First measurement of proton's charge form factor at very low Q2 with initial state radiation

Abstract

We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor (G p E ) at extremely small Q 2 . The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of G p E for 0.001 ≤ Q 2 ≤ 0.004 (GeV/c) 2 .

Lepton-proton two-photon exchange in chiral perturbation theory

“…However, in recent years the values of some low-energy physical quantities extracted from such experiments show discrepancies which are currently difficult to reconcile. The most contentious being the measurements of the rootmean-square (rms) proton radius extracted from ep scattering data versus the ones from the CREMA collaboration measurements, which are high-precision muonic hydrogen Lamb-shift determinations, leading to about 5σ discrepancy with the previous accepted proton rms value [1][2][3][4][5][6][7]. We note that a very recent hydrogen Lambshift measurement [8], however, reported a result consistent with the CREMA measurement [1,2].…”

“…6 Note the difference in the signs of the ±iη → ±i0 terms in the heavy proton propagators of the two amplitudes contributes to a residual imaginary part, which is, however, irrelevant in the present context of evaluation of the unpolarized cross section. 7 We do not apply SPA in the evaluation of the IR-finite seagull diagram. A naive application of SPA to this diagram leads to lepton self-energy-like contributions with spurious IR-divergent terms.…”

Electromagnetic and gravitomagnetic structure and radii of nucleons

“…Hence, the form of the analytic function can be determined in most part by the data at non-small momentum transfer. As a result, such a form at very small t can not satisfy the experimental data in the region of very small t. For example, the latest experimental data [8] show a large difference from the standard dipole form of the electromagnetic form factors at 0.01 < Q 2 < 0.016 GeV 2 . If we take the standard dipole form of the electromagnetic form factors Λ 4 /(Λ 2 − t) 2 with Λ 2 = 0.71 GeV 2 , the analytic calculations give < r 2 D >= 0.76 fm and < r 2 E >= 0.88 fm.…”

“…Such parametrization takes into account only the experimental data for the charge proton form factor measured in the electron-proton scattering. There is a difference between the value of the proton radius and the determination of < r 2 E > in the muon-nucleon bound state, which gives < r 2 E >= 0.84087(39) fm [10,11]. This discrepancy, known as the "proton radius puzzle", gave birth to a wide discussion and many approaches to explain this fact.…”

Reinterpretation of Classic Proton Charge Form Factor Measurements

“…offsets in the absolute normalization of the reported data. The tension between the data is reduced if the normalizations of the data sets are kept as free parameters, as is being done in modern analyses of form factor measurements [15,22,28], but does not disappear completely. Furthermore, introduction of additional five free parameters to the fits (normalizations) increases the variance of the extracted result and dilutes the significance of the extracted radius, which in the given case equals to 0.865(48) fm, see Fig.…”