# Strong Interactions and Nuclear Theory

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

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 of Stewart’s work 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 Iain 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, Philip Harris, 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.

### Lattice Gauge Theory

The low-energy physics of hadrons and nuclei arises from the same Standard Model that describes the quark interactions that are probed at high-energy colliders, but it requires different theoretical methods. William Detmold, John Negele, and Phiala Shanahan study QCD at low 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. Will 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. Will Detmold and Phiala Shanahan’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. John 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. Phiala Shanahan’s work has a particular focus on the gluonic structure of protons and nuclei, as will be probed experimentally at the future Electron Ion Collider. She is also pioneering the application of principled machine learning to lattice field theory, developing novel algorithms to enable calculations that are intractable by conventional means. Detmold, Negele and Shanahan 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. They 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.

### Hot QCD

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. Krishna Rajagopal is incorporating insights obtained via gauge/string duality, perturbative QCD methods, and hydrodynamics in modeling how jets produced in heavy ion collisions are modified via their passage through liquid quark-gluon plasma and how the wakes they leave behind in the droplet of liquid relax and evolve, discerning the most effective ways to use measurements of jets to probe the structure of this primordial liquid and understand how it forms and hydrodynamizes as remarkably quickly as it does. Jesse Thaler has proposed new data-driven jet analysis techniques to probe the dynamics of jet energy loss directly in LHC data. Krishna Rajagopal has also analyzed the critical point in the QCD phase diagram and the interplay of hydrodynamics and fluctuations near it. He 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 Hong Liu has 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”.