The ALICE upgrade
According to the long-term LHC schedule, 2018 is going to be the last year of LHC's second run. After that it will shut down for two years, during which both the Collider and the experiments are due to be upgraded. A project of this scale, as you can imagine, takes a lot of preparation and proper planning. We met with Werner Riegler, ALICE technical coordinator, to tell us more about the future improvement of ALICE.
What are the approval stages of an LHC detector upgrade?
Werner Riegler: Typically the first document to prepare is a letter of intent, where we describe what we want to do, what is the physics case for it, the overall technical implications, first ideas of time scale and cost. This is to propose the upgrade project as a whole. After the endorsement of this document by the LHCC, we are asked to prepare technical design reports, which detail the technical implementation costs and the schedule of the specific subsystems. Formally, once all the technical design reports are approved, the project is ready to go forward.
In the case of ALICE we have five technical design reports in total: on the Inner Tracking System (ITS), the Time Projection Chamber (TPC), the Muon Forward Tracker (MFT), the Readout and Trigger System and on the Online-Offline system. The last technical design report was approved in October of this year, and this concludes the approval process of the ALICE upgrade.
What is the schedule of the ALICE upgrade?
Werner Riegler: ALICE will take data until the end of 2018. The first three months of 2019 are then used to open the detector and to move the TPC to the large clean room at Point 2. It will stay there for 40 weeks, almost one year, for the upgrade of the readout chambers and the electronics. After that, towards the beginning of 2020, the TPC moves back into the cavern and the ITS, the MFT and the trigger detectors will be installed. In parallel to this activity, the electronics upgrade of all other subsystems and the implementation of the large O2 online farm are proceeding. Detector commissioning will start in late 2020 to be ready for beam in spring of 2021.
How much is the upgrade going to cost?
Werner Riegler: The cost of the original ALICE construction was in the order of 160 million Swiss francs. The cost volume of the upgrade is going to be around 55 million Swiss francs so it represents about one third of the original detector investment. So far we have the commitment from the major funding agencies for this upgrade.
What changes are going to be made to the detector?
Werner Riegler: The present ALICE detector can be read out at a rate of approximately 500 Hz. The major change for the upgrade will consist in an increase of this readout rate to 50 kHz in order to accept the full PbPb luminosity after LS2, so the readout capabilities of the ALICE subsystems have to be increased by a factor 100. This vast amount of data will be processed by the online-offline computing farm and all events will be written to permanent storage in a compressed form.
The Inner Tracking System will be completely replaced by a much more precise silicon tracker based on monolithic silicon sensors. These very thin sensors together with ultra-light support structures are boosting the tracking performance by significant factors.
This detector will be complemented by the Muon Forward Tracker that is based on the same technology and that will significantly increase the performance of the Muon System.
The other major component of the upgrade concerns the Time Projection Chamber (TPC). The field cage of the TPC will stay, but the readout chambers and readout electronics have to be exchanged. The new readout chambers will make use of the so called GEM technology.
That's the approximate scope of the ALICE upgrade, and with the finalization of the technical design reports we have defined the implementation in detail. We have 3 years to implement this ambitious programme, in parallel with the operation of the present ALICE detector and the scientific exploitation of the data we have taken. This huge programme clearly puts significant strain on the ALICE collaboration, but due to the excellence of the ALICE programme we are all highly motivated to bring this upgrade project to a successful conclusion.