With the choice of technology to build a high-energy linear collider, the world community of particle physicists moves a step ahead in its effort to answer some of the fascinating questions about the nature of the universe.
ON August 20, the International Committee for Future Accelerators (ICFA) took the first significant step in setting the stage for the world community of particle physicists to embark upon an international collaboration of unprecedented nature and scale towards realising the goal of understanding the fundamental nature of matter, energy, space and time. The ICFA announced its decision on the choice of technology to be adopted to build the future gigantic particle accelerator - the linear collider - which, physicists expect, will enable them to unravel the mysteries of nature at the sub-atomic level. Today, all data point to new physics lurking at energies beyond the reach of existing particle accelerators.
Accelerators impart energy to sub-atomic particles with magnetic fields and radio-frequency (RF) fields and accelerate them so that they travel at high speeds in evacuated chambers as collimated and focussed beams. They are essentially of two kinds. In one kind, they move in circular enclosures and the machines are variously called cyclotrons, synchrotrons or storage rings, depending on the energies achieved and the particles that are accelerated. In the other, particles are accelerated in linear structures called linear accelerators (LINACs).2
Two opposing LINACs make up a linear collider (L.C). The proposed L.C. will bring intense high-energy beams of electrons (e-) to collide headon with positrons (e+, the antimatter counterpart of electrons) travelling with equal energy in the opposite direction (see schematic diagram). The envisaged total energy that would be available for particle production through electron-positron annihilations is 500 billion or giga electron volt (GeV) in the first phase to over 1000 GeV or one tera electron volt (TeV) in the second phase. Acceleration up to these energies will be achieved in 30-40 km long LINACs housed in tunnels. (One TeV is about the energy of motion of a flying mosquito. What makes this energy so extraordinary is that, in an accelerator, it is squeezed into a space about a million million times smaller than a mosquito.) However, it would be too costly ($ 7-8 billion) for a single nation to undertake such a huge project, especially after the experience of the cancellation by the Clinton administration of the superconducting supercollider (SSC) project midway in1993. The L.C. has, therefore, been mooted as a global venture and will be called the International Linear Collider (ILC).
From two mature technologies, developed over the last nearly 12 years by two different high-energy physics groups - a Japanese-U.S. collaboration that promoted the "warm" technology and a European collaboration that promoted the "cold" technology - the ICFA made the difficult but necessary choice to enable the international effort to take off. It has picked the "cold" technology, in which the basic accelerating structure will use superconducting niobium cavities where the electric field due to L-band RF field of 1.3 gigahertz will give energy kicks to electron bunches. This technology has been perfected at the German accelerator laboratory DESY in Hamburg as a collaborative venture (called TESLA) of 55 institutes in 12 countries. On the other hand, the "warm" technology, which the committee has rejected, uses copper accelerating cavities at room temperature operating at a higher (X-band) RF of 11.4 GHz. This has been perfected at the Japanese accelerator laboratory KEK in Tsukuba in collaboration with the Stanford Linear Accelerator Centre (SLAC). An order of magnitude lower energy (50 GeV) electron-positron linear collider has been running at the SLAC since 1989.
The ICFA's announcement was an endorsement of the recommendation made by 12 `wise men', on the International Technology Recommendation Panel (ITRP), which was constituted in January 2004 under the chairmanship of Barry Barish of Caltech. "Both the `warm' X-band technology and the `cold' superconducting technology would work for a linear collider," said Barish. "Each offers its own advantages and each represents many years of R&D... At this stage it would be too costly and time consuming to develop both technologies toward construction. The decision was not an easy one... and we knew the selection would have significant consequences for the participating laboratories," Barish added. Indeed, a rational selection could not have been made unless both `cold' and `warm' technologies had been developed to become mature and viable alternatives.
"The decision is of great importance for DESY and its international partners since they developed this technology," a DESY press release said. In any case, DESY had decided to paticipate in the ILC even before the technology recommendation. "With this recommendation, it becomes clearer how DESY could make useful contribution to the ILC," remarked Albrecht Wagner, chairman of the DESY directorate. "Scientists at DESY are currently preparing the European X-ray free electron laser (XFEL), which will also be based on the TESLA technology," Wagner said. XFEL is intended to be used as a probe for testing various aspects of LINAC, including beam diagnostics.
"We are certainly disappointed that our technology was not selected," SLAC director Jonathan Dorfan, who is also the ICFA chair, said. "As the only laboratory to have built an L.C, we have the experience and expertise in most areas critical to the L.C. design which transfers naturally and powerfully onto the design based on cold cavities," he added. KEK's director Yoji Totsuka has welcomed the decision and said that KEK looks forward to participating in the truly global project. Incidentally, Hirotaka Sugawara, KEK's former director, was a member of the ITRP.
"A decade ago such a high-energy linear collider was just a dream - a vision for a revolutionary tool to answer some of the most fascinating and compelling questions about the nature of our universe," said Cornell University's Maury Tigner, the chair of the International Linear Collider Steering Committee (ILCSC) that appointed the ITRP. "It will not only investigate new frontiers of physics and technology but also in international science collaboration. The decision opens the way for the world particle physics community to unite behind one technology and concentrate our combined resources on the design of the collider," he added. The Barish panel, it should be pointed out, has only recommended the technology, and not the design, though TESLA has developed and tested a design for the collider. "We expect the final design to be developed by a team drawn from the combined `warm' and `cold' L.C. communities, taking full advantage of the experience and expertise of both," the ITRP panel has stated in its report to the ILSC.
The ITRP was charged with two important criteria. One, the ILC construction should begin by 2010 (for it to be up and running by 2015). Two, there should be sufficient temporal overlap between the Large Hadron Collider (LHC), which is being built at CERN - the European Centre for Particle Physics in Geneva - and the proposed ILC so that the physics derived from the two machines can complement each other. Colliders take a long time to plan and be executed. The LHC, which is expected to be ready by 2007, was being planned over 15 years.
In the LHC, circulating beams of protons will hit each other at 14 TeV energy. However, because protons are made up of more fundamental constituents called quarks and gluons, only about 2 TeV is available for each quark-quark interaction that will give rise to particle production. In the electron-positron annihilation interaction of L.C., on the other hand, the entire energy is available for particle generation. Also, L.C. interactions are much cleaner than LHC interactions as the latter will have the problem of large background. Other L.C. parameters that the ITRP considered in arriving at its decision included high beam luminosity (a measure of collision rate) of 1034 particles per cm2 per second and easy upgradability of the machine from 500 GeV to 1 TeV.
Besides the operating frequencies and temperatures, the two technologies differ in the following key aspects. A tunnel of about 40 km will be required for the `cold' technology as against a 30-km tunnel for the `warm' one. This arises basically because the warm technology, with its much higher RF frequency, can accommodate higher accelerating fields in its cavities and thus achieve a greater energy gain for a fixed length. Also the damping rings, which enable bunching of electrons before injection into the LINAC, and the positron source are simpler.
The `cold' technology, on the other hand, is a more efficient - nearly 100 per cent because of superconducting cavities - transmitter of power from the RF source. In the `warm' case, it is only about 30 per cent because of dissipation in the cavity walls. In order to reach the collision rates required, both the LINACs will have to be erected and aligned with extreme precision in the interaction region. For L-band or `cold' technology, it is around 0.5 mm whereas X-band requires a precision of few hundredths of a mm. Both technologies have wider impact beyond particle physics: `cold' technology has applications in accelerator-based research, while `warm' technology has applications in medicine and other areas.
According to the Executive Summary report of the ITRP, superconducting technology was chosen because of "attractive features" that follow from the lower RF and will facilitate future design. Having zeroed in on the `cold' technology, the report said: "A TeV scale electron-positron linear collider is an essential part of a grand adventure that will provide new insights into the structure of space, time, matter and energy. We believe that the technology for achieving this goal is now in hand, and the prospects for its success are extremely bright."
Since the mid-1990s, partly triggered by the SSC's demise, the world particle physics community has been veering around to the idea of a TeV scale electron-positron linear collider as an international endeavour. In 1995, under the ICFA's initiative, the first ILC Technical Review Committee Report (known as the Greg Loew Report) came out, which resulted in the idea gathering momentum worldwide. In 1996, the Asian Committee for Future Accelerators (ACFA), led by KEK, got into the act and issued an action plan for promoting an L.C. project. The original plan envisaged setting up an International Linear Collider Centre with the machine being built by 2001 in the Asia-Pacific region with KEK as the nodal laboratory. In 1997, the ACFA stated that Japan would host and fund such a centre. In 1999, the ICFA called for a worldwide R&D on TeV scale electron-positron collider, that would compliment the upcoming LHC. Consequent to planning exercises in Europe (involving the European ECFA), ACFA and the U.S. High Energy Physics Advisory Panel (HEPAP), a common conclusion emerged in 2002. The next major high-energy facility should be an electron-positron collider with an initial energy of 500 GeV, running in parallel with the LHC, and later upgradable to a TeV and more, to be established as a truly global project. In fact, both the ECFA and the HEPAP recommended this as top priority. Following this, the ICFA constituted the international steering committee and came out with the Second Loew report in 2003. The ACFA too came up with a `road map' in February 2003 spelling out its technology options based on work on `warm' technology carried out jointly by the KEK and the SLAC.
"To succeed in this vision requires a new paradigm and key to that paradigm is our need to come together with a common set of technical decisions as the basis of an L.C. design that truly has the collective ownership of the partners," ICFA stated. And one saw political consensus too emerging with the ministerial statement from Organisation of Economic Cooperation and Development (OECD) in January 2004 endorsing the plan for global collective initiative on L.C. The time seemed ripe for making the technology choice and the ITRP was constituted.
A sense of urgency is clearly evident in this global effort. The ITRP took only eight months - during which six meetings were held and DESY, SLAC and KEK were visited and over 3,000 pages of documents were studied - to arrive at the decision. However, there will be many other issues to be resolved before a construction decision can be made, including the choice of a site, which could become contentious, and the mechanism of international funding, but the ITRP's decision provides a firm basis to move forward.
A meeting to discuss the funding aspects too was held recently at the CERN on September 16 and 17. The international thermonuclear fusion project ITER could provide a model for ILC as well. While Germany's Ministry of Research and Education has made it clear that it is not keen on having the site in Germany, both the U.S. and Japan are keen that the ILC be located on their soils. Indeed, HEPAP in its long-range planning report has made a strong bid for its location in the U.S. And this also is perhaps why the U.S. has been supporting the location of the ITER fusion reactor in Japan - clearly Japan cannot afford both, having already spent $1.5 billion on its collider complex.
INDIA is one of the few developing countries that has, in recent years, made a reasonable mark in high-energy physics, particularly in international collaborative experiments. At the CERN, Fermilab and Brookhaven, U.S., India has made significant contributions in software as well as hardware, particularly particle detectors. In fact, the Department of Atomic Energy (DAE) has set up an accelerator technology development group, which has a major cooperation agreement with the CERN for collaborating on the LHC. The India-CERN cooperation agreement was first signed in 1991 with a view to participating in CERN's technical projects. This was followed by signing a protocol in 1996 for specific participation in the LHC under which nearly 20 projects have been assigned to Indian groups. The major ones among these are the setting up of two of the detectors in the LHC, namely, the Compact Muon Solenoid (CMS) and A Large Ion Collider Experiment (ALICE). Indian scientists have been contributing in a big way in LHC-related software as well. In recognition of its contribution to the CERN's activities, India was granted an observer status in the council of the CERN in December 2002.
With growing interest worldwide in the linear collider, Indian scientists have been carrying out studies under the Indian Linear Collider Working Group (ILCWG). The ILCWG is a multi-institutional forum that includes scientists from the DAE and non-DAE institutions as well as universities. It has been contributing to the deliberations of the ACFA as well as to the technical sub-committees of the ILCSC. In April 2003, the DAE organised an India-ACFA Meeting at the Centre for Advanced Technology (CAT), Indore. An India-U.S. Interaction Meeting on Linear Collider, organised by the Department of Science and Technology, was held in Delhi in November 2003. These culminated in India hosting the 6th ACFA Workshop on Physics and Detectors for Linear Collider in December 2003 at the Tata Institute of Fundamental Research (TIFR), Mumbai. Indeed, some Indian research contributions to L.C. physics have formed a significant component of the international effort in setting benchmarks for the collider, which have served as important inputs for the technology choice.
Indian participation in this global venture thus appears to be very much on the cards. In principle, the participation has been approved by both the DAE and the DST. In fact, a representative had been sent for the meeting on collider funding held at the CERN. "While Indian scientists (mainly phenomenologists) have been involved in LC-related activities right from inception, it is necessary to develop a strategy quickly for involvement as a nation in a formal, institutionalised manner," points out Rohini Godbole, a professor at the Indian Institute of Science (IISc), Bangalore.
Sometime ago, a section of scientists had felt that, since a good number of them would be engaged in the detector (ALICE and CMS) development for the LHC for the next few years, detector R&D for L.C. may not be feasible, unless young researchers are specifically identified and recruited for the purpose. "But with good part of the LHC work already over, the situation may be more conducive today than it was four years ago," says Godbole. However, according to V.S. Ramamurthy, Secretary, DST, the thrust of Indian participation for the present will be in the development of accelerator technology in view of the DAE's plans for Accelerator Driven Systems (ADS) related activities in the near future. Apparently the U.S. had shown interest in this area during the Indo-U.S. meeting.
Most important, since there already exists a successful model of cooperation with the CERN - which does not involve financial commitment but only hardware and software development - a similar arrangement could possibly be evolved for the ILC project as well. India has developed some expertise in fabricating superconducting cavities, albeit of low current carrying capacity for ion accelerators at the Nuclear Science Centre (NSC), Delhi, and TIFR. This could be an area where India can possibly contribute if R&D in the area is initiated and the expertise consolidated upon.
However, if India is to contribute meaningfully in the L.C. technology development, first it should have an independent management structure, with appropriate funding, and identify key leaders as part of it. Though this idea was mooted a year ago, things had not moved as quickly as one would have liked. Recent developments have, however, instilled some sense of urgency and the DAE is likely to put in place such a structure soon.