Observation of ψ(3770)→γχc1→γγJ/ψ

From ${e}^{+}{e}^{\\ensuremath{-}}$ collision data acquired with the CLEO detector at the Cornell Electron Storage Ring, we observe the non-$D\\overline{D}$ decay $\\ensuremath{\\psi}(3770)\\ensuremath{\\rightarrow}\\ensuremath{\\gamma}{\\ensuremath{\\chi}}_{c1}$ with a statistical significance of 6.6 standard deviations, using the two-photon cascades to $J/\\ensuremath{\\psi}$ and $J/\\ensuremath{\\psi}\\ensuremath{\\rightarrow}{\\ensuremath{\\ell}}^{+}{\\ensuremath{\\ell}}^{\\ensuremath{-}}$. We determine $\\ensuremath{\\sigma}\\mathbf{(}{e}^{+}{e}^{\\ensuremath{-}}\\ensuremath{\\rightarrow}\\ensuremath{\\psi}(3770)\\mathbf{)}\\ifmmode\\times\\else\\texttimes\\fi{}\\mathcal{B}\\mathbf{(}\\ensuremath{\\psi}(3770)\\ensuremath{\\rightarrow}\\ensuremath{\\gamma}{\\ensuremath{\\chi}}_{c1}\\mathbf{)}=(18.0\\ifmmode\\pm\\else\\textpm\\fi{}3.3\\ifmmode\\pm\\else\\textpm\\fi{}2.5)\\text{ }\\text{ }\\mathrm{pb}$ and branching fraction $\\mathcal{B}\\mathbf{(}\\ensuremath{\\psi}(3770)\\ensuremath{\\rightarrow}\\ensuremath{\\gamma}{\\ensuremath{\\chi}}_{c1}\\mathbf{)}=(2.8\\ifmmode\\pm\\else\\textpm\\fi{}0.5\\ifmmode\\pm\\else\\textpm\\fi{}0.4)\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}3}$. We set 90% C.L. upper limits for the transition to ${\\ensuremath{\\chi}}_{c2}$ $({\\ensuremath{\\chi}}_{c0})$: $\\ensuremath{\\sigma}\\ifmmode\\times\\else\\texttimes\\fi{}\\mathcal{B}l5.7\\text{ }\\text{ }\\mathrm{pb}$ ($l282\\text{ }\\text{ }\\mathrm{pb}$) and $\\mathcal{B}l0.9\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}3}$ ($l44\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}3}$). We also determine $\\ensuremath{\\Gamma}\\mathbf{(}\\ensuremath{\\psi}(3770)\\ensuremath{\\rightarrow}\\ensuremath{\\gamma}{\\ensuremath{\\chi}}_{c1}\\mathbf{)}/\\ensuremath{\\Gamma}\\mathbf{(}\\ensuremath{\\psi}(3770)\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}{\\ensuremath{\\pi}}^{\\ensuremath{-}}J/\\ensuremath{\\psi}\\mathbf{)}=1.5\\ifmmode\\pm\\else\\textpm\\fi{}0.3\\ifmmode\\pm\\else\\textpm\\fi{}0.3$ ($g1.0$ at 90% C.L.), which bears upon the interpretation of $X(3872)$.