We are about to enter a new era of discovery in particle physics. The Large Hadron Collider (LHC) at CERN will start physics operation in 2008 and is expected to make major discoveries that will have a profound impact on our understanding of the universe in which we live. Specifically, the Standard Model of particle physics has major omissions, such as the lack of a quantum theory of gravity, inconsistencies and missing elements, which the LHC is expected to explore. However, the history of particle physics shows that the major discoveries that have shaped our understanding of the physics of the basic constituents of the universe have been shared between hadron machines, similar to LHC, and electron-positron annihilation machines; specifically, the precision studies that have had such a profound effect in shaping and testing our current theories have been predominantly carried out at electron-positron machines such as the proposed International Linear Collider (ILC). The importance of such a machine was recognised in the European Strategy for Particle Physics promulgated last year by the CERN Council, in which it was characterised as “fundamental to complement the results of the LHC” and “a unique scientific opportunity”. The decision of ESFRI to incorporate the CERN Council Strategy into its Road Map thus underlines the importance of the ILC in the future landscape of European research infrastructures.
A challenging machine
The ILC is a 500 GeV centre-of-mass-energy linear electron-positron collider, so constructed to avoid the very large energy losses caused by synchrotron radiation in circular colliders of this energy. Although a linear collider is essential to reach this collision energy, it has drawbacks. In particular, the beams can only interact once and are then dumped, in contrast to the situation at a circular collider such as LHC, where the beams will interact many millions of times per second over periods of hours. This means that in order to achieve the required luminosity, a measure of the probability that electrons and positrons will annihilate, to deliver the physics goals of the ILC, the size and divergence of the beams must be reduced to an unprecedented level, orders of magnitude smaller than achieved in linear colliders to date.
On the road to industry
The ILC is based on superconducting radio-frequency accelerating structures, as pioneered by the TESLA Technology Collaboration predominantly based in Europe. This technology has been very successfully developed over the past few years within Europe, substantially through both the European XFEL proposal and the European Framework VI programmes CARE and EUROTeV. If the ILC is to be affordable, it is essential to maximise the accelerating gradient in the cavities, since it is directly proportional to the length of the collider required to achieve a specified energy. This improvement in gradient constitutes the main difference with respect to the cavities of the XFEL. Although many cavities with the appropriate gradient have now been produced in laboratories worldwide, the spread in the maximum gradient is still much too wide; and the cavities must be produced in industrial conditions. The achievement of this industrial reproducibility is the major technical goal of this project and is central to the production of the final engineering design of the ILC.
Finding a home
The ILC will probe the innermost structure of matter with unprecedented precision and will offer European scientists unique facilities to pursue their science. Only one linear collider of this energy range will be constructed in the world. Each of the main regions in particle physics has aspirations to play host to the machine. The organisation to allow site development and selection must be specified in the next few years; government involvement in the governance of the project will grow in the period leading up to the time of project submission, currently planned for 2012.