Resonance Electroproduction and the Origin of Mass

One of the greatest challenges within the Standard Model is to discover the source of visible mass. Indeed, this is the focus of a “Millennium Problem”, posed by the Clay Mathematics Institute. The answer is hidden within quantum chromodynamics (QCD); and it is probable that revealing the origin of mass will also explain the nature of confinement. In connection with these issues, this perspective will describe insights that have recently been drawn using contemporary methods for solving the continuum bound-state problem in relativistic quantum field theory and how they have been informed and enabled by modern experiments on nucleon-resonance electroproduction.

Empirical Consequences of Emergent Mass

The Lagrangian that defines quantum chromodynamics (QCD), the strong interaction piece of the Standard Model, appears very simple. Nevertheless, it is responsible for an astonishing array of high-level phenomena with enormous apparent complexity, e.g., the existence, number and structure of atomic nuclei. The source of all these things can be traced to emergent mass, which might itself be QCD’s self-stabilising mechanism. A background to this perspective is provided, presenting, inter alia, a discussion of the gluon mass and QCD’s process-independent effective charge and highlighting an array of observable expressions of emergent mass, ranging from its manifestations in pion parton distributions to those in nucleon electromagnetic form factors.

On SU(3)F positive-parity octet and decuplet baryons

A continuum approach to the three valence-quark bound-state problem in quantum field theory, employing parametrisations of the necessary kernel elements, is used to compute the spectrum and Poincarö- covariant wave functions for all flavour-SU(3) octet and decuplet baryons and their first positive-parity ex citations. Such analyses predict the existence of nonpointlike, dynamical quark-quark (diquark) correlations within all baryons; and a uniformly sound description of the systems studied is obtained by retaining flavour- antitriplet-scalar and flavour-sextet-pseudovector diquarks. The analysis predicts the existence of positive- parity excitations of the 𝚵, 𝚵*, Ω baryons, with masses, respectively (in GeV): 1.84(08), 1.89(04), 2.05(02). These states have not yet been empirically identified. This body of analysis suggests that the expression of emergent mass generation is the same in all u, d, s baryons and, notably, that dynamical quark-quark correla tions play an essential role in the structure of each one. It also provides the basis for developing an array of predictions that can be tested in new generation experiments.

Strong Running Coupling from the Gauge Sector of Domain Wall Lattice QCD with Physical Quark Masses

We report on the first computation of the strong running coupling at the physical point (physical pion mass) from the ghost-gluon vertex, computed from lattice simulations with three flavors of domain wall fermions. We find α_{MS[over ¯]}(m_{Z}^{2})=0.1172(11), in remarkably good agreement with the world-wide average. Our computational bridge to this value is the Taylor-scheme strong coupling, which has been revealed of great interest by itself because it can be directly related to the quark-gluon interaction kernel in continuum approaches to the QCD bound-state problem.

Heavy-light mesons in a contact interaction

We present a unified formalism for the analysis of mesons. It is provided by a symmetry preserving Schwinger-Dyson-Bethe-Salpeter-Equation (SDBSE) treatment of a vector-vector contact interaction. The contact interaction (CI) model offers a simple-to-implement alternative to perform exploratory studies of QCD, within the SDBSE framework. Within the limitation of this model, we calculate the spectrum of D and B mesons. Our results are in agreement within 2% when compare with experimental data, lattice QCD and other SDBSE calculations involving sophisticated interaction kernels.