Similar to the previous event, except that a photon (quantum of electromagnetic radiation) is also produced. This is not visible light, as the wavelength is so small, but a high-energy gamma ray. The photon, being uncharged, does not produce a track in the central part of the detector, but is detected in the electromagnetic calorimeter, where it gives off its energy, as seen in the green bar.

Notes for experts: Radiative dimuon event with a 30 GeV photon seen in the barrel electromagnetic calorimeter (HPC).