Polarization transfer observables in elastic electron-proton scattering at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>Q</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>=</mml:mo><mml:mn>2.5</mml:mn></mml:mrow></mml:math>, 5.2, 6.8, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>8.5</mml:mn><mml:mo> </mml:mo><mml:msup><mml:mrow><mml:mi>GeV</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msup></mml:math>
Bibliographic record
Abstract
Background: Interest in the behavior of nucleon electromagnetic form factors at large momentum transfers has steadily increased since the discovery, using polarization observables, of the rapid decrease of the ratio ${G}_{E}^{p}/{G}_{M}^{p}$ of the proton's electric and magnetic form factors for momentum transfers ${Q}^{2}\ensuremath{\gtrsim}1$ ${\mathrm{GeV}}^{2}$, in strong disagreement with previous extractions of this ratio using the traditional Rosenbluth separation technique.Purpose: The GEp-III and $\mathrm{GEp}\text{\ensuremath{-}}2\ensuremath{\gamma}$ experiments were carried out in Jefferson Laboratory's (JLab's) Hall C from 2007 to 2008, to extend the knowledge of ${G}_{E}^{p}/{G}_{M}^{p}$ to the highest practically achievable ${Q}^{2}$ given the maximum beam energy of 6 GeV and to search for effects beyond the Born approximation in polarization transfer observables of elastic $\stackrel{P\vec}{e}p$ scattering. This article provides an expanded description of the common experimental apparatus and data analysis procedures, and reports the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance dataset of the $\mathrm{GEp}\text{\ensuremath{-}}2\ensuremath{\gamma}$ experiment.Methods: Polarization transfer observables in elastic $\stackrel{P\vec}{e}p\ensuremath{\rightarrow}e\stackrel{P\vec}{p}$ scattering were measured at central ${Q}^{2}$ values of 2.5, 5.2, 6.8, and 8.54 ${\mathrm{GeV}}^{2}$. At ${Q}^{2}=2.5\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{2}$, data were obtained for central values of the virtual photon polarization parameter $\ensuremath{\epsilon}$ of 0.149, 0.632, and 0.783. The Hall C High Momentum Spectrometer detected and measured the polarization of protons recoiling elastically from collisions of JLab's polarized electron beam with a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scattered electrons in coincidence to suppress inelastic backgrounds.Results: The final GEp-III data are largely unchanged relative to the originally published results. The statistical uncertainties of the final $\mathrm{GEp}\text{\ensuremath{-}}2\ensuremath{\gamma}$ data are significantly reduced at $\ensuremath{\epsilon}=0.632$ and 0.783 relative to the original publication.Conclusions: The final GEp-III results show that the decrease with ${Q}^{2}$ of ${G}_{E}^{p}/{G}_{M}^{p}$ continues to ${Q}^{2}=8.5\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{2}$, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At ${Q}^{2}=8.5\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{2}$, ${G}_{E}^{p}/{G}_{M}^{p}$ remains positive but is consistent with zero. At ${Q}^{2}=2.5\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{2}$, ${G}_{E}^{p}/{G}_{M}^{p}$ derived from the polarization component ratio $R\ensuremath{\propto}{P}_{t}/{P}_{\ensuremath{\ell}}$ shows no statistically significant $\ensuremath{\epsilon}$ dependence, as expected in the Born approximation. On the other hand, the ratio ${P}_{\ensuremath{\ell}}/{P}_{\ensuremath{\ell}}^{\mathrm{Born}}$ of the longitudinal polarization transfer component to its Born value shows an enhancement of roughly 1.7% at $\ensuremath{\epsilon}=0.783$ relative to $\ensuremath{\epsilon}=0.149$, with $\ensuremath{\approx}2.2\ensuremath{\sigma}$ significance based on the total uncertainty, implying a similar effect in the transverse component ${P}_{t}$ that cancels in the ratio $R$.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.006 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".