Phys. Rev. Lett. 109, 042003 (2012)

While the Large Hadron Collider at CERN pushes the energy frontier for particle collisions up to the scale of several teraelectronvolts, colliders elsewhere in the world are exploiting lower-energy interactions to fill in more details of the present standard model. The BESIII collaboration — using the upgraded Beijing Electron Positron Collider (BEPCII) at the Intitute of High Energy Physics, Beijing, China — now presents the first measurement of a magnetic dipole transition of the ψ(3686) particle, a so-called charmonium (charm–anticharm quark) state, and a useful guide in understanding more about the strong interactions of quarks.

The collaboration pinpoints the radiative transition of the ψ(3686) to a photon and an ηc(2S) particle (the first radial excitation of the ηc state), followed by the decay of the ηc(2S) to a certain mix of kaons and pions (which are mesons made up of lighter up, down and strange quarks), using a modified kinematic fit to guard against 'fake' photons in the detector that would skew the analysis. The measured branching fraction for the magnetic dipole transition turns out to be a good match for theoretical calculations.