Since the 1960s, most particle accelerators built accelerated particles along a closed, roughly circular path. This is a good design technique, as, firstly it allows to focus the particle and anti-particles using the same magnets; secondly the particles can be made to go round the accelerator several times to attain very high energies, and thirdly the beams can be made to cross several times, to obtain a maximal number of collisions.[1fgt accelerators]
But this design is limited by brehmsstrahlung radiation, the process by which a charged particle radiates a photon when its path is bent—which is nearly constant in a ring-shaped particle accelerator. The amount of emitted brehmsstrahlung is inversely proportional to the mass of the particles –so light particles such as electrons (e-), positrons (e+) and muons(μ) are much more affected. LEP(Large Electron-Positron collider), at 60GeV, attained a limit for electrons-positrons, after which the particles loose too much energy in each turn.
The only way to accelerate e+ and e- at significantly higher energy appears to be the construction of linear colliders, where particles are accelerated along a linear path.
There are several compelling arguments[1worldwide LC study] for building a linear collider with energy range ~100GeV to 1TeV in the next few years. This is the timescale which will see the construction of the LHC (Large Hadron Collider ) at CERN in the LEP tunnel, which will attain energies of ~14TeV. The main aim for both a linear collider and the LHC is to find the mechanism which gives mass to gauge bosons and fermions, conjectured in the standard model to be the Higgs boson.
At the energies in question, the LHC will suffer from large backgrounds in Higgs production processes, as a consequence of being a hadron collider. This experimental difficulty will make the measurements of Higgs quark couplings, and Higgs self-couplings difficult. Electrons and positrons, as point-like particles, give a much cleaner signal, enabling the direct measurement of Higgs quantum numbers, and of its finer characteristics and threshold (the only parameter not predicted by the standard model is its mass, hints of which have been detected at LEP between 144 and ~200 GeV.) It is expected that a Higgs boson could be produced with only 2-3 other products in a linear e+e- collider, as opposed to numbers of the order of 100 in LHC.
Fig a: Interesting cross sections at a 1TeV linear Collider [jlc homepage]
In effect, the role of an e+e- collider is complementary to that of a hadron collider, as LEP has proved, enabling for example the in-depth investigation of W and Z bosons; as such, the building of a linear collider in the same time as LHC would give a complete toolset for particle physics in the next decade.
Beyond the standard model, super symmetry (SUSY) theories predict the existence of supersymmetric sparticles, mirroring particles in supersymmetric space and time dimensions. Effects are predicted in the 100-1000GeV range, either directly from sparticles of in this mass range, or indirectly from heavier sparticles; linear collider measurements could give information to select good SUSY theories.
As part of its programme, and particularly relevant for this project, a linear collider would be used for top quark physics (precise measurement of all top quark parameters), and W and Z boson precision measurements.
This project is mainly intended as a study for TESLA the next generation linear accelerator planned in DESY, Hamburg. TESLA (Tera-electron volt Energy Superconducting Linear Accelerator), is one of the contenders for a next generation linear collider. The main others are NLC (Next Linear Collider, Stanford[nlc homepage]), JLC (Japan[jlc homepage]) and in the longer term, CLIC at CERN[clic homepage](at a higher 3-5 TeV energy range.)
Fig b:TESLA overall layout sketch, [1tesla tdr], II-7.
TESLA is planned to accelerate electrons and positrons to a center of mass energy of 500GeV at first. In a second phase, energies of 800GeV-1TeV will be attained. The setup is over 30km in length, and the acceleration is carried out using superconducting niobium RF cavities, cooled at 2K by liquid helium. These allow very high acceleration gradients ( more than 30MV/m), and allow the acceleration and conservation of very small bunches.
A major challenge of TESLA is to produce high-luminosity beams: this means achieving very small spot sizes by concentrating the particles into bunches of length of a few hundred μm, and of widths one thousand times less. The bunch populations would average 1010 particles, and the bunches have to be damped to reduce emittance in large damping rings, as in fig. b. opposite.
In addition to e+e- collisions, TESLA has the capability of colliding e-e-(which can be used to search for heavy Majorana neutrinos), as well as γe- and γγ photon collisions, which would test fundamental QCD predictions of the F2γ photon structure function.