Beam Splash Events in the CMS Muon End Caps

Excitement returns to CMS this month, as the LHC begins to circulate beam. There are many good sources of information, for example, the online commentary by Darin Acosta, among others.

My team from Northwestern University is busy providing prompt feedback on the response of the cathode strip chambers (CSCs) from the CMS experiment. On 9-November, we observed the beam splash events produced when Beam 2 struck collimators and a wall of muons passed from the -Z to the +Z side of CMS. Here is a depiction of the charge measured on the radial strips of the CSCs:

Beam 2 splash event in the CSCs

The arrow indicates the direction of Beam 2, and one sees clearly more charge on the strips on the upstream side compared to the downstream side. The red fans show the inner set of chambers, while the blue fans show the outer. (There is one pair of green fans, but they are too small and to faint to make out in this picture.)

Here is a new event from this evening, 20-November, in which Beam 1 produces a splash in the CSCs:

Beam 1 splash event in the CSCs

Comparing to the picture above, it is clear that the two muon endcaps have exchanged rolls (and indeed, Andy reversed the direction of the arrow).

It is worth noting that the HV is set to stand-by values. The flux of muons is so great, on the order of 5 muons per cm², that we nonetheless see a tremendous about of charge compared to what we expect for a normal single muon, such as a cosmic ray or one coming from a pp collision.

A more conventional, and colorful, view of these kinds of events is given by the official iSpy event display program. Here is an example:

Beam Splash event as depicted by iSpy

The purple parts in the end caps are the CSCs, obviously registering lots of charge while many other subdetector systems are off.

As I write this post, the LHC operators are `capturing’ the beam, which means that the protons’ orbit is determined by the RF cavities that are turned on. This is a major milestone on the way to collisions.