Archive for November 21, 2009
Yesterday we enjoyed our second set of beam splash events, generated with beam one in contrast to earlier splash events generated with beam two.
Today we are thrilled to see beautiful beam halo events in CMS, like this one:
You can clearly see a trajectory (in red) extending across the CMS detector based on short track segments (light blue dashes) reconstructed in the muon endcap systems. (The grey cylinder in the middle indicates the tracking volume.) This picture was made by my graduate student as part of our effort to understand and validate the performance of the CMS cathode strip chambers (which are traced here in dark blue).
Muons in beam halo events run parallel to the beam and typically have a few hundred GeV, according to simulations. (We would love to test that with the real data…) They are generated when protons from the beam pass out of the beam pipe and strike some object near by, leading to an energetic hadronic shower out of which emerge one or more muons. This shower occurs many tens or hundreds of meters away from the experiment, so any muons that reach the apparatus have hardly any angle with respect to the beam.
These beam halo muons may be a nuisance one day, but right now they are a novelty. In fact, they are quite useful for the end cap muon systems, since they provide straight lines through the muon chambers which can be used to refine the chamber alignment, and their arrival time is well-defined thanks to the bunched nature of the LHC proton beams. (For some nice illustrations, see the postings by Darin Acosta.) This strobe-like signal helps us refine the synchronization of the chamber signals, which is important for recording them properly and for defining an accurate and efficiency trigger.
Two months ago we were busy analyzing cosmic rays. Two weeks ago, and two days ago, we were extracting information from beam splash events. Today, and for the next few days, we are looking at beam halo tracks. What comes after that? collisions!.