To take the full advantage of the ψ(3686) data set collected at e+e- collider at τ-charm energy region, a tag method is developed to directly measure the J/ψ meson decay branching fractions absolutely. The J/ψ meson decay can be measured with the J/ψ sample tagged by the two soft charged pions from the decay ψ(3686)→J/ψπ+π-. This method is illustrated by comparing the input and output branching fractions of J/ψ→γη with 106 million inclusive ψ(3686) Monte Carlo samples. The consistent result confirms the ...
