See CMS e-commentary for live updates.

“Beam-splashes” are when a beam is threaded part-way through the LHC ring, then deliberately collided with an absorbing block of tungsten to stop it, upstream of a detector. Many particles are created in this collision, most of them are absorbed, with the exception of the muons and neutrinos. CMS can detect muons, and what it sees is a huge splash of activity, shown in this event display from September, 2008.

The blue bars indicate huge deposits of energy in the calorimeters. They seem to project from the center of the detector, but this is an artifact of the software, which was designed to visualize collisions from the center. The calorimeter cells measure energy, not direction, so when it sees energy coming from a flood of particles arriving from the right, it draws them as though they came from the center.

You can also see little parallel lines surrounding the central burst like a school of fish. These are individual muons seen by the barrel muon detectors, which do measure direction.

Update: here it is, the first CMS beam-splash of 2009 (from the e-commentary page)!

The little red lines are reconstructed muon tracks, blue dots are raw hits, and the yellow/blue starburst in the center is the calorimeter energy. You can tell that the beam is coming from the right-hand side of the detector (“LHC beam-1”, the clockwise direction around the ring).

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