Sunday, November 23, 2003
Beauty Conference Highlights Possible New Physics
Almost 40 years ago, a group of physicists identified CP Violation, a new phenomenon that threatened to rewrite the rules governing how sub-atomic particles interact. This past October at the 9th International Conference on B-Physics at Hadron Machines (called Beauty 2003 for short), scientists from around the globe met at Carnegie Mellon University to discuss recent data dealing with CP Violation that once again might change the study of particle physics.
Our Current Understanding: The Standard Model
Today, the study of particle physics is shaped by research of the Standard Model. Initially conceived more than 30 years ago, the Standard Model describes what fundamental particles exist, how they interact and how they group together to form other, more complicated, particles. Unlike many discoveries attributed to one person or one date, the Standard Model is a constantly refined combination of theory and experiments contributed by thousands of physicists around the globe. The Beauty conferences are designed to bring the theorists and experimentalists together to discuss elements of the Standard Model relating to B-Mesons. These particles include at least one b-quark, one of the 12 fundamental particles that make up all known matter in the universe.
"Our conference goal was to foster communication between theorists and experimentalists," said Manfred Paulini, associate professor of physics at Carnegie Mellon and chair of the local organizing committee. "The participants got a lot of food-for-thought."
Attendees spent their time digesting recent experimental results about CP Violation and how it might transform our knowledge of the universe. Physicists hope that an understanding of CP Violation will help solve perhaps the greatest cosmic conundrum – why the universe is made up of mostly matter and not antimatter. While matter is composed of particles, antimatter is composed of antiparticles, or particles with an opposite charge. For instance, a positron, an antimatter particle, is a positively charged electron. While the universe had equal portions of matter and anti-matter shortly after the Big Bang, matter somehow came to dominate, which is what we see today when we view our universe.
A History of CP Violation
CP Violation can be thought of as a "break" in how physical laws are thought to work. Theory dictated that an interaction between particles would follow the same rules if one were to view the mirror reflection of that interaction, a property called parity (P). In 1957 scientists discovered that some particle interactions violated this symmetry between left and right. To accommodate this experimental result, scientists integrated a new theory into an early predecessor of the Standard Model. This revised theory postulated that, although parity can be violated, particle interactions would not change if both the charges of the particles (C) and the parity (P) were changed. But this theoretical adjustment, known as CP invariance, did not weather experiment. In 1964, physicists observed a violation of CP invariance at an experiment at Brookhaven National Laboratory in New York. This violation – CP violation – was only seen in a rare form of particle called the K meson.
"There were no other observations in any other particles for the next 30 years," said Lincoln Wolfenstein, emeritus university professor of physics at Carnegie Mellon. In recent years however, scientists have found evidence of CP Violation in a new exotic particle called the B meson.
"Now we see a large effect," explained Wolfenstein, a member of the local organizing committee for Beauty 2003. "The whole field is opening up."
A Framework for Measuring CP-Violation
Wolfenstein has been involved in studies and formulations of theories related to K Mesons, B Mesons and CP Violation for more than 40 years. Wolfenstein's research provided a framework for experimentalists to conceptualize how quarks interact, or couple, with each other and has helped both theoretical and experimental physicists characterize CP violation in particle interactions.
Wolfenstein's method involves a restatement of a central theoretical construct of the Standard Model, called the CKM matrix. Wolfenstein approximated the matrix's elements and restated these values in terms of the sides and angles of a special triangle, called the Unitarity Triangle.
"The name of the game," said Helmut Vogel, professor of physics at Carnegie Mellon, "is to determine the lengths of the sides and the angles of the Unitarity Triangle."
The angles correspond to the amount of CP Violation, or asymmetry, that occurs in particle interactions, and the lengths correspond to how much the quarks interact with each other. Vogel described current research as "zeroing in on the allowable locations" of the three points of the triangle. Using a process akin to the Pythagorean Theorem, scientists can deduce the location of the points of the triangle from the lengths of its sides and its angles. These values are then reintegrated into the CKM matrix to describe how different quarks decay into each other within particle interactions.
Footprints of a New Physics
The study of B mesons will augment the current understanding of the Standard Model and might even help identify new theories of physics that could change the way we view our universe.
"What we are seeing are footprints of a new physics governing our world," said Fred Gilman, Buhl Professor of Theoretical Physics and head of the Physics Department at Carnegie Mellon.
These "footprints" are seen in interactions that are not predicted or that violate laws of the Standard Model. In the case of CP Violation, one of these possible "footprints" might be seen in the measurements of the parameter sin 2Beta (one of the angles of the Unitarity Triangle). So far, most measurements of sin 2Beta have agreed to each other within accepted statistical errors.
However, at the Beauty 2003 Conference, Yuval Grossman of the Stanford Linear Accelerator and the Technion in Haifa, Israel cited recent experiments at Belle and BaBar, two "B-Factories" in Japan and at Stanford (respectively) dedicated to research of B-Mesons, that suggest deviations from current known values of sin 2Beta. Any further evidence of this kind would indicate the existence of new physics that supersedes the Standard Model. This would require a reconciliation of theoretical calculations with the new experimental data.
Hurdles to the New Physics
Although physicists compare data to draw conclusions, there is no truly agreed-upon theoretical value for what the asymmetry value should be. Only physical experiments and computational models, the most famous of which is called Lattice QCD, provide possible theoretical values. Many experimentalists believe that Lattice QCD calculations need to be more reliable. When Lattice QCD calculations first became available in the early 1990s, scientists were optimistic that they would soon have credible data to compare with their experimental results. However, in the years that followed, many new QCD calculations contradicted previous ones. Because of a lack of trusted theoretical values, some scientists such as Robert Cahn, a theorist turned experimentalist at Lawrence Berkeley National Laboratory in California, suggested at the conference that computational calculation projects should be given more money and therefore have review and funding committees, just like current physical experiments.
"Theory must become as rigorous as experiment," Cahn said in his talk called "B Physics in the LHC Era," referring to the new particle accelerator (the Large Hadron Collider) being built at CERN, the European Organization for Nuclear Research. These calls for accountability were not without controversy. Members of the Lattice QCD community attributed changes in data over time to the emergence of better algorithms and faster computers. Recent Lattice QCD calculations are up to par with current theory, they argued. On the other hand, experimentalists said they need confidence in the Lattice QCD values to apply them to the design of their experiments.
The growing interdependence between theory and experiment was evident in a talk given by David Asner of the University of Pittsburgh. He described how results at the CLEO-c experiment at Cornell were combined with Lattice QCD calculations to help design and analyze experiments at the B-Factories and at the Tevatron at Fermi National Laboratory in Batavia, Illinois. During a panel discussion at the end of the conference, moderated by Joel Bulter of Fermilab, many scientists, including Cahn, pressed for this larger accountability and acknowledgement of interdependency. By the end of the conference it was clear that theorists and experimentalists would have to reach a common ground in to move the field of B-Meson physics forward.
The Future of B-Physics
The future of B-Meson physics and CP Violation undoubtedly is nearing a transition.
"There is room for some improvement, but we are bumping up against systematic errors, errors that cannot be improved by better detectors or electronics," said Daniel Marlow (S'76), chair of the Department of Physics at Princeton University. When scientists switch on the LHC in 2007, B-Physics will shift perspective, going from research to understand and account for violations in the Standard Model to incorporating results of B-Physics and quark interactions in new theories of how the laws of our universe fit together.
Results over the next four years will chart the course of how this research will be conducted, according to Beauty participants. "We are looking into the future of high-energy physics," suggested Paulini. "There could be quite a few more surprising results in the next couple of years."
By: Michael Katz- Hyman