Resonant Few-Body Systems from the Lattice

George Washington University, Washington DC

Investigators

Abstract

Strong forces are responsible for the formation of matter as we know it, but the precise mechanism of how this occurs remain a mystery. Strong forces bind nucleons together in atomic nuclei, but they also lead to the emergence of resonant phenomena at higher energies that are studied, e.g., at the Thomas Jefferson National Accelerator Facility (JLab). The experimental and theoretical quest for resonances to understand the strong force focuses on few-body systems and their dynamics. This project aims at closing the gap between theoretical calculations on few-body systems, called lattice QCD, and phenomenology. The main technical challenge addressed in this proposal concerns the problem that these calculations are performed in a small cubic volume instead of infinite space, and how to remove the corresponding artifacts so that lattice QCD can be compared to Nature. Deviations of these predictions from measurements are hints for new physics. The project also aims to conceptually improve the analysis of three-body data from experiment. The PI will in parallel develop simulations of simple quantum mechanical systems to illustrate the undergraduate and graduate curriculum along the lines of standard textbooks to help develop a diverse, globally competitive STEM workforce. The theory underlying the strong force, Quantum Chromodynamics (QCD), can be calculated using Lattice QCD. However, finite-volume effects for the case of three particles are poorly understood. This project aims to further develop three-body methods to enable the extrapolation of Lattice QCD results to the physical world and to help answer fundamental questions regarding QCD predictions for the properties of excited mesons and baryons. For this, the principle of unitarity, related to the conservation of probability in nuclear reactions, can be efficiently used as the guiding principle, and can be extended to the three- or more particle sector. In this project, methods will be conceptually upgraded to be applied to emblematic resonances. To highlight one, the Roper resonance has been under investigation for decades due to its unusually light mass and its debated nature as radial excitation of the nucleon or hadronic molecule. Furthermore, there is no clear signal for it in ab-initio lattice QCD calculations. The focus of this project is to quantify the finite-volume effects to solve this puzzle. Other lines of investigation address axial resonances that, in their three-body dynamics, resemble the supposed exotic mesons – a new type of particle sought for at JLab and other experiments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.