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W-boson helicity fractions in top decay as probes of dimension-six and dimension-eight SMEFT operators

Afsaneh Kianfar1, Gholamhossein Haghighat1, and Mojtaba Mohammadi Najafabadi1,2

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Phys. Rev. D 113, 113004 – Published 10 June, 2026

DOI: https://doi.org/10.1103/3v41-xkzj

Abstract

Precision measurements of top-quark decays provide powerful probes of physics beyond the Standard Model (SM). While the impact of dimension-six operators in the SM effective field theory (SMEFT) has been extensively studied, the role of dimension-eight contributions remains largely unexplored, despite their potential importance as experimental precision improves. In this work, we present a combined analysis of dimension-six and a representative subset of dimension-eight SMEFT effects using the W-boson helicity fractions in top-quark decays. We compute the leading-order contributions of these operators to the top-quark decay width and helicity fractions, and perform one-parameter and selected two-parameter χ2 fits to the combined ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) measurements at a reference scale Λ=1  TeV. From the fit results, we find that the inclusion of dimension-eight contributions affects the allowed parameter space of several dimension-six coefficients through nontrivial correlations and degeneracies. Since the leading dimension-eight contributions enter at the same order O(Λ−4) as the squared dimension-six terms retained in our analysis, this highlights the importance of a consistent treatment of the EFT expansion when interpreting SMEFT constraints.

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References (45)

  1. W. Buchmuller and D. Wyler, Effective Lagrangian analysis of new interactions and flavor conservation, Nucl. Phys. B268, 621 (1986).
  2. B. Grzadkowski, M. Iskrzynski, M. Misiak, and J. Rosiek, Dimension-six terms in the standard model Lagrangian, J. High Energy Phys. 10 (2010) 085.
  3. I. Brivio and M. Trott, The standard model as an effective field theory, Phys. Rep. 793, 1 (2019).
  4. S. Hamoudou, J. Kumar, and D. London, Dimension-8 SMEFT matching conditions for the low-energy effective field theory, J. High Energy Phys. 03 (2023) 157.
  5. M. Beneke, I. Efthymiopoulos, M. L. Mangano, J. Womersley, A. Ahmadov, G. Azuelos, U. Baur, A. Belyaev, E. L. Berger, W. Bernreuther et al., Top quark physics, Report No. CERN-2000-004, CERN, Fermilab, 2000, p. 419, 10.5170/CERN-2000-004.419.
  6. W. Bernreuther, Top quark physics at the LHC, J. Phys. G 35, 083001 (2008).
  7. CMS Collaboration, Measurement of the W boson helicity fractions in the decays of top quark pairs to lepton+jets final states produced in pp collisions at s=8  TeV, Phys. Lett. B 762, 512 (2016).
  8. ATLAS and CMS Collaborations, Combination of the W boson polarization measurements in top quark decays using ATLAS and CMS data at s=8  TeV, J. High Energy Phys. 08 (2020) 051.
  9. ATLAS Collaboration, Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at s=13  TeV with the ATLAS experiment, Phys. Lett. B 843, 137829 (2023).
  10. S. M. Etesami, M. Khatiri Yanehsari, and M. Mohammadi Najafabadi, The effects of standard model extensions on W-boson helicity ratios in top quark decay, Int. J. Theor. Phys. 51, 3694 (2012).
  11. M. Mohammadi Najafabadi, Noncommutative standard model in top quark sector, Phys. Rev. D 77, 116011 (2008).
  12. C. Zhang and S. Willenbrock, Effective-field-theory approach to top-quark production and decay, Phys. Rev. D 83, 034006 (2011).
  13. R. Boughezal, C.-Y. Chen, F. Petriello, and D. Wiegand, Top quark decay at next-to-leading order in the standard model effective field theory, Phys. Rev. D 100, 056023 (2019).
  14. C. Zhang, Effective field theory approach to top-quark decay at next-to-leading order in QCD, Phys. Rev. D 90, 014008 (2014).
  15. A. Buckley, C. Englert, J. Ferrando, D. J. Miller, L. Moore, M. Russell, and C. D. White, Global fit of top quark effective theory to data, Phys. Rev. D 92, 091501 (2015).
  16. N. P. Hartland et al., A Monte Carlo global analysis of the standard model effective field theory: The top quark sector, J. High Energy Phys. 04 (2019) 100.
  17. J. Gao, C. S. Li, and H. X. Zhu, Top quark decay at next-to-next-to leading order in QCD, Phys. Rev. Lett. 110, 042001 (2013).
  18. M. Mohammadi Najafabadi, Secondary particles spectra in decay of polarized top quark with anomalous tWb coupling, J. Phys. G 34, 39 (2007).
  19. E. Boos and V. Bunichev, Symbolic expressions for fully differential single top quark production cross section and decay width of polarized top quark in the presence of anomalous Wtb couplings, Phys. Rev. D 101, 055012 (2020).
  20. E. E. Abasov, E. E. Boos, V. E. Bunichev, L. V. Dudko, D. E. Gorin, A. A. Markina, M. A. Perfilov, O. S. Vasilevskii, P. V. Volkov, G. A. Vorotnikov et al., Separation of left-handed and anomalous right-handed vector operators contributions into the Wtb vertex for single and double resonant top quark production processes using a neural network, Phys. Part. Nucl. 56, 447 (2025).
  21. J. L. Birman, F. Déliot, M. C. N. Fiolhais, A. Onofre, and C. M. Pease, New limits on anomalous contributions to the Wtb vertex, Phys. Rev. D 93, 113021 (2016).
  22. J. A. Aguilar-Saavedra and J. Bernabeu, W polarisation beyond helicity fractions in top quark decays, Nucl. Phys. B840, 349 (2010).
  23. F. Hubaut, E. Monnier, P. Pralavorio, K. Smolek, and V. Simak, ATLAS sensitivity to top quark and W boson polarization in tt¯ events, Eur. Phys. J. C 44S2, 13 (2005).
  24. M. Mohammadi Najafabadi, Probing of Wtb anomalous couplings via the tW channel of single top production, J. High Energy Phys. 03 (2008) 024.
  25. J. A. Aguilar-Saavedra, J. Carvalho, N. F. Castro, F. Veloso, and A. Onofre, Probing anomalous Wtb couplings in top pair decays, Eur. Phys. J. C 50, 519 (2007).
  26. J. A. Aguilar-Saavedra, J. Carvalho, N. F. Castro, A. Onofre, and F. Veloso, ATLAS sensitivity to Wtb anomalous couplings in top pair decays, Eur. Phys. J. C 53, 689 (2008).
  27. S. Kala, L. Kolay, L. Mukherjee, and S. Nandi, Constraining anomalous Wtb and related SMEFT couplings using low-energy and electroweak precision observables, J. High Energy Phys. 11 (2025) 071.
  28. S. Banerjee, R. Gupta, J. Reiness, and M. Spannowsky, Resolving the tensor structure of the Higgs coupling to Z-bosons via Higgs-strahlung, Phys. Rev. D 100, 115004 (2019).
  29. S. Alioli et al., Theoretical developments in the SMEFT at dimension-8 and beyond, arXiv:2203.06771.
  30. D. Gillies, A. Banfi, A. Martin, and M. A. Lim, Dimension-8 operators in W+W− production via gluon fusion, J. High Energy Phys. 06 (2025) 111.
  31. M. Flores-Hernandez and A. Martin, Higgs decays to four leptons to O(1/Λ4) in SMEFT, arXiv:2602.12326.
  32. H. El Faham, G. Ventura, and E. Vryonidou, Diboson production in the SMEFT at dimension-8, J. High Energy Phys. 04 (2026) 050.
  33. D. Gillies, A. Banfi, and A. Martin, Probing EFT breakdown in the tails of W+W− observables, arXiv:2601.19495.
  34. G. Pelliccioli and E. Re, SMEFT effects on spin correlations and entanglement at NLO QCD in di-boson production at hadron colliders, arXiv:2601.09540.
  35. T. Corbett, J. Desai, O. J. P. Eboli, and M. C. Gonzalez-Garcia, Dimension-eight operator basis for universal standard model effective field theory, Phys. Rev. D 110, 033003 (2024).
  36. T. Corbett, J. Desai, O. J. P. Éboli, M. C. Gonzalez-Garcia, M. Martines, and P. Reimitz, Impact of dimension-eight SMEFT operators in the electroweak precision observables and triple gauge couplings analysis in universal SMEFT, Phys. Rev. D 107, 115013 (2023).
  37. R. Boughezal, Y. Huang, and F. Petriello, Renormalization-group running of dimension-8 four-fermion operators in the SMEFT, Phys. Rev. D 110, 116015 (2024).
  38. L. P. G. De Assis, Constructing dimension-8 SMEFT from conserved currents, Phys. Rev. D 113, 056004 (2026).
  39. C. W. Murphy, Dimension-8 operators in the standard model effective field theory, J. High Energy Phys. 10 (2020) 174.
  40. J. A. Aguilar-Saavedra, A minimal set of top anomalous couplings, Nucl. Phys. B812, 181 (2009).
  41. H. L. Li, Z. Ren, J. Shu, M. L. Xiao, J. H. Yu, and Y. H. Zheng, Complete set of dimension-8 operators in the standard model effective field theory, Phys. Rev. D 104, 015026 (2021).
  42. A. Czarnecki, J. G. Korner, and J. H. Piclum, Helicity fractions of W bosons from top quark decays at NNLO in QCD, Phys. Rev. D 81, 111503 (2010).
  43. S. Navas et al. (Particle Data Group), Review of particle physics, Phys. Rev. D 110, 030001 (2024).
  44. M. Fischer, S. Groote, J. G. Körner, and M. C. Mauser, Complete angular analysis of polarized top decay at O(αs), Phys. Rev. D 65, 054036 (2002).
  45. A. Kianfar, Source code for the calculations presented in this work, https://github.com/afsanehkianfar/Paper-SMEFT-twb (2025).

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Phys. Rev. D 113, 113004– Published 10 June, 2026
  • Received 2 January 2026
  • Accepted 1 May 2026
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