CMS has been taking collision data since the 13TeV startup of the LHC on 3 June. During this period, the CMS magnet has been kept off due to an issue with the cooling system, so the beams have been used to calibrate and time-in the electronics of the various parts of the detector. These operations, which are largely independent of the magnetic field, are now complete. Meanwhile, the data collected with zero magnetic field can be used for fundamental research, like the measurement of the multiplicity of charged particles produced at the new collision energy of 13 TeV.
The issue with the magnet cooling system was identified in the final preparatory phase leading to collisions in the LHC. While preparing for beam in CMS, a problem was found in the system that feeds liquid helium to the CMS superconducting magnet. The problem was diagnosed to be due to oil, which is used in the initial compression stages, reaching the so-called 'cold-box’ of the cryogenic system. The cold-box is a complex system with several sets of filters protecting three turbines along the path of the helium towards the magnet. In order to clean the oil contamination essentially all components of the cold-box have been extracted and replaced.
Analysis confirms that there is no oil contamination in the CMS magnet itself or risk to its operation during 2015. The cold-box of is now being stabilised after the cleaning intervention and is being brought back to operational conditions.
CMS is confident that, following the LHC technical stop and the beam conditioning run that will start at the end of this week, after the low-intensity and commissioning period, the full magnetic field will be available for the 13 TeV LHC run.