The LHC has delivered more than twice as many collisions to the ATLAS and CMS experiments this year as it did in all of 2011. On 4 August, the integrated luminosity recorded by each of the experiments passed the 10 fb–1 mark. Last year, they each recorded data corresponding to around 5.6 fb–1. On 22 August this year, the more specialized LHCb experiment passed 1.11 fb–1, the same as its entire data sample for 2011.
The LHC’s peak luminosity had been running 5–10% lower following June’s technical stop. This was mainly owing to a slight degradation in beam quality from the injectors – an issue that was resolved at the beginning of August. The LHC had also been suffering from occasional beam instabilities, which have resulted in significant beam losses. A solution to this second problem lay in finding new optimum machine settings with the polarity of octupole magnets reversed relative to that of recent years. (The octupole magnets are used to correct beam instabilities.)
This reversal, accompanied by an adjustment of the settings in the sextupole magnets, was studied over several days in August. These changes paid off and the beams became more stable when brought into collision, so the bunch currents could be increased from 1.5 × 1011 to 1.6 × 1011 protons per bunch. With this increased bunch intensity, the peak luminosity in ATLAS and CMS reached more than 7.5 × 1033 cm–2 s–1, compared with the maximum peak luminosity of 3.6 × 1033 cm–2 s–1 in 2011.
In addition, successful commissioning of injection and RF-capture using new Super Proton Synchrotron optics (called Q20 optics) has opened the way for even higher bunch intensities. This new optics system has yet to be used operationally.
During the summer runs, the machine regularly enjoyed long fills in the 12- to 15-hour range. This showed the benefits of the extensive consolidation work to mitigate the effects of radiation to electronics in the LHC tunnel and the continuing efforts to improve overall reliability. The LHC is well on its way towards its goal of delivering in the order of 15 fb–1 in 2012. Indeed, at the beginning of September, CMS and ATLAS had already recorded more than 13 fb–1.
