Abstract
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.
Highlights
One of the greatest challenges within the Standard Model is to discover the source of visible mass
In the Standard Model, me is rightly attributed to the Higgs boson; but what is the source of the enormous enhancement to produce mp? This is the crux: the source of the vast majority of visible mass in the Universe is unknown
Followed logically to its origin, this question leads to an appreciation that the existence of our Universe depends critically on, inter alia, the following empirical facts: (i) the proton is massive, i.e. the mass-scale for strong interactions is vastly different to that of electromagnetism; (ii) the proton is absolutely stable, despite being a composite object constituted from three valence-quarks; and (iii) the pion, responsible for long-range interactions between nucleons, is unnaturally light, possessing a leptonlike mass despite being a strongly interacting composite object built from a valence-quark and valence-antiquark
Summary
The natural energy-scale for strong interactions is characterised by the proton mass: mp ≈ 1 GeV ≈ 2000 me ,. Followed logically to its origin, this question leads to an appreciation that the existence of our Universe depends critically on, inter alia, the following empirical facts: (i) the proton is massive, i.e. the mass-scale for strong interactions is vastly different to that of electromagnetism; (ii) the proton is absolutely stable, despite being a composite object constituted from three valence-quarks; and (iii) the pion, responsible for long-range interactions between nucleons, is unnaturally light (not massless), possessing a leptonlike mass despite being a strongly interacting composite object built from a valence-quark and valence-antiquark These are basic emergent features of Nature; and QCD must somehow explain them and many other high-level phenomena with enormous apparent complexity. The explanation of these features of Nature must lie in the dynamics responsible for the emergence of mp as the natural massscale for nuclear physics; and one of the most important goals in modern science is to explain and elucidate the entire array of empirical consequences of this dynamics
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