Résumé

ALICE regarde vers le futur

Après plus de trois ans de fonctionnement très satisfaisant, le détecteur ALICE va être soumis à tout un programme de consolidation et d’amélioration à l’occasion du premier grand arrêt du complexe d’accélérateurs du CERN. Ce programme fait suite à une première période d’exploitation, très intense, où le LHC s’est illustré en battant continuellement de nouveaux records. L’arrêt permet d’avoir du temps pour traiter l’ensemble de données recueillies. Mais entre la poursuite de l’analyse, le programme de travail chargé dans la caverne de l’expérience et la planification de futures améliorations, les membres de la collaboration ont du pain sur la planche.

After more than three years of highly successful operation, the ALICE detector is about to undergo a major programme of consolidation and upgrade during the Long Shutdown 1 (LS1) of CERN’s accelerator complex. This follows an intense first running period characterized by the continuous record-breaking performance of the LHC. While the shutdown provides time to take stock of the wealth of data collected, the ongoing analysis, the busy programme of work in the experiment’s cavern at Point 2 and the planning for future upgrades will ensure that everyone in the collaboration is kept busy.

The ALICE detector is specially designed for heavy-ion collisions, which are foreseen as part of the LHC programme for four weeks a year. The LHC delivered an integrated lead–lead luminosity of 150 μb–1 during heavy-ion periods in 2010 and 2011, as well 30 nb–1 of proton–lead luminosity in 2013. Together with data collected during normal proton–proton running, as well as in a dedicated five-day proton–proton run in 2011 at the equivalent lead-nucleon energy, these three data sets have provided an excellent basis for an in-depth look at the physics of quark–gluon plasma. With the recent successful conclusion of the proton–lead programme in particular, where the LHC and injectors once again showed their amazing capabilities, the physics-analysis teams in ALICE are certainly not on standby but are more active and excited than ever.

Down the cavern

As soon as LHC beam operations ended on 14 February, the occupation of the car park at the ALICE experimental site began to rise sharply, indicating the start of the major shutdown activities. (Long-term observations have shown that there is good proportionality between the number of parked cars and activities in the cavern.) The first of these, as in any ALICE shutdown, concerns the removal of hundreds of tonnes of shielding blocks from the access shaft and the cavern. This is to allow the opening of the large doors of the solenoid magnet and give access to the ALICE detector. This sequence is now well established because even during the short winter stops of 2010/2011 and 2011/2012, the ALICE detector was opened for installation of the electromagnetic calorimeter (EMCAL) and modules of the transition radiation tracker (TRD).

So what are the major plans for ALICE during LS1? The main activity on the detector will be concerned with the installation of the dijet calorimeter (DCAL), an extension of the existing EMCAL system that adds 60° of azimuthal acceptance opposite the existing 120° of the EMCAL’s acceptance. This new subdetector will be installed on the bottom of the solenoid magnet, which currently houses three modules of the photon spectrometer (PHOS). An entirely new rail system and cradle will be installed to support the three PHOS modules and eight DCAL modules, which together weigh more than 100 tonnes.

The removal of the present structures and the installation of the new services, support structures and then the DCAL and PHOS modules will take up most of this year. The installation of five modules of the TRD will follow and so complete this complex detector system, which consists of 18 units. This work is complicated by the installation path being obstructed by major support-structures for services that will have to be temporarily supported by different structures.

In addition to these mainstream detector activities, all of the 18 ALICE subdetectors will undergo major improvements and consolidation efforts during LS1. The computers and discs of the online systems have reached their end of life and will also have to be replaced, followed by upgrades of the operating systems and online software.

A major part – indeed, most – of the shutdown cost and human resources will go into the consolidation and upgrade of the ALICE infrastructure. The four levels of ALICE counting rooms, which house the data-acquisition, high-level trigger, detector control-system and most of the detector read-out electronics, have an electrical infrastructure that was installed during the times of the Large-Electron–Positron collider and is now outdated. The renewal of this infrastructure and the installation of a new and significantly more powerful uninterruptible power-supply system form a key element in ensuring the correct operation of ALICE after LS1.

Major safety systems will also have to be installed during LS1. An area of racks under the large dipole magnet, which is inaccessible in the event of a fire, will be equipped with a CO2 extinguishing system and the entire volume inside the solenoid magnet will be equipped with a nitrogen extinguishing system.

The production of chilled water will also undergo a major upgrade as a result of increased demands on cooling and ventilation for ALICE and the LHC. The need for doubling the cooling air-flow inside the solenoid magnet to 10,000 m3/h requires the addition of a new ventilation machine and large ventilation ducts from the surface to the cavern.

The shutdown activities have all been formulated in work packages, analysed for safety aspects and scheduled in detail. In addition, the extraction of LHC magnets through the ALICE shaft, as well as a large number of visitor groups that will come to see the experiment, will pose a big challenge to day-to-day planning for LS1.

All of these efforts will ensure that ALICE is in good shape for the three-year LHC running period after LS1, when the collaboration looks forward to heavy-ion collisions at the top LHC energy of 5.5 TeV/nucleon at luminosities in excess of 1027 Hz/cm2.

However, the LS1 efforts go beyond the hardware activities that are currently under way. The ALICE collaboration has plans for a major upgrade during the next long shutdown, LS2, currently scheduled for 2018. Then the entire silicon tracker will be replaced by a monolithic-pixel tracker system; the time-projection chamber will be upgraded with gaseous electron-multiplier (GEM) detectors for continuous read-out; and all of the other subdetectors and the online systems will prepare for a 100-fold increase in the number of events written to tape. With only five years to go before this major upgrade, the ALICE collaboration is also busy on this front, preparing technical design reports for submission later this year.

With a fantastic set of data already in hand, well prepared activities for LS1 underway and the prospect of a major upgrade during LS2, the ALICE collaboration is in good health and is pursuing with unwavering enthusiasm its exploration of the mysteries of the QCD phase transitions, in a scientific programme that will extend well into the next decade.