Observation of S* production at 1.98 GeV/c

The S* meson has been studied using data from the reactions π-p → π+π-n and π-p → K+K- at 1980 MeV/c. The parameters of the S* pole were found by fitting the dimeson mass spectra from these reactions to a Briet-Wigner resonance formula with a coherent background. The dimeson mass was calculated from the beam and neutron four-vectors, the latter being measured by a time-of-flight technique. The mass resolution is sigma = 4 MeV/c2. Dimeson production was identified by analyzing the final-state meson trajectories with a magnetostrictive wire spark-chamber spectrometer. The events in our data sample were required to have a slow forward-going neutron which restricted the allowed range of four-momentum tranfers to absolute value (t - t/sub min/) < 0.003 (GeV/c)2 in the S* region. From a sample of 21 700 π+π-n events and 52 K+K-n events, the position of the S* pole was found to be (970 +- 5) - i (22 +- 5) MeV/c2 as compared to the present world average value of (993 +- 4) - i (20 +- 4) MeV/c2