Study of the Magnetic Dipole Transition of J/ψ →γηc via ηc →p p ¯

Using (10.087±0.044)×109 J/ψ events collected with the BESIII detector at the e+e- BEPCII collider, we present the first amplitude analysis of J/ψ→γpp¯ with the pp¯ invariant mass in the ηc mass region [2.70,3.05] GeV/c2. The product branching fraction B(J/ψ→γηc)×B(ηc→pp¯) is determined to be (2.11±0.02stat±0.07syst)×10-5 with precision improved by 1 order of magnitude. Combining with the product branching fractions B(ηc→pp¯)×B(ηc→γγ) and B(J/ψ→γηc)×B(ηc→γγ), the branching fractions of B(J/ψ→γηc) and B(ηc→γγ) are calculated to be (2.29±0.01stat±0.04syst±0.18opbf)% and (2.28±0.01stat±0.04syst±0.18opbf)×10-4, respectively, which are consistent with the latest lattice quantum chromodynamics calculations. Here, opbf is the uncertainty from the other product branching fractions used in the calculation.