Cambridge Linear Collider Group - Home Page

Maurice Goodrick, Bart Hommels, John Marshall, Mark Thomson, David Ward

Linear Collider Projects

Whatever the findings of the LHC, there is generally held belief that a future electron-positron collider (or possibly a muon collider) will be important in order fully to understand the findings of the LHC. Although the LHC is very likely to discover new physics, a lepton collider will permit more precise measurements which will be required in order to properly understand the new physics.

The International Linear Collider (ILC) is intended to be an electron-positron linear collider producing collisions in the energy range 0.5-1 TeV - the same energy regime as addressed by LHC. The ILC is widely acknowledged to be one of the highest priorities for the future of Particle Physics. Several fully developed proposals for such a machine, were put forward in recent years (e.g. TESLA at DESY). All regions of the world have now combined behind a common technology choice,the ILC, based on the superconducting r.f. accelerating cavities of TESLA, and are carrying out a full technical design.

An alternative accelerating technology is offered by the CLIC project at CERN. In the longer term CLIC offers the prospect of a higher energy electron-positron collider reaching energies of 3 TeV or higher. Our research addresses this possibility as well.

Physics at a linear collider

Either the ILC or CLIC will complement the LHC program very effectively. Whatever new discoveries may be made at LHC (the Higgs boson, supersymmetry etc), an electron-positron collider will be able to study the new physics with much smaller backgrounds and hence much greater precision. This will be crucial in establishing any theory beyond the present standard model.

Many of the physics processes to be studied at such a machine require the measurement of the momenta of quarks, which will be detected as jets. Typically, interesting events will contain multiple (4 or 6) jets. The accurate measurement of the energy of a jet requires combining the measurements of different types of particles in different parts of the detector (tracking detectors and calorimeters). Experience from LEP tells us that good spatial resolution in the calorimetry is a crucial ingredient in combining all the information optimally. It is clear that an integrated approach to the detector design will be needed to achieve the best performance.

This approach to jet reconstruction is referred to as the "particle flow paradigm". Currently the most widely used and successful particle flow algorithm is PandoraPFA which was developed in Cambridge. This has shown that the design goals for the ILC (for example to distinguish can between the hadronic decay modes of the Z and W bosons) can certainly be achieved. Work is now ongoing to study the applicability of similar ideas at the higher energies of CLIC, and first results are quite encouraging.

CALICE - Calorimetry for an ILC detector

The Cambridge group joined the CALICE collaboration in 2002. CALICE is an umbrella for several R&D projects investigating high resolution calorimetry for a linear collider. The activities are focussed on a series of beam tests being performed in 2006-9 in which a series of prototype calorimeter modules were exposed to a variety of hadron and electron beams. The prototypes are all directed towards the construction of calorimeters with high granularity, dictated by the requirements of particle flow.

The first prototype electromagnetic calorimeter was a silicon-tungsten sampling calorimeter, with readout through 1x1cm² silicon pads. The 18x18x18cm³ prototype has approximately 10⁴ readout channels. See this paper for details. In parallel, a calorimeter is being tested in which small scintillator strips, of dimensions 1x4 cm, are used instead of the silicon.

The first hadronic calorimeter prototype was of the iron-scintillator sampling type, with dimensions 1x1x1m³ and sampling using 3x3cm² tiles in the shower core. Later, the scintillator will be replaced by RPCs and GEMs with digital readout in pads of size 1x1cm². The electromagnetic and hadronic calorimeters were tested together in CERN in 2006-7, and in Fermilab from 2008.

As well as yielding information about various detector technologies, the beam tests will play a crucial role in validating the Monte Carlo simulation programs which will be used to optimise the design of a full detector. In particular, simulations of hadronic shower processes in calorimeters are notoriously problematic, and good data are essential before credible simulation results can be delivered.

Cambridge and a Linear Collider

• The Cambridge group has played a leading role in the CALICE data analysis and simulation effort in the UK. This involved technical studies, comparing the simulation of key physics processes in various Monte Carlo packages, which then led into comparisons with test beam data. The latest version of GEANT4 was found to give satisfactory agreement with the electron beam data, while older versions were not acceptable. This work is still being coordinated by a Cambridge physicist.
• We also provided monitoring software for use in the CALICE test beam runs.
• We are carrying out studies of algorithms for jet reconstruction, and their implications for detector design. The problem of pattern recognition in highly granular calorimeters is a challenging one, and the quality of jet reconstruction largely relies on being able to separate the energy depositions from different particles and associate them with charged particles as appropriate. The world's most performant algorithm Pandora-PFA has been developed at Cambridge for this purpose. A current activity is to apply these ideas to higher energy jets in CLIC and other contexts.
• Global detector design studies are being carried out worldwide, in the context of three conceptual detector designs for the ILC. Cambridge is strongly involved with one of these: ILD , and also participates in the detector studies for CLIC.
• We have carried out longer term electronics R&D for granular calorimeters. The physics requirements demand that the calorimeter be very compact. Signals from tens of millions of sensors have to be read out in minimal space, with minimal power dissipation. This forms the basis for a challenging R&D programme. The immediate outcome of this work has been the provision of detector interface cards (DIFs) for the second generation CALICE Si-W ECAL, under the aegis of the EUDET project, and the development of techniques for the interconnection of modules into long detector slabs.