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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27 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

Bibliographical note

Funding Information:
Article funded by SCOAP3.

Funding Information:
We thank Kaladi Babu and Pedro Machado for discussions. WA and SG thank the Mainz Institute for Theoretical Physics (MITP) for its hospitality and support during parts of this work. The work of WA and SG was in part performed at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611. The research of WA is supported by the National Science Foundation under Grant No. PHY-1720252. SG is supported by a National Science Foundation CAREER Grant No. PHY-1654502. MJB and AT would like to thank Fermilab for its kind hospitality and support during the early stages of this project. Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. MJB was supported by the German Research Foundation (DFG) under Grant Nos. KO 4820/1-1 and FOR 2239, by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 637506, “νDirections”), by Horizon 2020 INVISIBLESPlus (H2020-MSCA-RISE-2015-690575) and by the Swiss National Science Foundation (SNF) under contract 200021-175940. The work of AT was supported under the International Cooperative Research and Development Agreement for Basic Science Cooperation (CRADA No. FRA-2016-0040) between Fermilab and Johannes Gutenberg University Mainz, and partially by the Advanced Grant EFT4LHC of the European Research Council (ERC) and the Cluster of Excellence Precision Physics, Fundamental Interactions and Structure of Matter (PRISMA — EXC 1098).


  • Beyond Standard Model
  • Heavy Quark Physics

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