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Exotic Hybrid Mesons (Nuclear Glue and Confinement)
One of the main scientific questions that remains unanswered is the nature and behaviour of the "glue" which holds the world's elementary building blocks (protons and neutrons, or their constituent quarks) together. The current understanding in subatomic physics is that the quarks inside protons and neutrons are held together by exchanging a "force mediator" (glue) particle, appropriately named "gluon". Theory stipulates that eight types of gluons exist which bind "coloured" (red, green or blue) types of quarks into colourless objects by having three quarks (one red, one green and one blue) coupled together, or alternatively a quark of one colour coupled to a quark of its anti-colour. The proton and neutron are manifestations of such quark triplets, whereas the quark-antiquark pairs are collectively known as mesons. The important and puzzling feature of this construction of nature is that quarks are never found free, but only in the aforementioned triplet or pair configurations, a phenomenon known as confinement.
Indeed, the quarks inside protons would fly apart were it not for the strong nuclear force which is carried by gluons just as the electromagnetic force is carried by photons. Photons have no charge and cannot associate together; hence there are no atoms of light. But gluons carry a type of charge (so-called "colour" charge) and so can clump together. The result of such an amalgamation is a glueball, a particle made up of nothing more than the force that holds nuclei together. Physicists have long sought experimental evidence for glueballs and for exotic combinations of gluons and quarks into a new form of matter, known as "hybrids".
An understanding of the confinement mechanism in QCD requires a detailed mapping of the spectrum of hybrid mesons. There is good reason to expect beams of photons to yield hybrid mesons with quantum numbers not possible within the conventional picture of mesons as bound states. At Jefferson Lab in Newport News, VA exciting plans are underway to upgrade the energy of the electron accelerator to 12 GeV. Along with this energy upgrade, a hermetic detector housed in a new experimental hall (Hall D) will be used to collect data on photo-produced mesons with unprecedented statistics.
In the early 1970's Nambu postulated that quarks inside mesons are tied together by 'strings' in order to explain the increase of meson mass with internal angular momentum. We know now that QCD describes the strong interaction between quarks. Modern lattice gauge theory (LGT) calculations show that indeed a string-like chrom-oelectric flux tube forms between distant static charges. This flux tube leads to quark confinement and to a potential energy between the quarks that increases linearly with the distance between them. Infinite energy is needed to separate the quarks to infinity.
In particular, in a region around 2 GeV/c2, a new form of hadronic matter must exist in which the gluonic degree of freedom of mesons is excited. We refer to these mesons as gluonic excitations. The smoking gun characteristic of these new states is that the vibrational quantum numbers of the string, when added to those of the quarks, can produce a total angular momentum J, a total parity P, and a total charge conjugation C not allowed for ordinary states. These unusual combinations, like 0+-, 1-+, and 2+-, are called exotic, and the states are referred to as exotic hybrid mesons. The unique feature of the hybrids is that exotic quantum numbers are possible.
The Hall D Detector, that is is being designed for the GlueX Experiment , has at its "heart" the barrel calorimeter (BCAL), and the design and construction of this device has been undertaken by the SPARRO Group. This device will be responsible for the detection, identification and total energy measurement of both neutral (photons, neutrons) and charged (protons, pions) particles.
Specifically, the BCAL must provide excellent energy and timing resolution, low threshold of detection, and the ability to completely contain the electromagnetic showers resulting from the conversion of photons. The dynamic energy range for photons over the complete list of induced reactions in the liquid hydrogen target spreads from as low as 20 MeV to 1 GeV. At the higher end of this range, the BCAL must be at least 15 radiation lengths thick, in order to capture enough of the produced photon shower to allow the correct reconstruction of the photon's energy. This requirement is coupled to the minimum inner radius of the BCAL which will allow for the placement of the interior subsystems (chambers, start/vertex counter and target), in setting the radial dimensions of the BCAL. It's longitudinal dimension is largely dictated by the length of the solenoidal magnet, resulting in a length of 4.5 m.
A design that is particularly suited for this experiment, is one of a lead and scintillating fiber combination (Pb/SciFi), with the fibers embedded in lead sheets that have been plastically deformed ("swaged") and aligned parallel to the central axis of the Hall D detector. The construction of the BCAL took place at the University of Regina. For more information, please refer to our BCAL web site.
Electromagnetic Form Factor of the Proton and the Neutron
Precise measurements to this end require the use of electromagnetic probes, which are perfect surgical tools for the task at hand due to the weakness of their interaction. An electron accelerator in this field must offer high energy, high current, large duty factor, and sophisticated, large solid angle detectors. The Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, fulfills these requirements.