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September 18, 2004

Scientific discoveries highlight U of C summer

Over the summer, three University of Chicago researchers developed a model that accurately reproduces periodic population surges and explains why these surges, known as outbreaks, have historically been unpredictable.

Outbreaks typically occur at long, irregular intervals, and the population suddenly rises and declines. The team, which includes Assistant Professor of ecology Greg Dwyer and Ph.D. candidate Susan Harrell Yee, used the gypsy moth as an example. Outbreaks of the moth, which was introduced to America in the 19th century, have laid waste to large areas of forests. About 80 species of butterflies and moth populations experience outbreaks, as do some small mammals. Dwyer says that this model can probably describe any of these species.

Dwyer, who collaborated with Jonathan Dushoff of Princeton University, created a model which combines additional data with two prevailing theories on insect outbreaks: the host-pathogen theory, which attributes outbreaks to the absence of disease in insects; and the predator theory, which attributes an explosion in insect population to a decline in predators.

Dwyer's model, the "host-pathogen-plus-predator model," describes how the decline of disease and predators upsets the formerly stable equilibrium of the insect-population dynamics; weather variability, competition of the insects' predators, and adaptation of insects' immunity further complicate the model.

The host-pathogen-plus-predator model, introduced in the July 15th issue of Nature, is the most accurate model explaining outbreaks. Unlike previous models, this one does not concentrate on the few species whose outbreaks occur at regular intervals; it takes more kinds of insect populations into consideration.

The model also explains the mystery of spatial synchrony, a phenomenon that occurs when outbreaks of the same species happen at the same time in locations hundreds of miles apart. The study's authors, though unsure why their model accounts for spatial synchrony, believe that the long time interval between outbreaks minimizes the effect of the environmental differences between the two affected areas.

Quark

University of Chicago scientists Edward Blucher, Richard Kessler, and former U of C scientist Sasha Glazov have solved an incongruence between experiment and theory that has bothered particle physicists for 20 years. They demonstrated that theory was right and past experiments have been wrong, using data collected at the Fermi National Accelerator Laboratory that are more precise than past measurements.

The three papers authored by the team and signed by the 55 other members of the Kaons at the Tevatron collaboration at the Fermi lab are on the subject of how some quarks interact in the beta decay of particles. However, the 1997 experiments that led to the papers were conducted on an entirely different phenomenon called CP violation.

CP violation is a process that causes nature to produce more matter than antimatter. In 1999, the group announced that they had definitively observed a new type of CP violation that had not been seen since an experiment in 1964. The 1964 experiment earned its authors a Nobel Prize in physics.

James Cronin, one of the recipients of the Prize and a professor emeritus in physics at Chicago, told the University of Chicago Chronicle that the success of the articles reflects "the enormous care with which Ed Blucher and his colleagues created this data…. Everything fits together in a really perfect way."

Blucher's team's experiments dealt with weak nuclear force, one of the four fundamental forces in the universe, which governs the emission of radioactive beta particles and also powers the sun. In previous experiments on weak nuclear force, scientists observed how up quarks were coupled to down quarks and strange quarks. The six types of quarks are thought to be the smallest particles that make up the universe.

Theory suggests when these quarks couple to one another, the measurements should add up to one. However, experiments kept showing a lower number.

The 1997 experiment, which produced strange quarks as a component of neutral kaon particles, allowed scientists to observe that up quarks couple to strange quarks by decaying into each other. The team found that the coupling strength of strange quarks to up quarks was 3 percent higher than what was previously determined.

To bolster the findings and assure their accuracy, the team created computer simulations that thoroughly tested the functioning of their particle detector. In this way, Blucher told the Chronicle, the team was able to correct the detector's small flaws. These experiments are also the first time that all of the relevant measurements were made during one modern experiment.

Four papers—the three written by University affiliated researchers and one written by Troy Andre, a graduate student who performed the calculations on the validity of the measurements—will be published on the experiment, created a sizable change in the physics world. The U.S. Department of Energy, the National Science Foundation, and the Ministry of Education and Science of Japan supported the research.Saturn

For six and a half years, it sat aboard an unmanned craft hurtling through space on a 2.2-billion-mile journey. But finally, last June, as NASA's Cassini spacecraft entered Saturn's orbit, the University of Chicago-designed dust collector aboard the spacecraft began its mission to solve the mystery of the planet's famed rings.

The University of Chicago-designed instrument, dubbed the High Rate Detector, has been collecting and recording the occasional dust particles that have hit the Cassini spacecraft while it traveled on its six-year journey to Saturn. Last July, when Cassini became the first spacecraft ever to enter Saturn's orbit, the High Rate Detector began its main mission of measuring and analyzing dust particles ranging in size, according to The University of Chicago Chronicle, "from twice the diameter of a single strand of human hair to 100 times smaller."

Capable of detecting 100,000 particles per second as they collide with two small detectors, the university's instrument will collect data for the next four years as

Cassini orbits the planet 76 times and interacts with its 31 known moons.

Built by Anthony Tuzzolino, a senior scientist at the University's Enrico Fermi Institute, the High Rate Detector is a component of a larger instrument, the German Cosmic Dust Analyzer. The two instruments will observe the physical, chemical, and active properties of Saturn's dust and its interactions with the planet's rings, frozen moons and magnetosphere. They are two of 18 instruments on Cassini and its Huygens probe.

The $3-billion Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency, involving 260 scientists from the United States and 17 European nations.

Already, Cassini has sent back high-resolution images of Saturn's rings and captured pictures of two previously undocumented moons during its first orbit.

The High Rate Detector is the latest in a line of University of Chicago instruments used to detect small particles. Last January, an instrument similar to the Cassini detector collected and measured comet dust aboard the Stardust spacecraft during its encounter with Comet Wild 2. Another Chicago dust detector flew aboard an Air Force satellite to study orbital debris from 1999 to 2002.String Theory

University of Chicago physicists joined scientists from around the world on August 12 to celebrate the 20th anniversary of the first superstring revolution. Jeffrey Harvey, the Enrico Fermi Distinguished Service Professor in Physics at the University of Chicago, presented the symposium's opening address, titled "The impact of the '84 revolution on physics, or how I learned to stop worrying and love superstring theory," to the 75 physicists who gathered at the Aspen Center for Physics in Colorado to mark the event.

String theory is a physical model whose fundamental starting points are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles), which were the basis of most earlier physics. Strings are believed to measure one millionth of a billionth of a billionth of a billionth of a centimeter across, beyond the means of any current particle accelerator to measure.

For many reasons, string theories are able to avoid some of the problems associated with the presence of point-like particles in physical theory. String theory has been able to unify gravity with other fundamental forces, such as electromagnetism.

Detailed studies of string theories reveal that they describe not only strings but also other objects, including points, membranes, and objects of higher dimensions. A rising numbers of physicists see superstring theory as a promising tool that will allow them to put together a set of natural laws that describes every object and force in the universe in one unifying theory.

After the discovery of string theory, Harvey, Emil Martinec (a physics professor at Chicago), and two of their colleagues published a paper postulating "heterotic" string theory, a hybrid theory of two prominent string theories. The four scientists, then working at Princeton University, were lightheartedly nicknamed the Princeton String Quartet.

The history of string theory has seen a series of confirmations of past theories and signs that all prevailing work may be pieces of a single larger theory, sometimes called M-theory.

While there is still no experiment verifying string theory, scientists may see evidence that supports it when the Large Hadron Collider (LHC) begins operating in 2007 or 2008 at CERN, the European particle physics laboratory. This machine will be the first ever able to measure things as small as strings. Such evidence might lead to confirmation of "supersymmetry"—new particles that would be partners of known elementary particles.