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A proton is a bound state of a strong interaction, governed by Quantum Chromodynamics (QCD). The electric charge radius of a proton, denoted by r E p , characterizes the spatial distribution of its electric charge carried by the quarks. It is an important input for bound-state Quantum Electrodynamic (QED) calculations of the hydrogen atomic energy levels. However, physicists have been puzzled by the large discrepancy between r E p measurements from muonic hydrogen spectroscopy and those from e p elastic scattering and ordinary hydrogen spectroscopy for over a decade. Tremendous efforts, both theoretical and experimental, have been dedicated to providing various insights into this puzzle, but certain issues still remain unresolved, particularly in the field of lepton scatterings. This review will focus on lepton-scattering measurements of r E p , recent theoretical and experimental developments in this field, as well as future experiments using this technique. [ABSTRACT FROM AUTHOR]

A novel photon-based proton model is developed. A proton’s ground state is assumed to be coherent to the degree that all of its mass-energy precipitates into a single uncharged spherical structure. A quantum vortex, initiated by the strong force, but sustained in the proton’s ground state by the circular Unruh effect and a spherical Rindler horizon, is proposed to confine the proton’s mass-energy in its ground state. A direct connection between the circular Unruh effect, the zitterbewegung effect, spin, and general relativity is proposed. Such a structure acts as an uncharged zitterbewegung fermion, and may explain neutrino mass. A ground-state proton’s central zitterbewegung fermion is assumed to be surrounded by a halo of charge shells of both signs. Virtual photon standing waves are assumed to synchronize the inner shell with the central zitterbewegung fermion. The charge shells are proposed to be associated with isospin and proton g-factor. There are only two model inputs—proton mass and quantized electronic charge—and just one adjustable parameter. The adjustable parameter, reduced only by about 0.4% from an initial estimate, provides the proton’s experimentally determined magnetic moment to arbitrary precision. The resulting modeled proton charge radius agrees very well with the 2018 CODATA value. Magnetic moment and charge radius are calculated algebraically in a manner easily understood by undergraduate physics students. This proposed ground-state proton model could be considered a low-energy approximation to a full quantum chromodynamical proton model. [ABSTRACT FROM AUTHOR]

The proton radius puzzle has motivated several new experiments that aim to extract the proton charge radius and resolve the puzzle. Recently, PRad, a new electron–proton scattering experiment at Jefferson Lab, reported a proton charge radius of 0. 8 3 1 ± 0. 0 0 7 statistical ± 0. 0 1 2 systematic . The value was obtained by using a rational function model for the proton electric form factor. We perform a model-independent extraction using z -expansion of the proton charge radius from PRad data. We find that the model-independent statistical error is more than 50% larger compared to the statistical error reported by PRad. [ABSTRACT FROM AUTHOR]

The "proton charge radius puzzle" arose because the root-mean-square charge radius of the proton was very accurately measured by two different methods that disagreed by many standard deviations. Electron scattering measurements of the charge radius are strongly supported by this new proton model, resolving the proton charge radius puzzle. Most of the mass of the visible Universe consists of protons and neutrons. The origin of hadronic mass remains a mystery. We show, by applying old physics in new ways, how to assemble a proton whose calculated charge radius, mass radius, magnetic moment, and magnetic-moment-weighted charge radius agree accurately with experiment. This new physics proves that fractionally charged constituents (quarks) play no role in explaining the mass and electromagnetic structure of an isolated proton. The confinement mechanism is not due to postulated gluons but from conservation laws acting inside a new vacuum phase. In essence, a proton is a bubble of a new vacuum phase that is more than 12 orders of magnitude hotter than the conventional vacuum. [ABSTRACT FROM AUTHOR]

We briefly review the so-called 'proton puzzle', i.e., the disagreement of the newly extracted value of the proton charge radius r from muonic hydrogen spectroscopy with other extractions, its possible significance and related problems. After describing the conventional theory to extract the proton radius from atomic spectroscopy we focus on a novel consistent approach based on the Breit equation. With this new tool, we confirm that the radius has indeed become smaller compared to the value extracted from scattering experiments, but the existence of different theoretical approaches casts some doubt on the accuracy of the new value. Precision measurements in atomic physics do provide the opportunity to extract light nuclear radii but the accuracy is limited by the methods of incorporating the nuclear structure effects. [ABSTRACT FROM AUTHOR]

The root-mean-square (rms) charge radius r of the proton has so far been known only with a surprisingly low precision of about 1% from both electron scattering and precision spectroscopy of hydrogen. We have recently determined r by means of laser spectroscopy of the Lamb shift in the exotic 'muonic hydrogen' atom. Here, the muon, which is the 200 times heavier cousin of the electron, orbits the proton with a 200 times smaller Bohr radius. This enhances the sensitivity to the proton's finite size tremendously. Our new value r = 0.84184 (67) fm is ten times more precise than the generally accepted CODATA-value, but it differs by 5 standard deviations from it. A lively discussion about possible solutions to the 'proton size puzzle' has started. Our measurement, together with precise measurements of the 1S-2S transition in regular hydrogen and deuterium, also yields improved values of the Rydberg constant, R = 10,973,731.568160 (16) m. [ABSTRACT FROM AUTHOR]

Abstract: A measurement of the Lamb shift of 49,881.88(76) GHz in muonic hydrogen in conjunction with theoretical estimates of the proton structure effects was recently used to deduce an accurate but rather small radius of the proton. Such an important shift in the understanding of fundamental values needs reconfirmation. Using a different approach with electromagnetic form factors of the proton, we obtain a new expression for the transition energy, , in muonic hydrogen and deduce a proton radius, . [Copyright &y& Elsevier]

Abstract: The unitary and analytic model of nucleon electromagnetic structure, describing all existing nucleon form factor data, is briefly reviewed. Then in the framework of this model the problem of inconsistency of older proton electric form factor data in a space-like region (obtained from the process by the Rosenbluth technique) with recent Jefferson Lab data on the ratio (measured in a precise polarization experiment) is suggested to be solved in favor of the latter data which, however, unlike older data imply the existence of a form factor zero, i.e. a diffraction minimum in around . The new behavior of with the zero gives a modified proton charge distribution with an enlarged value of the mean square charge radius. [Copyright &y& Elsevier]

We report on a windowless, high-density, gas-flow target at Jefferson Lab that was used to measure r p , the root-mean-square charge radius of the proton. The target achieved its design goal of an areal density of 2 × 1018 atoms/cm 2 , with the gas uniformly distributed over the 4 cm length of the cell and less than 1% residual gas outside the cell. This design eliminated scattering from the end caps of the target cell, a problem endemic to previous measurements of the proton charge radius in electron scattering experiments, and permitted a precise, model-independent extraction of r p by reaching unprecedentedly low values of Q 2 , the square of the electron's transfer of four-momentum to the proton. [ABSTRACT FROM AUTHOR]

Abstract: A new experimental method for a precise determination of the proton charge radius with elastic electron proton scattering at low momentum transfers is shown to be feasible, using a novel technique where the recoil protons are detected in a set of Si detectors. [Copyright &y& Elsevier]

High‐resolution laser spectroscopy serves the purpose of determining the energy difference between states of atoms and molecules with the best possible accuracy. Therefore, one may face the problem of finding the center of a symmetric line within a small fraction of the line width, or one needs to extract the energy difference from an asymmetric line without a uniquely defined center. Multiplets of atomic resonance lines are subject to mutual line pullings and give rise to asymmetric line distortions due to quantum interference. This paper reviews the treatment of these distortions for dipole‐allowed one‐photon transitions. Specific examples are given for hydrogen and helium spectroscopy. [ABSTRACT FROM AUTHOR]

High resolution spectroscopy of the hydrogen atom takes on particular importance in the new SI, as it allows to accurately determine fundamental constants, such as the Rydberg constant and the proton charge radius. Recently, the second most precisely measured transition frequency in hydrogen, 1S−3S, is obtained by our group. In the context of the Proton Radius Puzzle, this result calls for further investigation. [ABSTRACT FROM AUTHOR]