Light resonances and the low-q2 bin of \( {R}_{K^{*}} \)

  • Wolfgang Altmannshofer
  • Michael J. Baker
  • Stefania Gori
  • Roni Harnik
  • Maxim Pospelov
  • Emmanuel Stamou
  • Andrea Thamm
Open Access
Regular Article - Theoretical Physics


LHCb has reported hints of lepton-flavor universality violation in the rare decays BK(*)+, both in high- and low-q2 bins. Although the high-q2 hint may be explained by new short-ranged interactions, the low-q2 one cannot. We thus explore the possibility that the latter is explained by a new light resonance. We find that LHCb’s central value of \( {R}_{K^{*}} \) in the low-q2 bin is achievable in a restricted parameter space of new-physics scenarios in which the new, light resonance decays preferentially to electrons and has a mass within approximately 10 MeV of the di-muon threshold. Interestingly, such an explanation can have a kinematic origin and does not require a source of lepton-flavor universality violation. A model-independent prediction is a narrow peak in the differential BK*e+e rate close to the di-muon threshold. If such a peak is observed, other observables, such as the differential BKe+e rate and R K , may be employed to distinguish between models. However, if a low-mass resonance is not observed and the low-q2 anomaly increases in significance, then the case for an experimental origin of the lepton-flavor universality violating anomalies would be strengthened. To further explore this, we also point out that, in analogy to J/ψ decays, e+e and μ+μ decays of ϕ mesons can be used as a cross check of lepton-flavor universality by LHCb with 5 fb−1 of integrated luminosity.


Beyond Standard Model Heavy Quark Physics 


Open Access

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  1. [1]
    LHCb collaboration, Test of lepton universality using B +K + + decays, Phys. Rev. Lett. 113 (2014) 151601 [arXiv:1406.6482] [INSPIRE].
  2. [2]
    LHCb collaboration, Test of lepton universality with B 0K *0 + decays, JHEP 08 (2017) 055 [arXiv:1705.05802] [INSPIRE].
  3. [3]
    G. Hiller and F. Krüger, More model-independent analysis of bs processes, Phys. Rev. D 69 (2004) 074020 [hep-ph/0310219] [INSPIRE].
  4. [4]
    M. Bordone, G. Isidori and A. Pattori, On the Standard Model predictions for R K and \( {R}_{K^{*}} \), Eur. Phys. J. C 76 (2016) 440 [arXiv:1605.07633] [INSPIRE].ADSCrossRefGoogle Scholar
  5. [5]
    W. Altmannshofer, P. Stangl and D.M. Straub, Interpreting hints for lepton flavor universality violation, Phys. Rev. D 96 (2017) 055008 [arXiv:1704.05435] [INSPIRE].ADSGoogle Scholar
  6. [6]
    B. Capdevila, A. Crivellin, S. Descotes-Genon, J. Matias and J. Virto, Patterns of new physics in bsℓ + transitions in the light of recent data, JHEP 01 (2018) 093 [arXiv:1704.05340] [INSPIRE].ADSCrossRefGoogle Scholar
  7. [7]
    G. D’Amico et al., Flavour anomalies after the \( {R}_{K^{*}} \) measurement, JHEP 09 (2017) 010 [arXiv:1704.05438] [INSPIRE].CrossRefGoogle Scholar
  8. [8]
    L.-S. Geng, B. Grinstein, S. Jäger, J. Martin Camalich, X.-L. Ren and R.-X. Shi, Towards the discovery of new physics with lepton-universality ratios of bsℓℓ decays, Phys. Rev. D 96 (2017) 093006 [arXiv:1704.05446] [INSPIRE].ADSGoogle Scholar
  9. [9]
    M. Ciuchini et al., On flavourful easter eggs for new physics hunger and lepton flavour universality violation, Eur. Phys. J. C 77 (2017) 688 [arXiv:1704.05447] [INSPIRE].CrossRefGoogle Scholar
  10. [10]
    D. Bardhan, P. Byakti and D. Ghosh, Role of tensor operators in R K and \( {R}_{K^{*}} \), Phys. Lett. B 773 (2017) 505 [arXiv:1705.09305] [INSPIRE].ADSCrossRefGoogle Scholar
  11. [11]
    K. Fuyuto, W.-S. Hou and M. Kohda, Z-induced FCNC decays of top, beauty and strange quarks, Phys. Rev. D 93 (2016) 054021 [arXiv:1512.09026] [INSPIRE].ADSGoogle Scholar
  12. [12]
    A. Datta, J. Liao and D. Marfatia, A light Zfor the R K puzzle and nonstandard neutrino interactions, Phys. Lett. B 768 (2017) 265 [arXiv:1702.01099] [INSPIRE].ADSCrossRefGoogle Scholar
  13. [13]
    F. Sala and D.M. Straub, A new light particle in B decays?, Phys. Lett. B 774 (2017) 205 [arXiv:1704.06188] [INSPIRE].ADSCrossRefGoogle Scholar
  14. [14]
    D. Ghosh, Explaining the R K and \( {R}_{K^{*}} \) anomalies, Eur. Phys. J. C 77 (2017) 694 [arXiv:1704.06240] [INSPIRE].ADSCrossRefGoogle Scholar
  15. [15]
    A.K. Alok, B. Bhattacharya, A. Datta, D. Kumar, J. Kumar and D. London, New physics in b + μ after the measurement of \( {R}_{K^{*}} \), Phys. Rev. D 96 (2017) 095009 [arXiv:1704.07397] [INSPIRE].Google Scholar
  16. [16]
    F. Bishara, U. Haisch and P.F. Monni, Regarding light resonance interpretations of the B decay anomalies, Phys. Rev. D 96 (2017) 055002 [arXiv:1705.03465] [INSPIRE].ADSGoogle Scholar
  17. [17]
    A. Datta, J. Kumar, J. Liao and D. Marfatia, New light mediators for the R K and \( {R}_{K^{*}} \) puzzles, arXiv:1705.08423 [INSPIRE].
  18. [18]
    K.S. Babu, A. Friedland, P.A.N. Machado and I. Mocioiu, Flavor gauge models below the Fermi scale, JHEP 12 (2017) 096 [arXiv:1705.01822] [INSPIRE].ADSCrossRefGoogle Scholar
  19. [19]
    LHCb collaboration, Angular analysis of the B 0K *0 e + e decay in the low-q 2 region, JHEP 04 (2015) 064 [arXiv:1501.03038] [INSPIRE].
  20. [20]
    LHCb collaboration, Search for hidden-sector bosons in B 0K *0 μ + μ decays, Phys. Rev. Lett. 115 (2015) 161802 [arXiv:1508.04094] [INSPIRE].
  21. [21]
    HFLAV collaboration, Y. Amhis et al., Averages of b-hadron, c-hadron and τ-lepton properties as of summer 2016, Eur. Phys. J. C 77 (2017) 895 [arXiv:1612.07233] [INSPIRE].
  22. [22]
    M. Pepe Altarelli and F. Teubert, B physics at LHCb, Int. J. Mod. Phys. A 23 (2008) 5117 [arXiv:0802.1901] [INSPIRE].ADSCrossRefGoogle Scholar
  23. [23]
    P. Ilten, J. Thaler, M. Williams and W. Xue, Dark photons from charm mesons at LHCb, Phys. Rev. D 92 (2015) 115017 [arXiv:1509.06765] [INSPIRE].ADSGoogle Scholar
  24. [24]
    LHCb collaboration, LHCb detector performance, Int. J. Mod. Phys. A 30 (2015) 1530022 [arXiv:1412.6352] [INSPIRE].
  25. [25]
    D.M. Straub et al., Flavio,, Zenodo, (2017).
  26. [26]
    Belle collaboration, O. Lutz et al., Search for \( B\to {h}^{\left(\ast \right)}\nu \overline{\nu} \) with the full Belle Y(4S) data sample, Phys. Rev. D 87 (2013) 111103 [arXiv:1303.3719] [INSPIRE].
  27. [27]
    BaBar collaboration, J.P. Lees et al., Search for \( B\to {K}^{\left(\ast \right)}\nu \overline{\nu} \) and invisible quarkonium decays, Phys. Rev. D 87 (2013) 112005 [arXiv:1303.7465] [INSPIRE].
  28. [28]
    APEX collaboration, S. Abrahamyan et al., Search for a new gauge boson in electron-nucleus fixed-target scattering by the APEX experiment, Phys. Rev. Lett. 107 (2011) 191804 [arXiv:1108.2750] [INSPIRE].
  29. [29]
    R. Essig et al., Working group report: new light weakly coupled particles, in Proceedings, Community Summer Study on the Future of U.S. Particle Physics: Snowmass on the Mississippi (CSS2013), Minneapolis MN U.S.A., 29 July–6 August 2013 [arXiv:1311.0029] [INSPIRE].
  30. [30]
    H. Merkel et al., Search at the Mainz microtron for light massive gauge bosons relevant for the muon g − 2 anomaly, Phys. Rev. Lett. 112 (2014) 221802 [arXiv:1404.5502] [INSPIRE].ADSCrossRefGoogle Scholar
  31. [31]
    BaBar collaboration, J.P. Lees et al., Search for a dark photon in e + e collisions at BaBar, Phys. Rev. Lett. 113 (2014) 201801 [arXiv:1406.2980] [INSPIRE].
  32. [32]
    J. Alexander et al., Dark sectors 2016 workshop: community report, arXiv:1608.08632 [INSPIRE].
  33. [33]
    HyperCP collaboration, H. Park et al., Evidence for the decay Σ+ + μ , Phys. Rev. Lett. 94 (2005) 021801 [hep-ex/0501014] [INSPIRE].
  34. [34]
    LHCb collaboration, Evidence for the rare decay Σ+ + μ , LHCb-CONF-2016-013, CERN, Geneva Switzerland, (2016).
  35. [35]
    J.A. Dror, R. Lasenby and M. Pospelov, New constraints on light vectors coupled to anomalous currents, Phys. Rev. Lett. 119 (2017) 141803 [arXiv:1705.06726] [INSPIRE].ADSCrossRefGoogle Scholar
  36. [36]
    J.A. Dror, R. Lasenby and M. Pospelov, Dark forces coupled to nonconserved currents, Phys. Rev. D 96 (2017) 075036 [arXiv:1707.01503] [INSPIRE].ADSGoogle Scholar
  37. [37]
    P.J. Fox, J. Liu, D. Tucker-Smith and N. Weiner, An effective Z′, Phys. Rev. D 84 (2011) 115006 [arXiv:1104.4127] [INSPIRE].ADSGoogle Scholar
  38. [38]
    W. Altmannshofer, S. Gori, M. Pospelov and I. Yavin, Quark flavor transitions in L μ -L τ models, Phys. Rev. D 89 (2014) 095033 [arXiv:1403.1269] [INSPIRE].ADSGoogle Scholar
  39. [39]
    A. Bharucha, D.M. Straub and R. Zwicky, BVℓ + in the Standard Model from light-cone sum rules, JHEP 08 (2016) 098 [arXiv:1503.05534] [INSPIRE].ADSCrossRefGoogle Scholar
  40. [40]
    J.A. Bailey et al., BKℓ + decay form factors from three-flavor lattice QCD, Phys. Rev. D 93 (2016) 025026 [arXiv:1509.06235] [INSPIRE].ADSMathSciNetGoogle Scholar
  41. [41]
    BESIII collaboration, M. Ablikim et al., Precision measurements of ℬ[ψ(3686) → π + π J/ψ] and ℬ[J/ψ →  + ], Phys. Rev. D 88 (2013) 032007 [arXiv:1307.1189] [INSPIRE].
  42. [42]
    LHCb collaboration, LHCb dimuon and charm mass distributions, LHCb-CONF-2016-005, CERN, Geneva Switzerland, (2016).
  43. [43]
    LHCb collaboration, Search for D (s)+ → π + μ + μ and D (s)+ → π μ + μ + decays, Phys. Lett. B 724 (2013) 203 [arXiv:1304.6365] [INSPIRE].
  44. [44]
    Particle Data Group collaboration, C. Patrignani et al., Review of particle physics, Chin. Phys. C 40 (2016) 100001 [INSPIRE].
  45. [45]
    KLOE collaboration, F. Ambrosino et al., Measurement of the leptonic decay widths of the ϕ-meson with the KLOE detector, Phys. Lett. B 608 (2005) 199 [hep-ex/0411082] [INSPIRE].
  46. [46]
    R.R. Akhmetshin et al., Measurement of ϕ(1020) meson leptonic width with CMD-2 detector at VEPP-2M collider, Phys. Lett. B 695 (2011) 412 [arXiv:1010.4878] [INSPIRE].ADSCrossRefGoogle Scholar
  47. [47]
    D.R. Earles et al., Photoproduction of muon pairs with invariant masses between 930 and 1770 MeV, Phys. Rev. Lett. 25 (1970) 1312 [INSPIRE].ADSCrossRefGoogle Scholar
  48. [48]
    S. Hayes, R. Imlay, P.M. Joseph, A.S. Keizer and P.C. Stein, Branching ratio for the decay of ϕ mesons into lepton pairs, Phys. Rev. D 4 (1971) 899 [INSPIRE].ADSGoogle Scholar
  49. [49]
    BaBar collaboration, B. Aubert et al., Study of inclusive B and \( {\overline{B}}^0 \) decays to flavor-tagged D, D s and Λc+, Phys. Rev. D 75 (2007) 072002 [hep-ex/0606026] [INSPIRE].
  50. [50]
    LHCb collaboration, Measurement of the B 0K *0 e + e branching fraction at low dilepton mass, JHEP 05 (2013) 159 [arXiv:1304.3035] [INSPIRE].
  51. [51]
    W. Altmannshofer, S. Gori, M. Pospelov and I. Yavin, Neutrino trident production: a powerful probe of new physics with neutrino beams, Phys. Rev. Lett. 113 (2014) 091801 [arXiv:1406.2332] [INSPIRE].ADSCrossRefGoogle Scholar
  52. [52]
    CCFR collaboration, S.R. Mishra et al., Neutrino tridents and W-Z interference, Phys. Rev. Lett. 66 (1991) 3117 [INSPIRE].
  53. [53]
    S.N. Gninenko, N.V. Krasnikov and V.A. Matveev, Muon g − 2 and searches for a new leptophobic sub-GeV dark boson in a missing-energy experiment at CERN, Phys. Rev. D 91 (2015) 095015 [arXiv:1412.1400] [INSPIRE].ADSGoogle Scholar
  54. [54]
    W. Altmannshofer, J. Brod and M. Schmaltz, Experimental constraints on the coupling of the Higgs boson to electrons, JHEP 05 (2015) 125 [arXiv:1503.04830] [INSPIRE].ADSCrossRefGoogle Scholar
  55. [55]
    NA64 collaboration, D. Banerjee et al., Search for invisible decays of sub-GeV dark photons in missing-energy events at the CERN SPS, Phys. Rev. Lett. 118 (2017) 011802 [arXiv:1610.02988] [INSPIRE].
  56. [56]
    NA64 collaboration, D. Banerjee et al., Search for vector mediator of dark matter production in invisible decay mode, arXiv:1710.00971 [INSPIRE].
  57. [57]
    BaBar collaboration, J.P. Lees et al., Search for invisible decays of a dark photon produced in e + e collisions at BaBar, Phys. Rev. Lett. 119 (2017) 131804 [arXiv:1702.03327] [INSPIRE].

Copyright information

© The Author(s) 2018

Authors and Affiliations

  • Wolfgang Altmannshofer
    • 1
  • Michael J. Baker
    • 2
  • Stefania Gori
    • 1
  • Roni Harnik
    • 3
  • Maxim Pospelov
    • 4
    • 5
    • 7
  • Emmanuel Stamou
    • 6
  • Andrea Thamm
    • 7
  1. 1.Department of PhysicsUniversity of CincinnatiCincinnatiU.S.A.
  2. 2.Physik-InstitutUniversität ZürichZürichSwitzerland
  3. 3.Theoretical Physics DepartmentFermilabBataviaU.S.A.
  4. 4.Department of Physics and AstronomyUniversity of VictoriaVictoriaCanada
  5. 5.Perimeter Institute for Theoretical PhysicsWaterlooCanada
  6. 6.Enrico Fermi InstituteUniversity of ChicagoChicagoU.S.A.
  7. 7.Theoretical Physics DepartmentCERNGenevaSwitzerland

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