A STUDY OF THE REACTIONS PSI' MESON ---> GAMMA, GAMMA, PSI.

摘要

A large solid angle (98% of 4(pi) steradians) array of thallium-doped sodium iodide crystals, together with spark- and multiwire proportional chambers for charged particle tracking, is used to study the decays of the (psi)' meson. This apparatus (the "Crystal Ball detector") is situated in the East Interaction Region of the SPEAR electron-positron collision ring at the Stanford Linear Accelerator Center. From the fall of 1978 through the spring of 1979, the detector recorded 8 x 10('5) (psi)' decays utilizing a highly efficient trigger based on the total energy measured for each event.; The (psi)(3.095) and (psi)'(3.684) mesons are thought to represent the two lowest energy triplet-S levels of the quasi-atomic system formed by a bound charmed-quark/charmed-antiquark pair. As such, singlet-S and triplet-P levles ((chi) states) should be observed with masses between those of the (psi) and (psi)' by means of radiative transitions (initial observations of such states have been made by previous experiments). Cascade reactions of the form (psi)'(--->)(gamma)(chi), (chi)(--->)(gamma)(psi), (psi)(--->)(e('+)e('- )-or-(mu)('+)(mu)('-)) are used in this experiment to observe the intermediate states and to measure their spin by analyzing the angular correlations among the final-state particles. In addition, the multipole coefficients describing the individual radiative transitions are measured using the angular correlations.; Values of J = 2 and J = 1 are obtained for the (chi) states with masses of 3.55 and 3.51 GeV, respectively; these values support the basic charmonium model. Radiative transitions to and from the (chi) states are found to be dominantly E1 in nature, suggesting that the anomalous magnetic moment of the charmed-quark is small. The well-established J = 0 (chi)(3.41) state is observed in a cascade reaction for the first time, with a branching ratio BR((psi)'(--->)(gamma)(chi)(--->)(gamma)(gamma)(psi)) = (0.06 (+OR-) 0.02 (+OR-) 0.01)%; the first error describes uncertainties arising from statistics and acceptance corrections, while the second error is systematic. Branching ratios of (1.26 (+OR-) 0.08 (+OR-) 0.20)% for (chi)(3.55) and (2.38 (+OR-) 0.12 (+OR-) 0.38)% for (chi)(3.51) are consistent with those obtained in previous experiments. Natural line-widths of (4 (+OR-) 1) MeV for (chi)(3.55) and a full width consistent with the resolution of the apparatus for (chi)(3.51) are obtained. This is the first reported measurement of the width of any of the (chi) states.; An intermediate singlet-S state (the (eta)(,c)') which is predicted to have a mass of about 3.6 GeV and has been reported by other experiments at masses of 3.46 and 3.59 GeV is not observed in a cascade reaction by this experiment. A 90% confidence level upper limit of 0.04% is placed on the cascade braching ratio BR((psi)'(--->)(gamma)(eta)(,c)'(--->)(gamma)(gamma)(psi)).; The (gamma)(gamma)(e('+)e('-)-or-(mu)('+)(mu)('-)) final state is also found to result from the processes (psi)'(--->)((eta)-or-(pi)(DEGREES))(psi), with branching ratios BR((psi)'(--->)(eta)(psi)) = (2.18 (+OR-) 0.14 (+OR-) 0.35)% and BR((psi)'(--->)(pi)(DEGREES)(psi)) = (0.08 (+OR-) 0.02 (+OR-) 0.01)%. Isospin symmetry is violated in the latter decay at a level which is well described by theories of (eta)-(eta)'-(pi) mixing and a direct symmetry-breaking coupling. The Crystal Ball and Mark-II experiments at SPEAR (1978-79) measured consistent branching ratio values for the (eta) and (pi)(DEGREES) decay modes; however, the value for BR((psi)'(--->)(eta)(psi)) obtained from these experiments is found to be almost a factor of two smaller than those obtained in all earlier experiments.