Probing the fundamental structure of nuclei at Jefferson Laboratory
We use the intense and energetic beams available at the Thomas Jefferson National Laboratory (JLab) to probe the internal structure of nuclei. We use the CLAS12 spectrometer to perform experiments in which an electron scatters off a quark that then propagates through nuclei. We seek to understand how the nucleus reacts to an energetic quark passing through it. Answering this question will reveal mechanisms by which quarks are confined inside nucleons, and also constrain the position and momentum of quarks and gluons inside nuclei.
The Electron-Ion Collider
A long-term interest of our group is the future Electron-Ion Collider (EIC) at Brookhaven National Laboratory. The EIC will use electrons to image the quarks and gluons inside nuclei with unprecedented precision. Acting like a powerful electron-microscope, it will allow us to take tomographic 3D images of the atomic nucleus. By doing so, it will help us find the origin of the mass and spin of protons and neutrons, which are the building blocks of the all visible matter in the universe. The EIC has also the potential to discover a hypothetical new form of “gluon matter”, and reveal the mystery of why quarks are never observed in isolation.
UCR groups are working in R&D and the conceptual design of future EIC experiments. UCR is part of the University of California EIC consortium, which includes 4 UC campuses and all three UC-managed National Laboratories.
Why are the constituents of the proton confined?
All of the phase transitions that we are able to observe so far on earth involve the electromagnetic interaction, for example, the melting of ice, the formation of steam, or the magnetization of metals. We are seeking to see, for the first time on earth, an entirely new form of phase transition, that involving the Strong Interaction or quantum chromodynamics (QCD) in which a gas of quarks and gluons called a “Quark-Gluon Plasma” (QGP) condenses to form protons, neutrons and other hadrons. This is a remarkable transition because we believe that it also is the transition which gives rise to the protons and neutrons that make up most of the matter we see here on earth. Another curious thing occurs as protons and neutrons condense out of the hot vacuum that makes up the QGP that is presumably created in the most violent collisions.
Shortly after the big bang the universe went through a period of fantastic expansion, dubbed inflation. What could cause such a cataclysmic event? Just as the explosive power of turning water into steam powered much of the industrial revolution, it is believed that a phase transition of some sort – no one is sure what – powered inflation.
Where does the spin of the proton come from?
By some mysterious mechanism, having to do with the complex behavior of QCD, the gluons which have one unit of spin and quarks which have 1/2 a unit of spin, bind together to form something that has exactly 1/2 of a unit of "spin".
Spin is a property of particles as fundamental as charge and mass. The spin of the proton was first determined in 1927, yet we still do not know what makes up the spin of the proton. It was believed that the spin was carried by the quarks that make up the proton. However, experiments in the 1980’s led to the startling discovery that quarks contribute very little to the proton spin, setting off the “proton spin crisis”. It is now theorized that the spin is carried by gluons, which hold the proton together. Spin measurements have historically yielded surprising results and are a stringent test to theories as spin is an intrinsically relativistic and quantum mechanical aspect of particle interactions.