Three decades after the establishment of QCD as the theory of quarks and gluons, understanding QCD works remains one of the great puzzles in nuclear physics. A major challenge arises from the fact that the degrees of freedom observed in Nature are bound objects composed of quarks (and gluons): hadrons such as protons and neutrons. The remarkable and unique feature of QCD at large distances --- quark confinement --- prevents the individual quark and gluon constituents making up a hadronic bound state to be removed from the system and examined in isolation. As a corollary, quarks move with relativistic speeds inside the hadrons and this motion makes up 99.9% of the human mass.
At the other extreme, the property of QCD known as asymptotic freedom, in which quarks interact weakly at (very) short distances, allows one to easily describe certain properties of hadrons and atomic nuclei at extremely high energies in terms of quarks and gluons.
Despite the apparent differences between the hadronic and quark-gluon regimes, in Nature there exist instances where the behavior of low-energy hadronic cross sections, averaged over appropriate energy intervals, closely resembles that at asymptotically high energies, calculated in terms of quark-gluon degrees of freedom. This phenomenon is referred to as quark-hadron duality, and reflects the relationship between confinement and asymptotic freedom in QCD. Surprisingly, this phenomenon already takes place at rather low energies and averaged over relatively small energy intervals (or a few hadronic states). This proposal investigates electron scattering reactions off protons, neutrons and nuclei in an effort to understand the origin of this phenomenon.
The proposal encompasses several experiments scattering electrons off a proton target to push and understand the limits where quark-hadron duality arises. It also includes the development of a novel detector to as closely as possible mimic an experiment scattering electrons off a neutron target. According to many theories the different onset of quark-hadron duality for protons and neutrons will point to the origin of the duality phenomenon. Lastly, interesting questions also arise in atomic nuclei: does a proton bound in a nucleus change structure due to its underlying quark-gluon structure? Within QCD, there is no way to derive anything like a nucleus in which the constituents (in normal life, the protons and neutrons) do not change as the mean density increases. Similarly, can one witness the underlying quark-gluon structure of hadrons when they traverse a nucleus?
The proposing physicists have spearheaded the research thrust of quark-hadron duality and more generally the transition between the hadronic and quark-gluon descriptions of protons and nuclei. The approved project will allow this research to come to fruition and will simultaneously provide advanced, internationally-competitive, scientific training for underrepresented minorities. Two of the proposers, and two Ph.D. students, are female. In addition, one of the students is Afro-american and two are African.
