Revealing dressed quarks via the proton's charge distribution

Abstract

The proton is arguably the most fundamental of nature’s readily detectable building blocks. It is at the heart of every nucleus and has never been observed to decay. It is nevertheless a composite object, defined by its valence-quark content: u+u+d—i.e., two up (u) quarks and one down (d) quark; and the manner by which they influence, inter alia, the distribution of charge and magnetization within this bound state. Much of novelty has recently been learned about these distributions; and it now appears possible that the proton’s momentum-space charge distribution possesses a zero. Experiments in the coming decade should answer critical questions posed by this and related advances; we explain how such new information may assist in charting the origin and impact of key emergent phenomena within the strong interaction. Specifically, we show that the possible existence and location of a zero in the proton’s electric form factor are a measure of nonperturbative features of the quark-quark interaction in the standard model, with particular sensitivity to the running of the dressed-quark mass.