Experimental Study of the Quark-Gluon Structure of the Nucleon

My research is focused on the elucidation of how the basic constituents of the nucleon, quarks, are bound in a gluonic field. At present, we still have only basic one-dimensional momentum distributions of the quarks and gluons. Recent advances, both experimental and theoretical, have opened the possibility of learning about the true three-dimensional structure.

My present research program is based on using high-energy polarized beams to explore the nucleon's structure. A key quest is to understand in detail how the nucleon's spin of 1/2 hbar is derived from the quarks and gluons. The intrinsic spins of the quarks was once thought to be the complete explanation, but now we know that only about 30% of the nucleon spin comes from the quark spins. Another possible contribution could arise from the angular momentum of the gluonic field. Finally, the quarks could actually be moving in orbits inside the nucleon.

To study the contribution of quark spins to the nucleon, my group has participated in the HERMES experiment[1], using the 30 GeV polarized electron (or sometimes positron) beam at the Deutsches Elektronen Synkrotron (DESY) in Hamburg, Germany. We have studies the deeply inelastic scattering of these electrons from the quarks in polarized nucleons. Combined with the detection of hadrons created in the reaction, we have been able to determined how much each flavor quark contributes to the nucleon spin[2,3]. In addition, we have developed several new techniques, based on quantum interference between different final states, to study structures that result from orbital quark motion[4,5] and the dependence of the quark momentum on position within the nucleon[6,7]. For a nice review see Ref. 8; all the HERMES publications and their summaries for the general scientific public can be found at this link.

The contribution of gluon "spin" to the nucleon is being studied at the Brookhaven National Lab on Long Island, NY, using the colliding polarized proton beams available at the Relativistic Heavy Ion Collider (RHIC). Recent results [9] from the PHENIX experiment (in which we are involved) suggest that the contribution from the glue may be quite small, but there is still further investigation required in order to make a definitive statement. Another study at this lab (to be carried out in the near future) is to determine the contributions from virtual quark spins within the nucleon. Using the highest energy beams we can use the creation of the heavy electroweak vector bosons, W+, W- and Z0, to allow a direct measurement of the virtual quark spins using the weak interaction. A nice review of the PHENIX/RHIC spin program is available in Ref. 10.

In the more distant future, we have been working on developing a future experimental facility known as the Electron-Ion Collider. Recently, this facility received strong endorsement from the Nuclear Science Advisory Committee in its Long Range Plan.

If you have further questions, please don't hesitate to ask!