Belle II Does Not See Lepton Universality Violations In Select B Meson Decays

A new search for lepton universality violations by the Belle II collaboration did not find any sign of lepton universality violations. As the introduction to the new paper explains:

The decays B → K`+` −, where ` stands for an electron or a muon, are flavor-changingneutral-current processes. Being governed by a b → s quark-level transition, these decays are forbidden at tree level in the standard model (SM) of particle physics, but can proceed through b → s`+` − loop amplitudes at lowest order. Various SM extensions predict new particles that contribute to the processes, altering the values of observables from their SM predictions. These possibilities make B → K`+` − decays a sensitive probe for beyond-the-SM physics. 

One of the key predictions of the SM is that the coupling strengths of electroweak gauge bosons to charged leptons e, µ, and τ are the same, a property known as lepton-flavor universality (LFU). Accordingly, the ratio of branching fractions of B → Kµ+µ − to B → Ke+e − decays, called RK, is expected to be close to unity. A recent measurement of RK by the LHCb Collaboration reported a 3.1σ discrepancy with respect to its SM prediction in the range of dilepton mass squared q^2 ∈ (1.1, 6.0) GeV^2/c^4. On the other hand, the measurements reported by the Belle experiment are consistent with both the SM and the LHCb result, albeit with significantly less precision compared to the latter. 

In this report, we describe measurements of branching fractions B (B → J/ψK), LFU ratios 

RK(J/ψ) = B (B → J/ψ(µ +µ −)K)/ B (B → J/ψ(e +e −)K), (1) 

and isospin asymmetries 

AI = Γ[B0 → J/ψ(` +` −)K0 ] − Γ[B+ → J/ψ(` +` −)K+] Γ[B0 → J/ψ(` +` −)K0 ] + Γ[B+ → J/ψ(` +` −)K+] , (2) 

performed using data recorded by the Belle II experiment. 

In contrast to suppressed, charmless B → K`+` − decays, the B → J/ψ(` +` −)K decays involve a favored b → c tree-level transition. Hence, contributions from beyond-the-SM physics are expected to have a negligible impact. 

Since the branching fraction of B → J/ψ(` +` −)K is two orders of magnitude larger than that of B → K`+` − decays and both channels share the same final-state particles, the former decays constitute an excellent control sample for studies of the latter. Therefore, the measurement of RK (J/ψ) and its consistency with unity would be a strong validation of the future RK measurement in the charmless counterpart of B → K`` decays. Throughout the report, charge conjugate processes are implicitly included.

I believe the Belle paper consistent with the SM referenced is this one.

The bottom line result is that:

This makes it increasingly more difficult to come up with a model that sees lepton universality violations in the circumstances where they are observed, while not seeing them where they are not observed to high precision, even though this doesn't directly contradict the previous indications of lepton universality violations at the LHCb experiment at the Large Hadron Collider.

The paper and its abstract are as follows:

Belle II Collaboration, "Measurements of the branching fraction, isospin asymmetry, and lepton-universality ratio in B→J/ψK decays at Belle II" arXiv:2207.11275 (July 22, 2022).