The Qweak Experiment: First Determination of the Weak Charge of the Proton
- Univ. of Virginia, Charlottesville, VA (United States); Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
The $$Q_{weak}$$ Collaboration has completed a challenging measurement of the parity-violating asymmetry in elastic electron-proton ($$\vec{e}$$p) scattering at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The initial result reported here is extracted from the commissioning part of the experiment, constituting about 4% of the full data set. The parity-violating asymmetry at a low momentum transfer $Q^2$=0.025 GeV$^2$ is $$A_{ep}$$ = -279 $$\pm$$ 35 (stat) $$\pm$$ 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in $$\vec{e}$$p scattering. This result allowed the first determination of the weak charge of the proton $$Q_W^p$$ from a global fit of parity-violating elastic scattering (PVES) results from nuclear targets, where earlier data at higher $Q^2$ constrain uncertainties of hadronic structure. The value extracted from the global fit is $$Q_W^p$$ (PVES) = 0.064 $$\pm$$ 0.012, in agreement with the standard model prediction $$Q_W^p$$ (SM) = 0.0710 $$\pm$$ 0.0007. The neutral weak charges of up and down quarks are extracted from a combined fit of the PVES results with a previous atomic parity violation (APV) measurement on $$^{133}$$Cs. The analysis of the full $$Q_{weak}$$ data is ongoing and expected to yield a value for the asymmetry within 10 ppb of precision. Because of the suppression of $$Q_W^p$$, such a high precision measurement will place significant constraints to models of physics beyond the standard model.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Nuclear Physics (NP)
- DOE Contract Number:
- AC05-06OR23177
- OSTI ID:
- 1330674
- Report Number(s):
- JLAB-PHY-15-2201; DOE/OR/23177-3638; arXiv:1509.08829
- Resource Relation:
- Conference: CIPANP 2015, Vail, Colorado, May 19-24, 2015
- Country of Publication:
- United States
- Language:
- English
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