2nd report from the LHC performance workshop

Tuesday's sessions were spent looking ahead to the possibilities for 2012. The morning started with the experiments' desiderata for the year with the key requirement being either discovery of Higgs or exclusion at the 95% confidence level down to 115 GeV. To achieve this Atlas and CMS will need an integrated luminosity in the order of 15 fb^-1.

Potential improvements to performance and machine availability were then discussed with presentations on maximising the time the LHC is delivering collisions to the experiments, and the potential of the injectors to provide bunches with higher intensities and the smallest possible beam size (these fold directly into higher collision rates). Machine performance will also be improved in 2012 thanks to a number of mitigation measures taken during the Christmas stop aimed at reducing the effects of radiation on the electronics situated in the LHC tunnel.

The possibility for running at a beam energy of 4 TeV was put on the table. Discussions will continue and a final decision will be a clear outcome of the workshop. One of the big successes of 2011 was the squeeze - the reduction of the beam size at the interaction point - which was pushed in the latter part of the year. Squeezing further in 2012 might be possible in combination with the use of tighter collimator settings. This could give a peak luminosity of around 6x10³³ cm^-2s^-1 to be compared with a maximum of 3.6x10³³ cm^-2s^-1 in 2011.

With a bunch spacing of 50 ns, a total of 1380 bunches (as in 2011), 15 fb^-1 seems to be in reach if the tighter collimator settings prove to be operationally robust and the impressive performance of the LHC's many hardware systems continues.

by Bulletin's correspondent from Chamonix