Recently reported anomalies in various B meson decays and also in the anomalous magnetic moment of muon (g − 2) motivate us to consider a particular extension of the standard model incorporating new interactions in lepton and quark sectors simultaneously. Our minimal choice would be leptoquark. In particular, we take vector leptoquark (U 1 ) and comprehensively study all related observables including (g − 2) µ , R K ( * ) , R D ( * ) , B → (K) where are various combinations of µ and τ , and also lepton flavor violation in the τ decays.

We apply the Festina Lente (FL) bound on a hidden sector with U (1) gauge symmetries. Since the FL bound puts a lower bound on masses of particles charged under the U (1) gauge symmetries, it is possible to constrain the hidden sector even with a tiny coupling to the Standard Model. In particular, we focus on the phenomenological implications of the FL bound on milli-charged particles, which naturally arise when kinetic mixing between the photon and the hidden photon is allowed. It turns out that the milli-charged particle with the mass M 5 meV is prohibited by the FL bound in the case of a single hidden U (1), independent of the value of kinetic mixing. This bound is crucial when bosonic dark matter is taken in consideration in this framework: the fuzzy bosonic dark matter models requesting minuscule masses are ruled out by the FL bound if the longevity of dark matter is protected by the hidden gauge symmetry.

Recently reported anomalies in various B meson decays and also in the anomalous magnetic moment of muon (g − 2)μ motivate us to consider a particular extension of the standard model incorporating new interactions in lepton and quark sectors simultaneously. Our minimal choice would be leptoquark. In particular, we take vector leptoquark (U1) and comprehensively study all related observables including ${(g-2)_{\mu }},\ R_{K^{(*)}},\ R_{D^{(*)}}$, B → (K)ℓℓ′ where ℓℓ′ are various combinations of μ and τ, and also lepton flavor violation in the τ decays. We find that a hybrid scenario with additional $U(1)_{B_3-L_2}$ gauge boson provides a common explanation of all these anomalies.

We investigate various search strategies for light vector boson X in $$ \mathcal{O} $$ O (10) MeV mass range using J/Ψ associated channels at BESIII and Belle II: (i) J/Ψ → ηcX with 1010J/Ψs at BESIII, (ii) J/Ψ(ηc + X) + $$ \mathrm{\ell}\overline{\mathrm{\ell}} $$ ℓ ℓ ¯ production at Belle II, and (iii) J/Ψ + X with the displaced vertex in X → e+e− decay are analyzed and the future sensitivities at Belle II with 50 ab−1 luminosity are comprehensively studied. By requiring the displaced vertex to be within the beam pipe, the third method results in nearly background-free analysis, and the vector boson-electron coupling and the vector boson mass can be probed in the unprecedented range, 10−4 ≤ |εe| ≤ 10−3 and 9 MeV ≤ mX ≤ 100MeV with 50 ab−1 at Belle II. This covers the favored signal region of 8Be* anomaly recently reported by Atomki experiment with mX ≃ 17 MeV.

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