L'avenir se fera ensemble

Tandis que progresse la R&D sur les options de collisionneur e+e pour explorer la physique des particules á des ènergies de quelque 1 TeV ou plus, de rècentes rèunions tenues au CERN semblent indiquer qu'une seule communautè travaillera sur le collisionneur linèaire mondial pour les accèlèrateurs et pour les dètecteurs. Au CERN, le 12 juin, le Comitè exècutif du Projet mondial de conception du collisionneur linèaire international, le Comitè de pilotage de l'Étude du collisionneur linèaire compact et le Directoire du CERN ont tenu leur première rèunion conjointe. Rècemment, le CERN s'est ègalement joint aux travaux de dèveloppement de dètecteurs á l'èchelle mondiale dans le cadre du rècent Projet de dètecteurs pour collisionneur linèaire.

There are two major efforts underway to develop a linear electron–positron collider to complement CERN's LHC in the exploration of physics in the region of the "terascale" – energies of around 1 tera-electron-volt (TeV) and higher. The concept pursued by the International Linear Collider (ILC) Global Design Effort (GDE) is based on superconducting RF technology for collisions up to 1 TeV in energy (CERN Courier December 2005 p24). The Compact Linear Collider Study (CLIC) on the other hand, is developing a novel technological approach based on two-beam acceleration, which is potentially capable of achieving collisions at energies of multi-tera-electron-volts (CERN Courier September 2008 p15). Now these two efforts are coming closer together with the aim of combining resources on areas of common interest.

On 12 June the first joint meeting of the ILC GDE executive committee, the CLIC steering committee and the CERN directorate took place at CERN. The GDE's executive committee consists of the GDE's director, Barry Barish, together with three regional directors (for the Americas, Asia and Europe), three project managers and three accelerator experts, who include the chairman of the CLIC steering committee (Jean-Pierre Delahaye). The CLIC steering committee comprises accelerator, detector and particle-physics experts as well as the chairman of the CLIC/CTF3 collaboration board (Ken Peach) and the ILC representative (Brian Foster).

The meeting proved to be a successful start to bringing the ILC and CLIC efforts closer together, particularly in areas linked to the construction and implementation of a future collider (see box). Following the meeting, a statement of common CLIC/ILC intent is under discussion. The aim is to promote and develop scientific and technical preparations for a linear collider as well as exploit possible synergies that enable the design concepts for the ILC and CLIC to be prepared efficiently in the best interest of linear colliders and more generally of high-energy physics.

Higher energies

One area of common ground is the development of suitable detectors for the particular environment of a terascale e+e linear collider. CERN joined this worldwide detector--development effort through its newly established Linear Collider Detector (LCD) project, which targets physics and detectors at a future collider, be it ILC or CLIC.

Currently most of the effort at CERN is going into the preparation of the conceptual design report for CLIC, which will be delivered by the end of 2010. Earlier studies have shown that the layout of an experiment exploiting the physics potential of a 3 TeV CLIC machine is in many ways similar to an experiment designed for sub-tera-electron-volt energies. Therefore, the ILC detector concepts (named ILD and SiD) form an excellent starting point for the CLIC study. Adaptations concentrate on a few essential differences: the higher CLIC energy, the increased beam-induced background rates and the ultra-fast 0.5 ns bunch spacing.

Compared with the ILC, outgoing particles will generally have higher energies at CLIC and will often group closely together in highly boosted jets. To preserve a good performance level this normally calls for an increase in lever-arm of the tracking system and more depth for the calorimeters. In practice this increase in size can be limited by optimizing the choice of detector materials and granularity, thereby restricting the corresponding increase in the inner radius of the detector's solenoid coil.

The electron- and positron-beam bunches in CLIC are extremely small, with sizes of just 40 nm wide and 1 nm high. Their close encounter gives rise to strong electric fields, leading to the emission of numerous beamstrahlung photons, most of which will leave the detector through the outgoing beam pipe. Some subsequent secondary-beamstrahlung products will nonetheless enter the main detector volume. Because bunch crossings take place every 0.5 ns, the resulting background hits in the detector will overlay quickly while genuine high-energy e+e-physics interactions will take place at a much smaller rate. This means that to preserve the capability to recognize the physics signatures with good precision, the electronic-signal readout of most detectors at CLIC will require time stamping. Current studies indicate that a time--stamping resolution in the 20 ns range will be sufficient.

Low power consumption will be a "must" for all future linear-collider detectors because this allows for low-mass detectors and, therefore, excellent track and vertex precision. Turning the power of the detectors on and off at the pace of the incoming bunch-trains can potentially reduce the on-detector power dissipation by nearly two orders of magnitude. The corresponding power-pulsing rate will be 5 Hz for the ILC and 50 Hz for CLIC.

Given the similarity between the ILC and CLIC detectors, the new LCD-physics and detector project is an important cornerstone of the ILC–CLIC collaboration. It integrates fully into the worldwide detector and physics studies – and profits from the tremendous developments made for the ILC and its predecessors. It uses the ILC-experiment concepts and detector technologies as a basis and makes use of the same simulation tools. As of 2010, hardware R&D will start in a number of critical areas for a CLIC detector, such as very dense hadron calorimetry, time stamping of tracking and calorimeter signals, power pulsing of detector electronics and reinforced conductors for a large solenoid.

Motivated by the case of an e+e collider as the next machine to explore particle physics at the terascale, work towards a common linear-collider-physics community is underway. The LHC results will tell us whether this will be a sub-tera-electron-volt machine (ILC) or whether an energy reach to multi-tera-electron-volts is needed (CLIC).

• For further information about ILC, see www.linearcollider.org/. For details about CLIC, see http://clic-study.web.cern.ch; and for more about LCD, see http://lcd.web.cern.ch/LCD/.