May was an eventful month both for ALICE and for the Large Hadron Collider (LHC). Despite the excellent start by the operators of the LHC during the start-up and the unprecedented way in which they were able to increase the luminosity, operations were interrupted severely on two occasions. On 29 April a small beech marten caused a short circuit in a large, open-air electrical transformer, situated above ground at CERN. That led to a power perturbation of the 18 kV and 66 kV grids of the LHC, causing a one-week interruption. A further delay was caused a week later by problems with the power converter of the Proton Synchrotron (PS).
With the first data collected during stable beams, the ALICE team was able to test the performance of the new readout cards of the TPC, as well as to check the data quality after the replacement of the gas of the TPC. These dedicated runs were carried out at higher than normal collision rates during the intensity ramp-up phase of the LHC. This allowed maximum read-out rates for the TPC, which will also be encountered during the p-Pb run later this year.
During the early period of the intensity ramp up the magnetic field of the solenoid was temporarily lowered from the nominal field of 0.5 T to 0.2 T in order to collect more data for the production of low-mass particles. During periods when there were no beams in the machine, ALICE performed cosmic runs, both with and without magnetic field.
On 17 May the LHC provided dedicated fills for the Van der Meer scans, which are used by all four experiments for luminosity and length scale calibrations. “The scan for ALICE consisted of eight successive scans, which lasted three and a half hours. During the van der Meer scans of the other three experiments the special beam optics applicable to IP2 allowed for data taking, including also the Zero Degree Calorimeter (ZDC) detectors. That resulted in 15 million ultra-diffractive minimum bias events. These exceptional optic settings of the beams entail a reduced external crossing angle of +160 µrad. These conditions allowed the inclusion of the ZDCs, which are positioned close to the beam pipe – about 116 m on either side of the interaction point. In normal production runs the larger external crossing angle of the beams of +200 µrad prevents the operation of the ZDC during pp data taking. While the ZDC is excluded during the pp campaign, it will play its role during the p-Pb run.” – explained Siegfried Förtsch, ALICE run coordinator.