Nuclear Physics at Richmond The Frontier One of the major frontiers in nuclear physics is understanding why quarks and gluons act the way they do to create everyday matter. We still do not have a quantitative understanding of how quarks unite to form nuclei. No one has ever seen a lone quark. This property, called confinement, means quarks cannot be isolated. The quarks are bound with other quarks by the color force to form pairs or triplets. The force between quarks increases as the distance between them increases (see left-hand figure below), so no quarks can be found alone. The reasons for quark confinement are complicated. The best theory is quantum chromodynamics (QCD) and there is no analytic proof that QCD should be confining, but intuitively confinement is due to the force-carrying gluons having color charge. As two electrically-charged particles separate, the electric fields between them diminish quickly, allowing electrons to become unbound. However, as two quarks separate, the force experienced by the quark remains constant regardless of its distance from the other quark. The total energy increases linearly with distance. This is one of the major challenges in nuclear physics since quarks can not be studied independently. Thomas Jefferson National Accelerator Facility, Newport News Va Thomas Jefferson National Accelerator Facility (JLab) is a US Department of Energy national laboratory and the newest precision accelerator in the United States. It is a giant electron microscope used for illuminating the core of an atom. Its focus is on determining the nature of quarks and how they bind together to make neutrons and protons and most of the matter we see today. The Continuous Electron Beam Accelerator Facility (CEBAF) is the central particle accelerator at JLab. CEBAF is capable of producing electron beams of 2-6 GeV. The racetrack shaped accelerator is about 7/8 mile around and 25 feet underground. The electrons are accelerated through the straight sections and then bent around the curves using magnets. The electrons travel up to five times around the track at relativistic speeds and are then sent to one of the three experimental halls where the beam collidess with the target. What is the Universe Made Of? The Standard Model is a theory that describes and explains subatomic particles and interactions. Matter as we know it is comprised of six quarks, six leptons and the force carriers. Quarks and leptons are called fermions with half-integer spin and the force carriers are known as bosons with integer spin. What is Nuclear Physics? CLAS – The CEBAF Large Acceptance Spectrometer, located in Hall B at JLab, is used to detect electrons, proton, pions and other subatomic particles. CLAS is a 45- ton, $50-million radiation detector with around 40,000 detecting elements and nearly a 4π coverage. CLAS is able to detect most particles scattered off in a nuclear collision because it covers a large range of angles. The particles go through each region of CLAS leaving behind signals that are collected and stored on tape. Using the information gained from the layers, the collisions are reconstructed and the intensity pattern reveals the forces and structures of the colliding particles. CLAS is able to determine the mass and velocity of the particles and can be used to reconstruct the events so that we can better understand what is happening. The event rate is about 3000 Hz (or 15 MegaBytes/s). The initial analysis is done at JLab and we analyze those results more deeply at the University of Richmond. Bosons are force carrier particles which create the interactions between the matter particles and defines the properties of matter. There are three known forces; Strong or Color, Electroweak, and Gravity. The strength of the force depends on the mass of the force carrier particle as well as the distance between particles. Gluons act on quarks and other gluons and bind atomic nuclei together. They keep the quarks from being able to be separated by conserving color charge. Quarks are confined by a force equivalent to 22 tons. This allows no quark to be seen alone. The Electroweak force acts on quarks and leptons. It causes heavier particles to decay into the lighter ones. This explains why most of our matter is comprised of up and down quarks (including neutrons and protons) and electrons. Gravity, though most familiar to people, is the particle we know the least about. It is the weakest force, (Think about it, you need something as big as a planet to notice a difference) and the Graviton, though speculated to exist, has never been observed. Fermions make up most of the matter around us. There are two groups, the first is comprised of leptons and their corresponding neutrinos. The neutrinos are much smaller than the other leptons and interact with other matter via the weak force and gravity. There are 6 quarks that make up matter. As far as we know, quarks can only be found in groups called hadrons. There are two known types of hadrons, baryons which are made from three quarks, and mesons which are made from a quark and an anti- quark. An example of a baryon is a neutron or a proton, however it is interesting to note that their mass may be made up mostly from the forces that act on the quarks and not the bare quarks themselves. These two diagram show the structure within the atom. The left shows how much smaller a quark is than an atom. The top tier is the atom, which is 100,000 times bigger than the nucleus. The quarks inside the protons and neutrons within the nucleus are at least 100,000 times smaller. The figure above shows how the quarks make up the nucleons and how the nucleons form to make the nucleus and atom. Bottom: CLAS Detector in Hall B. Upper Right: UR students visiting JLab: Rusty Burrell, Nate Lawrence, Brent Follin and Kuri Gill. The Real Table of the Elements. Hall B Racetrack-shaped Accelerator (underground) Lattice QCD has confirmed the existence of flux tubes between static massive charges. The potential between two quarks rises linearly as a function of separation. This allows the quarks not to be separated Nuclear science began by studying the structure and properties of atomic nuclei, the cores of atoms. Today, its reach extends from the quarks and gluons that form the substructure of the once- elementary protons and neutrons, to the most dramatic of cosmic events— supernovae. At its heart, nuclear physics attempts to understand the composition, structure, and properties of atomic nuclei.