The proton radius puzzle

Abstract

The proton size, specifically its charge radius, was thought known to about 1% accuracy. Now a new method probing the proton with muons instead of electrons finds a radius about 4% smaller, and to boot gives an uncertainty limit of about 0.1%. We review the different measurements, some of the calculations that underlie them, some of the suggestions that have been made to resolve the conflict, and give a brief overview new related experimental initiatives. At present, however, the resolution to the problem remains unknown.

Baryons as relativistic three-quark bound states

“…A closely related issue that also depends on the magnitude of TPE corrections is the proton radius puzzle. We will only briefly sketch the problem here and refer to [1,643,[671][672][673][674] for detailed reviews. The basic outline is the same as in Fig.…”

“…However, the resulting value for E TPE is too small to resolve the problem -to obtain a proton radius consistent with the CODATA value, the TPE contributions would have to be larger by an order of magnitude (requiring E TPE ∼ 0.360 meV). Therefore, at present it seems unlikely that TPE effects can resolve the proton radius puzzle unless one is willing to account for 'haywire' hadronic effects [1,690]. In any case, given that our overall knowledge of the CS amplitude is still rather sparse more detailed investigations are certainly desirable.…”

“…2, are in striking contrast to the ratio obtained via LT or Rosenbluth separations, showing an approximately linear decrease of R with Q 2 which is in strong violation of the Q 2 scaling behavior (see also Refs. [1,2,28,29]). The discrepancy between the LT and PT measurements of G E /G M has stimulated considerable activity, both theoretically and experimentally, over the past decade.…”

“…They are highly nontrivial objects governed by the strong interaction. Their complicated internal structure is far from understood and even supposedly trivial properties like the charge radius of the proton pose disturbing puzzles [1]. Historically, baryons have been the focus of both experimental and theoretical interest long before the dawn of Quantum Chromodynamics (QCD) and will likely continue to be so for many decades to come.…”

Point-particle effective field theory III: relativistic fermions and the Dirac equation

“…In what follows we specialize for simplicity to parity-preserving interactions and spinless compact central objects, and so strictly speaking the interactions we find suffice in themselves to describe finite-size effects in the He + ion or muonic states in even-even nuclei [24,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The effects we find also apply to nuclei with spin (such as hydrogen) once the effective theory of the first-quantized source is supplemented by the extra interactions that a nuclear spin allows.…”