N ov 1 99 6 ππ S - WAVE INTERACTION AND 0 + + PARTICLES

From a re-analysis of ππ → ππ and ππ → KK̄ data, we found for ππ S-wave interaction below 2 GeV, σ(400), f0(980), f0(1500) and f0(1780) clearly show up. f0(1370) can be included with a very small branching ratio to ππ. f0(1300) and f0(1590) are not real resonances and are due to interference effects of above resonances. The σ(400) with a width about 700 MeV is mainly produced by t-channel exchange force. Nearly all known mesons are bound states of a quark and an antiquark. The qq̄ mesons containing u, d and s quarks can be ascribed into various LJ SU(3) nonets according to their spin S, orbital angular momentum L and total angular momentum J. Among the lowest nonets, S0, S0 and P2 nonets are well established; P1 and P1 are also settled although there are still some uncertainties [1]; only P0 nonet is very problematic: it has two opening positions for its isoscalar part, while there are too many candidates, but none of them can fill in without controversy. Now let’s have a look at these candidates. • σ(300 ∼ 800) with width Γ = 200 ∼ 1000MeV [2]. It was listed in old PDG booklets[2] more than twenty years ago, but has been dropped out in newer versions. However it is needed in the σ model, the extended Nambu-Jona-Lasinio model and the nucleon-nucleon scattering models; it is also needed to explain the low energy enhancement in ππ invariant mass spectra from various production processes. • f0(980) with a peak width about 50 MeV[1]. Due to its very narrow peak width, it was regarded to be difficult to be ascribed as a qq̄ state. Many exotic explanations have been proposed for it, such as KK̄ molecule[3, 4], multiquark state[5], and Gribov’s minion [6], etc. ∗Talk presented at 34th Course of International School of Subnuclear Physics, Erice, Sicily, July 3-12,1996