Atelier sur le LHC haute luminosité

Avec la récente mise à jour de la stratégie européenne pour la physique des particules, le projet LHC haute luminosité (HL-LHC) prend un nouvel essor. C’est ainsi qu’est née l’idée d’un atelier sur les expériences auprès du HL-LHC organisé par le Comité européen sur les futurs accélérateurs. Il s’agissait de fournir aux collaborations des expériences un lieu d’échange, où elles pourraient mettre en commun les résultats et explorer les synergies, et également renforcer les liens avec la machine et les communautés de la théorie. Lors de trois journées intensives, du 1er au 3 octobre, au Centre des congrès d’Aix-les-Bains, des membres des quatre grandes collaborations LHC se sont réunis pour préparer l’avenir à haute luminosité.

Following the presentations at the Open Symposium in Cracow in September 2012 and a great deal of work by the European Strategy Group for Particle Physics, the update to the 2006 European Strategy for Particle Physics was published in 2013 and adopted at a special European Strategy Session of CERN Council in Brussels on 30 May (CERN Courier July/August 2013 p9). In developing its vision for the future, the updated strategy took full account of the massively important discovery of a Higgs boson at the LHC in 2012 and of the global research landscape. For the programme at CERN, it contains the clear message: "Europe’s top priority should be the exploitation of the full potential of the LHC, including the high-luminosity upgrade of the machine and detectors with a view to collecting ten times more data than in the initial design, by around 2030. This upgrade programme will also provide further exciting opportunities for the study of flavour physics and the quark–gluon plasma."

The priority given to the high-luminosity upgrade, dubbed the High-Luminosity LHC (HL-LHC), underlines the importance of the ongoing machine and detector developments for this facility, including supporting studies on performance and physics reach. Indeed, there has been highly active R&D in the required accelerator and detector technologies, following the recommendations of the 2006 strategy document. Much of this work has been conducted within the four large LHC experimental collaborations or – for the accelerator complex – within the framework of the EU-funded HiLumi LHC Design Study (CERN Courier March 2012 p19).

A three-day forum

With the recent update of the European Strategy, the HL-LHC project is expanding rapidly and the idea of an HL-LHC Experiments Workshop sponsored by the European Committee for Future Accelerators (ECFA) was conceived to offer a forum for the experimental collaborations to share results, explore synergies and to strengthen links with the machine and theoretical communities. After a concerted effort, colleagues in theory, the four big LHC collaborations and the accelerator community – co-ordinated through eight preparatory groups – organized three intensive days of workshop at the Centre des Congrès, Aix-les-Bains, on 1–3 October.

After an opening on behalf of ECFA by its chair Manfred Krammer, CERN’s Frédérick Bordry presented the latest plans for the accelerator upgrade. The ALICE, ATLAS, CMS and LHCb collaborations then gave overviews of their strategy to follow the planned increase in machine luminosity. This will proceed with staged upgrades, installed across a decade during end-of-year technical stops and two long access periods (long shutdowns) required for the major modifications. Many of the detailed plans are already documented in reports to CERN’s LHC Committee (LHCC) and more are in advanced stages of preparation. To round off the first morning, CERN’s director-general, Rolf Heuer, gave the laboratory’s perspective on the HL-LHC programme, underlining planning for the next 20 years at the LHC and the thinking on future directions, taking CERN forward to its centenary celebrations in 2054.

The experimental collaborations presented many updates to the studies on physics’ prospects that were documented at the Cracow Open Symposium and at the "Snowmass" meeting in Minneapolis in summer 2013, based on a better understanding of the expected experimental performance. This was complemented with a broad theoretical survey of the rich physics programme at the energy frontier offered by the HL-LHC facility. The extremely high number of collisions to be recorded in a year at the HL-LHC provides the opportunity to look for rare processes, study systems with high mass and make high-precision measurements.

The HL-LHC is designed to deliver in every year of operation 10 times the number of collisions collected at the LHC to date, yielding 10 times more data by the end of HL-LHC operation than the LHC is expected to have delivered by around 2022. This gives unprecedented sensitivity in measurements of a range of properties of the newly found Higgs boson, as well as in searches for new high-mass particles, and allows precision studies of a variety of fundamental particles and processes. In addition, should the 13–14 TeV running this decade lead to further discoveries of new particles, the HL-LHC will be essential to measure their properties.

Discussion then focused on areas where the machine and experiment teams need to work most closely: beam parameters, instrumentation and interfaces, shutdown planning and radiation protection. There were presentations of exciting new ideas that might allow the inherent problem of high-luminosity operation – the huge number of interactions every bunch crossing – to be mitigated by extending the interaction region along the beam direction.

This "pile-up" of interactions, the high data rates and the level of integrated radiation doses, will be the major experimental challenges for operation in the HL-LHC’s beam conditions. For the workshop, the areas of detector-upgrade preparations were split into those relating to tracking, calorimetry, muon systems, read-out electronics and triggering, data acquisition, offline software and computing. Each topic was covered in a dedicated session, where joint presentations across the four big experiments addressed the motivation, requirements and conceptual designs for upgrades, as well as the ongoing R&D programmes to provide efficient and cost-effective technical solutions.

For HL-LHC operation, major activities in ATLAS and CMS are related to the replacement of the tracker, owing to the high number of tracks per bunch crossing, the read-out bandwidth limitations and the integrated radiation levels that go far beyond the capabilities of available technologies at the time of their original construction. The much higher data rates also motivate a number of upgrades to other parts of the experiments, especially to their read-out electronics. In particular, the complexity of the collision events will complicate greatly the ability of the vital on-detector data-reduction (triggering) to retain only those events that are interesting to physics. Many improvements are aimed at refining this online selection. The detector, electronics, trigger and data-acquisition upgrades in ATLAS and CMS have been designed to optimize the physics acceptance, especially for the key decay channels of the Higgs boson, including those rare decays that can be reached only at the HL-LHC.

The rich programmes in flavour and heavy-ion physics were discussed from the perspective of all four experiments, but the focus for upgrades was on the dedicated experiments, LHCb and ALICE, which are designed to optimize their sensitivity to these areas of physics. Detector upgrades will extend that sensitivity and allow a greatly increased number of collisions to be recorded, improving the statistical precision for measurements and studies of rare processes significantly. These upgrades do not rely on implementing the HL-LHC machine upgrades and so can be undertaken earlier to bring these improvements sooner.

There were a number of closing presentations emphasizing the key themes from the workshop, which were formulated in a short report to ECFA at its meeting on 21–22 November. This report reflects the interest of those organizing the sessions in seeing more specialist follow-up meetings and a similar plenary meeting, possibly in autumn 2014.

The organizers would like to thank all those who contributed to the work of the preparatory groups, the speakers and chairs, the conference support from CERN and particularly the ATLAS and CMS secretariats. The success of the event was a great testament to the enthusiasm of the 326 registered participants and the many more researchers worldwide working on R&D towards this major further step in the LHC’s unique adventure at the high-energy frontier.