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EXOTIC RELATIVES OF PROTONS AND NEUTRONS DISCOVERED BY CARNEGIE MELLON
14 March 2007 - Carnegie Mellon Universtity
| The Department of Energy's Fermi National Accelerator Laboratory Collider Detector at Fermilab, a collaboration that includes researchers from the University of Pittsburgh and Carnegie Mellon University, announced yesterday the discovery of two rare types of particles, exotic relatives of the more common proton and neutron. |
"These particles, named Sigma-b [Σb], are exotic composites, new ways of putting together the heavy and light quarks. We infer their existence from observing their decay products, and we're only seeing them now because even the decay products are pretty rare," said Joseph Boudreau, associate professor of physics and astronomy at Pitt, who was involved in the research with Paul Shepard, professor of physics and astronomy at Pitt. Manfred Paulini, associate professor of physics at Carnegie Mellon, and James Russ, professor of physics at Carnegie Mellon, are also members of Fermilab's CDF collaboration; their research focuses on the study of particles containing heavy quarks. "Ordinary matter consists of only two of the lightest quarks plus the lightest lepton, known as the electron," Boudreau added. "The heavier forms of matter exist only for short periods of time, but they're just as crucial to our understanding as the stable forms. You can't explain the proton, the neutron, or the electron without explaining the heavier members of the family." "The proton and neutron family tree has several different branches," said Russ. "These new particles are very similar to other relatives of the proton studied at Fermilab and elsewhere in a variety of experiments. The masses of these new states fit in beautifully with the pattern that we expect from those of charm baryons." Baryons, derived from the Greek word barys, meaning "heavy," are particles that contain three quarks, the most fundamental building blocks of matter. The CDF collaboration discovered two types of Sigma-b particles, each one about six times heavier than a proton. There are six different types of quarks: up, down, strange, charm, bottom, and top (u, d, s, c, b, and t). The two types of baryons discovered by the CDF experiment are made of two up quarks and one bottom quark (u-u-b), and two down quarks and one bottom quark (d-d-b). For comparison, protons are u-u-d combinations, while neutrons are d-d-u. The new particles are extremely short-lived and decay within a tiny fraction of a second. "These particles are like rare jewels that we mined out of our data," said CDF spokesperson Jacobo Konigsberg of the University of Florida. "Piece by piece, we are developing a better picture of how matter is built out of quarks. We learn more about the subatomic forces that hold quarks together and tear them apart. Our discovery helps complete the 'periodic table of baryons.'" Using Fermilab's Tevatron collider, the world's most powerful particle accelerator, physicists can recreate the conditions present in the early formation of the universe, reproducing the exotic matter that was abundant in the moments after the big bang. While the matter around us comprises only up and down quarks, exotic matter contains other quarks as well. The Tevatron accelerates protons and antiprotons close to the speed of light and makes them collide. In the collisions, energy transforms into mass, according to Einstein's famous equation E=mc2. To beat the low odds of producing bottom quarks, which in turn transform into the Sigma-b, according to the laws of quantum physics, scientists take advantage of the billions of collisions produced by the Tevatron each second. The CDF experiment identified 103 u-u-b particles, positively charged Sigma-b particles (Σ+b), and 134 d-d-b particles, negatively charged Sigma-b particles (Σ-b). In order to find this number of particles, scientists culled through more than 100 trillion high-energy proton-antiproton collisions produced by the Tevatron over the last five years. Boudreau has contributed heavily to the reconstruction software for CDF; it follows charged particle trajectories through tracking detectors. "The discovery of the Sigma-b is at the end of a long chain of reconstruction: Tracks are inferred from a few coordinates; heavier and less stable forms of matter are inferred from the tracks; and in this way we work our way back through a decay chain, generally towards heavier and more exotic forms of matter, and generally by putting together decay products," said Boudreau. Paul Shepard constructed the silicon vertex detector, a precise instrument that surrounds the particle interaction region and measures tiny amounts of ionization resulting from the passage of charged particles. "The Sigma-b discovery would have been out of the question without that detector, which not only measures charged particle tracks but also selects them in real time from among an overwhelmingly large background," said Boudreau. James Russ was involved in an earlier CDF study of exotic baryons that tested many of the techniques of analysis used in the current study. Manfred Paulini, together with Carnegie Mellon postdoctoral researcher Soon Jun, are leading the CDF detector simulation project. "Simulating particle decays using computer programs and tracing particle trajectories through a computer image of the CDF detector are essential for understanding where in the real detector we might have missed recording Sigma-b particles," Paulini explained. In a scientific presentation on Friday, Oct. 20, CDF physicist Petar Maksimovic, professor at Johns Hopkins University, presented the discovery to the particle physics community at Fermilab. He explained that the two types of Sigma-b particles are produced in two different spin combinations, J=1/2 and J=3/2, representing a ground state and an excited state, as predicted by theory. Quark theory predicts six different types of baryons with one bottom quark and spin J=3/2 (see graphic at www.fnal.gov/pub/presspass/images/sigma-b-baryon-images.html). The CDF experiment now accounts for two of these baryons. "Our data samples continue to increase, and I expect to see even more discoveries of this kind in the near future," said Boudreau. Also involved in the research from Pitt were postdoctoral research fellows Azizur Rahaman and Karen Gibson and graduate students Chunlei Liu and Mark Hartz. CDF is an international experiment of 700 physicists from 61 institutions and 13 countries. It is supported by the Department of Energy, the National Science Foundation, and a number of international funding agencies. (The full list can be found at www-cdf.fnal.gov/collaboration/Funding_Agencies.html.) Using the Tevatron, the CDF and DZero collaborations at Fermilab discovered the top quark, the final and most massive quark, in 1995. Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated under contract by Universities Research Association, Inc.
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About: Carnegie Mellon Universtity
The Carnegie Institution of Washington (www.carnegieinstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science. Since its founding in 1900 by industrialist and philanthropist Andrew Carnegie, Carnegie Mellon University has been a pragmatic institution, adapting rapidly to change. In fewer than 100 years it has changed its name three times--each transition marking a milestone in the institution's 20th century evolution. Whether it was Carnegie Technical Schools, as it was in its first 12 years, Carnegie Institute of Technology, its name from 1912 to 1967, or Carnegie Mellon University, three primary purposes formed its foundation. Throughout this century, Carnegie Mellon has focused on delivering distinctive and first-quality education, fostering research, creativity and discovery, and using the new knowledge created on campus to serve our larger society. When Arthur A. Hamerschlag served as the school's first president, Carnegie Technical Schools' 12 professors and six administrators sought to educate the sons and daughters of Pittsburgh workers for employment in the region's growing industries. These educators served the vision of Carnegie by organizing into four faculties: the School of Science and Technology, the School of Fine and Applied Arts, the School of Apprentices and Journeymen, and the Margaret Morrison Carnegie School for Women. In its earliest years, the institution served primarily part-time and undergraduate students. The faculty, many of whom did not have doctor's degrees, focused on teaching and curriculum development. But research efforts began as early as 1916 when the Division of Applied Psychology of the Carnegie Institute of Technology developed rating scales for job placement. This rating system was used to classify two million men for placement in the armed forces during World War I. Research bureaus were organized in coal mining, nuclear physics, applied chemistry and metallurgy. And by granting the nation's first undergraduate degree in drama in 1917, the institution began a tradition of leadership in the arts that spanned the century. Through research and the education of its students during the administration of President Thomas S. Baker in the 1920s and '30s, the institution began its strong tradition of transferring knowledge and skills to industry and government. Building on this firm foundation, the administration of President Robert E. Doherty introduced a new approach to education that would be used as a model by similar institutions around the nation. The Carnegie Plan for Professional Education, initiated in 1939-40, required engineering and science students to take a quarter of their courses in a new Humanistic and Social Relations sequence. In addition, its curriculum focused on teaching students problem-solving techniques, a hallmark of the Carnegie Mellon educational experience today. While the Doherty administration has been credited with this educational innovation, it also oversaw growth in the institution's research capability. Between 1936 and 1950, the number of graduate students grew from 36 to more than 260. The research budget ballooned from $156,000 to $1 million. In the 1950s, the newly formed Graduate School of Industrial Administration, endowed by William Larimer Mellon, emerged as one of the three or four best business schools in the nation. (In 2004 the school was renamed the David A. Tepper School of Business after benefactor and alumnus David Tepper (MBA '82).) Today, the school is recognized as a pioneer in the field of management science and one of the top business schools in the world. The Warner administration oversaw the institution's burgeoning research enterprise. This period of research growth was aided by the work of the institution's Computation Center, founded in 1956 to provide computing services to the campus. A major grant from benefactor Richard K. Mellon in 1965 aided the establishment of a Computer Science Department, a department which would be the genesis of Carnegie Mellon's worldwide reputation in computer science. By the end of the Warner administration and the start of the administration of President H. Guyford Stever in 1966, Carnegie Tech had most elements of a university. Its merger in 1967 with the Mellon Institute created Carnegie Mellon University and brought a $60 million endowment, extensive research facilities and renowned research personnel to the institution. Five years later, President Richard M. Cyert (1972-90) began a tenure that was characterized by unparalleled growth and development. The university's research budget soared from about $12 million annually in the early 1970s to more than $110 million in the late 1980s. The work of researchers in new fields such as robotics and software engineering helped the university build on its reputation for innovative ideas and pragmatic solutions to the problems of industry and society. Carnegie Mellon began to be recognized as a truly national research university able to attract students from across the nation and around the world. The Cyert administration stressed strategic planning and comparative advantage, pursuing opportunities in areas in which Carnegie Mellon could outdistance its competitors. An archetypal example of this approach was the introduction of the university's "Andrew" computing network in the mid-1980s. This pioneering network, which linked all computers and workstations on campus, set the standard for educational computing and firmly established the university as a leader in the uses of technology in education and research. Education and teaching also benefited in this period with the establishment of a University Teaching Center to improve faculty teaching and the renovation of many of the university's classrooms. Cognizant of the university's heritage, President Robert Mehrabian (1990-97) invited alumni from the era of the institution's first president, Arthur A. Hamerschlag, to attend his inauguration in 1990. President Mehrabian emphasized Carnegie Mellon's traditional strengths in education, research and service to society while focusing on initiatives for leadership in the 21st century. With the appointment of the university's first Vice Provost for Education, President Mehrabian placed renewed emphasis early in his administration on the quality of undergraduate education. He also moved aggressively to complete the most ambitious campus building plan since the Warner era. The University Center, which opened in August 1996, and the Purnell Center for the Arts, to be completed by the fall of 1999, are keys to enhancing the quality of life on campus, another priority of the Mehrabian administration. Confronted by shrinking governmental support of university research, President Mehrabian diversified the university's research agenda. He stressed the need to build strong relationships with the business world, matching industry's needs with the university's areas of research strength. He also put new emphasis on productivity, improvement of administrative services and strategic management of university resources. President Mehrabian established strong, new partnerships with the greater Pittsburgh community. He led a community-wide economic development initiative, spurred collaboration with primary and secondary schools, and worked closely with local community groups. On April 15, 1997, Jared L. Cohon, former dean of Yale University's School of Forestry and Environmental Studies, was elected by the university's Board of Trustees to succeed President Mehrabian, who resigned to spend more time with his family in California. "Since I was chosen, since this wonderful event has occurred, it has made me reflect on why you are choosing me," President Cohon said in his first speech to the university community. "And I've said to people since this was announced that the more I think about it, the more I realize how well I think this institution and I fit together. We'll see if that's true. I think it is. "When I was at Johns Hopkins we used to always hold up Carnegie Mellon as an example," Cohon said. "So, for many years I've ... been jealous of what has been accomplished here across departmental lines. I celebrate that. I think it is so valuable in every aspect of this university and it will position Carnegie Mellon to be even better...." During Cohon's presidency, Carnegie Mellon has continued its trajectory of innovation and growth. Today, President Cohon is leading implementation of a comprehensive strategic plan that aims to leverage the university's existing strengths to benefit society in the areas of biotechnology and the life sciences, information and security technology, environmental science and practices, the fine arts and humanities. The university is also committed to broadening and enhancing undergraduate education to allow students to explore various disciplines while maintaining a core focus in their primary area of study. Realizing that today's graduates must understand international issues, Carnegie Mellon is committed to providing a global education for its students and is striving to expand its international offerings and to increase its presence on a global scale. Increasing diversity, in all aspects, and fostering the economic development of southwestern Pennsylvania, are also top priorities. Over the years Carnegie Mellon's leaders have reflected Andrew Carnegie's original dedication and commitment to this institution. In his 1900 letter to the mayor of Pittsburgh establishing Carnegie Technical Schools, Andrew Carnegie wrote, "My heart is in the work." These words have been echoed by students, faculty and administrators throughout this century and they live on the Carnegie Mellon campus today. |
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