Grant: Subatomic Physics Envelope - Individual

During the past century, a revolution has occurred in the understanding of the structure of matter.  We have moved well into examination of the myriad of complexity contained inside atoms - proposed as "indivisible" in 1805 by Dalton - from molecular structures, to electron orbitals around a nuclear center, to the structure of the atomic nucleus itself (where lies the interest of this research proposal).  In 1932, the discovery of the neutron brought acceptance to the idea that the atomic nucleus is composed of elementary particles called neutrons and protons.  Further, during the 1970's, experimental results made it clear that even these neutrons and protons are individually made of smaller, more fundamental, particles called quarks and gluons.  Modern nuclear physics researchers now strive to gain clear insight into the detailed interactions between neutrons and protons within nuclei in order to understand the relation of these interactions to the underlying structure of quarks and gluons.  Further, considerable effort is also pointed toward understanding the interaction between these quarks and gluons themselves, and how these interactions give rise to the internal structure properties of protons and neutrons. This research project exploits the technological advances which make possible detailed structure measurements of the proton and light nuclei using intermediate energy electron beams at the Thomas Jefferson National Accelerator Facility ("Jefferson Lab") in the USA.  One specific question being addressed, through measurement of polarization observables in the scattering of electrons from liquid hydrogen, is a precise determination of whether a particular excited state of the proton (the so-called "Roper resonance") represents a "breathing mode" excitation of the proton, in which the internal quarks effectively slosh in-and-out against each other ... or whether this state arises from a complicated interaction of the quarks and mesons within the proton.  Similar methods will be used in the disintegration of deuterium to study a low-energy regime where well-developed nucleon/meson theories of this simple nucleus should be accurate, but seem to have difficulty describing existing data.