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Title: Towards a Resolution of the Proton Form Factor Problem: New Electron and Positron Scattering Data

Journal Article · · Physical Review Letters
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There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p). from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (epsilon) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing epsilon at Q(2) = 1.45 GeV2. This measurement is consistent with the size of the form factor discrepancy at Q(2) approximate to 1.75 GeV2 and with hadronic calculations including nucleon and Delta intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV2.

Research Organization:
Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States); Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Nuclear Physics (NP); Instituto Nazionale di Fisica Nucleare (INFN); Centre National de la Recherche Scientifique (CNRS); National Science Foundation (NSF)
Contributing Organization:
CLAS Collaboration
Grant/Contract Number:
AC05-06OR23177; FG02-96ER40960; AC02-06CH11357; AC05-060R23177
OSTI ID:
1169688
Alternate ID(s):
OSTI ID: 1180924; OSTI ID: 1251618
Report Number(s):
JLAB-PHY-14-1960; DOE/OR/23177-3300; arXiv:1411.6908; PRLTAO; ArticleNumber: 062003
Journal Information:
Physical Review Letters, Vol. 114, Issue 6; ISSN 0031-9007
Publisher:
American Physical Society (APS)Copyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 68 works
Citation information provided by
Web of Science

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Cited By (9)

Evaluation of the Proton Charge Radius from Electron–Proton Scattering journal September 2015
Two-Photon Exchange: Future Experimental Prospects journal July 2018
The structure of the nucleon: Elastic electromagnetic form factors journal July 2015
Hard two-photon contribution to elastic lepton-proton scattering determined by the OLYMPUS experiment journal February 2018
Measurement of two-photon exchange effect by comparing elastic e±p cross sections text January 2017
Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment text January 2016
Codata Recommended Values Of The Fundamental Physical Constants: 2014 text January 2015
Evaluation of the strength of electron-proton scattering data for determining the proton charge radius text January 2015
Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment text January 2016

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