The LHC has devoted the past month to luminosity production and the gradual increase of bunch intensities. Impressive results have been achieved; for example, the LHC was able to run, with stable beams for 26 hours with one fill; and deliver an integrated luminosity of 435 pb-1 in just one week. This is almost nine times higher than the entire harvest from the previous year.
It might be said that ALICE is not able to fully enjoy these rich conditions: as, unlike the other experiments, ALICE is not designed to work at such high intensities or in a high pile-up environment, with several interactions per bunch crossing. Given this, we have limited our target luminosity to the level of 2 Hz/µb - which is three orders of magnitude smaller than in ATLAS and CMS.
In the current filling scheme we only have 39 colliding bunches in ALICE; more than two-thousand bunches - from both beams - do not collide at Point 2, resulting in a high rate of beam-gas interactions. Of course, the background strongly depends on vacuum conditions in the LHC beam pipe around point 2. Usually the vacuum is spoiled during injections, but improves slowly with stable beams. For some fills, bad vacuum conditions result in very high beam-background interaction rates - of up to 500 kHz – which prevents us from using ALICE’s gaseous detectors.
Over the past few weeks we have often spent several hours at the beginning of each fill running only with the Inner Tracking System and the calorimeters, waiting for the interaction rate to calm down to 200 kHz – when we can bring up the gaseous detectors. This situation is being investigated at the moment and is probably related to the increased bunch intensities.
In ALICE, data taking was mostly dedicated to the rare triggers: such as single muon and dimuon triggers in the muon arm; and high-pt triggers in the Electromagnetic Calorimeter. We've also recently started collecting high-pt photon triggers with the Photon Spectrometer.
In addition, we have had a number of test runs devoted to preparing for heavy-ion data taking and the further development of ALICE’s trigger capabilities. For example: we have tested the performance of the Time Projection Chamber’s multi-event buffer, which is required for optimization of our data-taking rates during heavy-ion runs. We have also been working on the commissioning of level 1 triggers, which should, in the future, enrich our physics with rare probes.
Several pauses in the LHC programme gave us the chance to fix issues with the Time Projection Chamber, the Transition Radiation Detector and the Silicon Strip Detector. At the moment, ALICE is in good shape, but we are still working out some minor problems to improve our data-taking efficiency.