by Virginia Greco. Published: 12 April 2017

The interventions on the detector are almost complete and the massive doors of the L3 magnet have been closed. Now it is time for commissioning and technical runs.

Technicians putting back air cooling pipes after tclosing the ALICE L3 magnet. [Photo credits: Klaus Barth].

After completing the planned main interventions on the ALICE detector, at the end of March the huge doors of the L3 magnet were closed again. The closing of the doors, each one weighing 350 tons, is a critical task that takes some days. Once this was done, the transport crew and the ALICE technicians started putting back in place the big concrete blocks that form a shielding wall. These 7.5-metric-ton heavy blocks are moved in two steps: first they are lowered into the experimental cavern through the access shaft and then arranged in a wall in front of the detector. Even heavier blocks (around 40 tons each) will be used to seal the access shaft. The detector is still accessible, but most of the operations on the hardware have been concluded. The groups in charge of the different sub-detectors and systems are now testing the functionality of the whole instrumentation.

Crane lifting two 7.5-metric-ton heavy blocks of concrete to be lowered in the experimental cavern [Photo credits: Roberto Divià].

The Run Coordinator, Grazia Luparello, has already scheduled a long list of tests necessary to guarantee high performance and good data quality at the restart of the LHC. The first step of the re-commissioning consists in global technical runs during which the experts of each sub-detector, as well as of the central systems (Data Acquisition, Detector Control System, Data Quality Monitoring, Trigger and High Level Trigger) can test the recent developments in the data acquisition chain even though no data are really recorded.

Once this first stage is fully completed, cosmic ray data will be taken. This will allow studying the alignment of the detectors of both the central barrel and the muon spectrometer.

Towards the end of April - beginning of May the accelerator experts will switch on the LHC and start commissioning. One of the first tests consists in the set up of the transfer lines between the SPS and the LHC. During this operation proton beams at 450 GeV are dumped on the TED absorber, situated at the end of the injection line from the SPS to LHC.

The ALICE experiment is close enough to the injection point of the beam 1 to be able to detect the secondary particles produced in these interactions, mainly muons. These collisions, which are referred to as “splash events”, offer a good opportunity for timing alignment of trigger detectors.

Accelerator tests will continue for about 4-5 weeks. During this period few low intensity proton bunches will collide in non-stable conditions, the so-called “quiet beams”. ALICE will profit from these collisions for final commissioning of some sub-detector hardware and additional timing measurements.

Once the LHC experts have finished the commissioning phase, they will work to provide stable beams for the regular data taking. The number of bunches in the beams and their intensity will be gradually increased until reaching the nominal conditions. Fills with a reduced number of bunches will be used by ALICE to take data in special conditions - such as with lower magnetic field than the nominal one - which will extend the physics reach of some specific analysis.

If everything goes as expected, the first stable beam will be declared at the very beginning of June and the data-taking will start.