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Flavour-changing neutral currents making and breaking the standard model


The standard model of particle physics is our best description yet of fundamental particles and their interactions, but it is known to be incomplete. As yet undiscovered particles and interactions might exist. One of the most powerful ways to search for new particles is by studying processes known as flavour-changing neutral current decays, whereby a quark changes its flavour without altering its electric charge. One example of such a transition is the decay of a beauty quark into a strange quark. Here we review some intriguing anomalies in these decays, which have revealed potential cracks in the standard model—hinting at the existence of new phenomena.

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Figure 1: Examples of FCNC decays.
Figure 2: Observation of the decay  → μ+μ.
Figure 3: Measurements of the observable in B → K+ decays.
Figure 4: Illustration of an effective field theory for b → sℓ+ decays.
Figure 5: Global fit of rare beauty decays.


  1. ATLAS Collaboration. The ATLAS experiment at the CERN Large Hadron Collider. J. Instrum. 3, S08003 (2008)

  2. CMS Collaboration. The CMS experiment at the CERN LHC. J. Instrum. 3, S08004 (2008)

  3. Evans, L. et al. LHC machine. J. Instrum. 3, S08001 (2008)

    Article  Google Scholar 

  4. LHCb Collaboration. The LHCb detector at the LHC. J. Instrum. 3, S08005 (2008)

  5. Bjorken, B. J. & Glashow, S. L. Elementary particles and SU(4). Phys. Lett. 11, 255–257 (1964)

    CAS  ADS  MathSciNet  Article  Google Scholar 

  6. Glashow, S. L., Iliopoulos, J. & Maiani, L. Weak interactions with lepton–hadron symmetry. Phys. Rev. D 2, 1285–1292 (1970).Proposed a mechanism for the suppression of FCNCs, and predicted the existence of a fourth quark—the charm quark

    ADS  Article  Google Scholar 

  7. Aubert, J. J. et al. Experimental observation of a heavy particle J. Phys. Rev. Lett. 33, 1404–1406 (1974)

    CAS  ADS  Article  Google Scholar 

  8. Augustin, J. E. et al. Discovery of a narrow resonance in e+e annihilation. Phys. Rev. Lett. 33, 1406–1408 (1974)

    CAS  ADS  Article  Google Scholar 

  9. BaBar Collaboration. The BaBar detector. Nucl. Instrum. Methods A 479, 1–116 (2002)

  10. Belle Collaboration. The Belle detector. Nucl. Instrum. Methods A 479, 117–232 (2002)

  11. Ellis, J. Beyond the standard model with the LHC. Nature 448, 297–301 (2007)

    CAS  ADS  Article  Google Scholar 

  12. Altmannshofer, W. & Straub, D. M. New physics in BKμμ? Eur. Phys. J. C 73, 2646 (2013)

    ADS  Article  Google Scholar 

  13. Bobeth, C., Ewerth, T., Kruger, F. & Urban, J. Analysis of neutral Higgs-boson contributions to the decays B s+ and → Kℓ+. Phys. Rev. D 64, 074014 (2001)

    Google Scholar 

  14. Babu, K. & Kolda, C. F. Higgs-mediated B0μ+μ in minimal supersymmetry. Phys. Rev. Lett. 84, 228–231 (2000)

    CAS  ADS  Article  Google Scholar 

  15. Huang, C.-S., Liao, W. & Yan, Q.-S. Promising process to distinguish super-symmetric models with large tanβ from the standard model: BX s μ+μ. Phys. Rev. D 59, 011701 (1999)

    ADS  Article  Google Scholar 

  16. Rai Choudhury, S. & Gaur, N. Dileptonic decay of B s meson in SUSY models with large tanβ. Phys. Lett. B 451, 86–92 (1999)

    CAS  ADS  Article  Google Scholar 

  17. Bobeth, C. et al. B s,d+ in the standard model with reduced theoretical uncertainty. Phys. Rev. Lett. 112, 101801 (2014)

    ADS  Article  Google Scholar 

  18. HPQCD Collaboration. B and B s meson decay constants from lattice QCD. Phys. Rev. D 86, 034506 (2012)

  19. Fermilab Lattice & MILC Collaborations. B- and D-meson decay constants from three-flavor lattice QCD. Phys. Rev. D 85, 114506 (2012)

  20. RBC–UKQCD Collaborations. B-meson decay constants with domain wall light quarks and nonperturbatively tuned relativistic b-quarks. AIP Conf. Proc. 1560, 368 (2013)

  21. Cabibbo, N. Unitary symmetry and leptonic decays. Phys. Rev. Lett. 10, 531–533 (1963)

    ADS  Article  Google Scholar 

  22. Kobayashi, M. & Maskawa, T. CP violation in the renormalizable theory of weak interaction. Prog. Theor. Phys. 49, 652–657 (1973)

    CAS  ADS  Article  Google Scholar 

  23. CMS Collaboration & LHCb Collaboration. Observation of the rare μ+μ decay from the combined analysis of CMS and LHCb data. Nature 522, 68–72 (2015).After three decades of searching, the first observation of the very rare decay  → μ+μ, made by the CMS and LHCb collaborations

  24. ATLAS Collaboration. Study of the rare decays of and B0 into muon pairs from data collected during the LHC run 1 with the ATLAS detector. Eur. Phys. J. C 76, 513 (2016)

  25. LHCb Collaboration. Differential branching fractions and isospin asymmetries of BK()μ+μ decays. J. High Energy Phys. 6, 133 (2014)

  26. LHCb Collaboration. Angular analysis and differential branching fraction of the decay φμ+μ. J. High Energy Phys. 9, 179 (2015)

  27. LHCb Collaboration. Measurements of the S-wave fraction in B0K+πμ+μ decays and the B0K(892)0μ+μ differential branching fraction. J. High Energy Phys. 11, 47 (2016)

  28. Bobeth, C., Hiller, G. & Piranishvili, G. Angular distributions of → λ++ decays. J. High Energy Phys. 12, 040 (2007)

    Google Scholar 

  29. Bordone, M., Isidori, G. & Pattori, A. On the standard model predictions for R K and R K . Eur. Phys. J. C 76, 440 (2016)

    ADS  Article  Google Scholar 

  30. LHCb Collaboration. Test of lepton universality using B+ → K++ decays. Phys. Rev. Lett. 113, 151601 (2014).Most precise measurement of the lepton universality ratio R K

  31. BaBar Collaboration. Measurement of branching fractions and rate asymmetries in the rare decays BK()+. Phys. Rev. D 86, 032012 (2012)

  32. Belle Collaboration. Measurement of the differential branching fraction and forward-backward asymmetry for BK()+. Phys. Rev. Lett. 103, 171801 (2009)

  33. Belle Collaboration. Improved measurement of the electroweak penguin process BX s +. Phys. Rev. D72, 092005 (2005)

  34. BaBar Collaboration. Measurement of the BX s + branching fraction and search for direct CP violation from a sum of exclusive final states. Phys. Rev. Lett. 112, 211802 (2014)

  35. LHCb Collaboration. Angular analysis of the B0K0μ+μ decay using 3 fb−1 of integrated luminosity. J. High Energy Phys. 2, 104 (2016).Most precise results yet from angular analysis of the decay B0 K0μ+μ, showing a discrepancy with standard model predictions for the observables that describe the angular distribution, including the observable

  36. Belle Collaboration. Lepton-flavor-dependent angular analysis of BK+. Phys. Rev. Lett. 118, 111801 (2017)

  37. Blake, T., Lanfranchi, G., Straub, D. M. & Rare, B. Decays as tests of the standard model. Prog. Part. Nucl. Phys. 92, 50–91 (2017)

    CAS  ADS  Article  Google Scholar 

  38. Descotes-Genon, S., Hofer, L., Matias, J. & Virto, J. On the impact of power corrections in the prediction of BKμ+μ observables. J. High Energy Phys. 12, 125 (2014)

    ADS  Article  Google Scholar 

  39. Pich, A. Effective field theory: course. In Probing the Standard Model of Particle Interactions: Proc. Summer School in Theoretical Physics (eds Gupta, R. et al.) 949–1049 (Elsevier, 1998); preprint at

  40. Wilson, K. G. & Zimmermann, W. Operator product expansions and composite field operators in the general framework of quantum field theory. Commun. Math. Phys. 24, 87–106 (1972)

    ADS  MathSciNet  Article  Google Scholar 

  41. Descotes-Genon, S., Hurth, T., Matias, J. & Virto, J. Optimizing the basis of BK+ observables in the full kinematic range. J. High Energy Phys. 5, 137 (2013).First proposal to use angular observables such as, which have reduced uncertainties from strong-force interactions

    ADS  Article  Google Scholar 

  42. Descotes-Genon, S., Matias, J. & Virto, J. Understanding the BKμ+μ anomaly. Phys. Rev. D 88, 074002 (2013)

    ADS  Article  Google Scholar 

  43. Altmannshofer, W., Gori, S., Pospelov, M. & Yavin, I. Quark flavor transitions in L μL τ models. Phys. Rev. D 89, 095033 (2014)

    ADS  Article  Google Scholar 

  44. Mahmoudi, F., Neshatpour, S. & Virto, J. BK μ+μ optimised observables in the MSSM. Eur. Phys. J. C 74, 2927 (2014)

    ADS  Article  Google Scholar 

  45. Crivellin, A., D’Ambrosio, G. & Heeck, J. Explaining hμ±τ, BKμ+μ and B+μ/BKe+e in a two-Higgs-doublet model with gauged L μL τ . Phys. Rev. Lett. 114, 151801 (2015)

    ADS  Article  Google Scholar 

  46. Descotes-Genon, S., Hofer, L., Matias, J. & Virto, J. Global analysis of bsℓℓ anomalies. J. High Energy Phys. 6, 92 (2016)

    ADS  Article  Google Scholar 

  47. Hurth, T., Mahmoudi, F. & Neshatpour, S. On the anomalies in the latest LHCb data. Nucl. Phys. B 909, 737–777 (2016)

    CAS  ADS  Article  Google Scholar 

  48. Jäger, S. & Martin Camalich, J. On BVℓℓ at small dilepton invariant mass, power corrections, and new physics. J. High Energy Phys. 5, 43 (2013)

    ADS  Article  Google Scholar 

  49. Beaujean, F., Bobeth, C. & van Dyk, D. Comprehensive Bayesian analysis of rare (semi)leptonic and radiative B decays. Eur. Phys. J. C 74, 2897 (2014); erratum 74, 3179 (2014)

  50. Hurth, T. & Mahmoudi, F. On the LHCb anomaly in BK+. J. High Energy Phys. 4, 97 (2014)

    ADS  Article  Google Scholar 

  51. Gauld, R., Goertz, F. & Haisch, U. An explicit Z′-boson explanation of the BKμ+μ anomaly. J. High Energy Phys. 1, 69 (2014)

    ADS  Article  Google Scholar 

  52. Datta, A., Duraisamy, M. & Ghosh, D. Explaining the BKμ+μ data with scalar interactions. Phys. Rev. D 89, 071501 (2014)

    ADS  Article  Google Scholar 

  53. Lyon, J . & Zwicky, R. Resonances gone topsy turvy - the charm of QCD or new physics in bsℓ+? Preprint at (2014)

  54. Altmannshofer, W. & Straub, D. M. New physics in bs transitions after LHC Run 1. Eur. Phys. J. C 75, 382 (2015).Global analysis of FCNC anomalies involving the beauty quark, hinting at a significant contribution from a new-physics particle.

    ADS  Article  Google Scholar 

  55. Altmannshofer, W., Gori, S., Profumo, S. & Queiroz, F. S. Explaining dark matter and B decay anomalies with an L μ− L τ model. J. High Energy Phys. 12, 106 (2016)

    ADS  Article  Google Scholar 

  56. Pati, J. C. & Salam, A. Lepton number as the fourth color. Phys. Rev. D 10, 275–289 (1974); erratum 11, 703 (1975)

    CAS  ADS  Article  Google Scholar 

  57. Bauer, M. & Neubert, M. Minimal leptoquark explanation for the, R K, and (g − 2)µ anomalies. Phys. Rev. Lett. 116, 141802 (2016)

    Google Scholar 

  58. Fajfer, S. & Konik, N. Vector leptoquark resolution of R K and puzzles. Phys. Lett. B 755, 270–274 (2016)

    CAS  Google Scholar 

  59. Gripaios, B., Nardecchia, M. & Renner, S. A. Composite leptoquarks and anomalies in B-meson decays. J. High Energy Phys. 5, 6 (2015)

    ADS  Article  Google Scholar 

  60. Khodjamirian, A., Mannel, T. & Wang, Y. BK + decay at large hadronic recoil. J. High Energy Phys. 2, 10 (2013)

    ADS  Article  Google Scholar 

  61. Khodjamirian, A., Mannel, T., Pivovarov, A. & Wang, Y.-M. Charm-loop effect in BK()+ and BKγ. J. High Energy Phys. 9, 89 (2010)

    ADS  Article  Google Scholar 

  62. Belle II Collaboration. Belle II technical design report. Preprint at (2010)

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K.A.P. acknowledges support from the European Science and Technology Facilities Council under grant number ST/K001256/1; F.A. acknowledges support from the Netherlands Foundation for Fundamental Research of Matter (FOM) and the Netherlands Foundation of Scientific Research Institutes (NWO-I); M.-O.B. acknowledges support from CERN; and P.O. acknowledges support from the Swiss National Science Foundation under grant number BSSGI0_155990.

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Archilli, F., Bettler, MO., Owen, P. et al. Flavour-changing neutral currents making and breaking the standard model. Nature 546, 221–226 (2017).

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