The first and the last OPAL event Between 1989 and 2000 the OPAL experiment at the LEP accelerator has collected data from millions of electron-positron interactions.
Every single interaction recorded by the detector is called an 'event'.
The data recorded for an event can be visualised by an event display. The event displays which you find on this page always show the plane perpendicular to the beam. You see the small beam pipe in the centre. Surrounding it are the tracking chambers which detect charged particles. The curvature of the tracks is a measure of their momentum.