Measured proton electromagnetic structure deviates from theoretical predictions

Abstract

The visible world is founded on the proton, the only composite building block of matter that is stable in nature. Consequently, understanding the formation of matter relies on explaining the dynamics and the properties of the proton's bound state. A fundamental property of the proton involves the system's response to an external electromagnetic (EM) field. It is characterized by the EM polarizabilities 1 that describe how easily the charge and magnetization distributions inside the system are distorted by the EM field. Moreover, the generalized polarizabilities 2 map out the resulting deformation of the densities in a proton subject to an EM field. They reveal essential information regarding the underlying system dynamics and provide a key for decoding the proton structure in terms of the theory of the strong interaction that binds its elementary quark and gluon constituents together. Of particular interest is a puzzle in the proton's electric generalized polarizability that remains unresolved for two decades 2 . Here we report measurements of the proton's EM generalized polarizabilities at low four-momentum transfer squared. We show evidence of an anomaly to the behaviour of the proton's electric generalized polarizability that contradicts the predictions of nuclear theory and we derive its signature in the spatial distribution of the induced polarization in the proton. The reported measurements suggest the presence of a novel, not yet understood dynamical mechanism in the proton and present significant challenges to the nuclear theory.

The Proton and Occam’s Razor

“…Recent measurements by this method are reported in references [26,12]: their authors obtain 1.2−1.3 fm proton size. This size is signicantly larger than the above-mentioned Zitterbewegung radius measurements, and the origin of such discrepancy has not been understood in preceding works.…”

Proton electromagnetic generalized polarizabilities

“…The measurements suggest a local enhancement of α E (Q 2 ) in the measured region, at the same Q 2 as previously reported by [54,55,60], but with a smaller magnitude than what was originally suggested. The Q 2 -dependence of the electric GP was Frontiers in Physics frontiersin.org explored using two types of approaches [64], namely, methods that use traditional fits to the data using predefined functional forms and methods that are based on data-driven techniques that assume no direct underlying functional form. Both methods point to a Q 2dependence for α E (Q 2 ) that is consistent with the presence of a structure in the measured region, in sharp contrast with the current theoretical understanding that suggests a monotonic dependence of α E (Q 2 ) with Q 2 .…”

“…Here, the method follows an extension of the formalism to extract the light-front quark charge densities [72] from the proton form factor data. The induced polarization in the proton from the GP world data was extracted by [64]. In this work, an accurate parametrization of the polarizabilities was first derived from a fit to the experimental data, and from that, the induced polarization in the proton, P 0 , defined by Eqs 19, 20 was extracted based on the formalism discussed by [71], i.e.,…”

Unbound Low-Energy Nucleons As Semiclassical Quantum Networks

“…Nucleon modeling via lattice QCD may be conceptually correct, but is extremely computationally expensive [24], and it is not known how bare current quarks transition into dressed constituent quarks [25,26]. The precision of chiral effective field theory (EFT) [27] has generally been good, but recent discrepancies have emerged at the lowest energies [28][29][30][31][32][33]. This paper offers an alternative approach to modeling unbound ground-state nucleons that seamlessly merges with chiral EFT [27] and lattice QCD [24,34] at higher energies.…”