The high energy regime for electron positron colliders (TeV scale) requires the construction of a linear collider, since the energy losses in a ring system like LEP would be prohibitive. The result of the e+e- annihilation is a fireball of energy, which can materialise in the form of conventional or novel particle-antiparticle pairs, such as quark-antiquark or (if they exist) supersymmetric particles. Such a machine will also be a prolific 'factory' for Higgs bosons (again, if they exist). These possibly exotic states have several distinguishing characteristics. The tree of quarks form jets which are sufficiently clean that one can almost reconstruct the uderlying Feynman diagrams. This contrasts with hadron colliders like LHC, where the products of colliding two bags of quarks and gluons is much more messy. However, seeing the jets is not enough. To sort out the physics, one needs to label each jet, as far as possible, according to its leading quark flavour. This is in principle possible for beauty and charm quarks, and also for top, which usually decays via beauty.
The technology of 'flavour identification' for heavy quarks relies on high precision vertex detectors, which can measure the tracks of particles close to the interaction point (IP) with few-micron precision. By reconstructing the tree of vertices, primary, secondary and tertiary (due to charm decays from beauty parents) it is possible to identify the leading quark in most jets, and hence really get back to the underlying Feynman diagrams.
RAL has had a long history of developing these vertex detectors. Starting with charm in CERN in the mid-80s, the group built vertex detectors for the world's first linear collider, the SLAC Linear Collider (SLC) in California, which ran on the Z peak (0.1 TeV) through the '90s. The technology chosen was charge-coupled devices (CCDs) with 300 million tiny pixels (20x20 microns square), arranged in concentric cylinders round the IP. It is believed that this technology can be developed for the more challenging TeV regime, where a 5-layer detector of 1 gigapixel scale is suggested (see figure).
Such a detector requires a major R&D programme, which is the work of the LCFI collaboration (Linear Collider Flavour ID), consisting of 6 UK universities plus RAL. The work of our group comprises multiple aspects of detector development, as well as ongoing physics studies so as to evaluate the tradeoffs between different technical options, as the work progresses.
The goal is to determine whether the CCD technology can be developed to do the job required for the physics. In parallel, there are other collaborations round the world pushing on this and other options. Within the next 4 years, it should become clear which if any of these options can be made to work. By building on our previous track record, we are optimistic that the UK will have an important role to play in the construction of a vetex detector for the future linear collider, and that this instrument will prove a vital tool for extracting the exciting physics that is waiting to be discovered in the TeV energy regime.
Since we work with pixel detectors which are also sensitive to visible light and X-rays, the development of fast CCDs is not surprisingly of considerable inter-disciplinary interest (in astronomy, SR facilities for X-ray diffraction instruments, and many other). Students from our group have subsequently branched out into several such areas of science and technology.
For more information the Linear Collider Group homepage is:
http://hepwww.rl.ac.uk/lcfi/