Komitee für ElementarTeilchenphysik KET Particle Physics in Germany Status and Perspectives Particle physics explores the innermost structures of matter, space and time and investigates the fundamental forces in the universe. In recent decades, it has provided the basis of our picture of the physical world. Nevertheless, some essential questions still remain unanswered. The present publication offers a graphic insight into the current state of elementary particle physics, highlighting questions that remain open and grand objectives. In turn, it makes a number of recommendations — embedded within a long-term strategy — regarding the future development of particle physics in Germany. This study is directed at people from the worlds of science and politics, at young up-and-coming scientists from all disciplines, at journalists, and generally at anyone with an interest in particle physics from whatever sector of society. With this publication, the Committee for Elementary Particle Physics KET would like to convey some of the fascination of fundamental research as well as continue the dia- logue between the scientific community and the public at large that latterly received new momentum with the initiative “2000 — The Year of Physics.” The Committee for Elementary Particle Physics KET November 2002 STRATEGY AND RECOMMENDATIONS FOR THE DEVELOPMENT OF PARTICLE PHYSICS IN GERMANY Particle physics explores the innermost structure of mat- SLAC and KEK-B at the KEK research centre in Tsukuba, ter, space and time and the fundamental forces of the uni- Japan: both are electron-positron accelerators that run at a verse. Initially an outgrowth of atomic and nuclear physics, lower energy suited to their specific physics goals. Ground- it developed into a research field on its own through the breaking theoretical insights, analyses and precision construction of large particle accelerators in the second calculations have played an equally important role in the half of the 20th century. Particle physics has a long and successful development of particle physics. The results of successful tradition in Germany and, based on current the close collaboration between experiment and theory technological and scientific developments, offers outstan- are manifested in the Standard Model of particle physics, a ding prospects for the future. 20th-century achievement the significance of which ranks Experiments using high-energy particle beams have alongside that of Maxwell’s theory of electrodynamics and uncovered the intriguing facts that, in addition to the Einstein’s theory of relativity. electrons in the atomic shell, other electron-related leptons Driven by the extreme technological demands of high- exist, and that the protons and neutrons, which form the energy particle accelerators and highly sensitive particle atomic nucleus, are made up of smaller objects, the quarks. detectors, experimental particle physics quickly organised According to our understanding, leptons and quarks are and coordinated itself at an international level in order to the elementary building blocks of matter as we know it. An make effective and targeted use of the resources available. equally astounding breakthrough occurred in the study of This international cooperation not only brought the the fundamental forces. In addition to the long-known desired success in deciphering the laws of nature. It also gravitational and electromagnetic forces, two other forces led to the development of new methods and technologies, were discovered: the weak force, which is responsible for with a broad potential for applications in other branches the decay of radioactive nuclei, and the strong nuclear of science, the economy and everyday life. force, which binds quarks together inside protons and As knowledge and technological know-how have neutrons, and which binds the protons and neutrons to increased, a phenomenon could be observed in particle each other inside the nuclei. Furthermore, precision physics that is typical for the fundamental sciences. Each measurements of the interactions of leptons and quarks new piece of knowledge generates new and deeper led to the impressive confirmation of the theoretical pre- questions and new theoretical and experimental challen- diction that the electromagnetic and weak forces are ges. These are the motor for developing new ideas, theo- actually different manifestations of the same force. The ries and technologies, for planning and building new knowledge of the outstanding role of symmetry and research projects and for training new generations of symmetry breaking in the microscopic world has equally highly motivated and qualified scientists. profound and far-reaching consequences. They not only Despite the great advances in our understanding of the provide a decisive key to explaining the properties of fascinating world of leptons and quarks, particle physics is leptons and quarks and their interactions but also, in their anything but a completed field of science. Some of the interplay, to the fundamental understanding for the central open questions being asked today, which touch on inexhaustible variety of phenomena in the macroscopic the very foundations of our world view, are as follows: world. ◆ How do elementary particles acquire their mass? Research into the elementary building blocks of matter and the fundamental forces in the universe has revolution- ◆ Is there one universal interaction from which all ised our understanding and knowledge of the origin, the known fundamental forces, including gravity, derive? structure and the future evolution of our world. A major ◆ Are there forms of matter undiscovered up to now, factor in this success has been the high-energy accelera- such as a whole new world of supersymmetric tors: over the past ten years, these have been largely the particles? Could this be the explanation of the “dark LEP electron-positron accelerator at the European research matter”, whose existence is suggested by the large scale laboratory CERN in Geneva, the SLC electron-positron col- structure formation of galaxies and the movement of lider at the Stanford Linear Accelerator Center SLAC in the stars inside galaxies? California, the HERA electron-proton facility at the ◆ Deutsches Elektronen-Synchrotron DESY in Hamburg, and What is the nature of the “dark energy” that causes the the Tevatron proton-antiproton accelerator at the US universe to expand at an ever-increasing rate? research centre Fermilab in Chicago. More recently they ◆ Are there hidden dimensions in addition to the three were joined by the so-called “B-meson factories” PEP-II at spatial dimensions that we are familiar with? I Compelling theoretical arguments and indirect experi- demonstrated by the LEP and SLC experiments, for mental indications suggest that the answers to at least instance, in the indirect determination of the mass of the some of these questions ought to lie in the mass range top quark, later confirmed by direct measurement at between a few hundred giga-electronvolts (GeV) and Fermilab. The prerequisite for such extrapolations, how- several tera-electronvolts (TeV). That equates to more than ever, is a far more accurate analysis of particle properties a hundred to a thousand times the proton mass and, for the and processes than the LHC experiments alone are capable most part, lies outside the reach of today’s accelerators. of providing. Moreover, a high level of sensitivity is Comprehensive and precise investigation of physics at the required to corral the new physics beyond the Standard TeV energy scale is the major challenge for particle physics Model which cannot be investigated by the LHC, or at least in the coming 15 to 20 years — a challenge that can only be not sufficiently. Detailed studies performed in interna- taken up through worldwide cooperation. tional cooperation have shown that the required measure- This document presents recommendations for the ments can only be carried out at an electron-positron future development of particle physics in Germany in the linear collider. For this reason it has been decided by next 15 to 20 years, based on the current status of knowl- worldwide consensus within the particle physics commu- edge, the theoretical perspectives and the experimental nity, that the next major high-energy physics project options, while taking into account the available resources. should be the construction of a linear collider with a total The LHC proton-proton collider, with a centre-of-mass energy of 0.5 to 1 TeV. energy of 14 TeV, is presently being built at CERN in The new data and knowledge that can be expected Geneva. Such a high energy is required in order to attain from this linear collider will be greatly complementary to energies in the TeV range for the collisions of the elemen- the results obtainable at the LHC. From the scientific view- tary building blocks within the proton. In parallel, high- point, an overlap in time with the physics programme at energy physicists throughout the world are constructing the LHC is imperative. Experience with electron-positron components for four large particle detectors for the LHC facilities such as LEP and the corresponding hadron experiments. Operation should begin in 2007. For the time accelerators such as the Tevatron demonstrate impressively being, the LHC project has the highest potential for the synergy effects that can be derived from such con- ground-breaking discoveries that will shed light on the current experimentation. open questions outlined above and to be