Light resonances and the low-q 2 bin of RK*

Wolfgang Altmannshofer, Michael J. Baker, Stefania Gori, Roni Harnik, Maxim Pospelov, Emmanuel Stamou, Andrea Thamm

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29 Scopus citations


LHCb has reported hints of lepton-flavor universality violation in the rare decays B → K(*)+, 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 RK* 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 B → K*e+e rate close to the di-muon threshold. If such a peak is observed, other observables, such as the differential B → Ke+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.

Original languageEnglish (US)
Article number188
JournalJournal of High Energy Physics
Issue number3
StatePublished - Mar 1 2018
Externally publishedYes

Bibliographical note

Funding Information:
Article funded by SCOAP3.

Publisher Copyright:
© 2018, The Author(s).


  • Beyond Standard Model
  • Heavy Quark Physics


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