First Observation of psi(3770) --> gamma chi_{c1} --> gamma gamma J/psi

TL;DR

This paper reports the first observation of the decay psi(3770) to gamma chi_c1, using e+e- collision data, with detailed measurements of the decay rate and upper limits for related transitions, providing new insights into charmonium states.

Contribution

The study presents the first experimental observation of the non-D Dbar decay psi(3770) to gamma chi_c1, including precise measurements of its branching fraction and transition rates.

Findings

Observation of psi(3770) -> gamma chi_c1 with 6.6 sigma significance

Measured branching fraction BR(psi(3770)->gamma chi_c1)=(2.8+-0.5+-0.4) x 10^-3

Set upper limits for transitions to chi_c2 and chi_c0

Abstract

From e+e- collision data acquired with the CLEO detector at CESR, we observe the non-D Dbar decay psi(3770)->gamma chi_c1 with a statistical significance of 6.6 standard deviations, using the two-photon cascades to J/psi and J/psi->l+l-. We determine sigma(e+e- -> psi(3770)) x BR(psi(3770)->gamma chi_c1) =(18.0 pm3.3 pm2.5) pb and branching fraction BR(psi(3770)->gamma chi_c1)=(2.8+-0.5+-0.4) x 10^-3. We set 90% C.L. upper limits for the transition to chi_c2 (chi_c0) : sigma x BR < 5.7 pb (< 282 pb) and BR < 0.9 x 10^-3 (<44 x 10^-3). We also determine Gamma(psi(3770)->gamma chi_c1)/Gamma(psi(3770)->pi+pi-J/psi) =1.5+-0.3+-0.3 (>1.0 at 90 % C.L.), which bears upon the interpretation of X(3872).