Matter's Basic Building Blocks
Scientists use telescopes to study stars and microscopes to study living cells. At Jefferson Lab, they use the CEBAF accelerator and its three experimental halls to study the stuff of stars, cells and everything in between. At its heart, all the matter in the universe is made-up of particles called quarks and gluons. In simplest terms, quarks and gluons combine to form the protons and neutrons in the nucleus, which, together with electrons, make up atoms. The CEBAF accelerator can "see" things a million times smaller than an atom, allowing scientists to study both the nucleus of the atom and the particles that comprise it.
The 12 GeV Upgrade will allow Jefferson Lab to pursue with even greater precision and resolution its primary mission of studying these basic building blocks of matter. It will make it possible for physicists to employ new methods to study the basic properties of these particles, how they are formed, how they interact, and the forces that mediate these interactions. Through experiments aimed at revealing these and other secrets of matter, physicists will use an upgraded CEBAF to contribute to Jefferson Lab's goal to push our knowledge of nuclear and particle physics well beyond its current level.
With the 12 GeV Upgrade, scientists plan to address one of the great mysteries of modern physics -- the mechanism that "confines" quarks to exist only together with other quarks and never alone. According to a fundamental theory of physics, the force that binds quarks together -- the strong force -- is so powerful that quarks cannot exist in isolation. Until recently, quarks had only been seen in pairs (in particles called mesons) and in triplets (in particles called baryons: protons and neutrons are baryons). But other quark combinations are theoretically possible. The new experimental hall planned for the Upgrade (Hall D) aims to produce exotic or hybrid mesons and study their properties. These exotic mesons consist of a quark and anti-quark held together by gluons, but unlike conventional mesons, the gluons are excited. The gluons are the carriers of the strong force (the "glue") that binds the quark and anti-quark together. Although predicted by theory, there is only tentative experimental evidence for these exotic mesons, and it is essential that we produce many of them and study their properties in detail if we are to fully understand the strong force and how it confines quarks.
The Upgrade will also enable scientists to research the fundamental structure of protons and neutrons (collectively called nucleons). It's now known that nucleons are basically made up of three quarks held together by gluons. But nucleons also contain a 'sea' of quarks and gluons that pop into and out of existence rapidly. The increased energy and new detectors of the 12 GeV Upgrade will allow scientists to probe how these particles interact to determine nucleons' basic properties -- their mass, their spin and how they interact with other particles around them.
How protons and neutrons bind together to form the nucleus is another puzzle physicists want to solve. One idea is that when nucleons are close together, the strong force binding quarks together inside a nucleon can "leak out", and even the "leaked" force is so strong that it can overcome the electric repulsion between the protons to bind the nucleons together. Another idea is that protons and neutrons swap quarks, and this exchange provides the force that binds nucleons together. The Upgrade will allow scientists to study new aspects of how nucleons interact and thereby determine the mechanism that binds them together.
Finally, an upgraded CEBAF will allow physicists to study the limits of the "Standard Model," a theory that describes the fundamental particles and their interactions. So far, tests of this model have confirmed its validity (except for the neutrino sector). The Upgrade will open new opportunities for probing the model's limits. If the Standard Model predictions are confirmed, we will have extended our knowledge of its range of applicability and its accuracy. If we discover that it fails, we will have the first important clues necessary to develop a new or revised theory of matter that will be even more accurate and inclusive and provide deeper insights into the nature of the world we live in.