Since the first stable beams were injected and collided in the LHC, ALICE started to take data with special detector and trigger configurations. A few tests were also performed.
A proton-proton collision event at a centre-of-mass energy of 13 TeV, recorded by ALICE on 30 April 2018, one of the first with proton beams containing 1200 bunches. Particle tracks (multiple colours) and energy deposits (yellow and orange blocks) can be seen in the event. The large number of tracks is due to many simultaneous collisions taking place from the same crossing of two bunches of protons; the many vertices, corresponding to the various simultaneous collisions, can be seen in the right-hand-bottom image [Credit: ALICE/CERN].
On 17 April, the LHC operators declared stable beams for the first time in 2018 and ALICE, as the other experiments, started taking data. The conditions of running, though, were far from being the nominal ones, since there were only three proton bunches per beam and the luminosity was low. In the following days, the number of bunches was stepwise increased and on 28 April, 13 days ahead of schedule, 1200 bunches of protons per beam were successfully injected and collided. This was a crucial step in the intensity ramp up of the LHC towards the optimal running configuration – which foresees 2556 bunches per beam – because it formally marked the beginning of the 2018 physics season.
Over the last month, ALICE carried out a few tests, starting from the timing calibration of some of the subdetectors. The first days of stable beams were particularly suitable for this, since the few bunches injected (first 3 and then 12) were well separated in time.
When the number of bunches increased, some days were dedicated to stability studies of one of the subdetectors, the Time-Projection Chamber (TPC), which, during the end-of-year shutdown, had undergone some interventions to minimize the risk of current leakage in its field cage at high luminosity. In particular, it had been cleaned up of some dust deposited in the inside [see previous article – add link] and the gas mixture had been changed by replacing neon with argon and by adding water, because this composition appeared to reduce the risk of overcurrent.
The objective of these tests was to measure the maximum luminosity that the TPC can handle with the present configuration, in anticipation of the Pb-Pb run scheduled for the end of 2018 and of the future Run 3, which will take place after the second long shutdown (2019-2020). In ALICE, the proton beams are normally off-centred in order to reduce the luminosity with respect to that made available by the LHC, thus it is sufficient to slightly move and overlap more the beams to get a higher luminosity. The study was performed increasing the luminosity in following steps, starting from 15 Hz/ubarn – which corresponds to 10 kHz/ubarn in Pb-Pb collisions – up to 20 and 30 Hz/ubarn. The TPC ran stably up to 20 Hz/ubarn, over which some leakage current development was observed. This is a satisfactory result, which shows that the interventions produced positive effects. Nevertheless, the TPC team will plan further tests to push the detector towards its limits.
A voltage scan was also performed on the TPC to test it for spatial distortions, which could increase because of the use of argon. Charged particles passing through the chamber ionize the gas and this creates distortion in the electromagnetic field. Static distortions are not problematic because, once measured and mapped, can be compensated for when data are analyzed. On the contrary, dynamic distortions might create troubles. These can be mitigated, though, by applying alternating potential to the cover electrodes of the readout chamber, so it is important to find the best configuration. The data taken during this test are now being analyzed. The results will give indications on the most convenient running conditions to adopt.
Once these studies were concluded, a special physics run was put in place using a low magnetic field. The ALICE solenoid normally generates a field of 0.5 T, but the latter was lowered to 0.2 T to satisfy a request coming from one of the physics groups. 500 million minimum bias events were taken over 10 days with low B-field, which will be used for measuring the electron-positron pair production in the low invariant mass region.
Once the quantity of events requested for this purpose had been reached, and even overcome, the magnetic field was increased back to the nominal value. In the meanwhile, the LHC operators continued the intensity ramp-up and, on 7 May, reached the nominal running conditions with 2556 bunches per beam.
Currently, in ALICE data are being taken with a set of triggers and special setups, as requested by some subdetector groups: in particular, the Electromagnetic calorimeter (EMCal) and the Di-Jet Calorimeter (DCal).
A few more special setups will be used in the following weeks, before starting a long data taking period with a fixed trigger configuration, which will hopefully last for some months.