Chinese-American Collaboration in Hadron Physics Bears Fruit

Monday, September 19, 2011

At right: Prof. Gao and her former Ph.D. student Xin Qian, currently a Millikan Fellow at Caltech. Xin gave an invited talk at this workshop.

Prof. Haiyan Gao, the new chair of Duke Physics, is working on several fronts to encourage collaboration among physicists in American and China—particularly among physicists who study hadrons, particles that interact through the strong force. The time is ripe because students and young scientists in China are jumping at the chance to do research in the United States, and scientific funding agencies in China—whose budgets are growing—are eager to support international collaboration in physics. Just this year, the National Science Foundation of China awarded two grants to support research being done at the Thomas Jefferson National Accelerator Facility  in Virginia by Gao and a team of 130 collaborators from 40 institutions and 8 countries—including China.

A paper describing some of that research was published in Physical Review Letters August 10, 2011. Xin Qian, who earned his PhD at Duke last year, is the lead author on the paper, which reports the results of an experiment exploring the three-dimensional motion of quarks inside a neutron. Qian is currently a Millikan Fellow at the California Institute of Technology. He is also the winner of the 2011 Jefferson Lab Thesis Prize. Gao, who was Qian’s advisor at Duke, is also one of the authors of the paper.

While researchers at other laboratories have previously explored the three-dimensional motion of quarks inside protons, this was the first time a team had looked at the motion of quarks inside a neutron. The team used a helium-3 nucleus as the target for the 6 GeV electron beam. A polarized helium-3, which has two protons and one neutron, is an effective polarized neutron target because the spins of the two protons are opposite and “cancel” each other out, leaving the spin of the lone neutron. The polarization is perpendicular to the plane formed by the beam and the electrons that scatter off the target, and it can be arranged so that it is pointing up or down.

When the electron beam hits the helium-3 nucleus, the virtual photon exchanged between the electron and the target couples to a quark inside directly. The struck quark combines with another quark outside the nucleus to form a particle called a pion. By measuring the azimuthal angular distribution of such pions relative to the plane of the electron, physicists can infer information about the movement of the quarks inside the neutron before the beam hit. The team of scientists plans to continue this line of research, with some important improvements. The Jefferson Lab is undergoing a major upgrade of the energy of the electron beam from 6GeV to 12 GeV. Gao is leading a major effort to build a new $25-million spectrometer that will take the advantage of higher electron beam energies. The Jefferson Lab energy upgrade, which is  funded by the Department of Energy, is  expected to be complete in 2013. “We have a very good plan in place for the next decade for a comprehensive major program at Jefferson Lab using 12 GeV and the new spectrometer,” Gao says. “We’re also going to build a new polarized proton target.”

Prof. Gao led an evening discussion during the 2011 US-China Hadron Workshop in Weihai, China. Also in the photo are: Prof. Fan Wang from Nanjing Uinversity, and Prof. Robert D. McKeown, Deputy director of Jefferson Lab.

In addition to encouraging U.S.-Chinese collaboration at Jefferson Lab, Gao has also co-organized an annual workshop on hadron physics in China for the past several years. Participants are primarily from China and the U.S., but there are some from Europe as well. The first workshop was in 2006, and it has been an annual affair since 2009. The 2012 workshop is already planned for Beijing. “Before this experimental program and the workshop, the collaboration was on the informal side,” Gao says. The workshops allow physicists and representatives from funding agencies to come together to plan more formal collaborations, such as the research and the upgrade at Jefferson Lab. Here at Duke, Gao was recently asked to be on the China Faculty Council set up by the Provost’s office. Council members are charged with facilitating research connections and new collaborations for faculty or units interested in working in China and/or linking ongoing projects to China.  Duke is building a campus near Shanghai, which is where Gao grew up. With all these projects underway, Gao says she sees a bright future for collaborative work among Chinese and American physicists.