Speaker
Summary
The observed invariant mass and angular distributions reveal the ABC-effect to
be a $\sigma$ channel phenomenon associated with the formation of a
$\Delta\Delta$ system in the intermediate state. The most pronounced feature
is the enormeous low-mass enhancement in the observed $\pi^0\pi^0$ invariant
mass distributions, which in the $\pi^0\pi^0$ channel is larger than in the
$\pi^+\pi^-$ channel, since the latter also contains isovector contributions
\cite{bash}. In contrast to previous inclusive measurements and theoretical
predictions we observe no high-mass enhancement.
The differential distributions for the
$\pi^0\pi^0$ channels can be well described, if a strong attraction between
two $\Delta$s in the intermediate state or even a bound $\Delta\Delta$ system
is assumed. Such a boundstate situation had been predicted previously
\cite{oka,wang}. This ansatz is capable of
describing also the results of previous inclusive measurements on $^4$He as
well as the resonance-like energy dependence of the total cross sections. The
latter are in favor of a substantial binding between the two $\Delta$s.
- supported by BMBF, DFG (Europ. Graduate School) and COSY-FFE
\bibitem{bash}
M.~Bashkanov et al.,
Phys.~Lett.~{\bf B637}~(2006)~223.
\bibitem{oka}
M.~Oka, K.~Yazaki, Prog.~Theor.~Phys.~{\bf 66}~(1981)~572; J. Ping et
al., Phys.~Rev.~{\bf C64}~(2002)~044003
\bibitem{wang} J. Ping, H. Pang, F. Wang, T. Goldmann, Phys. Rev. {\bf
C64},
044003 (2002) and references therein
