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Beyond the Large Hadron Collider

November 18, 2002

A briefing note based on a seminar and discussion held at the Institute of Physics on Thursday 3 October 2002.

This seminar is part of a series of evening seminars and discussions that highlight exciting and important new areas of research in physics and their applications. Topics at previous seminars have included Photonics, e-Science, Climate Change and Quantum Information   (accounts are available on the Institute¢s website at http://policy.iop.org). Seminars on Novel Fission Reactors, Spintronics and Nanotechnology are being considered for the near future. For further information, contact the Public Relations Department at the Institute: Dianne Stilwell, tel; 020 7470 4875, e-mail: dianne.stilwell@iop.org.

Introduction

The Large Hadron Collider (LHC) is a high-energy particle machine being built at CERN, the European Laboratory for Particle Physics in Geneva, by a global collaboration of scientists including many from the UK. It will offer a new level of understanding of the matter and forces in the Universe. Although the LHC will not start operation until 2007, there is currently intense international discussion about the next generation of particle accelerators to follow the LHC, and the science they should explore. The UK is also considering its scientific role and funding policy in these future particle physics experiments.

The seminar, chaired by Professor Brian Foster (University of Bristol and Chair of the European Committee for Future Accelerators), explored these issues, reviewing the underlying science and the case for the two main proposals. The first is the linear collider, of which TESLA (Tera Electron Volt Energy Superconducting Linear Accelerator) to be based at the DESY (Deutsche Elektronen-Synchrotron) Laboratory in Hamburg is a leading contender (described by Professor Robert Klanner, DESY Director of Research). The second is a neutrino factory (presented by Professor Ken Peach, Director of Particle Physics at the Rutherford Appleton Laboratory).

The successful Standard Model of Particle Physics

One of the outstanding achievements of the 20th century has been to develop a coherent description of the way nature works at the most basic level. Particle physicists have constructed a mathematical picture of the fundamental building blocks of the Universe called the Standard Model. It describes matter, and the forces through which matter interacts, in terms of a small number of elementary particles. The matter particles are divided into two types, quarks and leptons, of which there are six, each divided into three ?generations? of increasing mass. The first generation consists of everyday stable particles: the up and down quarks, which make up protons and neutrons of atomic nuclei; and the two leptons, the electron and the electron neutrino. The second generation consists of the charm and strange quarks, and the muon and the muon neutrino; while the third generation comprises the bottom and top quarks, and the tau and tau neutrino. These heavier generations of particles are generally not long-lived and are found either in particle accelerator experiments or in cosmic rays. Each particle also has an antimatter partner which has opposite properties like electrical charge.

There are also four types of interactions - the electromagnetic force, the weak force, the strong force and gravity. These are mediated by force carriers, or bosons: the electromagnetic force is carried by the photon; the weak force by the W and Z particles; and the strong force by the gluon. Gravity is not included in the Standard Model although theorists are working on concepts to unify all the forces.

There is also an additional particle now included in the Standard Model, which plays a crucial role. The masses of the elementary particles cover a huge range - from the neutrino, which has no mass according to the Standard Model, to the top quark, which has about the same mass as a gold atom. To explain the origin of this mass, Peter Higgs of Edinburgh University suggested a new type of interaction called the Higgs field which gives the other particles mass, and generates a new particle, the Higgs boson. We have not yet found the Higgs in particle experiments - and this is one of the reasons the LHC is being constructed.

A deeper understanding needed

Our current understanding of the Standard Model has been built on previous, fantastically successful collider experiments at CERN and elsewhere. Nevertheless, the Standard Model is mathematically incomplete. It describes the current experimental data with remarkable precision, but it does not explain its most obvious features: the pattern of masses and forces, the different generations of particles, and the relationship between the quarks and leptons. Theorists are working on ideas going beyond the Standard Model which unify all of the forces at high energies. (It is thought that the forces were unified at the unimaginable energies existing at the beginning of creation just after the Big Bang). So far, however, we have experimental evidence for only the unification of the electromagnetic and weak forces. The currently favoured idea in unification is ?supersymmetry?, in which each of the known particles has a heavier supersymmetric partner (the mathematics of particle physics is built on the concept of fundamental symmetries in nature). Another idea unifying the forces involves the existence of more dimensions in our Universe than the three of space and one of time we know about.

The role of the Large Hadron Collider

The LHC, when it starts in 2007, will shed light on these problems. It will look for the Higgs boson, and may well also find new physics such as evidence of supersymmetry or even ?hidden dimensions?. The machine is designed to collide protons at energies in the range of teraelectronvolts, TeV (million million electronvolts), but since the actual elementary particles colliding are quarks (protons are made up of three quarks) the useful energy available for creating new particles is much lower. Data from previous particle experiments indicate that the Higgs has a mass (which can be expressed as energy in electronvolts) of at least 120 gigaelectronvolts, GeV (thousand million electronvolts), so the LHC should find it.

Why we need the Linear Collider

The proton-proton collisions in the LHC offer only a coarse microscope, sweeping over a broad range of energies but with limited precision. Once new phenomena are observed, a totally different kind of collider is then needed to focus in on specific details such as the mass of any new particles discovered.

Studies have been going on over the past decade to consider how best to build on the new physics that will undoubtedly be uncovered at the LHC. The unanimous choice of the world¢s high-energy physics community is a TeV linear collider (LC) colliding beams of electrons and their antimatter partner, positrons. Electron and positron collisions give very clean results, unlike proton collisions. This is because they are elementary particles (they are not made up of smaller particles), whereas protons are not elementary. This is essential to home in on the conditions needed, for example, to measure the mass of the Higgs accurately, or identify the expected supersymmetric particles. The hope is that the operation of the LC will overlap with that of the LHC so that discoveries at the LHC can inform the running of the LC, and vice versa.

The candidates for the Linear Collider

The collider would be 25 to 30 kilometres long and run initially at 500 GeV, eventually going up to 800 GeV. This configuration presents major technological challenges because enormously high voltage gradients are needed to accelerate the charged particles - an electric field of at least 22.5 megavolts per metre to achieve 500 GeV, and 35 megavolts to reach 800 GeV! If you were to produce electrons of this energy using a cathode ray tube ? which is the kind of accelerator you have in a television ? you would need a voltage 20 million times greater than is found in the average television. Acceleration is attained using radiofrequency waves created inside special devices called radiofrequency (RF) cavities; these are one of the critical components for technological innovation. Another challenge is to steer the particle beams, only a few billionths of a metre across, so that they meet. This requires special focusing magnets.

At the moment, there are three candidates for the LC, but only one of them would go ahead:
* TESLA, being developed at DESY in Hamburg (it already hosts an international collider project, the Hadron Electron Ring Accelerator, HERA);
* the Next Linear Collider at the Stanford Linear Accelerator Center in California, SLAC (which has considerable experience in building linear accelerators);
* the JLC Electron-Positron Linear Collider project in Japan.

CERN also has a project, the Compact Linear Collider, CLIC, which is of more novel design and is at an earlier stage in development.

TESLA¢s advanced technology

The TESLA R&D is the most advanced, according to Professor Klanner. The Hamburg team has already had a test facility up and running for four years, the components being tested under realistic conditions. TESLA will use novel superconducting RF cavities that are expected to reach the required 800 GeV of energy. Site preparation is well advanced in terms of environmental impact studies and seeking local planning permissions.

In addition, TESLA will exploit the electron beam, in combination with a special magnetic device called an undulator. This will create an oscillating electromagnetic field that will generate a so-called free electron laser (FEL). This will emit X-rays a billion times brighter than are available today, offering a unique tool for structural research in physics, chemistry, molecular biology and materials science. Since the wavelength of the X-rays is comparable to interatomic distances, the FEL will be able to probe chemical reactions as they happen and take rapid snapshots of the structure of large biological molecules.

International collaboration

Professor Klanner affirms a project of the size of the linear collider can be realised only within an international collaboration. He anticipates that the host country would have to cover half the basic cost, with the international partners covering the rest. Germany has expressed interest in hosting a major project like TESLA, and the German Government is expected to make a decision before the end of 2003. Even more important is international collaboration in terms of the technology. ?No one country has the brain power or manpower to build the LC,? says Klanner. He hopes that national research groups would take responsibility for building and operating particular components as part of their research programme - they together would have a share of the linear collider.

With the appropriate agreements, construction could start in 2005, with first operation in 2012 so that the experimental programme overlaps with the LHC operation.

Why neutrinos are important

Another completely different particle physics experiment also under consideration is the so-called neutrino factory. It would generate intense beams of neutrinos, which is not easy to do! Why neutrinos? Although these ghostly, neutral particles hardly make their presence known in the everyday world, they are the second most abundant particle (after photons) in the Universe, and it¢s probable that they play a vital role in its evolution. Particle physicists and cosmologists are also particularly excited about neutrinos at the moment because of some recent dramatic observational results.

The Standard Model assumes that neutrinos do not have mass. However, data from a major international experiment, the Sudbury Neutrino Observatory (SNO) in Canada, which is detecting neutrinos from the Sun, indicate otherwise. Previous solar neutrino detectors had revealed that fewer solar neutrinos were reaching the Earth than expected from theories of how the Sun shines. Only electron neutrinos are emitted in solar nuclear reactions, and it was suggested that they were changing into something else on their way to Earth, perhaps ?oscillating? into muon and tau neutrinos. Previous experiments had only detected electron neutrinos, hence the flux deficit. SNO, however, detects all neutrinos through two types of interaction which distinguish between the electron neutrino flux and the total neutrino flux. The results so far do seem to indicate that neutrinos undergo oscillations between their different types. This is supported by Japanese studies with the Super-Kamiokande experiment detecting atmospheric muon neutrinos, which also show a deficit - probably because they change into tau neutrinos.

Neutrinos and the Universe

What does this mean? Well, neutrinos can oscillate only if they have mass, so theorists are having to consider how to extend the Standard Model to incorporate this idea. Cosmologically, even a small neutrino mass could account for a significant proportion of the ?dark matter? which is thought to comprise at least 30 per cent of the mass-energy in the Universe - as predicted by the gravitationally governed dynamics of galaxies and galaxy clusters. (The lightest supersymmetric particle could make up the rest of dark matter, which is another reason for building the LC).

Another reason that neutrinos are important is that they may explain why there is mainly only matter in the Universe and not antimatter. Both would have been created in equal amounts in the Big Bang. A possible answer is that some kinds of particle do not behave in exactly the same way as their antiparticle partners - as would be expected from mathematical arguments about symmetry. This is called CP-violation and may provide the mechanism for why the antimatter was destroyed in the early Universe. The discovery that neutrinos have mass opens up the possibility that they could show CP-violation - as happens with quarks but at a level 10 orders of magnitude too small to explain the observed asymmetry in the Universe.

The neutrino factory

A neutrino factory would be able to probe the details of neutrino oscillations, and CP-violation in neutrinos. A neutrino factory would be complex to build, consisting of a series of interlinked accelerators. The first would be a very high-power accelerator system called a proton driver, which would fire an intense beam of protons at a target to create particles called pions. The pions are then focused and captured before they decay into muons. These particles, which decay into electrons and neutrinos in two microseconds, are produced with a wide range of energies and angles, and need to be collected together - ?cooled? - into compact bunches, rather like ?muon snowballs?. They are then re-accelerated in a series of linear accelerators and stored in a triangular-shaped ring before they decay to produce the neutrino beams. The muon beam generated in this way would be 10,000 times more intense than previous beams. The muon storage ring would have a special design to ensure than it emitted a tight beam of neutrinos at a target thousands of kilometres away (neutrinos can travel a long way through the Earth without interacting with matter).

There are technical challenges - designing the proton source and the target, the muon capture, cooling, and acceleration and storage. While the neutrino factory could be built somewhere like CERN, the Rutherford Appleton Laboratory (RAL) in the UK is also well positioned to host it. The UK could design and build the proton driver based on world-leading technology already developed for the neutron spallation source called ISIS, at RAL. UK researchers have already carried out a number of design studies for the various components, and are proposing, with international collaborators, an experiment called the Muon Ionisation Cooling Experiment, MICE, to test ideas for cooling muons, perhaps to be hosted at RAL. Professor Peach said that the neutrino factory was about five years behind the LC in development and would probably follow on afterwards.

The UK perspective

Over the past two decades, UK particle physics experimental work has concentrated on building and operating detectors to take data at overseas facilities, rather than on developing accelerator technology for domestic particle physics facilities. In 1999, an initiative was started, supported by the Particle Physics and Astronomy Research Council (PPARC) and the Council for the Central Laboratory of the Research Councils, CCLRC (RAL and Daresbury), to revive accelerator R&D in the UK for particle physics. ?It has been extraordinarily successful in demonstrating that we have the relevant expertise in UK universities and at the national laboratory,? says Peach. Nevertheless, the level of support for particle physics research would have to be raised. Peach hopes that UK groups would be able to offer a ?bespoke service?, perhaps building subsystems such as damping rings for the linear collider. He applauds the recent PPARC/DTI approval for a Faraday Partnership in RF power. This approach will also benefit UK industry and stimulate new spin-out technology.

In the meantime, international teams are hammering out a global model for supporting and developing the next generation of particle experiments needed if we are to answer the fundamental scientific questions of how the Universe, and ourselves, came to exist.

Institute of Physics