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 RK, 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

This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.


  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

Personalised recommendations