ATLAS experimenters celebrated today as the first beams circulated the Large Hadron Collider in both directions. While everyone was cheering in the LHC control room, the cheers were echoed in the ATLAS and other control rooms, and in several auditoriums around CERN.

While no collisions are planned for a few weeks, the ATLAS detector lit up as a flood of particles traversed the detector when the beam was occasionally directed at a target near ATLAS. This allowed ATLAS physicists to study how well the various components of the detector were functioning in preparation for the forthcoming collisions. The screen displays of these events in the ATLAS control room led to even greaters cheers.

For physicists and many news media, it was fantastic to be here at CERN to witness the startup of the LHC. At 10:25am (CERN time), after only an hour of tuning the beam, they circulated the beam all the way around the 27 km ring. This was seen as a cross-section of the beampipe in which one dot was the injection into the ring and a second dot was the beam returning to that same location (see the photo at left).

Now, after twenty years of preparation, ATLAS is ready and anticipates incredible discoveries about our Universe in the coming months and years.