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      Pion Polarizabilities and Hybrid Meson Structure at CERN COMPASS


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          CERN COMPASS can investigate pion-photon interactions, to achieve a unique Primakoff Coulomb physics program, centered on pion polarizability and hybrid meson structure studies. COMPASS uses 100-280 GeV beams (muon, pion) and a virtual photon target, and magnetic spectrometers and calorimeters to measure the complete kinematics of pion-photon reactions. The COMPASS experiment is scheduled to begin data runs in 2001. Pion polarizabilities and hybrid mesons can be studied via the Primakoff reactions pion + gamma ---> pion + gamma and pion + gamma ---> Hybrid. The electric (alpha) and magnetic (beta) pion and Kaon polarizabilities characterize their deformation in an electromagnetic field, as occurs during gamma-pion Compton scattering. They depend on the rigidity of their internal structures as composite particles, and are therefore important quantities to test the validity of theoretical models. The polarizability measurement will provide an important new test of QCD chiral dynamics. The studies of quark-antiquark-gluon hybrid mesons would improve our understanding of these exotic mesons. COMPASS may improve previous Primakoff polarizability and Hybrid studies by two to three orders of magnitude. Appendixes A (Pion and Kaon Polarizabilities at COMPASS) and B (Hybrid Meson Structure at COMPASS) of this contribution include evaluations submitted to the APS DNP Town Meeting White Paper Committee. These summarize (1) the fundamental scientific issues addressed, (2) major achievements since the last DNP long range plan, (3) the short and long term U.S. outlook, (4) comparison of U.S. and global effort, (5) other issues.

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          Light Meson Spectroscopy

          We survey the current status of light meson spectroscopy. We begin with a general introduction to meson spectroscopy and and its importance in understanding the physical states of Quantum Chromo Dynamics (QCD). Phemenological models of hadron spectroscopy are described with particular emphasis on the constituent quark model and the qualitative features it predicts for the meson spectrum. We next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids glueballs}, as well as multiquark states. The established meson states are compared to the quark model predictions and we find that most meson states are well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. We end with a brief description of future directions in meson spectroscopy.

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            24 December 2000
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            Tel Aviv University Preprint TAUP 2661-2000
            13 pages, 3 figures, Invited Talk at the APS DNP Town Meeting on Electromagnetic & Hadronic Physics, Dec. 2000, Jefferson Lab., including evaluations submitted to APS DNP Town Meeting White Paper Committee


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