Elementary Particle Physics
Experimental Particle Physics Overview Particle physics has been a major research area since the early 1950s. Wisconsin physicists were pioneers in tests of symmetry principles, hadron spectroscopy, neutrino interactions and the structure of hadrons. More recently, they have participated in the discovery of the W and Z bosons, the discovery of the Top quark, and the measurement of the properties of the decays of heavy quarks. Today, the Wisconsin group has made important contributions to colliding beam experiments at four different laboratories: Fermilab and SLAC in the U.S. as well as the German Synchrotron Laboratory (DESY) and the CERN laboratory in Switzerland. There are also fixed target experiments in the mix. This program, together with major participation in the preparations for the next generation colliders, provides graduate students interested in this area with a wide choice of research topics, and access to most of the world’s finest facilities. Work in this field involves mounting large and complex experiments as part of multi-institutional collaborations at sites that are remote from campus. The design and construction of equipment takes place largely in Madison, and analysis of data is done both at the laboratories and on campus. Fermilab is located less than three hours’ drive from Madison which is home to the world’s most energetic collider where proton-antiproton collisions at 2 TeV in the center of mass are observed. Graduate students involved in experimental particle physics investigations may expect to spend two or more years at the accelerator laboratory, once they have completed their course work in Madison. ALEPH Detector (CERN) A collaboration containing a Wisconsin group operates the ALEPH detector, one of four experiments mounted at the electron-positron collider LEP. The research program at LEP is unusually rich and varied, ranging from precision measurements of electroweak parameters to searches for unexpected phenomena. For example, the mass of the Z boson, cousin to the ordinary photon, is now one of the most accurately known parameters of the “Standard Model”. Predictions of this model include a longitudinal polarization of fermions produced in decays of Z bosons, which was verified by the ALEPH experiment. As the luminosity and energy have been increased, the focus has shifted to the search for the elusive Higgs boson. One of the most topical areas of research is the physics of hadrons containing the b-quark. The LEP experiments recently have made several important contributions to this field, beginning with the first observation of heavy hadrons, followed by measurements of lifetimes and masses. Subtle, but significant quantum mechanical effects have been observed bearing on the time-dependent mixing of the b-hadron flavor. Wisconsin students participate in and contribute to all of these research projects and, in some cases, have made the first presentation of the latest results. Working with the ALEPH group is an exciting and educational experience. BaBar Detector (SLAC) BaBar is a newly constructed detector for the B-Factory positron-electron storage ring at the Stanford Linear Accelerator Center (SLAC). The storage ring is designed to produce B Mesons at a rate an order of magnitude greater than existing electron-positron colliders, yielding 30 million pairs of B mesons per year. This high yield is required to study CP violation, the fundamental asymmetry between matter and antimatter. CP violation has been observed in the K meson system, but to challenge the Standard Model in a definitive way requires measurements of the K meson’s heavy “brother”, the B meson. Together with a similar new facility in Japan, the BaBar detector will give us some quantitative insight into the nature of CP violation and the asymmetry between matter and antimatter, a relic of the Big Bang. The BaBar detector is a state of the art detector consisting of a silicon vertex detector, a drift chamber, a particle identification system, a Cesium Iodide electromagnetic calorimeter, and a superconducting magnet with an instrumented flux return. Collider Detector Facility (Fermilab) The CDF experiment observes proton-antiproton collisions at the Fermilab Tevatron, currently the world’s highest energy collider. It supports a broad range of studies of electroweak and QCD processes and searches for new phenomena, particularly in the production of W and Z bosons and heavy quarks. The discovery of the top quark was the main achievement of the last experimental run. KTeV Experiment (Fermilab) The KTeV experiment at Fermilab includes the study of fundamental symmetries, rare decay processes, weak interactions, and polarization phenomena in the systems. A striking asymmetry of our world is the fact that the universe appears to be composed entirely of matter and no astronomical object made of anti-matter has ever been detected. This asymmetry is thought to be a consequence of CP violation, which so far has been observed in the neutral K meson system where it is a tiny effect (about 1 part in 500) in certain decays of . The KTeV experiment seeks to determine whether or not this effect can be fully understood in the context of the present picture of matter (the “Standard Model”). During the first phase of the experiment (during 1997) KTeV has observed a new type of asymmetry in the system, “direct CP violation,” that cannot be explained by the “Standard Model!” Furthermore, KTeV has already discovered six new decay processes that had never before been seen and is systematically studying them. MINOS Experiment (Fermilab) The MINOS experiment at Fermilab and at the Sudan Mine in Minnesota will study neutrino interactions produced by 120 GeV/c protons from the Main Injector at Fermilab onto a carbon target. A beam of muon neutrinos with a very small admixture of electron neutrinos is produced by meson decays at the proton target. This beam is directed towards the Sudan mine in Minnesota, where part of the MINOS detector will be located. The goal of the MINOS experiment is to detect the phenomenon of neutrino oscillations. The definitive observation of oscillations will prove that neutrinos have mass. ZEUS Experiment (DESY laboratory in Hamburg, Germany) This detector, which is located at the unique electron proton collider HERA, is used to probe the structure of the proton with unprecedented resolution. This facility provides a laboratory in which to test in detail the predictions of Quantum Chromodynamics. The observation of a strong increase in the density of gluons at small distances in the proton was a surprise. Searches for leptoquarks and other new phenomena are continuously updated as luminosity increases. Other investigations relate to the diffractive production of particles and the determination of the characteristics of the “Pomeron” and to the measurement of photon interactions with the proton. Wisconsin students have participated in all facets of the collection and analysis of the data. With the planned increase in luminosity (2000) and other upgrades, the future of continued discovery is bright. CMS Experiment (CERN) The Wisconsin group is participating in the move to the new energy frontier represented by the proton-proton collider now under construction at the CERN laboratory in Geneva, Switzerland. This machine will collide two 17 TeV beams at extraordinarily large luminosity. New particles and interactions, which are expected in this energy range, will be investigated. The remaining facet of the “Standard Model” — the Higgs sector — will be exhaustively explored. We will also search for the extension of the model called supersymmetry. Wisconsin groups participate in both the large general purpose detector collaborations, CMS — Compact Muon Solenoid — and ATLAS. Research and Development for Detectors and Accelerator Facilities. The Wisconsin High Energy Group has several research projects for the development of advanced technologies for future colliders. Among past construction projects have been the design and construction of a state-of-the-art calorimeter trigger for the ZEUS detector. The forward muon detector at the Collider Detector Facility (CDF) at FNAL was built at the UW as was the intermediate muon detector for the CDF upgrade. Future projects include design, construction and commissioning of the calorimeter trigger electronics and the muon detector system for the CMS detector, and contributions to the silicon tracking detector for ATLAS. The group is also involved in advanced research and development of polarized electron sources for the NLC collider project currently in the design phase at SLAC. The Institute for Elementary Particle Physics Research The faculty, staff, and students of the Institute for Elementary Particle Physics Research, also known as the Phenomenology Institute [or even as “Phenoland”], conduct an active research program in elementary particle theory. Hot areas of research include the origin of mass and the search for Higgs bosons, low-energy supersymmetry (SUSY) and grand unification of forces, physics with extra dimensions in superstring theories, neutrino oscillations, CP violation, particle cosmology, and strong interaction phenomena such as QCD bound states and hadronic jets. The research program emphasizes aspects of particle theory that can be tested experimentally, and there is strong interest in fundamental theoretical ideas. The Institute has a long history of leadership in theoretical research and is one of the largest research groups of its kind in the world. Institute theorists made important contributions to the developments which led to and established the Standard Model of elementary particles. As examples, Institute researchers invented search strategies used in the discoveries of the W boson and top quark, made seminal studies of the quark and gluon content of the nucleon and of scattering processes in QCD, developed the theory of neutrino oscillations, and made pioneering contributions in neutrino astronomy. This leadership continues. While the Standard Model successfully describes experiments up to the highest energies available today, the predicted Higgs boson is yet to be found and there are theoretical reasons to believe that the theory is incomplete and needs to be extended to incorporate supersymmetry and possibly the extra dimensions that appear in superstring theories. Institute researchers have recently played leading roles in theoretically exploring Higgs boson physics and SUSY extensions of the Standard Model, in developing search strategies for finding the Higgs boson and SUSY particles, and in developing strategies for exploring the physics of extra dimensions. The Institute’s extensive computing facilities and expertise provide a valuable resource for theoretical research. In addition, the Institute maintains close ties with theorists and experimenters who work at the major laboratories and other universities and sponsors an annual conference in Madison on physics at high energy colliders. Since 1984, when the Institute was founded, 55 Ph.D.’s have been trained by Institute faculty and 35 postdocs have been involved in the research program. A large fraction of these Ph.D.’s and postdocs have found permanent positions in particle physics research. There are currently 9 graduate students, 7 postdoctoral level staff and 7 senior faculty in the Institute. Approximately 50 scientists visit annually from other institutions for periods ranging from a few days to a semester. The institute has exchange programs with Brazil and Japan. All of these activities provide exceptional opportunities for students to be directly involved in forefront research and gain exposure to the larger particle physics community. Graduate students participate in a large fraction of the Institute’s 50+ research publications each year. The research activities of the Institute are funded primarily by the Department of Energy. The next decade promises to be one of the most exciting in the history of particle physics because of the incoming data from high energy experiments. Institute researchers are well positioned to continue making fundamental contributions in the quest to uncover the ultimate nature of the Universe. |
