Strong Interactions

The challenge of understanding strong interactions is a unifying theme that cuts across many areas of CTP research and also plays a central role in aspects of the physics of condensed matter and ultracold atoms. The interactions among quarks and gluons, described by Quantum Chromodynamics (QCD), are particularly important because they exhibit many characteristic and challenging features of a strongly coupled theory while at the same time they are described at short length scales by a well- understood and well-tested theory, QCD, which is a central part of the Standard Model.

Understanding strong QCD interactions is crucial to interpreting collider searches for new short distance physics, within and beyond the Standard Model, as well as to understanding the properties of the hot matter that filled the microseconds old universe and the dense matter in the centers of neutron stars. They are also the key to understanding how quarks and gluons form protons, neutrons, and other hadrons – which were the earliest complex structures formed in the universe – and subsequently nuclei. QCD provides a defining example of a theory in which the entities and phases that it describes do not resemble the elementary constituents of which they are made. This feature is characteristic of strongly interacting systems in many areas of physics and makes them both interesting and challenging. Effective field theory provides a crucial tool both for probing the fundamental description of nature embodied by the electroweak part of the Standard Model, and for understanding QCD. In recent years the number of physical phenomena successfully described by effective field theory methods has been rapidly expanding, and MIT faculty have made crucial contributions to these developments. This includes Iain Stewart's co-invention of the soft-collinear effective theory, which provides a rigorous description of the energetic jets formed by high energy quark and gluon collisions. This formalism has enabled improvements in precision by a factor of ten for cross section calculations, and has made a broader range of sophisticated reactions theoretically tractable. Other examples include probing fundamental symmetries like the Standard Model description of CP violation and weak decays, and precisely determining essential parameters like the strong coupling constant. Recently there has also been a renaissance in the realm of jet physics, including both our understanding of jet properties and the invention of new jet observables, where both Jesse Thaler and Stewart have played important roles, for example providing theoretical tools and results that are now used in new physics searches and Higgs analyses at the LHC, including those carried out by Markus Klute and Christoph Paus as part of the CMS collaboration. Thaler has also been a key pioneer involved in developing methods that for the first time make it possible to use the substructure of a jet to determine its parentage, for example whether it came from a gluon, a quark, a boosted W boson, or even a supersymmetric squark.

The physics of hadrons and nuclei arises from the same Standard Model that is probed at colliders, but it requires different theoretical methods. William Detmold and John Negele study QCD at lower energies from first principles using a lattice field theory approach and thereby understand how QCD, whose fundamental degrees of freedom are quarks and gluons, gives rise to the rich and complex structure of protons, neutrons, and eventually nuclei. By employing innovative analytic and computational methods, they are able to make fundamental progress in solving complex problems in QCD that are not amenable to other techniques. Detmold's research centers on obtaining quantitative understanding of how the complexity of nuclei emerges from their underlying quark and gluon degrees of freedom, and of the dynamics of the rearrangement of the light quarks and gluons that occurs when a heavy quark decays, for example at particle colliders such as the LHC where LNS colleague Mike Williams measures these decays using the LHCb experiment. Detmold's advances in the QCD study of nuclei have the potential for transforming nuclear physics as they provide a path towards ab initio calculations of nuclear processes with fully quantifiable uncertainties. Negele's research focuses on understanding the underlying structure of the proton. His calculations are now elucidating the contributions of quarks and gluons to the spatial, momentum, and spin structure of protons and neutrons. Both Detmold and Negele also perform carefully quantified calculations of unmeasured properties of nucleons and nuclei that are needed in experimental searches for dark matter and other new physics. Detmold and Negele are key members of a national initiative exploiting the country's most powerful computers for lattice QCD and also use large-scale resources at MIT.

At high enough temperature and/or density, QCD describes various phases of matter in which the quarks and gluons do not coalesce into hadrons or nuclei. Understanding these liquids requires linking particle and nuclear physics, cosmology, astrophysics and condensed matter physics. Experimentalists including Yen-Jie Lee, Gunther Roland and Bolek Wyslouch have used heavy ion collision experiments at RHIC and the LHC to show that the hot quark-gluon plasma that filled the microseconds-old universe is a strongly coupled liquid. Understanding the properties of this new phase of matter and how it emerges from QCD is a central challenge for the coming decade. Together with Hong Liu, Krishna Rajagopal is using gauge/string duality to study similar fluids from first principles and to discern the most effective ways to use measurements of jets and heavy mesons in heavy ion collisions at the LHC to probe the liquid quark-gluon plasma. For example, he has shown how a high energy quark plowing through this liquid can lose substantial energy and yet emerge looking similar a jet in vacuum, with small modifications as seen in the data. Rajagopal has also analyzed the critical point in the QCD phase diagram and has proposed signatures for its experimental detection, showing how to use the collision-energy scan now underway at the Relativistic Heavy Ion Collider (RHIC) to search for the critical point in a large region of the QCD phase diagram. Rajagopal and Frank Wilczek have previously analyzed the properties of the superfluid, color superconducting, quark matter that may lie at the centers of neutron stars, providing a clear understanding of the properties of such matter at very high densities.

The longer term challenge to theorists is to use the data to gain an understanding of how a strongly coupled liquid, which shows no signs of the individual particles of which it is made, can emerge from QCD. This quest resonates with challenges that are central to contemporary condensed matter physics, where Allan Adams and Hong Liu have used gauge/string duality techniques developed to study quark-gluon plasma to gain insights into superfluids, and some of the most interesting and most puzzling materials, including "strange metals".

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