Cosmic success at LHCb

LHCb has been touched by the cosmos. For the first time the team has measured cosmic rays hurtling through three of the experiment’s sub-detectors simultaneously, selected by muon triggers.

A snapshot of a reconstruction of a cosmic ray passing through LHCb’s HCAL, ECAL and muon chambers (crosses), alongside a photograph showing the layout of the real thing.

During a global commissioning run on 3 April the LHCb team used three of the experiment’s sub-detectors (the electromagnetic calorimeter (ECAL), the hadronic calorimeter (HCAL) and the muon system) to trace the paths of muons originating from cosmic rays. This achievement is important as it’s the closest thing to a dress rehearsal before the LHC is switched on and the LHCb’s detector is working for real.

The LHCb software enabled the collaborators to see 3D simulations of the paths taken by the muons passing through the sub-detectors by visualising the energy deposited in each of the activated calorimeter cells and the signals in the muon chambers. The pictures above show the results and are recorded by the HCAL (blue cubes), ECAL (red cubes) and the muon system (green crosses).

For the LHCb team these images represent the culmination of years of hard work. "Last week we had the ECAL, the HCAL and the muon stations running simultaneously," says Thomas Ruf, Physics Software Project Leader of LHCb. "It shows that each of the detectors work together, and are synchronised, all the software chain works, and the raw data makes sense. For us it was very exciting and we recorded many interesting events."

Tests using cosmic rays can only be carried out on certain sub-detectors, like the calorimetry and muon systems, which have a very large surface area and can detect particles coming from all directions. Cosmic rays, by definition, travel down vertically from the sky, which is not very convenient for LHCb as it is arranged in vertical planes along the beam line. This means that the smaller sub-detectors, positioned very close to the beam line, have very little chance of detecting cosmic rays as they only detect particles travelling at extremely small angles.

So what’s next for LHCb? Thomas Ruf is excited: "The teams from the various sub-detectors, the trigger and the online system that are working at the pit are doing a fantastic job. We’re going to check the signals from some of the other sub-detectors too," he explains. "We still have the scintillator pad detector and the pre-shower detector to test, for example. Then we want to add the outer tracker, a big tracking system that’s positioned after the magnet, and maybe even the Rich detectors, but this is then the maximum we can do with cosmic rays - we really have to wait for the LHC for the rest!"

A virtual simulation of the cosmic tracks can be found at: