Photon scores a conversion in ATLAS
A candidate for the predicted decay of the Higgs is into two photons. This is a slice of such an event. As a neutral particle, the photon leaves no track unless it "converts" into an electron and a positron, as happened in this event. For clarity though, a line has been added from the central vertex of the event (magenta dot) at 4 o'clock to show the path of the photon. This line stops where photon converts (at the brown dot) to an electron and a positron, seen as red and blue tracks, respectively. The actual signals in ATLAS are represented by the small red and blue dots in the tracker and the thick yellow energy deposit in the electromagentic calorimeter. The other lines and thick dots are all reconstructions from the data they represent.
Credit: ATLAS Experiment © 2011 CERN