The σ-meson production in excited Υ decay processes:: constraint from chiral symmetry

In the recent letter [T. Komada, M. Ishida, S. Ishida, Phys. Lett. B 508 (2001) 31], hereafter referred to as I, we have shown that the pi pi mass spectra in various excited Y decay processes are phenomenologically well reproduced by a coherent sum of a Breit-Wigner amplitude and of the direct 2 pi -production amplitude. On the other hand, the pi pi -production amplitude was widely believed to be suppressed generally near the threshold, because of the derivative coupling property of Nambu-Goldstone pi meson. In this Letter we show that the threshold-suppression does not hold in the process with large energy release, in conformity with the steep increase from the pi pi threshold observed in the transition Y(3S --> IS), and that the method of I is consistent with chiral symmetry. Furthermore, applying an effective amplitude explicitly consistent with chiral symmetry, we reanalyze the typical pi pi -spectra. (C) 2001 Elsevier Science B.V. All rights reserved.