skip to main content


Title: Light resonances and the low-q2 bin of RK*$$ {R}_{K^{*}} $$

LHCb has reported hints of lepton-flavor universality violation in the rare decays $$B \to K^{(*)} \ell^+\ell^-$$, both in high- and low-$q^2$ bins. Although the high-$q^2$ hint may be explained by new short-ranged interactions, the low-$q^2$ 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-$q^2$ 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 $$B \to K^* e^+e^-$$ rate close to the di-muon threshold. If such a peak is observed, other observables, such as the differential $$B \to K e^+e^-$$ rate and $$R_K$$, may be employed to distinguish between models. However, if a low-mass resonance is not observed and the low-$q^2$ anomaly increases in significance, then the case for an experimental origin of the lepton-flavor universality violating anomalies would be strengthened. Finally, to further explore this, we also point out that, in analogy to $$J/\psi$$ decays, $e^+e^-$ and $$\mu^+\mu^-$$ decays of $$\phi$$ mesons can be used as a cross check of lepton-flavor universality by LHCb with $5$ fb$$^{-1}$$ of integrated luminosity.
 [1] ;  [2] ;  [1] ;  [3] ;  [4] ;  [5] ;  [6]
  1. Univ. of Cincinnati, OH (United States). Dept. of Physics
  2. Univ. of Zurich (Switzerland). Physik-Inst.
  3. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States). Theoretical Physics Dept.
  4. Univ. of Victoria, BC (Canada). Dept. of Physics and Astronomy; ; Perimeter Inst. for Theoretical Physics, Waterloo, ON (Canada); European Organization for Nuclear Research (CERN), Geneva (Switzerland). Theoretical Physics Dept.
  5. Univ. of Chicago, IL (United States). Enrico Fermi Inst.
  6. European Organization for Nuclear Research (CERN), Geneva (Switzerland). Theoretical Physics Dept.
Publication Date:
Report Number(s):
arXiv:1711.07494; FERMILAB-PUB-17-523-T; CERN-TH-2017-240; MITP/-17-083; ZU-TH-35/17
Journal ID: ISSN 1029-8479; 1637364
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of High Energy Physics (Online)
Additional Journal Information:
Journal Name: Journal of High Energy Physics (Online); Journal Volume: 2018; Journal Issue: 3; Journal ID: ISSN 1029-8479
Springer Berlin
Research Org:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Country of Publication:
United States
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Beyond Standard Model; Heavy Quark Physics
OSTI Identifier: