First beam splashes at the LHC
After a two-year shutdown, the first beams of Run 2 circulated in the LHC last Sunday. On Tuesday, the LHC operators performed dedicated runs to allow some of the experiments to record their first signals coming from particles splashed out when the circulating beams hit the collimators. Powerful reconstruction software then transforms the electronic signals into colourful images.
“Splash” events are used by the experiments to test their numerous subdetectors and to synchronise them with the LHC clock. These events are recorded when the path of particles travelling in the LHC vacuum pipe is intentionally obstructed using collimators – one-metre-long graphite or tungsten jaws that are also used to catch particles that wander too far from the beam centre and to protect the accelerator against unavoidable regular and irregular beam losses. The particles sprayed out of the collision between the beam and the collimators are mostly muons. ATLAS and CMS recorded their first splash events of Run 2 on Tuesday, 7 April.
ATLAS
This picture shows the first collimator "splash" event seen by the ATLAS experiment in LHC Run 2 , on Tuesday, 7 April: event 16848, run 260466. The collimator position is 140 m in front of the ATLAS interaction point. The figure on the left shows an axial view of the various components of the ATLAS detector. The figure on the right shows the energy deposits in the cells of the ATLAS calorimeter.
CMS
CMS event display from LHC beam splash on Tuesday, 7 April. This is the first time the full detector has seen coincident particles since the end of Run 1 - over two years ago. In contrast to collisions where the particles come from the centre, in this splash event, particles traverse the detector horizontally from one side to the other. In the centre, the electromagnetic and hadron calorimeters show energy deposited according to the size of the colour emanating from the centre. The yellow in the top, bottom, left, and right indicate activity in the resistive plate chambers while the blue on the left and right indicate activity in the cathode strip chambers. The attention of the collaboration has been focused on understanding the features of these complicated events to confirm that the response is as expected after the extensive work performed during Long Shutdown 1. The collaboration’s appetite has been whetted for collisions at 13 TeV later this year!
