by Virginia Greco and Grazia Luparello. Published: 10 November 2017

The latest news from point 2.

An event from the first Xe-Xe run at LHC at the energy 5.44 TeV per nucleon pair registered by ALICE. [Credit: ALICE]

During the last months, the excellent performance of the LHC allowed ALICE to take many data of pp collisions at 13 TeV with high efficiency. Physics fills of about 12 hours each have been following one another, with a time window between the dump of a beam and the subsequent stable beam of approximately 3 hours.

This quiet flow of operations was interrupted twice in order to perform some short special runs. On October 12, for the first time, xenon (Xe) nuclei were injected and collided in the LHC. A run with xenon nuclei was planned for the NA61/SHINE fixed-target experiment at the Super Proton Synchrotron (SPS), thus it was decided to take the opportunity for a short run with xenon at the LHC. Xenon has atomic number 54 (it has 54 protons in the atom), and the isotope used has 75 neutrons. Compared with lead, which has atomic number 82 and, in the case of the isotope accelerated at the LHC, 126 neutrons, the xenon nuclei are quite lighter. As a consequence, the two systems (Pb-Pb and Xe-Xe collisions) are different. Even though no surprises are expected, it will be interesting and useful to analyse these data.

In order to minimize the time needed for the accelerator setup, the run with xenon was performed using the same beam optics as for pp collisions. This choice was unfavourable for ALICE, though, since the beam in our experiment is not very collimated. To compensate partially for this disadvantage, ALICE was provided with double colliding bunches (16 instead of 8) at the point of interaction.

A first fill of xenon nuclei, which lasted 9 hours, was used by the accelerator experts for setup and commissioning. Once stable beams were declared, they were immediately dumped because of an interlock of the machine to be masked. A new fill was injected immediately after and quickly taken to the status of stable beams, allowing ALICE and the other experiments to collect data for 6 hours. Then, some more tests were performed by the LHC team before going back to the injection of protons.

ALICE ran smoothly with an efficiency of 98%, resulting in a respectful quantity of data, even though the luminosity was low because of the beam configuration mentioned above.

High multiplicity tests were also performed on September 4. The goal was to run at the rate of pp collisions that produces a quantity of charged particles compatible with what is expected to be produced in 50kHz Pb-Pb collisions in the future LHC Run 3 starting from 2021. This is necessary in order to understand how the sub-detectors work in conditions similar to those expected in Run 3. The detectors presenting instabilities in such conditions were identified and mitigation measures will be adopted.

The increase of luminosity – and hence multiplicity - was made possible by moving the two beams closer to each other (in ALICE, they are normally quite displaced. With proton beams, ALICE normally runs at 2.6 Hz/ubarn. To perform this stability test, the luminosity was increased in small steps from 2.6 to 70 Hz/ubarn.

Starting from November 11 another special run dedicated to ALICE will take place. Even though there will be no Pb-Pb collisions, ALICE has been granted the possibility to take data with pp at 5 TeV, which is the same energy at which data were collected in p-Pb an Pb-Pb collisions in the past. This data will allow comparing results in different systems at the same energy. ALICE’s goal is to collect one billion of these reference events in total: 120 million were collected in 2015, the rest is expected to be taken in this upcoming run. Considering the rate of data taking of ALICE, such a request translates into 160 hours of stable beams. As a consequence, the accelerator team has decided to dedicate 11 days to this run, of which 2 will be for setup and 9 for data taking, when very long fills (hopefully of about 30 hours each) will be delivered.