DESY (Deutsches Elektronen-Synchrotron Laboratory)

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DESY (DEUTSCHES ELEKTRONEN-SYNCHROTRON LABORATORY)

DESY is a national research laboratory in Germany and, besides the European Laboratory for Particle Physics (CERN), the other major particle accelerator center in Europe. The name (German electron synchrotron) derives from the first accelerator constructed there. DESY is located in a suburban area of Hamburg. It has a staff of about 1,200 and a budget of about 150 M Euro.

The laboratory was established in 1959. Its principal funding agency is the federal ministry for education and research. The original mission was to design, construct, and run a high-energy particle accelerator, so that researchers from German universities and other research institutes, wishing to participate in the emerging field of particle physics, could conduct experiments there. Gradually, DESY attracted users from all over Europe and from the United States, Canada, Japan, and China. The laboratory, though formally remaining a national institution, factually became international.

From the beginning, DESY fostered an active program of synchrotron radiation applications. This work, including condensed matter physics and material science, geology, chemistry, life sciences, and medical applications has steadily grown in scope and importance and has gradually transformed the laboratory into an interdisciplinary research establishment.

The Past

The first accelerator constructed at DESY was an electron synchrotron of 7.5 GeV beam energy. Research interests included the structure of the nucleon investigated by elastic and inelastic electron-nucleon scattering, the production of hadrons and hadronic resonances by high-energy photons, and tests of quantum electrodynamics. The synchrotron was commissioned in 1964; experiments continued for a decade. Meanwhile DESY investigated the possibility of constructing electron-positron storage rings of high energy, building on the pioneering work at Stanford University and the Italian national laboratory in Frascati. The machine was completed in 1974 and was called DORIS (from the German Doppel-Ring-Speicher); it had a maximum energy of the colliding beams of initially 4.3 GeV, later upgraded to 5.6 GeV. It came into operation just after the discovery of the J/ψ resonance, the first charm-anticharm bound state, and was to become a significant player in unraveling the physics of the new quark states and of the τ lepton, discovered shortly afterward.

Notable observations at DORIS include the P-wave charm-anticharm states and the S-wave states of the bottom-antibottom quark states, the so-called upsilon resonances. In 1982 a comprehensive investigation of B mesons, the hadrons containing one heavy b quark or antiquark, was launched. In its course, the ARGUS collaboration discovered the quantum mechanical mixing of neutral B mesons with their anti-particles, the first such mixing case found since an analogous phenomenon had been discovered with K mesons more than two decades earlier. This observation led the way to exciting prospects in b quark research, in particular to the possibility of charge parity–parity (CP) violation by B mesons, a development that culminated in the construction of specialized B factories at the Stanford Linear Accelerator Center (SLAC) and the Japanese High-Energy Accelerator Research Organization (KEK). DORIS has been, since 1993, used as a dedicated synchrotron radiation source.

The next accelerator at DESY was an electron-positron collider as large as would fit on the site. The PETRA (Positron Electron Tandem Ring Accelerator) ring had a circumference of 2.3 kilometers and a collision energy in the center-of-mass of initially about 30 GeV. It was commissioned in 1978, and a few months after its start, the international Two-Arm Solenoid Spectrometer (TASSO) collaboration began to observe so-called three jet events. These are events in which out of the electron-positron collision, instead of just a lepton-antilepton or quark-antiquark pair, a quark-antiquark pair accompanied by an additional energetic "hard" gluon appeared, resulting in a third jet of hadrons. The interpretation followed readily, and the results were soon corroborated by the other three collaborations working at PETRA, each using their own detector. This was a crucial step toward establishing the theory of quantum chromodynamics.

PETRA was then gradually upgraded to 46 GeV energy in an attempt to find the top quark and/or supersymmetric particles. At the time, it was not clear how large the masses of these particles would be. On the way, many important observations were made at PETRA (and at the similar PEP machine at the Stanford Linear Accelerator Center). They concerned the radiation of hard gluons, the formation of jets of hadrons from quarks and gluons, and the properties of the gluons like spin and coupling strength to quarks; furthermore, they also concerned properties of b quarks such as the lifetime, the electroweak couplings of the heavy quarks and of the μ and τ leptons, tests of quantum electrodynamics at very small distances, and more. These results went a long way toward establishing the Standard Model as a viable description of the particles and their interactions down to distances of the order of 10^-15 cm. Experiments at PETRA were carried on until 1986. PETRA was then converted into an injector for HERA, the next big accelerator at DESY, and today (2002) it also serves as a synchrotron radiation source for hard X rays.

HERA

Meanwhile plans were made at DESY for a new type of accelerator—an electron-proton collider. This project, called HERA (Hadron Electron Ring Accelerator), apart from being a novelty in machine design, faced two further challenges. Since it would not fit on the DESY site, it was constructed below ground in an adjacent area, which was partly industrial and partly residential. And since both the human resources needed and the price tag were substantially higher than what the German funding agency was prepared to grant, it was funded and built in international collaboration. Remarkably, authorization to operate the accelerator 15 meters underground, directly under private homes, was obtained. The project was co-funded by the funding agencies of a number of foreign (that is, non-German) countries, and several foreign institutes sent substantial human resources to DESY to help construct HERA. It was completed in 1991.

HERA is housed in a 6.3-km-long tunnel, in which protons of up to 920 GeV energy and electrons of 28 GeV circulate. The electron energy is lower because of synchrotron radiation while the proton energy is limited by the magnetic guiding fields generated by superconducting magnets. Owing to the colliding beams, the lepton-nucleon interactions in HERA have more than ten times higher energy in the center of mass than in lepton-nucleon scattering experiments that employ fixed nuclear targets. The interactions are observed at two beam intersections where detectors called H1 and ZEUS have been set up by large international collaborations. As the scattering takes place in the unified electroweak region where the weak force is no longer small compared to the electromagnetic force, the finite range

effects in the electron-quark interactions were seen for the first time. The analysis of these interactions is yielding precise information on the structure of the nucleon in terms of the quarks and gluons down to scales of 10^-16 cm, about one thousandth of the nucleon size. A surprise was the very large density of "soft" gluons and quark-antiquark pairs found in the proton. Quantum chromodynamics interprets this as a consequence of a continuing emission and reabsorption of gluons by the quarks, with the gluons in turn radiating further gluons and generating quark-antiquark pairs which can again annihilate into gluons—a multistep process by which the interior of the nucleon becomes something like a permanently fluctuating, dense liquid of quarks, anti-quarks, and gluons. This state of the interior of the nucleon has so far defied quantitative description in terms of quantum chromodynamics. It presumably is closely related to the phenomenon of quark confinement.

Measurements of similar nature, at lower energy but with longitudinally polarized electrons or positrons scattered on polarized nucleons, are made in the experiment HERMES (HERA Measurement of Spin). It uses the electron beam of HERA, observing scatterings on a target of polarized gas molecules in a spectrometer. The aim is to unravel the spin structure of the nucleon in terms of the angular momenta of the constituent quarks, antiquarks, and gluons. In agreement with other experiments it is found that the three valence quarks are carrying only about one third of the nucleon's spin. Present effort is directed toward finding the contribution of the gluons to the angular momentum. There is a fourth large detector at HERA, called HERA-b, in which the final states from scattering of the 920-GeV proton beam on a fixed target are being measured; b quarks are produced whose rare decay modes will be analyzed in the HERA-b spectrometer. About 1,200 physicists are participating in the experiments at HERA. It is planned that this experimental program will continue until at least 2006.

Other Activities

In addition to accelerator and detector construction and particle physics synchrotron radiation experiments, theoretical studies in particle physics and cosmology take place at DESY. Furthermore, DESY has a branch institute in Zeuthen near Berlin that has evolved from the former East German Institute of High Energy Physics. Besides collaborating in the general research and development (R&D) program at DESY, physicists in Zeuthen have a major interest in neutrino astrophysics, aimed at the detection of cosmic neutrinos in underwater and under-ice Cherenkov telescopes—in particular, participation in the Antarctic Muon and Neutrino Detector Array (AMANDA) and IceCube projects at the South Pole. Another activity pursued at DESY-Zeuthen is lattice gauge theory including the development, together with the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, of parallel computers specially designed for the necessary calculations.

TESLA Project

Since 1992, DESY has been pursuing, on the initiative of its late director Bjorn Wiik and in the framework of an international collaboration, the design of a superconducting linear collider for electrons and positrons with center-of-mass energies of 500 to 800 GeV, named the Tera Electronvolt Superconducting Linear Accelerator (TESLA). Such a machine would be an ideal instrument for definitely establishing the Higgs mechanism and testing its various aspects. One needs to know whether the Higgs particle is standing alone, whether it is a supersymmetric Higgs, or whether it is the first sign of something completely new. To this end one must measure all of its interactions and decays precisely. If there is a light Higgs particle and if supersymmetric particles are found, the experiments at TESLA, combined with results from the LHC, may become for supersymmetry what optical spectroscopy was for quantum mechanics: the establishment of precision data from which the underlying theory can be developed. If no evidence for the Higgs mechanism is found at TESLA, it will be even more interesting to find out what takes its place. TESLA would be ideally suited also as driver of a self-amplifying spontaneously emitting free electron laser which could serve as a source of hard, coherent X rays. An X-ray laser of this sort would be far more powerful than any currently available X-ray source, opening unprecedented opportunities for X-ray-based research and for X-ray applications in a wide variety of fields, from materials research to life sciences.

In 2001, DESY and the TESLA collaboration presented a detailed proposal to construct TESLA and an associated X-ray laser laboratory. The machine would be housed in a 33-km-long underground tunnel. A possible site near DESY has been identified. The project would have to be an international collaboration and would involve founding a new administrative structure in which DESY would act as host and as one of the partners.