The ALICE lead-ion season is under way

Virginia Greco

On November 8 the ALICE experiment recorded the first lead-ion collisions of 2018. The beam intensity ramp-up was completed in the following few days and now the LHC is producing collisions at full speed. All of the ALICE experts are working hard to guarantee optimal performance of each system and subdetector.

Collision of Pb ions at LHC with energy of 5.02 TeV per nucleon pair recorded by ALICE on 08.11.2018. The coloured lines represent the reconstructed trajectories of charged particles produced from the collision. The orange towers represent energy measured by the electromagnetic calorimeters. The green trajectory in the forward region corresponds to a muon traversing the dedicated spectrometer. [Credit: ALICE/CERN]

The 2018 pp collision season was officially closed on October 24 to leave room for the heavy-ion run, which was scheduled to start on November 8 and to proceed for three-and-a-half weeks. ALICE completed its proton data taking reaching an integrated luminosity of 27.3 pb-1.

As soon as the last proton fill was dumped, the accelerator experts started a machine development period, which was used to perform beam studies and concluded on October 31. During the following three-day technical stop, activities took place in the caverns of the four LHC experiments in order to get ready for the new ion run.

In ALICE, many activities – including deeper checks and replacements of high voltage cables, optical fibres, ethernet switches, readout front-end cards, low voltage power supplies, electronics crates and hard disks in data acquisition system – were performed to prepare sub- and central systems. A new set of 7 IROC and OROC chambers for the TPC upgrade were installed in the cavern for testing, replacing those previously tested. In addition, the prototype of the Forward Calorimeter (FoCal) detector, under development for a possible future upgrade to perform during the third long shut down period (LS3), was taken out of the cavern since its test beam run was concluded successfully.

Having closed down all accesses to the underground areas, on November 3 the LHC started its engines again to inject lead ions into the accelerator complex. Switching from protons to heavy ions requires a number of operations necessary to “commission the beam”, in other words, the machine has to be reconfigured to accelerate and collide different particles. Once the settings were fixed, “non-stable” collisions were delivered, which were used to optimize the settings for the Diffractive Detector (AD) and the Zero Degree Calorimeter (ZDC). Collisions with stable beam were provided once the accelerator experts had completed all of the necessary checks and tests.

The 2018 lead ion season started officially on Tuesday, the 8thof November, and the first stable beam collisions in ALICE, at an energy of 5.02 TeV for each colliding pair of nucleons (protons and neutrons), were recorded at 9.19 pm (CET). To the great excitement of the crew, the experts, and of the run and trigger coordination, during the first two fills – which had 50 bunches per beam – our experiment performed very well.

In the following days, the beam intensity ramp-up was started, which means that an increasing number of bunches per beam were injected for each fill, up to the nominal value. The ramp-up was implemented in five steps: 64, 260, 484, 592 and, finally, 648 bunches per beam.

During the 260-bunch fill, the ALICE DAQ system showed some issues, which were taken care of by the team of experts. The configurations of each system were tuned within a few days, in order to adapt to the new running conditions.  The trigger rate for the TPC was also adjusted to prevent the TPC high voltage supply from tripping.

On the night of the 12thof November, a van der Meer (vdM) scan, which is very important to determine the luminosity of lead-lead collisions, was performed successfully for ALICE. 

Currently, the LHC is delivering collisions at full speed and the ALICE experiment is running smoothly. 

Event rate and data rate of the readout of the ALICE detector during the van der Meer scan.

On Thursday the 13thof November, a live broadcast from the ALICE Run Control Centre took place and could be followed on the CERN and ALICE Facebook pages. Hosted by scientific journalist Paola Catapano, the event was aimed at sharing with the interested public the excitement for the new lead ion run and the goal and expectations for this data taking period. Spokesperson Federico Antinori participated, together with Kristjan Gulbrandsen, ALICE Run Coordinator, and Ester Casula, who was Shift Leader at the time of the event.

If you missed it, you can watch its recording here.

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