An Upgraded Luminosity Leveling Procedure for the ALICE Experiment

Abstract

The Large Hadron Collider (LHC) Interface project (LHC_IF) ensures that the A Large Ion Collider Experiment (ALICE) can operate congruently and safely with the LHC in different machine configurations and beam modes. Measurements in proton–proton (pp) collisions are vital for ALICE and serve as a reference to calibrate the Pb ion–Pb ion (Pb–Pb) measurements. However, during the pp operation to limit the event pile up (number of pp collisions per bunch crossing) and ensure a high-quality data sample, the instantaneous luminosity must be limited in ALICE to ${5\times 10^{30} {\text {cm}}^{-2}\,\mathrm {{s}}^{-1}}$ . This is achieved by applying a beam–beam separation in the separation plane up to several beam ${\sigma }$ (beam size unit), which is known as luminosity leveling. Given that the luminosity in a collider is expressed by a well-known formula in terms of parameters such as the beam separation, bunch intensities, number of colliding bunches, and beam size, then it is possible to determine the beam separation if the other parameters are known for both the target luminosity and the measured instantaneous luminosity. From the difference of the two beam separations, different step sizes spanning from 0.5 ${\sigma }$ down to 0.025 ${\sigma }$ are calculated and transmitted to LHC that accordingly steers the beams until the instantaneous luminosity smoothly reaches the target within ± 5%. In this note, the results achieved with the new beam-leveling procedure over almost a year of operation, as well as comparisons with simulations, are presented.