The University of Connecticut
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It is still an open question whether the Standard Model of the strong nuclear force reduces to the Quark Model at low temperature and density. Experimental results on heavy quarkonia are in good agreement with the SM expectation for QM states, and so far no conclusive evidence of new degrees of freedom has emerged. However numerical calculations of the excitation spectrum of low-temperature QCD on a space-time lattice indicate that the additional degrees of freedom carried by the gluon field should be manifested by the appearance of so-called exotic (non-QM) states in the spectrum. Experimental factors (small cross sections, complicated decays) strongly favor conducting the search for exotic mesons in the light-quark sector. This presents challenges of its own. Theoretically, the interpretation of the light meson spectrum in terms of fundamental degrees of freedom requires solving the strong-coupling problem, which means massive lattice calculations. The nuclear physics theory community, including the theory group at Jefferson Lab, is presently mounting a major effort to take advantage of advances in computing to tackle this problem. On the experimental side, the challenge is to measure the light-quark meson spectrum with sufficient statistics over a sufficiently large mass region to conclusively test the predictions of lattice QCD. Past experiments, especially over the last decade, have shed a good deal of light on the light meson spectrum; two or three strong candidates for exotics are now in hand. If indications from the lattice are correct then many more such states lie in the region between 1.8GeV and the charm threshold. This region has not been intensely searched in the past. High-energy photoproduction experiments at Jefferson Lab offer a new window into this interesting region, and this program (Hall D) has been identified by the laboratory as its leading new initiative during the coming decade.
The experimental nuclear group at UConn is committed to playing a leading role in the Hall D experiment at Jefferson Lab. R. Jones is a member of the collaboration board of Hall D, and co-chairs the working group in charge of the photon source. He has primary responsibility for the design of the coherent bremsstrahlung facility in Hall D. The UConn group has assumed specific R&D responsibility for the photon beam collimator and the focal plane instrumentation. We have also developed the GEANT simulation for Hall D, and are building up a Beowulf cluster to serve as a simulation platform for the collaboration. These activities offer many opportunities for student research at both the undergraduate and graduate levels.
The Radphi experiment is the precursor to Hall D. It is mounted behind the CLAS detector at Jefferson Lab, and has access to a photon beam of half the energy expected for Hall D. At this limited energy, Radphi concentrates on the mass region in the meson spectrum around 1GeV, where the enigmatic states a0(980) and f0(980) are found. The φ meson located just above these states in mass is copiously producted in photoproduction, and the decay rates of the φ into the two scalar states provides a probe of their underlying structure. UConn joined Radphi (1996) after the experiment was approved (1994), and soon thereafter made the proposal to extend the forward coverage of the detector by adding the barrel calorimeter and trigger scintillator system from the Jetset experiment (formerly at CERN LEAR). This upgrade was carried out during 1999, primarily by a joint UConn - Indiana effort, and was proved successful during a summer 1999 test run. Summer 2000 the experiment completed its allocation of 700 hours of beam time, collecting over 800 million physics triggers. Starting September 2000, R. Jones was appointed data analysis coordinator for Radphi. During the first year of analysis the φ signal was found in the data. The search for the a0(980) and f0(980) is still underway. UConn is also responsible for the GEANT Monte Carlo simulation of Radphi. The program was developed by UConn students, and is now being actively used on the UConn cluster to study the major background reactions in the Radphi data. Graduate students at Indiana, William and Mary and UConn are preparing their PhD theses on this experiment.
The data from Jetset (data-taking completed 1995) on the search for narrow states produced in proton-antiproton annihillation has been analyzed. As analysis coordinator for Jetset, R. Jones oversaw the publication of the first results from the experiment. While at UConn he has also developed the partial-wave analysis of the φφ final state, and this analysis has been carried out independently at UConn and by A. Palano at the University of Bari. Some evidence of a state similar to the χ(2230) that has been seen by Mark III and BES in J/Ψ radiative decays is seen in the Jetset pwa results, and this result has been shown at conferences. Some minor differences between the results of the two analyses remain, and are being resolved.
A new effort with a different physics focus from the above program is the Qweak experiment (Jlab LOI-01-101, contact person R. Carlini) which has as its goal a 3% measurement of the weak charge of the proton at low Q2, as a search for new physics. At low Q2 the weak vector coupling of the proton is protected against strong renomalization effects by current conservation, and so can be rigorously related to the sum of the weak vector charges of the quarks. A comparison of the new measurement with existing measurements of the electroweak coupling constant at the Z0 pole provides a test of the running of the weak coupling constant with Q2 that is predicted by the SM. The value of Qweak is directly proportional to the parity-violating asymmetry in the elastic scattering of left- and right-handed electrons from an unpolarized proton target. This experiment complements a similar measurement being carried out at SLAC, which investigates the weak vector charge of the electron via parity-violating Moller scattering. UConn theorist M. Ramsey-Musolf has worked out several new physics scenarios which might be expected to produce an observable departure from the SM expectation for Qweak. At the 3% level, this measurement would be competitive in precision with other low-energy tests of the SM, such as atomic parity-violation. The measurement requires the highest current available at Jefferson Lab, but would be carried out at energies that are available now, before the 12GeV upgrade and Hall D construction are finished. Under one possible scenario, the Qweak experiment would make use of the existing G0 spectrometer after that experiment is completed. UConn is working with the nuclear group at Louisiana Tech to investigate the design of a better-optimized spectrometer for Qweak.
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