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  • Letter
  • Published:

Precision measurement of the weak charge of the proton

Nature volume 557, pages 207–211 (2018)Cite this article

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Abstract

Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory’s limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter–antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today’s high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton’s interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x, y, z) → (−x, −y, −z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton’s weak charge1. Here we report the value 0.0719 ± 0.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is −226.5 ± 9.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton’s weak charge is in excellent agreement with the standard model2 and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales.

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Fig. 1: Parity-violating electron scattering from the proton.
Fig. 2: The reduced asymmetry \({{\boldsymbol{A}}}_{{\bf{ep}}}/{{\boldsymbol{A}}}_{{\bf{0}}}={{\boldsymbol{Q}}}_{{\bf{w}}}^{{\bf{p}}}+{{\boldsymbol{Q}}}^{{\bf{2}}}{\boldsymbol{B}}\left({{\boldsymbol{Q}}}^{{\bf{2}}},{\boldsymbol{\theta }}{\bf{= 0}}\right)\) versus Q2.
Fig. 3: Variation of sin2θW with energy scale Q.
Fig. 4: Mass and coupling constraints on new physics.

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Change history

  • 22 May 2018

    In the originally published article, equation (6) was corrupted. This has now been corrected.

References

  1. Erler, J., Kurylov, A. & Ramsey-Musolf, M. J. Weak charge of the proton and new physics. Phys. Rev. D 68, 016006 (2003).

    Article  ADS  Google Scholar 

  2. Particle Data Group. Review of particle physics. Chinese Phys. C 40, 100001 (2016).

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  4. Allison, T. et al. The Q weak experimental apparatus. Nucl. Instrum. Meth. A 781, 105–133 (2015).

    Article  ADS  CAS  Google Scholar 

  5. Narayan, A. et al. Precision electron-beam polarimetry at 1 GeV using diamond microstrip detectors. Phys. Rev. X 6, 011013 (2016).

    Google Scholar 

  6. Smith, G. R. The Q weak target performance. Nuovo Cim. C 35, 159–163 (2012).

    Google Scholar 

  7. Young, R. D., Roche, J., Carlini, R. D. & Thomas, A. W. Extracting nucleon strange and anapole form factors from world data. Phys. Rev. Lett. 97, 102002 (2006).

    Article  ADS  CAS  Google Scholar 

  8. Young, R. D., Carlini, R. D., Thomas, A. W. & Roche, J. Testing the standard model by precision measurement of the weak charges of quarks. Phys. Rev. Lett. 99, 122003 (2007).

    Article  ADS  CAS  Google Scholar 

  9. Arrington, J. & Sick, I. Precise determination of low-Q nucleon electromagnetic form factors and their impact on parity-violating e-p elastic scattering. Phys. Rev. C 76, 035201 (2007).

    Article  ADS  Google Scholar 

  10. Hall, N. L., Blunden, P. G., Melnitchouk, W., Thomas, A. W. & Young, R. D. Quark-hadron duality constraints on γZ box corrections to parity-violating elastic scattering. Phys. Lett. B 753, 221–226 (2016).

    Article  ADS  CAS  Google Scholar 

  11. Blunden, P. G., Melnitchouk, W. & Thomas, A. W. New formulation of γZ box corrections to the weak charge of the proton. Phys. Rev. Lett. 107, 081801 (2011).

    Article  ADS  CAS  Google Scholar 

  12. Blunden, P. G., Melnitchouk, W. & Thomas, A. W. γZ box corrections to weak charges of heavy nuclei in atomic parity violation. Phys. Rev. Lett. 109, 262301 (2012).

    Article  ADS  CAS  Google Scholar 

  13. Gorchtein, M., Horowitz, C. J. & Ramsey-Musolf, M. J. Model dependence of the γZ dispersion correction to the parity-violating asymmetry in elastic ep scattering. Phys. Rev. C 84, 015502 (2011).

    Article  ADS  Google Scholar 

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

    Article  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  16. Green, J. et al. High-precision calculation of the strange nucleon electromagnetic form factors. Phys. Rev. D 92, 031501 (2015).

    Article  Google Scholar 

  17. Sufian, R. S. et al. Strange quark magnetic moment of the nucleon at the physical point. Phys. Rev. Lett. 118, 042001 (2017).

    Article  ADS  Google Scholar 

  18. Liu, J., McKeown, R. D. & Ramsey-Musolf, M. J. Global analysis of nucleon strange form factors at low Q 2. Phys. Rev. C 76, 025202 (2007).

    Article  ADS  Google Scholar 

  19. Erler, J. & Ramsey-Musolf, M. J. Weak mixing angle at low energies. Phys. Rev. D 72, 073003 (2005).

    Article  Google Scholar 

  20. Kumar, K. S., Mantry, S., Marciano, W. J. & Souder, P. A. Low-energy measurements of the weak mixing angle. Annu. Rev. Nucl. Part. Sci. 63, 237–267 (2013).

    Article  ADS  CAS  Google Scholar 

  21. Seiberg, N. Naturalness versus supersymmetric non-renormalization theorems. Phys. Lett. B 318, 469–475 (1993).

    Article  ADS  MathSciNet  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

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

    Article  ADS  Google Scholar 

  24. The NuTeV Collaboration. Precise determination of electroweak parameters in neutrino–nucleon scattering. Phys. Rev. Lett. 88, 091802 (2002); errratum 90, 239902 (2003).

    Article  Google Scholar 

  25. Bentz, W., Cloët, I. C., Londergan, J. T. & Thomas, A. W. Reassessment of the NuTeV determination of the weak mixing angle. Phys. Lett. B 693, 462–466 (2010).

    Article  ADS  CAS  Google Scholar 

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

    Article  ADS  Google Scholar 

  27. Davoudiasl, H., Lee, H.-S. & Marciano, W. J. Muon g−2, rare kaon decays, and parity violation from dark bosons. Phys. Rev. D 89, 095006 (2014).

    Article  Google Scholar 

  28. Erler, J., Horowitz, C. J., Mantry, S. & Souder, P. A. Weak polarized electron scattering. Annu. Rev. Nucl. Part. Sci. 64, 269–298 (2014).

    Article  ADS  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  30. Abramowicz, H. et al. Search for first-generation leptoquarks at HERA. Phys. Rev. D 86, 012005 (2012).

    Article  Google Scholar 

  31. Berger, N. et al. Measuring the weak mixing angle with the P2 experiment at MESA. J. China Univ. Sci. Tech. 46, 481–487 (2016).

    Google Scholar 

  32. Benesch, J. et al. The MOLLER experiment: an ultra-precise measurement of the weak mixing angle using Møller scattering. Preprint at https://arxiv.org/abs/1411.4088v2 (2014).

  33. Musolf, M. J. et al. Intermediate-energy semileptonic probes of the hadronic neutral current. Phys. Rep. 239, 1–178 (1994).

    Article  ADS  CAS  Google Scholar 

  34. Erler, J. & Ramsey-Musolf, M. J. Low energy tests of the weak interaction. Prog. Part. Nucl. Phys. 54, 351–442 (2005).

    Article  ADS  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  36. G0 Collaboration. Strange quark contributions to parity-violating asymmetries in the forward G0 electron–proton scattering experiment. Phys. Rev. Lett. 95, 092001 (2005).

    Article  Google Scholar 

  37. G0 Collaboration. Strange quark contributions to parity-violating asymmetries in the backward angle G0 electron scattering experiment. Phys. Rev. Lett. 104, 012001 (2010).

    Article  ADS  Google Scholar 

  38. HAPPEX Collaboration. Parity-violating electroweak asymmetry in ep scattering. Phys. Rev. C 69, 065501 (2004).

    Article  Google Scholar 

  39. HAPPEX Collaboration. Constraints on the nucleon strange form-factors at Q 2 ~ 0.1 GeV2. Phys. Lett. B 635, 275–279 (2006).

    Article  ADS  Google Scholar 

  40. HAPPEX Collaboration. Precision measurements of the nucleon strange form factors at Q 2 ~ 0.1 GeV2. Phys. Rev. Lett. 98, 032301 (2007).

    Article  Google Scholar 

  41. HAPPEX Collaboration. New precision limit on the strange vector form factors of the proton. Phys. Rev. Lett. 108, 102001 (2012).

    Article  Google Scholar 

  42. Spayde, D. T. et al. The strange quark contribution to the proton’s magnetic moment. Phys. Lett. B 583, 79–86 (2004).

    Article  ADS  CAS  Google Scholar 

  43. Maas, F. E. et al. Measurement of strange quark contributions to the nucleon’s form-factors at Q2 = 0.230 (GeV/c)2. Phys. Rev. Lett. 93, 022002 (2004).

    Article  ADS  CAS  Google Scholar 

  44. Maas, F. E. et al. Evidence for strange quark contributions to the nucleon’s form-factors at Q 2 = 0.108 (GeV/c)2. Phys. Rev. Lett. 94, 152001 (2005).

    Article  ADS  CAS  Google Scholar 

  45. Baunack, S. et al. Measurement of strange quark contributions to the vector form factors of the proton at Q 2 = 0.22 (GeV/c)2. Phys. Rev. Lett. 102, 151803 (2009).

    Article  ADS  CAS  Google Scholar 

  46. HAPPEX Collaboration. Parity-violating electron scattering from 4He and the strange electric form factor of the nucleon. Phys. Rev. Lett. 96, 022003 (2006).

    Article  Google Scholar 

  47. Balaguer Ríos, D. et al. Measurement of the parity violating asymmetry in the quasielastic electron-deuteron scattering and improved determination of the magnetic strange form factor and the isovector anapole radiative correction. Phys. Rev. D 94, 051101 (2016).

    Article  ADS  Google Scholar 

  48. SAMPLE Collaboration. Parity violating electron deuteron scattering and the proton’s neutral weak axial vector form-factor. Phys. Rev. Lett. 92, 102003 (2004).

    Article  Google Scholar 

  49. González-Jiménez, R., Caballero, J. A. & Donnelly, T. W. Global analysis of parity-violating asymmetry data for elastic electron scattering. Phys. Rev. D 90, 033002 (2014).

    Article  ADS  Google Scholar 

  50. Zhu, S., Puglia, S. J., Holstein, B. R. & Ramsey-Musolf, M. J. Nucleon anapole moment and parity-violating ep scattering. Phys. Rev. D 62, 033008 (2000).

    Article  ADS  Google Scholar 

  51. Kelly, J. J. Simple parametrization of nucleon form factors. Phys. Rev. C 70, 068202 (2004).

    Article  ADS  Google Scholar 

  52. Galster, S., Klein, H., Moritz, J., Schmidt, K. H. & Wegener, D. Elastic electron–deuteron scattering and the electric neutron form factor at four momentum transfers 5 fm−2 < q 2 < 14 fm−2. Nucl. Phys. B 32, 221–237 (1971).

    Article  ADS  CAS  Google Scholar 

  53. Venkat, S., Arrington, J., Miller, G. A. & Zhan, X. Realistic transverse images of the proton charge and magnetic densities. Phys. Rev. C 83, 015203 (2011).

    Article  ADS  Google Scholar 

  54. Rislow, B. C. & Carlson, C. E. Modification of electromagnetic structure functions for the γZ -box diagram. Phys. Rev. D 88, 013018 (2013).

    Article  ADS  Google Scholar 

  55. Grames, J. et al. Two Wien filter spin flipper. In Proc. 2011 Particle Accelerator Conference (eds Satogata, T. & Brown, K.) 862–864 (IEEE, New York, 2011).

  56. Arrington, J., Blunden, P. G. & Melnitchouk, W. Review of two-photon exchange in electron scattering. Prog. Part. Nucl. Phys. 66, 782–833 (2011).

    Article  ADS  CAS  Google Scholar 

  57. Qweak Collaboration. Beam normal single spin asymmetry measurements from Q weak. Preprint at https://arxiv.org/abs/1604.04602 (2016).

  58. Kargiantoulakis, E. A Precision Test of the Standard Model via Parity-Violating Electron Scattering in the Q weak Experiment. PhD thesis, Univ. Virginia (2015); https://misportal.jlab.org/ul/publications/view_pub.cfm?pub_id=14261.

  59. Agostinelli, S. et al. Geant4 – a simulation toolkit. Nucl. Instrum. Meth. A 506, 250–303 (2003).

    Article  ADS  CAS  Google Scholar 

  60. Abrahamyan, S. et al. New measurements of the transverse beam asymmetry for elastic electron scattering from selected nuclei. Phys. Rev. Lett. 109, 192501 (2012).

    Article  ADS  Google Scholar 

  61. Gorchtein, M. & Horowitz, C. J. Analyzing power in elastic scattering of the electrons off a spin-0 target. Phys. Rev. C 77, 044606 (2008).

    Article  ADS  Google Scholar 

  62. Hauger, M. et al. A high-precision polarimeter. Nucl. Instrum. Meth. A 462, 382–392 (2001).

    Article  ADS  CAS  Google Scholar 

  63. Magee, J. et al. A novel comparison of Møller and Compton electron-beam polarimeters. Phys. Lett. B 766, 339–344 (2017).

    Article  ADS  CAS  Google Scholar 

  64. Horowitz, C. J. Parity violating elastic electron scattering from 27Al and the QWEAK measurement. Phys. Rev. C 89, 045503 (2014).

    Article  ADS  Google Scholar 

  65. McHugh, M. A Measurement of the Transverse Asymmetry in Forward-Angle Electron-Carbon Scattering Using the Q weak Apparatus. PhD thesis, George Washington Univ. (2017); https://misportal.jlab.org/ul/publications/view_pub.cfm?pub_id=14918.

  66. Ferroglia, A., Ossola, G. & Sirlin, A. Bounds on M W, M t, \({\sin }^{2}{\theta }_{{\rm{e}}{\rm{f}}{\rm{f}}}^{{\rm{l}}{\rm{e}}{\rm{p}}{\rm{t}}}\). Eur. Phys. J. C 35, 501–507 (2004).

    Article  ADS  CAS  Google Scholar 

  67. Bethke, S. αs 2002. Nucl. Phys. B Proc. Sup. 121, 74–81 (2003).

    Article  ADS  CAS  Google Scholar 

Download references

Acknowledgements

This work was supported by the US Department of Energy (DOE) Contract number DEAC05-06OR23177, under which Jefferson Science Associates, LLC, operates the Thomas Jefferson National Accelerator Facility. Construction and operating funding for the experiment was provided through the US DOE, the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI), and the National Science Foundation (NSF) with university matching contributions from the College of William and Mary, Virginia Tech, George Washington University and Louisiana Tech University. We thank the staff of Jefferson Laboratory, in particular the accelerator operations staff, the target and cryogenic groups, the radiation control staff, as well as the Hall C technical staff for their help and support. We are grateful for the contributions of our undergraduate students. We thank TRIUMF for its contributions to the development of the spectrometer and integrated electronics, and BATES for its contributions to the spectrometer and Compton polarimeter. We are indebted to P. G. Blunden, J. D. Bowman, J. Erler, N. L. Hall, W. Melnitchouk, M. J. Ramsey-Musolf and A. W. Thomas for discussions. We also thank P. A. Souder for contributions to the analysis. Figure 2 was adapted with permission from ref. 3 (copyrighted by the American Physical Society).

Author information

Author notes

  1. Deceased: W. R. Falk, J. M. Finn, P. Solvignon.

Authors and Affiliations

  1. Department of Physics, University of Zagreb, Zagreb, Croatia

    D. Androić & T. Seva

  2. Department of Physics, College of William and Mary, Williamsburg, VA, USA

    D. S. Armstrong, T. Averett, K. Bartlett, R. D. Carlini, J. C. Cornejo, W. Deconinck, J. F. Dowd, J. M. Finn, V. M. Gray, K. Grimm, J. R. Hoskins, J. Leckey, J. H. Lee, J. A. Magee & S. Yang

  3. Division of Experimental Physics, A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute), Yerevan, Armenia

    A. Asaturyan, A. Mkrtchyan, H. Mkrtchyan, V. Tadevosyan & S. Zhamkochyan

  4. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

    J. Balewski, F. Guo, S. Kowalski & J. F. Rajotte

  5. Physics Division, Thomas Jefferson National Accelerator Facility, Newport News, VA, USA

    J. Beaufait, J. Benesch, R. D. Carlini, S. Covrig Dusa, D. Gaskell, M. Jones, D. Mack, D. Meekins, J. Mei, R. Michaels, B. Sawatzky, G. R. Smith, P. Solvignon & S. A. Wood

  6. Department of Physics and Astronomy, Ohio University, Athens, OH, USA

    R. S. Beminiwattha, P. M. King, J. H. Lee, J. Roche & B. Waidyawansa

  7. Department of Physics, Christopher Newport University, Newport News, VA, USA

    F. Benmokhtar

  8. Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada

    J. Birchall, W. R. Falk, M. T. W. Gericke, L. Lee, S. MacEwan, R. Mahurin, W. T. H. van Oers, S. A. Page, J. Pan, W. D. Ramsay, V. Tvaskis & P. Wang

  9. Department of Physics, University of Virginia, Charlottesville, VA, USA

    M. M. Dalton, C. Gal, D. Jones, M. Kargiantoulakis, V. Nelyubin, K. D. Paschke, R. Silwal & W. A. Tobias

  10. Science Division, TRIUMF, Vancouver, British Columbia, Canada

    C. A. Davis, L. Lee, W. T. H. van Oers & W. D. Ramsay

  11. Department of Physics, Hampton University, Hampton, VA, USA

    J. Diefenbach &  Nuruzzaman

  12. Department of Physics and Astronomy, Mississippi State University, Mississippi State, MS, USA

    J. A. Dunne, D. Dutta, A. Narayan, L. Z. Ndukum,  Nuruzzaman, M. H. Shabestari & A. Subedi

  13. Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

    W. S. Duvall, J. Leacock, A. R. Lee, J. Mammei, N. Morgan & M. L. Pitt

  14. Department of Natural Sciences, Southern University at New Orleans, New Orleans, LA, USA

    M. Elaasar

  15. Department of Physics, Idaho State University, Pocatello, ID, USA

    T. Forest

  16. Department of Physics, Louisiana Tech University, Ruston, LA, USA

    T. Forest, K. Grimm, H. Nuhait, N. Simicevic & S. P. Wells

  17. Accelerator Division, Thomas Jefferson National Accelerator Facility, Newport News, VA, USA

    J. Grames, M. Poelker & R. Suleiman

  18. Department of Physics, University of Connecticut, Storrs-Mansfield, CT, USA

    R. Jones

  19. Department of Physics, University of Northern British Columbia, Prince George, British Columbia, Canada

    E. Korkmaz

  20. Department of Physics, University of Winnipeg, Winnipeg, Manitoba, Canada

    J. W. Martin & V. Tvaskis

  21. Department of Physics, George Washington University, Washington, DC, USA

    M. J. McHugh, K. E. Mesick, A. Micherdzinska, A. K. Opper & R. Subedi

  22. Department of Physics, University of New Hampshire, Durham, NH, USA

    S. K. Phillips

  23. Physics Department, Hendrix College, Conway, AR, USA

    D. T. Spayde

  24. Department of Physics and Mathematical Physics, University of Adelaide, Adelaide, South Australia, Australia

    R. D. Young

  25. Department of Physics, Syracuse University, Syracuse, NY, USA

    W. R. Falk & P. Zang

Consortia

The Jefferson Lab Qweak Collaboration

  • D. Androić
  • , D. S. Armstrong
  • , A. Asaturyan
  • , T. Averett
  • , J. Balewski
  • , K. Bartlett
  • , J. Beaufait
  • , R. S. Beminiwattha
  • , J. Benesch
  • , F. Benmokhtar
  • , J. Birchall
  • , R. D. Carlini
  • , J. C. Cornejo
  • , S. Covrig Dusa
  • , M. M. Dalton
  • , C. A. Davis
  • , W. Deconinck
  • , J. Diefenbach
  • , J. F. Dowd
  • , J. A. Dunne
  • , D. Dutta
  • , W. S. Duvall
  • , M. Elaasar
  • , W. R. Falk
  • , J. M. Finn
  • , T. Forest
  • , C. Gal
  • , D. Gaskell
  • , M. T. W. Gericke
  • , J. Grames
  • , V. M. Gray
  • , K. Grimm
  • , F. Guo
  • , J. R. Hoskins
  • , D. Jones
  • , M. Jones
  • , R. Jones
  • , M. Kargiantoulakis
  • , P. M. King
  • , E. Korkmaz
  • , S. Kowalski
  • , J. Leacock
  • , J. Leckey
  • , A. R. Lee
  • , J. H. Lee
  • , L. Lee
  • , S. MacEwan
  • , D. Mack
  • , J. A. Magee
  • , R. Mahurin
  • , J. Mammei
  • , J. W. Martin
  • , M. J. McHugh
  • , D. Meekins
  • , J. Mei
  • , K. E. Mesick
  • , R. Michaels
  • , A. Micherdzinska
  • , A. Mkrtchyan
  • , H. Mkrtchyan
  • , N. Morgan
  • , A. Narayan
  • , L. Z. Ndukum
  • , V. Nelyubin
  • , H. Nuhait
  • ,  Nuruzzaman
  • , W. T. H. van Oers
  • , A. K. Opper
  • , S. A. Page
  • , J. Pan
  • , K. D. Paschke
  • , S. K. Phillips
  • , M. L. Pitt
  • , M. Poelker
  • , J. F. Rajotte
  • , W. D. Ramsay
  • , J. Roche
  • , B. Sawatzky
  • , T. Seva
  • , M. H. Shabestari
  • , R. Silwal
  • , N. Simicevic
  • , G. R. Smith
  • , P. Solvignon
  • , D. T. Spayde
  • , A. Subedi
  • , R. Subedi
  • , R. Suleiman
  • , V. Tadevosyan
  • , W. A. Tobias
  • , V. Tvaskis
  • , B. Waidyawansa
  • , P. Wang
  • , S. P. Wells
  • , S. A. Wood
  • , S. Yang
  • , R. D. Young
  • , P. Zang
  •  & S. Zhamkochyan

Contributions

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

Corresponding authors

Correspondence to R. D. Carlini or G. R. Smith.

Ethics declarations

Competing interests

The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Apparatus.

a, Schematic of critical accelerator components and the Qweak apparatus4. The electron beam is generated at the photocathode, accelerated by the Continuous Electron Beam Accelerator Facility (CEBAF) and sent to experimental Hall C, where it is monitored by beam position monitors and beam current monitors. The insertable half-wave plate (IHWP) provides slow reversal of the electron beam helicity. The data acquisition system records the data. b, Computer-aided design drawing of the experimental apparatus. c, The Qweak apparatus, before the final shielding configuration was installed. d, Interior of the hut shielding the detectors, showing two of the Cherenkov detectors (right) and a pair of tracking chambers (left).

Extended Data Fig. 2 Beamline background.

Determination of Abb, the false asymmetry arising from beamline background events. Uncertainties are 1 s.d. a, Correlation of the main detector asymmetry to that of the upstream luminosity monitors, measured when the signal from elastically scattered electrons in the main detectors was blocked at the first collimator. b, Correlation of asymmetries from the upstream luminosity monitors with one of the other background detectors (a bare PMT located in the detector shield house). c, Correlation of the unblocked main detector asymmetry to that of the upstream luminosity monitor for Run 2. Our Abb determination was based on this slope.

Extended Data Fig. 3 Rescattering bias.

a, Schematic illustrating the precession of longitudinally polarized electrons through the spectrometer magnet, generating sizeable transverse spin components upon arrival at the detector array (spin directions indicated by red and blue arrows for the two electron helicity states). An end-view of the detector array, indicating the right (R) and left (L) PMT positions, is shown on the left. b, Difference between the asymmetry measured by the two (R and L) PMT tubes versus the detector number (Run 2 data). c, Calculated rescattering bias Abias versus detector number, with the eight-detector-averaged value shown by the red lines. Uncertainties (1 s.d.) are systematic.

Extended Data Fig. 4 Electron beam polarization.

Measurements from the Compton (closed blue circles) and Møller (open red squares) polarimeters during Run 2. Inner error bars denote statistical uncertainties and outer error bars show the statistical and point-to-point systematic uncertainties added in quadrature. Normalization, or scale-type, uncertainties are shown by the solid blue (Compton) and red (Møller) bands. All uncertainties are 1 s.d. The yellow band shows the derived polarization values used in the evaluation of the parity-violating asymmetry Aep. The time dependence of the reported polarization is driven primarily by the continuous Compton measurements, with a small-scale correction (0.21%, not included in this figure) determined from an uncertainty-weighted global comparison of the Compton and Møller polarimeters.

Extended Data Fig. 5 Asymmetry from aluminium.

Parity-violating asymmetry from the aluminium alloy target versus the dataset number. All uncertainties are 1 s.d. The labels ‘IN’ and ‘OUT’ refer to the state of the insertable half-wave plate at the electron source, which generated a 180° flip of the electron spin when IN. The subscripts denote the setting of the Wien filter, with L and R corresponding to the presence and absence, respectively, of an additional 180° rotation of the spin direction of the electron beam. A period in which a further 180° flip was generated through (ge − 2) precession (ge, electron gyromagnetic ratio) via a modified accelerator configuration during Wien 6 is indicated. The combinations OUT–R and IN–L with no (ge − 2) spin flip reveal the physical sign of the asymmetry. Solid lines represent the time-averaged values, and the horizontal dashed line indicates zero asymmetry. The vertical dashed lines delineate particular data subsets with a given Wien filter setting.

Source Data

Extended Data Fig. 6 Asymmetry from the proton.

Observed parity-violating asymmetry Aep after all corrections, versus the dataset number (acquired in the double-Wien-filter configuration). The Wien filter reversed the beam helicity at approximately monthly intervals. The subscripts denote the setting of the Wien filter as L or R, corresponding to the presence or absence, respectively, of a 180° rotation of the spin direction of the electron beam. IN and OUT refer to the state of the insertable half-wave plate at the electron source, generating an additional 180° flip of the spin when IN. A period in which a further 180° flip was generated through (ge − 2) precession via a modified accelerator configuration is indicated. The combinations OUT–R and IN–L with no (ge − 2) flip reveal the physical sign of the asymmetry. Solid lines represent the time-averaged values and the dashed line indicates zero asymmetry. The uncertainties (1 s.d.) shown are those of the corresponding Amsr values (see text) only—that is, they do not include time-independent uncertainties—so as to illustrate the time stability of the results. The weighted mean and P-value of the upper OUT–L and IN–R data are 226.9 ± 10.2, P = 0.59 (upper solid line), respectively. For the opposite combination, OUT–R and IN–L, we find a weighted mean of −226.1 ± 10.5 and P = 0.36 (lower solid line).

Source Data

Extended Data Table 1 Helicity-correlated beam parameter differences and sensitivities
Full size table
Extended Data Table 2 Asymmetries and their corrections
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Extended Data Table 3 Raw asymmetries and their corrections
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Extended Data Table 4 Radiative corrections
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Source data

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The Jefferson Lab Qweak Collaboration. Precision measurement of the weak charge of the proton. Nature 557, 207–211 (2018). https://doi.org/10.1038/s41586-018-0096-0

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