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Linear Collider

27 Sep 2016

Linear Collider Studies at UCL

UCL has been working since 1991 towards construction of the 500 GeV to 1 TeV International Linear Collider (ILC). The programme is accelerating, and we have funding from PPARC, CCLRC and EUROTeV to strengthen the physics case and to develop technologies for the detector and for beam-monitoring.

An e+e- linear collider in the energy range 0.5–1 TeV will be essential to make precise measurements in the region of electroweak symmetry-breaking which will be opened up by the LHC. The "International Linear Collider" (ILC) project was created in 2004, with the key decision to pursue superconducting technology for the radio-frequency cavities which will accelerate the beam. The beams will require challenging monitoring and diagnostic technologies, and UCL is leading efforts to develop a system of spectrometers which will determine the absolute energy and the energy profile of the beams — essential to make precision measurments of the top mass and any new particles which may be discovered. The group is also an important part of the laser-wire project, which will scan a laser beam across the electron beams, measuring their profile and allowing them to be brought into collision more effectively.

The other aspect of our involvement in the ILC is our R&D work with the CALICE collaboration on a calorimeter for the detector at such a collider. To make the required precision measurements, the calorimeter at the ILC must have an unprecedented energy resolution, particularly for high-energy jets of particles. This has lead to the concept of a "tracking calorimeter" where very high spatial resolution allows different types of particles to be separated from each other, and thus the information about their energy can be more accurately deduced. To test this concept, prototypes are being tried out in test beams right now. UCL produced essential parts of the readout electronics, and is participating in data-taking and analysis.

These R&D projects are backed up by physics studies which will develop further to constrain the overall detector designs for the collider. Current work is concentrated on two topics: i) measurement of the mass of the top quark, ii) prediction of the QCD background to all physics channels from virtual photon-photon collisions.

It is expected that the mass of the top quark can be measured to a precision of much better than 1 part in a thousand by doing an energy scan of the colliding beams around the top-antitop production threshold. This will be essential if the top mass is not to remain the weakest constraint on the parameters of the Standard Model (or of its successor, SUSY for instance). Such precision will only be possible by combining accurate determination of the beam energies, using the spectrometers, with precise measurements of the acollinearity distribution of elastically scattered electrons and positrons (Bhabha scattering). The acollinearity measurements need to be done in the endcap region of the detector where the spatial resolution of CALICE will be important.

Even in the comparitively clean environment of an electron-positron collider, there will be large backgrounds from QCD events through virtual photon-photon interactons. Their rate of production will be important in the ability to discover new physics, particulary SUSY channels with small mass differences between superpartners. Using current data, and the group's experience from LEP and HERA, Monte Carlo models are being tuned to describe these processes and extrapolate to get the expected rates at the ILC.