Résumé

CMS prépare le futur

Il n’y aura pas de répit au point 5, site du détecteur CMS, au cours du premier long arrêt du LHC. Pendant que l’on prépare l’accélérateur à fonctionner, à partir de 2015, à une énergie plus élevée, et avec une luminosité plus forte, la collaboration CMS aura elle aussi du pain sur la planche. Des équipes procéderont à des travaux de maintenance et de consolidation sur les sous-détecteurs de CMS, afin de leur permettre de faire face à la performance améliorée du collisionneur. Il y aura notamment des opérations importantes sur le système de refroidissement pour le trajectographe et sur les détecteurs de muons des bouchons.

Three years after resuming operation at a centre-of-mass energy of 7 TeV in 2010 and ramping up to 8 TeV last year, the LHC is now taking a break for its first long shutdown, LS1 (CERN Courier March 2013 p26). During the long period of highly successful running, the CMS collaboration took advantage of the accelerator’s superb performance to produce high-quality results in a variety of physics analyses, the most significant of which being the joint discovery with ATLAS of a new, Higgs-boson-like particle in July 2012 (CERN Courier September 2012 p49).

Now, as the LHC teams prepare the machine for running from 2015 onwards at a higher centre-of-mass energy (13–14 TeV) and with increasing luminosity, the collaboration will continue to be busy maintaining and consolidating the CMS subdetectors and making sure that they can handle the collider’s improved performance. For several systems, this will involve making provision for upgrades to be implemented later in the detector’s lifetime. Point 5, the home of the CMS detector and control room, will see a busy LS1.

Tracker climate control

Perhaps the biggest priority for CMS is to reduce the effects of radiation damage on the performance of the Tracker. The CMS tracking system forms the innermost subdetector and fits snugly round the LHC beampipe. It must withstand an onslaught of some 1010 particles a second and the aggressive field of mixed radiation that this produces. The only way to mitigate against the progressive effects of this irradiation is to operate the Tracker at a lower temperature than the present few degrees Celsius – perhaps as much as 30° C lower. It is crucial that the Tracker will run under these conditions over the next decade, during which a replacement will be designed and built. The issue here is two-fold: on the one hand, the Tracker coolant must run at a lower temperature; on the other, there can be no condensation on the cooling circuits and detectors, which will be much colder than before, and that is a matter of controlling humidity.

Because the Tracker will not be in an hermetically sealed environment, despite an intensive programme of improvement, the humidity inside it will have to be controlled by blowing in dry gas to force out all of the water vapour. In addition to the Tracker itself, the nearby coolant pipes – which will also be at low temperature because of the coolant – are not well insulated. The collaboration will have to make sure that the detector and nearby pipework are dry, to avoid condensation and the growth of ice, which can inflict major damage.

CMS will require substantially more dry gas (nitrogen during operation, air during maintenance) than previously (up to a few hundred normal cubic metres per hour are envisaged) making it no longer cost-effective to purchase liquefied nitrogen. The collaboration has therefore procured an on-site plant that extracts the water vapour and, optionally, the oxygen from air, outputting a dry atmosphere with (optionally) 95% nitrogen. This plant is a relatively large piece of equipment that requires integration, installation and commissioning. It will be deployed in a few months’ time, after the detector is opened up, to confirm that the improved sealing system works well enough to allow the Tracker to run at a much reduced temperature after LS1 and beyond. This is the number one priority for CMS for the shutdown.

During the normal year-end technical stop of 2016–2017, the collaboration will install the Phase 1 upgrade of the CMS pixel tracker, which is the closest physics detector to the collision point. This will feature an additional, fourth layer, among other improvements. To get the first layer as close to the collision point as possible, a smaller-diameter beampipe will be installed during LS1, with an outer diameter of 45 mm – compared with the current 59.6 mm. The additional pixel layer will improve the CMS experiment’s ability to tell where a track comes from, which vertex it comes from or if, indeed, it comes from a primary vertex at all. Running under conditions of high pile-up, resolving which tracks and clusters belong to which vertices is absolutely crucial for the physics analyses.

Although replacing the pixel tracker will require a shutdown of only three to four months, installing a new beampipe will take significantly longer – more than a year – so this has to take place during LS1. It is a delicate operation that requires the detector to be in its most open condition with the pixels removed. Once the new beampipe is in place, the collaboration will conduct a dry run by installing a "3D print" of the new pixel detector: a shell that represents the volume of the detector. This is to make sure that the operation can be performed rapidly with the real object, that it does not jam anywhere and that the adjustment systems all work.

More for muons

Another major element of the CMS plans for LS1 features work to improve the muon detectors. The original design for the endcap part of this system had four triggering and measurement stations for muons but the fourth layer was not considered essential for initial operation. However, to function effectively in the future, the fourth layer is now needed to provide more discriminatory power between interesting muons and fake signatures from mismeasurement or background. Hundreds of detector components have to be built and installed. The biggest assembly site is in Building 904 on CERN’s Prévessin site, where teams from CERN and around the world, including the US, China, Russia, Korea, Pakistan and Italy, are halfway through the detector-construction project. Meanwhile, preparations are well advanced for a consolidation of the barrel part of the muon system; some key on-board electronics will be moved from the underground experimental cavern to the neighbouring service cavern, thus taking advantage of the accessibility of this latter cavern for maintenance activities even during LHC operation.

Associated with the installation of the fourth endcap layer is the refurbishment of chambers in the first layer. The inner wires of these chambers were read out in groups in the initial version of CMS. This was fine for lower collision rates but in future the full granularity of this detector layer will be required. In addition, the electronics are not optimal for the expected higher collision rates, so the collaboration is going to replace all of the on-board electronics. The electronics from the first layer will be reused to provide electronics for the outer layer, where it is easier to cope with the collision rate. A special operational support centre has been built at Point 5 specifically for this refurbishment task and for other detector activities, including cold-storage of the pixel tracker while the new beampipe is fitted. Because some elements to be stored or modified may have been activated by radiation, the centre includes a controlled workshop area.

New shielding discs, 10 cm deep, are to be installed outside the new fourth muon stations on the endcap yoke on either end of the detector. Each shielding disc is made of 12 iron sector-casings filled with a special concrete. Following manufacture and preassembly tests in Pakistan, these discs, whose preparation has taken five years, with the design finished only two years ago, are now being re-assembled and filled at CERN. The first has just been finished. The concrete, developed for this specific application by CERN’s civil engineers, is almost 50% denser than normal concrete – it is made using haematite (or ferric oxide) instead of the usual sand – and it is loaded with boron to absorb low-energy neutrons that would otherwise give rise to unwanted hits in the detector. The overall density of neutrons flying round the cavern will be decreased by having these massive 14-m-diameter shielding discs installed.

The new 100-tonne shielding discs represent the first large mechanical elements of CMS to be constructed entirely underground in the experimental cavern, because the heavy-duty cranes – used to lower each of the existing elements of CMS in their entirety – are no longer installed at Point 5. (The CMS experiment was unique in being constructed in massive "slices" above ground, see CERN Courier October 2008 p45 and CERN Courier April 2007 p6.) Each disc will have to be taken apart into its 12 component sectors for lowering and then be rebuilt in a vertical position underground. The shielding discs will have an installed clearance to the new detector layer of around 10–20 mm, so it will be a delicate operation and the logical course of action is to install the discs before the detectors.

The magnet and other systems

The consolidation and upgrade programme aims to equip CMS for running well into the 2030s, and a key element of operating for another two decades will be the CMS magnet – a unique object that is impossible to envisage replacing. Changes are being made to ensure that the experiment is not vulnerable to a major breakdown of the supporting cryogenic system, which could prevent CMS from running for a long time, or to avoid unnecessary on–off cycles, which could prematurely age the magnet.

It is important to remember that the detector was designed for 10 years of operation, with a cycle of 7 months for operation and 5 months of shutdown, and a technical stop every three weeks. In practice, there has been three years of continuous operation with only short winter stops – not long enough to open the detector up for thorough servicing – and a technical stop every 6, 8 or 10 weeks. This is a radically different scenario from the one for which CMS was built. Although, the detector has performed well, there is a pressing need to consolidate it for the new regime. For the magnet consolidation, the obvious change is to install a duplicate compressor plant, to mitigate against the failure of the existing plant at Point 5, which has compressors that have run well beyond the recommended service intervals of 40,000 hours without maintenance.

The electrical system is going to be completely revised so that the two levels of the underground service cavern will be supplied through the UPS (uninterruptible power supply), system to give better protection against power glitches. There will also be cooling modifications, not only to make the magnet more robust but also to accommodate the new detectors of the fourth muon layer, the new operating conditions for the Tracker and the future pixel tracker. Many of these modifications have to be put in place during LS1 because there will not be adequate time to do so later.

All of the photo-transducers of the Hadron Calorimeter (HCAL) are to be replaced. Although the work will only be finished during subsequent shutdowns, it is important to begin now while the CMS teams have access to detector components that will not be accessible later. For example, it might not be possible to access the outer HCAL (HO) on the central yoke wheel during LS2, whereas this can be done in the current shutdown.

To be sure that all systems are running well, the collaboration will repeat the Cosmic Run At Four Tesla (CRAFT) (actually 3.8 T) exercise in late summer of 2014, after closing the yoke and testing the magnet (CERN Courier January/February 2009 p8.). Although there will be no collisions, the detector will record valuable calibration and commissioning data from cosmic rays. If there is a problem with the new cooling systems or with the humidity control of the Tracker, for example, this should be detected promptly and should give the teams just enough time to open up the detector, do whatever needs to be done to fix it and close it again, before the designated end of the shutdown.

The schedule for 2013 is planned in fine detail with a list of hundreds of tasks that are currently being translated into day-to-day planning schematics, and with work packages that have to be understood, approved, checked for co-activity, possible radiological factors and so forth. In addition, amid all this important technical work, the CMS collaboration will attempt to welcome around 20,000 visitors to the site at Point 5 over the course of the year. The coming two years might be described as a shutdown period for the LHC and its experiments but life at Point 5 will be as busy as it has ever been.