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.
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Wright, A. Smooth transition. Nature Phys 8, 637 (2012). https://doi.org/10.1038/nphys2420
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DOI: https://doi.org/10.1038/nphys2420