Production and Neutral Decay of theηMeson inπ−pCollisions

Abstract

The reaction T-9 + qn has been observed through the decay mode 7 -t 27 a t T,-=592, 653, 704, 875, 975, 1117, and 1300 MeV. The detection apparatus was a cubic array of six steel-plate optical spark chambers that completely surrounded a liquid-hydrogen target. We identified events attributed to the decay of an 7 by the large c.m. opening angle between the two showers generated in the steel plates by the decay photons. We have calculated the total cross section for 7 production, which is proportional to the number of events under the large-angle peak in the opening-angle distribution. The total cross section rises steeply from threshold to a maximum of about 2.4 mb a t 650 MeV, and then falls gradually to about 0.66 mb a t 1300 MeV. The differential cross section was obtained by taking the coefficients of a Legendre-polynomial fit to the angular distribution of bisectors of selected two-shower events, and converting them to the coefficients of the 7 c.m. angular distribution. The differential cross section is found to be isotropic a t 592 MeV, to require terms through Pe(cos0,) between 655 and 975 MeV, and to have a forward peak fitted by terms through P~(CO&,) a t 1117 MeV and through P.%(CO&~) a t 1300 MeV. I t is suggested that production a t threshold is predominantly through an S state, with some PI and Da waves entering by 655 MeV.We suggest that all the absorption in the Sli T N state can be explained by the 7 production.

P -wave excited baryons from pion- and photo-induced hyperon production

“…Differential cross section were reported in [18,19,185,186,187,188]; however there are large discrepancies between different data sets at least above 1800 MeV. [19] and full squares the data from [18].…”

“…Here, we fit the precise data from the Crystal Ball collaboration [19]; for the higher energy region, we use the data from [18] since they are mostly compatible with the Crystal Ball data and can probably be trusted up to 1800 MeV. Fig.…”

“…The dynamical Breit-Wigner parameterization of the πN elastic amplitude can be written as: [112], c) of the differential cross section for the π − p → ηn [18], of the differential cross section for the π − p → KΛ, separated into lowenergy d) [20] and high-energy e) region [21,22]…”

Photoproduction of $ \eta$ -mesons off neutrons from a deuteron target

“…The K-matrix parameters for the πN and ηN channels are constrained from the fit of the elastic πN → πN data (extracted by [33]) and the fit of the π − p → ηn differential cross section [36], [37]. The description of these data is shown in Figs.…”

Production of η and η′ mesons on nucleons and nuclei

“…In contrast to the photoproduction case, most of the measurements of the near-threshold π − p → ηn differential cross section were carried out many years ago and some inconsistencies are apparent in the data base [127,128,129,130,131,132,133,134]. There are, however, two more recent experiments where the η meson was reliably identified through its 2γ or 6γ decay mode [135,136].…”