Physics Chair Sees Entire Universe in a Neutron

One summer evening, 12-year-old Haiyan Gao returned to her family's home in Shanghai a bit late after spending the day at a friend's house.  As she came in the door, her father asked what she had been doing. She told him she had been watching her friends play chess."So you think Madame Wu or Marie Curie would spend the summers wasting all that time watching others play, instead of trying to solve some physics or mathematics problems?" he asked her. It was the 1970s. Chinese citizens were just starting to hear about the accomplishments of their country's scholars. Gao, now the chair of Physics at Duke University, says her father regularly tried to inspire her and her sister by telling them stories of Chien-Shiung Wu, a female physicist. Wu came to the U.S. in the 1930s and helped scientists unravel the chain of reactions needed to build the atomic bomb and to harvest energy from nuclear reactions. She also ran the experiments that showed that if a mirror were held up to the universe, the reflection would not be identical.That night, "he made me feel a little bit bad, so I went back to books," Gao says, smiling at the memory.Gao went on to study physics at Tsinghua University, graduating first in her class. She came to the U.S. in 1989 to attend Caltech and pursue a Ph.D."I knew that to make new discoveries I would have to work with the spin of the neutron," and probably with particles that were polarized, meaning they all spin in the same direction, she says. Once in Pasadena, she met with theoretical and experimental nuclear and particle physicists.Gao recognized that going into particle physics would require work on large collaborative projects at high-energy accelerators such as Fermilab and CERN. But she wanted to "do something original, fundamental and not too overwhelmingly large," she says. "I wanted a project I could be on top of, one where I could know what's going on during the whole project, even lead it as a student." She chose to work with nuclear physicist Robert McKeown to probe the structure and spin of the neutron, and also interacted regularly with physicists Brad Filippone and Steve Koonin.The neutron had been discovered by James Chadwick in 1932 and Madame Wu, Marie Curie and others explored the particle's role creating nuclear energy, weapons and radiation. But physicists to this day cannot explain precisely what causes the particle to spin, even while using this basic property in applications such as MRIs, magnetic resonance imaging.McKeown first showed Gao how she could use Helium-3, a rare-Earth isotope of helium, to probe the neutron and better understand its spin. A free neutron decays too quickly to study it. But the Helium-3 atom has two protons and one neutron and does not decay, making its neutron easier to study. The neutron and the protons in He-3 spin individually like tops or planets rotating around an imaginary axis. At a conference on electroweak nuclear physics at Duke this week to honor McKeown's career, Gao will update colleagues on her work using Helium-3, to probe the neutron's spin.  "The neutron could be a wonderful laboratory where physicists discover new physics and new forces of nature," Gao says.In her experiments, Gao and her collaborators have found that a Helium-3 neutron's rotation comes mostly from an orbital motion of the quarks inside the particle.  The results of the experiment, however, are still just a snapshot of how the quarks turn, Gao says. She and other physicists need more data, almost like what you would see in a three-dimensional movie compared to the static image they now have, so she and her team are designing a new instrument to run at Jefferson Lab in Newport News, Va., where McKeown is now the deputy director for science. The instrument, called SoLID or the Solenoidal Large Intensity Device, will be the most sensitive yet to study how quarks spin within neutrons. It will cost more than $20 million dollars and be built as an international collaboration. To fund part of the experiment, Gao has been helping physicists in China apply to their equivalent of the National Science Foundation and other government agencies to obtain nearly $1 million dollars. The money will fund the design and construction of SoLID's Gas Electron Multiplier detectors. These detectors are the key pieces of equipment that will measure how quarks spin and how that affects the rotation of the neutron they are in, Gao says.But she is not only interested in studying the neutron to figure out what makes it spin. She has also been inspired by McKeown to use Helium-3 as a sub-atomic lab to look for new forces of nature. In her talk she will also describe another new experiment she is running in her lab here at Duke.The key structure in the experiment is a micrometer-thin glass cell filled with polarized Helium-3 gas. The atoms have a precise pattern to their spin, which Gao and her students can measure and track through time. In their experiment, she and her team will place a non-polarized ceramic block or liquid next to the glass cell. If they detect a change in the spin pattern of the He-3 atoms -- and they rule out all systematic errors -- Gao and her students could make the first, direct measurement of the short-range force.This force is still theoretical, but if discovered would become the fifth force of nature, joining the strong, weak, electromagnetic and gravitational forces. Physicists predict the short-range force to act at the micrometer or centimeter scale. And, if it exists, it may help scientists create a better model of the universe and also what makes up its mass.Gao's father can no longer say his daughter has been her wasting time. His inspiration didn't go unheeded, either.