In the nuclear physics, and atomic physics weak charge refers to the standard model vector coupling of nucleons to the Z boson, namely , where T3 is the weak isospin, Q the charge, and θ the weak mixing angle. Thus, it is approximately -0.99 for a neutron and +0.07 for a proton. Measurements in 2017 give the weak charge of the proton as 0.0719 ± 0.0045
It further affects parity violating electron scattering. The weak charge may be summed in atomic nuclei, so that the predicted weak charge for 133Cs (55 protons, 78 neutrons) is -73.23 , while the experimentally deduced value was -72.58.
A recent study using four even-numbered isotopes of ytterbium was found consistent, to a 0.1% level of accuracy, with the formula weak charge Qw = −0.989 N + 0.071 Z, the latter two variables corresponding to the respective numbers of neutrons and protons in 170Yb, 172Yb, 174Yb, and 176Yb. In the ytterbium system, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed. The specific transition observed was the forbidden transition from 6s2 1S0 to 5d6s 3D1 (24489 cm−1). The latter state was mixed, due to weak interaction, with 6s6p 1P1 (25068 cm−1) to a degree proportional to the nuclear weak charge.
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- Androić, D.; Armstrong, D.S.; Asaturyan, A.; et al. (2018). "Precision measurement of the weak charge of the proton". Nature. 557: 207–211. doi:10.1038/s41586-018-0096-0.
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- Antypas, D.; Fabricant, A.; Stalnaker, J. E.; Tsigutkin, K.; Flambaum, V. V.; Budker, D. (2018). "Isotopic variation of parity violation in atomic ytterbium". Nature Physics. arXiv:1804.05747. doi:10.1038/s41567-018-0312-8.
- "Atomic parity violation research reaches new milestone" (Press release). Universitaet Mainz. 2018-11-12. Retrieved 2018-11-13.