Hybrid Bubble Chamber Study of 11.5 Gev/c Pion - Interactions.
Results are presented on the analysis of the following two reaction channels:. (pi)('+)p (--->) p(,f)p(,s)(pi)('+)p (1). (pi)('+)p (--->) p(,f)(pi)('+)(pi)('+)(pi)('0)(pi)(' -) (2). where the subscripts f and s indicate the faster and the slower protons respectively. The faster proton is required by the detection trigger to have momentum > 4.5 GeV/c in the laboratory frame. Our data comes from an exposure of the Stanford 40" Hydrogen Hybrid Bubble Chamber to 11.5 GeV/c positive pions. 1.2 million pictures were taken, corresponding to 208 events/(mu)b at 100% efficiency. For Reaction (1), a LPS analysis indicates strong production of the (DELTA)('++)(1232) in the backward direction. The estimated cross-section for. (pi)('+)p (--->) p(,f)p(DELTA)('++) (1a). is (8.6+/-0.9)(mu)b. The associated (p(,f)p) system produced in the forward direction shows no indication of narrow structures ("baryonium" states) reported by other experiments. The estimated upper limit for the production of narrow (width < 20MeV) (p(,f)p) resonant states with mass < 2.4 GeV at the 4 standard deviation level is (sigma)(.) branching ratio = 180nb. A Reggeised phase space model assuming one pion exchange (OPE) can reproduce the shape of the (p(,f)p) mass spectrum quite well. However, the observed decay angular distributions cannot be explained by OPE alone; neither can they be explained by Stodolsky -Sakurai's rho-meson exchange model. An attempt is made to estimate the absolute cross-section of Reaction (1a) as a function of the (p(,f)p) invariant mass in the (VBAR)t'p( --->)(DELTA))(VBAR) < 0.1 region where OPE is expected to dominate. The method is based on a Born Term OPE model with vertex form factors together with previous measured cross-sections of the time reversal related process pp (--->) (pi)(pi). The result indicates that even at low (VBAR)t'(VBAR), OPE cannot fully account for the observed data. For Reaction (2), we report the first observation, in pion-nucleon interaction, of the backward production of the B(1235) meson and its subsequent decay to ((pi)('+)+(omega)). The estimated cross-section is (sigma)((pi)('+)p(--->)p(,f)B) = (1.15+/-0.17)(mu)b with (VERT.BAR)u'((pi)(--->)p(,f))(VBAR) < 1.5 GeV('2). A spin-parity analysis concludes the B(1235) to have J('P) = 1('+) or 2('-). Assuming the accepted spin-parity of 1('+), we find the D-wave to S-wave ratio of the B(1235) to be (0.37+/-0.12). Assuming nucleon exchange to dominate, we give estimates for the SU(3) mixing parameter F/D to be (0.53+/-0.07) as well as the coupling ratio, g(,Bpp)('2)/g(,(pi)pp)('2), to be (0.18+/-0.05). We see some indication of a narrow enhancement at M((pi)('+)(omega)) around 1.03 GeV but are unable to confirm this as a resonant state.
- Pub Date:
- Physics: Elementary Particles and High Energy