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Title: First Determination of the Weak Charge of the Proton

The Q weak experiment has measured the parity-violating asymmetry in polarized e-p elastic scattering at Q 2 = 0.025(GeV/c) 2, employing 145 microamps of 89% longitudinally polarized electrons on a 34.4cm long liquid hydrogen target at Jefferson Lab. The results of the experiment's commissioning run are reported here, constituting approximately 4% of the data collected in the experiment. From these initial results the measured asymmetry is A ep = -279 +- 35 (statistics) +- 31 (systematics) ppb, which is the smallest and most precise asymmetry ever measured in polarized e-p scattering. The small Q 2 of this experiment has made possible the first determination of the weak charge of the proton, Q p W, by incorporating earlier parity-violating electron scattering (PVES) data at higher Q 2 to constrain hadronic corrections. The value of Q p W obtained in this way is Q n W(PVES) = 0.064 +- 0.012, in good agreement with the Standard Model prediction of Q p W(SM) = 0.0710 +- 0.0007. When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted. That PVES+APV analysis reveals the neutron'smore » weak charge to be Q n W(PVES+APV) = -0.975 +- 0.010.« less
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Publication Date:
OSTI Identifier:
Report Number(s):
JLAB-PHY-13-1756; DOE/OR/23177-2619; arXiv:1307.5275
Journal ID: ISSN 0031-9007
DOE Contract Number:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 111; Journal Issue: 14
American Physical Society (APS)
Research Org:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Contributing Orgs:
Qweak Collaboration
Country of Publication:
United States