LHC Report: Back to record performance

The technical issues that caused a rocky re-start after the technical stop and a relatively low performance of the machine have been tackled and resolved in the past days. The LHC is back to record collision rates and matching the 2011 data set now seems within reach for the summer conferences.

The technical stop at the end of April was followed by a period in which increased beam losses were observed before the beams were brought into collision, causing reduced peak luminosities. This issue was tackled by optimising the beams in the injector chain, making a fine adjustment of the orbit during the energy ramp-up and ensuring precise bunch collisions in ATLAS and CMS. These measures brought the peak luminosity at the beginning of a fill up to record levels of 6 x10³³ cm^-2s^-1, but the fills were short as there were still sudden losses of some bunches that triggered the beam dump.

The lost bunches were identified as bunches that only collide in LHCb. These bunches see fewer collisions, so the “beam-beam effects” that can stabilise the beams are smaller and their collision parameters are different from those of the other bunches. It was then decided to change the filling scheme of the LHC to exclude almost all bunches which only collide in LHCb. This proved to be a winner and there was a series of nice long fills with the record fill staying in the machine for almost 20 hours and producing an integrated luminosity of more than 0.2 fb^-1 for each of the ATLAS and CMS experiments.

Assuming realistic machine availability with no major hardware problems, producing about 1 fb^-1 per week seems feasible. Compare this to the 5.6 fb^-1 produced over the whole of 2011. On Wednesday 23 May the integrated delivered luminosity were about 2.8 fb^-1, so we seem well on track to doubling the experiments' data-set in time for the summer physics conferences.

by Jan Uythoven for the LHC Team