On Sunday, 7.10.2001, a 1.3 mA proton beam in 10 bunches at 920 GeV and a 1.0 mA positron beam in 12 bunches at 27.5 GeV were stored in the upgraded HERA ring. A 'horizontal bump' was driven and at 16:55 a steep rise in the counting rate of the H1 Luminosity Photon Detector from a beam-gas background level of 0.7 kHz to 140 kHz was observed.
This is interpreted as a clear sign for first collisions in HERA II.
They occured almost to the day 10 years after the first ever collions in HERA (19.10.1991).
A few hours later the same beams were brought into collision again by reloading the magnet-setting file saved previously, producing a neat sharp step: