The LHC is preparing for a major high-luminosity upgrade (HL-LHC) with the objective to increase the instantaneous luminosity to around 2 × 1035 cm–2 s–1 for proton–proton (pp) collisions and 6 × 1027 cm–2 s–1 for lead–lead (Pb–Pb) collisions. To fully exploit this new and unique accelerator performance, the ALICE experiment has engaged an ambitious upgrade programme that will allow the inspection of Pb–Pb collisions at an expected rate of 50 kHz while preserving and even enhancing its unique capabilities in particle identification and low transverse-momentum measurements. This will open a new era in the high-precision characterisation of the quark–gluon plasma (QGP), the state of matter at extreme temperatures.
Measurements of pp collisions serve as vital reference measurements to calibrate the Pb–Pb measurements. However, to limit the event “pile-up” during pp collisions (i.e. the number of pp collisions per bunch crossing) and to ensure a high-quality data set, the instantaneous luminosity must be limited in ALICE at a value of 1030 cm–2 s–1. This is achieved by applying a beam–beam separation in the horizontal plane of up to several σ (beam-size units): first, once the beams are ready for physics, a controlled and automatic luminosity ramp-up sets in to reach the target luminosity defined by ALICE. Next, fine-tuning is carried out during the fill – a procedure known as luminosity levelling, which requires algorithms running synchronously on the ALICE and LHC sides.
Following detailed simulations and several tests at the LHC, a new luminosity levelling algorithm has been in operation since June this year. The algorithm calculates the beam separation for both the target luminosity and the measured instantaneous luminosity, and uses the difference of the two separations to calculate step sizes. These are then transmitted to the LHC, which steers the beams until the target luminosity is reached within ±5%. When the beams approach the final separation in the horizontal plane, much smaller step sizes are applied to ensure a smooth and precise convergence of the luminosity to the target (see figure). This automatic procedure speeds up the collider operation and also prevents luminosity overshooting, which can occur during manual operations. Thanks to this new procedure, ALICE has increased its data-taking efficiency and can safely change the target luminosity even during fills with thousands of colliding bunches, a necessary step in anticipation of the high luminosities to be delivered by the LHC in the near future.