by alanbarr scientist
Some of the scientists have had a chat about the object first labeled as a #muonjet. After some discussion we reckon it is probably an example of what is called "punch-through". This is rare, but not unknown.
Here's how it works (for the keen!)
High energy particles tend to get stopped later in the layers of the detector called calorimeters (green and red rings on the images). When they get stopped they shower into other particles (not shown as tracks, but shown as little yellow energy deposits in the images).
If by chance the shower starts late in the calorimeter, towards the outside, the resulting shower can pass right out the back and into the #muon chambers (blue outer layers). Since usually we only expect #muon objects to pass through the whole detector, it gets labelled as a large number of muons.
This is probably what happened in this image, but we're still investigating in more detail.