by Virginia Greco. Published: 11 June 2018

The 2018 LHC physics data-taking season is already in full swing and the accelerator is performing very well. During the past month, ALICE has alternated normal physics data-taking with some special runs and tests.

Since the official start in 2018 of the LHC physics data-taking season (marked by reaching 1200 bunches per beam), the accelerator has been performing well and remains ahead of schedule in delivered luminosity.

During the past month, ALICE has been alternating normal physics data-taking with some special runs and tests. First, the trigger mixture was modified to provide the Electromagnetic Calorimeter (EMCal) and the Di-Jet Calorimeter (DCal) groups with a good quantity of data necessary for their calibrations. This allowed for a precise energy calibration of each channel of the EMCal detector. 

A “quiet” period with the normal trigger configuration followed. This is the setting that will be used for most of the proton-proton collision data-taking, but of course the quota of available bandwidth devoted to each trigger can be varied during the year, according to the amount of the data acquired for each trigger type. 

On June 6, a Run 2 Pb-Pb equivalent-flux run was performed, which means that higher- multiplicity was allowed in the experiment. This is obtained by moving the beams closer together in order to get more collisions in the ALICE detector. Instead of levelling the multiplicity at the normal value of 2.6 Hz/μb, it was levelled at 11 and, later, at 15 Hz/μb. This exercise is required to test the detector in conditions similar to those in which it will run at the end of this year, when Pb-Pb collisions will be delivered by the LHC.

In general, an increase in luminosity always causes a higher number of issues, i.e. occasional problems that can reduce the efficiency of the experiment. But, while this is assumed as normal and manageable, possible major issues have to be foreseen and prevented. These tests are thus crucial to check the limits of all the subdetectors and related systems. It is also very important to perform them early in the data taking season, in order to give time to the various teams to analyse the data and to plan some intervention, if needed.

Shortly after this Run 2 Pb-Pb equivalent-flux run, a similar test was performed to simulate the Pb-Pb flux that ALICE will get after the Long Shutdown 2 (LS2), i.e. in Run 3. It is an especially delicate test, because it implies increasing the luminosity even more, all the way up to 70 Hz/ μb. Obviously, this study involved only the set of subdetectors that will actually exist after the LS2 and those that will be only partially upgraded – but, for example, the TPC and the Muon Spectrometer ran but did not take data at the highest luminosity, since either their readout and data acquisition systems or their chambers will be completely changed to cope with the increased rate of data in Run 3. If some issues become apparent as a result of these tests, a plan of interventions can be put together to fix them during the LS2.

The next two weeks will be a period of machine development, a technical stop, and special runs. First, the LHC experts will perform some tests and beam studies in order to improve the performance of the LHC. After the technical stop, a van der Meer scans will be carried out in the experiments, with the aim to calibrate precisely their luminosity measurements. 

Then, another special run will follow in which the beams will be un-squeezed (with a b* of 90 m) and, as a consequence, the protons will travel in a much more parallel fashion. These running conditions are meant to allow special physics studies to be carried out by CMS and TOTEM, as well as by ATLAS and ALFA.

Once these special runs are completed, normal operation will resume and ALICE will be able to run for some time with normal trigger conditions. New tests will be probably scheduled in the future, according with the results of those just performed in the latest days.