Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Measurement of parity violation in electron–quark scattering


Symmetry permeates nature and is fundamental to all laws of physics. One example is parity (mirror) symmetry, which implies that flipping left and right does not change the laws of physics. Laws for electromagnetism, gravity and the subatomic strong force respect parity symmetry, but the subatomic weak force does not1,2. Historically, parity violation in electron scattering has been important in establishing (and now testing) the standard model of particle physics. One particular set of quantities accessible through measurements of parity-violating electron scattering are the effective weak couplings C2q, sensitive to the quarks’ chirality preference when participating in the weak force, which have been measured directly3,4 only once in the past 40 years. Here we report a measurement of the parity-violating asymmetry in electron–quark scattering, which yields a determination of 2C2u − C2d (where u and d denote up and down quarks, respectively) with a precision increased by a factor of five relative to the earlier result. These results provide evidence with greater than 95 per cent confidence that the C2q couplings are non-zero, as predicted by the electroweak theory. They lead to constraints on new parity-violating interactions beyond the standard model, particularly those due to quark chirality. Whereas contemporary particle physics research is focused on high-energy colliders such as the Large Hadron Collider, our results provide specific chirality information on electroweak theory that is difficult to obtain at high energies. Our measurement is relatively free of ambiguity in its interpretation, and opens the door to even more precise measurements in the future.

This is a preview of subscription content

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Comparison of the present results with those of earlier experiments and predictions of the standard model.
Figure 2: Mass exclusion limits Λ on the electron and quark compositeness and contact interactions.


  1. Lee, T. D. & Yang, C.-N. Question of parity conservation in weak interactions. Phys. Rev. 104, 254–258 (1956)

    ADS  CAS  Article  Google Scholar 

  2. Wu, C. S., Ambler, E., Hayward, R. W., Hoppes, D. D. & Hudson, R. P. Experimental test of parity conservation in beta decay. Phys. Rev. 105, 1413–1415 (1957)

    ADS  CAS  Article  Google Scholar 

  3. Prescott, C. Y. et al. Parity nonconservation in inelastic electron scattering. Phys. Lett. B 77, 347–352 (1978)

    ADS  Article  Google Scholar 

  4. Prescott, C. Y. et al. Further measurements of parity nonconservation in inelastic electron scattering. Phys. Lett. B 84, 524–528 (1979)

    ADS  Article  Google Scholar 

  5. Cahn, R. N. & Gilman, F. J. Polarized-electron-nucleon scattering in gauge theories of weak and electromagnetic interactions. Phys. Rev. D 17, 1313–1322 (1978)

    ADS  CAS  Article  Google Scholar 

  6. Glashow, S. L. Partial symmetries of weak interactions. Nucl. Phys. 22, 579–588 (1961)

    Article  Google Scholar 

  7. Weinberg, S. A model of leptons. Phys. Rev. Lett. 19, 1264–1266 (1967)

    ADS  Article  Google Scholar 

  8. Salam, A. Weak and electromagnetic interactions. In Elementary Particle Theory: Relativistic Groups and Analyticity (ed. Svartholm, N. ) 367–377 (Almqvist and Wiksell, 1968)

    Google Scholar 

  9. Anthony, P. L. et al. Precision measurement of the weak mixing angle in Moller scattering. Phys. Rev. Lett. 95, 081601 (2005)

    ADS  CAS  Article  Google Scholar 

  10. Czarnecki, A. & Marciano, W. J. Electrons are not ambidextrous. Nature 435, 437–438 (2005)

    ADS  CAS  Article  Google Scholar 

  11. Abrahamyan, S. et al. Measurement of the neutron radius of 208Pb through parity-violation in electron scattering. Phys. Rev. Lett. 108, 112502 (2012)

    ADS  CAS  Article  Google Scholar 

  12. Armstrong, D. S. & McKeown, R. D. Parity-violating electron scattering and the electric and magnetic strange form factors of the nucleon. Annu. Rev. Nucl. Part. Sci. 62, 337–359 (2012)

    ADS  CAS  Article  Google Scholar 

  13. Alcorn, J. et al. Basic instrumentation for Hall A at Jefferson Lab. Nucl. Instrum. Methods A 522, 294–346 (2004)

    ADS  CAS  Article  Google Scholar 

  14. Subedi, R. et al. A scaler-based data acquisition system for measuring parity violation asymmetry in deep inelastic scattering. Nucl. Instrum. Methods A 724, 90–103 (2013)

    ADS  CAS  Article  Google Scholar 

  15. Martin, A. D., Stirling, W. J., Thorne, R. S. & Watt, G. Parton distributions for the LHC. Eur. Phys. J. C 63, 189–285 (2009)

    ADS  CAS  Article  Google Scholar 

  16. Androic, D. et al. First determination of the weak charge of the proton. Phys. Rev. Lett. 111, 141803 (2013)

    ADS  CAS  Article  Google Scholar 

  17. Wood, C. S. et al. Measurement of parity nonconservation and an anapole moment in cesium. Science 275, 1759–1763 (1997)

    CAS  Article  Google Scholar 

  18. Bennett, S. C. & Wieman, C. E. Measurement of the 6S7S transition polarizability in atomic cesium and an improved test of the Standard Model. Phys. Rev. Lett. 82, 2484–2487 (1999); erratum. 83, 889 (1999)

    ADS  CAS  Article  Google Scholar 

  19. Ginges, J. S. M. & Flambaum, V. V. Violations of fundamental symmetries in atoms and tests of unification theories of elementary particles. Phys. Rep. 397, 63–154 (2004)

    ADS  CAS  Article  Google Scholar 

  20. Dzuba, V. A., Berengut, J. C., Flambaum, V. V. & Roberts, B. Revisiting parity nonconservation in cesium. Phys. Rev. Lett. 109, 203003 (2012)

    ADS  CAS  Article  Google Scholar 

  21. Erler, J. & Su, S. The weak neutral current. Prog. Part. Nucl. Phys. 71, 119–149 (2013)

    ADS  Article  Google Scholar 

  22. Beise, E. J., Pitt, M. L. & Spayde, D. T. The SAMPLE experiment and weak nucleon structure. Prog. Part. Nucl. Phys. 54, 289–350 (2005)

    ADS  CAS  Article  Google Scholar 

  23. Eichten, E., Lane, K. D. & Peskin, M. E. New tests for quark and lepton substructure. Phys. Rev. Lett. 50, 811–814 (1983)

    ADS  CAS  Article  Google Scholar 

  24. Schael, S. et al. Electroweak measurements in electron-positron collisions at W-boson-pair energies at LEP. Phys. Rep. 532, 119–244 (2013)

    Article  Google Scholar 

  25. Chekanov, S. et al. Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA. Phys. Lett. B 591, 23–41 (2004)

    ADS  CAS  Article  Google Scholar 

  26. Aaron, F. D. et al. Search for contact interactions in e±p collisions at HERA. Phys. Lett. B 705, 52–58 (2011)

    CAS  Article  Google Scholar 

  27. Aad, G. et al. Search for contact interactions and large extra dimensions in dilepton events from pp collisions at with the ATLAS detector. Phys. Rev. D 87, 015010 (2013)

    ADS  Article  Google Scholar 

Download references


We thank the personnel of Jefferson Lab for their efforts which resulted in the successful completion of the experiment, and A. Accardi, P. Blunden, W. Melnitchouk and their collaborators for carrying out the calculations necessary for the completion of the data analysis. X.Z. thanks the Medium Energy Physics Group at the Argonne National Laboratory for support during the initial work on this experiment. J.E. was supported by PAPIIT (DGAPAUNAM) project IN106913 and CONACyT (México) project 151234, and acknowledges the hospitality and support by the Mainz Institute for Theoretical Physics (MITP) where part of his work was completed. This work was supported in part by the Jeffress Memorial Trust (award no. J-836), the US NSF (award no. 0653347), and the US DOE (award nos DE-SC0003885 and DE-AC02-06CH11357). This work was authored by Jefferson Science Associates, LLC under US DOE contract no. DE-AC05-06OR23177. The US Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for US Government purposes.

Author information

Authors and Affiliations



Authors contributed to one or more of the following areas: proposing, leading, and running the experiment; design, construction, optimization, and testing of the data acquisition system; data analysis; simulation; extraction of the physics results from measured asymmetries; and the writing of this Letter.

Corresponding author

Correspondence to X. Zheng.

Ethics declarations

Competing interests

The author declare no competing financial interests.

Additional information

J.E. is currently on sabbatical leave at the PRISMA Cluster of Excellence and MITP, Johannes Gutenberg University.

Supplementary information

Supplementary Information

This file contains Supplementary Methods, Supplementary References and Supplementary Tables 1-2. (PDF 244 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

The Jefferson Lab PVDIS Collaboration. Measurement of parity violation in electron–quark scattering. Nature 506, 67–70 (2014).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing