Lattice QCD studies of the spectrum and structure of hadron excitations

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A Project summary The physics of hadron excitations, such as the N* resonances, hybrid mesons and baryons in which the valence quarks are bound by an excited gluon ?eld, glueballs, doubly-charmed baryons, and the recently-discovered pentaquark systems, will be studied using computer simulations of quarks and gluons on a space-time lattice. Such systems should provide a wealth of information to help us understand the physics of con?nement and hadron formation in quantum chromodynamics, the theory of the strong interactions and one of the fundamental forces of Nature. These states are also currently of great interest to experimentalists, exempli?ed by the Hall B N* program and the GlueX experiment in the proposed Hall D at the Thomas Je?erson National Accelerator Facility, as well as the CLEO-c project at Cornell. Understanding the physics of con?ning gluons is an important intellectual challenge; in fact, it is one of the seven so-called Millenium Prize problems announced by the Clay Mathematics Institute of Cambridge, Massachusetts. One of the most reliable means of investigating such systems is using computer simulations of quarks and gluons on a space-time lattice. Until recently, these states were di?cult to simulate and hence, received little attention. Advances in lattice simulation technology (the use of better gauge and fermion actions on anisotropic lattices, the application of variational techniques with large correlation matrices, constrained curve ?tting using Bayesian priors) and computing technology (clusters, dramatically faster processors) make it possible not only to determine the spectra of such excitations, but also to probe their internal structures for the ?rst time. Commodity computing clusters at Carnegie Mellon University, the University of Maryland at College Park, the University of California at San Diego, and Je?erson Laboratory will be used to carry out these simulations. A major objective of this proposal is to calculate the mass spectrum of light-quark hybrid mesons and baryons, the N* excitations, doubly-charmed baryons, and pentaquark states using Monte Carlo methods with large sets of well-designed operators. Another key goal is to study the excitation spectrum and structures of the stationary states of gluons in the presence of a static quark-antiquark pair and three static quarks. The proposal also includes plans to determine the spatial sizes and internal structures of the low-lying pure-gauge glueballs, and to test various models of con?nement using the glueball spectrum. The proposed research incorporates the participation of three graduate students, and possibly one or more undergraduate students, whose educations will be enhanced not only by involvement in scienti?c projects at the forefront of nuclear theory, but also with high-technology training in the use of state-of-the-art parallel computing resources. The broader impacts of the proposed activities will be strengthened by posting results with simpli?ed background information on publicly accessible web pages.