First Determination of the Weak Charge of the Proton
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Author(s)
Balewski, Jan T.
Guo, Fang
Rajotte, Jean-Francois
Kowalski, Stanley B
Date Issued
October 2013
Journal
Physical Review Letters
Publisher
American Physical Society
Citation
Androic, D., D. S. Armstrong, A. Asaturyan, T. Averett, J. Balewski, J. Beaufait, R. S. Beminiwattha, et al. “First Determination of the Weak Charge of the Proton.” Physical Review Letters 111, no. 14 (October 2013). © 2013 American Physical Society
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Final published version
Abstract
The Q[subscript weak] experiment has measured the parity-violating asymmetry in e⃗ p elastic scattering at Q[superscript 2] = 0.025 (GeV/c)[superscript 2], employing 145 μA of 89% longitudinally polarized electrons on a 34.4 cm long liquid hydrogen target at Jefferson Lab. The results of the experiment’s commissioning run, constituting approximately 4% of the data collected in the experiment, are reported here. From these initial results, the measured asymmetry is A[subscript ep]= −279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in e⃗ p scattering. The small Q[superscript 2] of this experiment has made possible the first determination of the weak charge of the proton Q[p over W] by incorporating earlier parity-violating electron scattering (PVES) data at higher Q[superscript 2] to constrain hadronic corrections. The value of Q[p over W] obtained in this way is Q[p over W](PVES) = 0.064 ± 0.012, which is in good agreement with the standard model prediction of Q[p over 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’s weak charge to be Q[n over W](PVES + APV) = −0.975 ± 0.010.
MIT Department
Massachusetts Institute of Technology. Department of Physics
Massachusetts Institute of Technology. Laboratory for Nuclear Science
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DOI of Published Version
https://doi.org/10.1103/PhysRevLett.111.141803
