Résumé

Vers une luminosité plus élevée

Le LHC devrait rester l’accélérateur le plus puissant du monde pendant au moins deux décennies. Toutefois, au-delà de 2019-2021, il faudra faire tourner la machine plus de dix ans pour diviser par deux l’erreur statistique des mesures auprès du collisionneur, à moins d’augmenter sensiblement la luminosité nominale. Pour poursuivre le progrès scientifique et exploiter ses pleines capacités, il faudra considérablement améliorer la machine après 2020. Le but est d’accroître la luminosité d’un facteur 5 à 10 au-delà de la valeur nominale initiale et de fournir une luminosité intégrée de 3000 fb–1 en l’espace de 10 à 12 ans

The Large Hadron Collider (LHC) has been exploring the new high-energy frontier since 2009, attracting a global user-community of more than 7000 scientists. At the start of 2011, the long-term programme for the LHC had a minimum goal of an integrated luminosity (a measure of the number of recorded collisions) of at least 1 fb–1. Thanks to better-than-anticipated performance, the year ended with almost six times this amount delivered to each of the two general-purpose experiments, ATLAS and CMS.

The LHC is the pinnacle of 30 years of technological development. Set to remain the most powerful accelerator in the world for at least two decades, its full exploitation is the highest priority in the European Strategy for Particle Physics, adopted by the CERN Council and integrated into the European Strategy Forum on Research Infrastructures (ESFRI) Roadmap (CERN Courier September 2006 p37). However, beyond the run in 2019–2021, halving the statistical error in the measurements will require more than 10 years of running – unless the nominal luminosity is increased by a considerable amount. The LHC will need a major upgrade after 2020 to maintain scientific progress and exploit its full capacity. The aim is to increase its luminosity by a factor of 5–10 beyond the original design value and provide 3000 fb–1 in 10 to 12 years.

From a physics perspective, operating at a higher luminosity has three main purposes: to perform more accurate measurements on the new particles discovered at the LHC; to observe rare processes that occur at rates below the current sensitivity, whether predicted by the Standard Model or by the new physics scenarios unveiled by the LHC; and to extend exploration of the energy frontier, to increase the discovery reach with rare events in which most of the proton momentum is concentrated in a single quark or gluon.

Technological challenges

The LHC will also need technical consolidation and improvement. For example, radiation sensitivity of electronics may already be a limiting factor for the LHC in its current form. Transferring equipment such as power supplies from the tunnel to the surface requires a completely new scheme for "cold powering", with a superconducting link to carry some 150 kA over 300 m with a vertical step of 100 m – a great challenge for superconducting cables and cryogenics.

With such a highly complex and optimized machine, an upgrade must be studied carefully and will require about 10 years to implement (figure 1). This has given rise to the High-Luminosity LHC (HL-LHC) project, which relies on a number of key innovative technologies, representing exceptional technological challenges, such as cutting-edge 12 T superconducting magnets with large aperture, compact and ultraprecise superconducting cavities for beam rotation, new types of collimators and 300-m long, high-power superconducting links with almost zero energy dissipation.

The high-luminosity upgrade therefore represents a leap forward for key hardware components. The most technically challenging aspects of these cannot be done by CERN alone but will instead require strong collaboration involving external expertise. For this reason part of the HL-LHC project is grouped under the HiLumi LHC Design Study, which is supported in part by funding from the Seventh Framework programme (FP7) of the European Commission (EC).

Six work packages

HiLumi LHC comprises six work packages (WP), which are all overseen by the project management and technical co-ordination (WP1). Accelerator physics (WP2) is at the heart of the design study and it relates closely to the WPs that are organized around the main equipment on which the performance of the upgrade relies (figure 2). The first aim is to reduce β* (the beam focal length at the collision point), so the insertion-region magnets (WP3) that accomplish this function are the first set of hardware to consider. Crab cavities (WP4) will then make the decreased β* really effective by eliminating the reduction caused by geometrical factors; they will also provide levelling of the luminosity during the beam spill. Collimators (WP5) are necessary to protect the magnets from the 500 MJ stored energy in the beam – a technical stop to change a magnet would take 2–3 months. Superconducting links (WP6) will avoid radiation damage to electronics and ease installation and integration in what is a crowded zone of the tunnel. The remaining WPs of HL-LHC are not included in the FP7 Design Study as they refer to accelerator functions or processes that will be carried out within CERN (with the exception of the 11 T dipole project for collimation in the cold region of the dispersion suppressor, which is the subject of close collaboration with Fermilab).

The 20 participants within the HiLumi LHC Design Study include institutes from France, Germany, Italy, Spain, Switzerland and the UK, as well as organizations from outside the European Research Area, such as Russia, Japan and the US. As well as providing resources, participants are sharing expertise and responsibilities for the intellectual challenges.

The Japanese and US contributions constitute roughly one third of the manpower for the design study and are well anchored in existing partnerships formed during the construction of the LHC, namely the CERN-KEK collaboration and the US LHC Accelerator Research Program (LARP). Japan participates as a beneficiary without funding and the US laboratories are associates connected to the project via a memorandum of understanding. The participation of leading US and Japanese laboratories enables the implementation of the construction phase as a global project. The proposed governance model is tailored accordingly and could pave the way for the organization of other global research infrastructures.

The four-year HiLumi LHC Design Study was launched last November with a meeting attended by almost 160 participants, half of whom were from institutes beyond CERN (CERN Courier January/February 2012 p9). The meeting was held jointly with LARP because HL-LHC builds on both US and European activities. It included a meeting of the collaboration board, during which Michel Spiro, president of the CERN Council, presented the necessary steps for inclusion in the updated European Strategy for Particle Physics. CERN Council will discuss the updated strategy in March 2013 and plans to adopt it in a special session in Brussels in early summer 2013. Spiro’s presentation showed that with respect to the initially proposed timeline of HiLumi LHC, the Preliminary Design Report will now need to advance by one year to be ready by the end of 2012.

The FP7 HiLumi LHC Design Study thus combines and structures the efforts and R&D of a large community towards the ambitious HL-LHC objectives. It acts as a catalyst for ideas, helping to streamline plans and formalize collaborations. When evaluated by the EC, the design study proposal scored 15 out of 15 and was ranked top of its category, receiving funding of €4.9 million. "The appeal of the HiLumi LHC Design Study is that it goes beyond CERN and Europe to a worldwide collaboration," stated Christian Kurrer, EC project officer of HiLumi LHC at the meeting in November. "This will further strengthen scientific excellence in Europe."

• For more details about the High Luminosity upgrade and the HiLumi LHC Design Study, see http://cern.ch/HiLumiLHC.