Weak charge

In nuclear physics and atomic physics, weak charge refers to the standard model weak interaction coupling of the vector Z boson to nucleons; it indicates an effect on parity violating electron scattering. For any given nuclear isotope, the total weak charge is approximately −0.99 per neutron, and +0.07 per proton.[1]

Theoretical basis

The formula for the weak charge is derived from weak hypercharge, and is given by

where Qw is the weak charge, T3 is the weak isospin, Q is the electric charge, and θW the weak mixing angle.

Empirical formulas

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 value determined experimentally, from measurements of parity violating electron scattering, was −72.58 .[2]

Measurements in 2017 give the weak charge of the proton as 0.0719±0.0045 .[3]

A recent study used four even-numbered isotopes of ytterbium to test the formula Qw = −0.989 N + 0.071 Z , for weak charge, with N corresponding to the number of neutrons and Z to the number of protons. The formula was found consistent to 0.1% accuracy using the 170Yb, 172Yb, 174Yb, and 176Yb isotopes of ytterbium.[4]

In the ytterbium test, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed.[5] 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.[4]

See also

References
  1. Hagen, G.; Ekström, A.; Forssén, C.; Jansen, G. R.; Nazarewicz, W.; Papenbrock, T.; Wendt, K. A.; Bacca, S.; Barnea, N.; Carlsson, B.; Drischler, C.; Hebeler, K.; Hjorth-Jensen, M.; Miorelli, M.; Orlandini, G.; Schwenk, A.; Simonis, J. (2016). "Charge, neutron, and weak size of the atomic nucleus". Nature Physics. 12 (2): 186–190. arXiv:1509.07169. doi:10.1038/nphys3529.
  2. Dzuba, V. A.; Berengut, J. C.; Flambaum, V. V.; Roberts, B. (2012). "Revisiting Parity Nonconservation in Cesium". Physical Review Letters. 109 (20): 203003. arXiv:1207.5864. doi:10.1103/PhysRevLett.109.203003. PMID 23215482.
  3. Androić, D.; Armstrong, D.S.; Asaturyan, A.; et al. (2018). "Precision measurement of the weak charge of the proton". Nature. 557: 207–211. arXiv:1905.08283. doi:10.1038/s41586-018-0096-0.
  4. 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.
  5. "Atomic parity violation research reaches new milestone" (Press release). Universitaet Mainz. 2018-11-12. Retrieved 2018-11-13.


This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.