On March 25 the first proton beams for 2016 started circulating in the LHC, which marked the official end of the 13-week long year-end technical stop. The next four weeks are dedicated to beam commissioning, which will be followed by a ramp-up in intensity by increasing the number of protons per bunch, until the target intensity is reached. Stable beams at 13 TeV are expected around the end of April.
One of the first tests, performed before injecting the proton beams into the LHC, was to set up the transfer lines between the SPS and the LHC. During this operation every 30 seconds proton beams at 450 GeV were dumped on the TED absorber, situated at the end of the injection line from the SPS to the LHC. Since ALICE is located about 300 metres from the TED, it detects secondary particles from these interactions, mainly muons, which can be used to check the trigger detectors. ”We use the TED shots for timing purposes. What our detectors see are energetic muons, coming from the TED. Once we know the radio frequency of the beam, we can tune our detectors in accordance with it. We have set up a specific trigger configuration for the detectors, participating in these tests, and we try to check if they all trigger in coincidence” - explains Siegfried Förtsch, ALICE run coordinator.
Secondary particles coming from beams, stopped just before entering the LHC ring during transfer line tests can reach ALICE detector. In this event display, taken during one such test, clusters in the Inner Tracking System and Electromagnetic Calorimeter, as well as hits in the Muon Trigger chambers can be seen.
In the meantime the commissioning of most detectors in ALICE and the implementation of the updated detector control system (DCS) software were successfully completed. Since the beginning of March cosmic runs were performed, during which data were taken with a subset of detectors. Since it underwent a major intervention during the year-end technical stop, one of the last detectors to join the global runs was the time projection chamber (TPC). In order to increase the readout speed, all its RCU cards (readout control units) were replaced with new and improved RCU2 cards. In addition to that the gas of the TPC was purged and replaced with fresh gas. “We cannot do physics without the TPC. That is why most of the cosmic data we recorded in March will not be analysed. Instead these will be mainly used for checking the alignment and trigger efficiencies of the different detectors” – says Siegfried.