Higher genus surface operad detects infinite loop spaces

The operad studied in conformal field theory and introduced ten years ago b y G. Segal [S] is built out of moduli spaces of Riemann surfaces. We show h ere that this operad which at first sight is a double loop space operad is indeed an infinite loop space operad. This leads to a new proof of the fact that the classifying space of the stable mapping class group Z x B Gamma(i nfinity)(+), is an infinite loop space after plus construction [T2]. This n ew approach has various advantages. In particular, the infinite loop space structure is more explicid. Mathematics Subject Classification (1991): 55P47, 32G15, 81T40, 55R35.