Spinning Quarks Yield Clues to Orbital Motion

Monday, February 6, 2012

In a new Physical Review Letters paper (J. Huang et al., PRL108, 052001 (2012), [link], co-authored by members in Prof. Haiyan Gao's Medium Energy Physics Group, new measurements of novel spin phenomena provide the first experimental indication of a partial alignment of quark spin along the direction of motion of a neutron spinning perpendicularly to its direction of motion. Such an alignment, known as transversal helicity, can only be observed if the quarks undergo orbital motion inside the neutron. The experiment used a high-energy polarized electron beam at DOE's Thomas Jefferson National Accelerator Facility (Jefferson Lab), located in Newport News, VA. Electron spins were frequently flipped so that the spins of the incident electrons were parallel to their motion half of the time and anti-parallel in the other half. The electrons scattered off polarized neutrons in helium-3 nuclei, with neutron spins aligned perpendicular to the beam direction. Neutrons decay into protons and antineutrinos. Spin aligned helium-3 nuclei are what nature offers physicists as the best effective spin aligned neutrons.

The high-energy electrons penetrated deep inside the neutron, striking one of its quarks. Remnants of the recoiling quarks were observed in high-precision particle detectors as high-energy pions in coincidence with the scattered electrons. In this first measurement of its kind, the observed dependence of the reaction rate on the beam and target spin orientations allowed the physicists to probe the transversal helicity, an important aspect in the three-dimensional imaging of quark motion in the neutron. The success of this experiment has inspired a rich program of future experiments at energy upgraded Jefferson Lab that Prof. Gao is leading to further explore transversal helicity in the neutron and proton with much higher precision and larger kinematic coverage. This research was recently featured in the Duke Today article "Quarks make their world turn." Read it online here.