我们在RJ /ψ≡ℬℛBC+→J /ψτ+ντ/ℬℛBC+→J /ψμ+νμ$$ R \ left（J / \ psi \ right）\ equiv \ mathrm {\ mathcal {B} } \ mathrm {\ mathcal {R}} \ left（{B} _C ^ {+} \至J / \ psi {\ tau} ^ {+} {\ nu} _ {\ tau} \ right）/ \ mathrm {\ mathcal {B}} \ mathrm {\ mathcal {R}} \ left（{B} _C ^ {+} \ to J / \ psi {\ mu} ^ {+}