Run 2, which started in 2015, has come to an end. During the last month of operation, the LHC delivered Pb-Pb collisions at 5.02 TeV of energy per nucleon pair, allowing ALICE to collect 0.9 nb-1of integrated luminosity with lead ions.
Screen shot of the LHC page 1 showing the dump of the last beam of the LHC Run 2 on Monday 3 December 2018 (Image: CERN)
After three weeks and a half of fruitful data taking with Pb-Pb collisions, the ALICE experiment was switched off at the beginning of December and entrusted to the technical coordination team, who immediately started the upgrade operations. These new heavy-ion collision samples, together with the rest of the incredible amount of data collected during LHC Run 2 (2015-2018), will allow ALICE to move forward in many important analyses and to perform new measurements.
“Early this year we set a very ambitious goal for the 2018 lead-ion run: we wanted to get about 1 nb-1of integrated luminosity delivered,” explains 2018 Run Coordinator Kristjan Gulbrandsen. “When I first proposed this to the LHC experts, they considered it wildly impossible. But we actually got 0.9 nb-1, which is pretty close! Thus, we are really happy.”
Meeting this challenging goal required effort and thorough work by the accelerator operators as well as by the ALICE detector experts, who were able to push the current experiment performance beyond its design specifications.
The Pb-Pb run, the seventh one involving lead-ions since the start of the LHC, was carefully prepared in the previous months through a number of tests. Some adjustments, though, had to be done during the data taking to make the detector perform at its best. Particular attention was given to the Time Projection Chamber (TPC), since its readout multi-wire proportional chambers put limits on the data taking rate the TPC can cope with: specifically, the trigger configuration was adapted to improve its stability and avoid high-voltage trips. Also, the storage system had to be reset because of an issue with the data acquisition (DAQ).
The accelerator faced some difficulties as well. Shortly after the beginning of the run, the luminosity in ALICE was observed to be lower than expected, since the effective β* (a parameter related to the beam size and, thus, the interaction rate) was higher. The LHC experts were challenged by an issue with the skew quadrupoles on both sides of the interaction point, which was preventing the beams from overlapping correctly in ALICE. They fixed it when the polarity of the ALICE magnet was flipped and, from then on, the experiment was provided with the expected luminosity.
Once these issues were taken care of, the run proceeded quite smoothly and the data taking efficiency increased to a few percent less than that seen during the pp run.
“We managed to include triggers to select central and mid-central collisions – something that we did not have in the previous Pb-Pb run – so we collected very big data samples with these configurations relative to those in 2015: a factor of 8 for central and 4 for mid-central,” Kristjan explains.“And for the other trigger selections, we were able to collect about 2 times as much data as we did in 2015. There will be a pretty good volume of data to analyze in the years to come.”
ALICE stopped data taking at 5 pm on December 2, whereas the accelerator team continued working on machine development for 13 more hours. While shutdown operations for the experiments started immediately on December 3, the second-long shutdown (LS2) officially began on December 10, when the CERN Control Centre operators passed a symbolic key to the LS2 coordinators, who will keep it until early 2021.
“When it was over, it took me some time to realize that I could actually relax,” states Kristjan laughing. “It has been an intense year! Being Run Coordinator is certainly an interesting experience. You feel that you are in the thick of it, but it is also tiring because you need to always be on top of everything. I have to say that having Taku[(A/N) Taku Gunji, Deputy Run Coordinator] there, during the whole lead-ion run, was crucial. I couldn’t have done it without him. I would also like to thank run managers, trigger coordination, technical coordination, physics board members, and subsystem experts for the success of the Pb-Pb data taking.”