With heavy ion runs starting in less than a week, scientists and technicians at ALICE are busy readying the machine for the demands of lead-lead collisions within the Large Hadron Collider. Matters talked with ALICE’s run co-ordinator, Chilo Garabatos, about the preparations needed in the switch from proton-proton runs.
The focus at present is on the tuning of the Time Projection Chamber (TPC), which is being tested out in a manner that simulates what is expected to occur in a heavy ion run. “This is in order to evaluate the performance and find out the settings to run this detector for the forthcoming ion experiment,” Garabatos said.
ALICE's Control Room, at Point 2.
The main issue with lead-lead collisions will be the increase in the numbers of particles generated compared with proton-proton runs – a rise from around 10 to approximately 10,000 per collision. This is being approximated by overlapping several proton-proton collisions in the
TPC at high luminosity.
This increase will cause more electron clusters to be produced in the TPC. While this detector normally uses these to identify the presence of particles, too many can upset its stability and resolution. “Of course there are a lot of parameters that drive the response of the detector to these many tracks and this is what we are trying to tune,” Garabatos says.
The tests, which are ongoing, will cover all stages of the data gathering, from the detectors themselves all the way to the storage tapes - fine tuning to ensure the huge volume of information can be correctly gathered and stored for analysis.
The head-on collision of lead ions will allow ALICE to begin the task for which it was designed: the study of the organisation of quarks and gluons in conditions similar to those that existed microseconds after the big bang – in a state we call a quark-gluon plasma. While we cannot directly observe this state, ALICE’s physicists will be able to infer its properties by studying the observable particles that will form and disperse as the quarks and gluons cool and recombine.
Work also continues on finding ways to improve the efficiency of the data-taking of the proton-proton runs, which will resume mid-February next year. “We are constantly trying to improve the experiment... to simplify operations, fix bugs here and there, and make the lives of shifters easier - like implementing automated actions instead of manual,” Garabatos explained. “We are developing new triggers for proton-proton running. There are two which we are testing now (photon triggers by our electromagnetic calorimeter), and another which will be tested early next year (electron trigger by the Transition Radiation Detector).”
For the upcoming heavy ion collisions, however, running conditions will remain fixed. “No big change will be deployed before the heavy ion run… We want to run with the tools and systems we have now because we know that they work,” he added. The time-saving automations being developed now will instead be saved for deployment in the new year.
Additionally, the number of shifters on duty, which was lowered to six in May this year, will return to 24 for the lead-lead collisions, to ensure the detectors are well monitored.
Garabatos is enthusiastic about the ion runs. “There is nothing that excites a heavy ion physicist more than knowing heavy ions are about to come!” he said.