Evidence for the Direct Two-Photon Transition from $\psi(3686)$ to $J/\psi$

TL;DR

This paper reports the first measurement of a two-photon transition between charmonium states, specifically from $8(3686)$ to $J/A$ using BESIII data, including branching fractions and polarization studies.

Contribution

It provides the first experimental evidence and measurement of the two-photon transition in charmonium, expanding understanding of quantum chromodynamics in heavy quarkonium systems.

Findings

Branching fraction measured as (3.1 ± 0.6(stat) +0.8/-1.0(syst)) x 10^{-4}

Upper limit of 4.5 x 10^{-4} at 90% confidence level

Studied decay plane orientation and $J/A$ polarization

Abstract

The two-photon transition $\psi(3686)\to\gamma\gamma J/\psi$ is studied in a sample of 106 million $\psi(3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1\pm0.6(\unit{stat})^{+0.8}_{-1.0}(\unit{syst})) \times10^{-4}$ using $J/\psi\to e^+e^-$ and $J/\psi\to\mu^+\mu^-$ decays, and its upper limit is estimated to be $4.5\times10^{-4}$ at the 90% conference level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the $\psi(3686)$ decay plane and the $J/\psi$ polarization in this decay are also studied. In addition, the product branching fractions of sequential $E1$ transitions $\psi(3686)\to\gamma\chi_{cJ}, \chi_{cJ}\to\gamma J/\psi (J=0,1,2)$ are reported.