCD in extreme conditions requires linking usually disparate strands of theoretical physics, including particle and nuclear physics, cosmology, astrophysics and condensed matter physics. Krishna Rajagopal and Frank Wilczek study the properties of the cold dense quark matter that may lie at the centers of neutron stars. This stuff is the QCD analogue of a superconductor. However, if probed with ordinary light it looks like a transparent insulator and not like an electric conductor, as previously assumed. The properties of sufficiently dense quark matter have now been understood from first principles, but many interesting questions remain to be answered at lower densities. Progress requires coupled advances in theory, astrophysical observation, and experiments on analogue systems made of ultracold fermionic atoms. Robert Jaffe and Edward Farhi did the first work on quark matter in astrophysics. This work makes contact with research in Xray astronomy, condensed matter theory and ultracold cold atoms carried out elsewhere in our department.

Hong Liu and Krishna Rajagopal do research on hot quark matter, of the sort that is created in current experiments at the Relativistic Heavy Ion Collider (RHIC)external link icon. They are using gauge/gravity duality to understand properties of the strongly coupled, liquid-like, quark-gluon plasma, which these experiments tell us filled the universe for the first microseconds after the big bang. For example, they study how a high energy quark plowing through this liquid loses energy and under what conditions a pair of heavy quarks moving through this fluid can bind into a meson. Bolek Wyslouch and Gunther Roland are leading the related experimental effort at the Large Hadron Collider, where definitive measurements are anticipated. Rajagopal has also analyzed the critical point in the QCD phase diagram and has proposed signatures for its experimental detection, making it possible for experimentalists at RHICexternal link icon to do a definitive search for the critical point in a large region of the phase diagram.