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Recent experiments 1 " 8 have shown various resonances of multipion systems. We report here evidence for a di-pion resonance with isotopic spin T=l at 575±20 Mev.Single pion production in TT + -/> collisions at incident pion kinetic energies of 820, 900, and 1050 Mev were studied by means of the Saclay 3 5-cm hydrogen bubble chamber at the proton synchrotron Saturne. Experimental details and preliminary results have been described elsewhere. 9 The histograms in Fig. 1 show the distribution of events as a function of the kinetic energy Q^ in the rest system of the two pions produced in the reaction, 7r + +/) + 7r + + 7r°+/>. (1)A peak appears at all incident energies at a value of Qfl^ ~ 300 Mev. For comparison we show the predictions of the isobar model 10 as a dashed curve normalized to the total number of events. At the higher energies one also notes an excess of events for Q v1l ^ 400 Mev. This is probably due to the gradual appearance of the p meson, for which phase space increases with incident energy. This process obscures somewhat the 300-Mev peak at 1050 Mev. Indications of a peak around 300 Mev have also been found by Pickup et ak 1 and Erwin et al. 2The existence of the peak is confirmed by the variation as a function of Q n7r of the TT 0 and TT + forward-backward asymmetry in the center-ofmass system. The isobar angular distribution at 820-and 900-Mev incident energies is peaked backwards in the cm. system. 9 Furthermore, the isobar is predominantly doubly charged. Thus, if the main mechanism is isobar formation, one expects, in a qualitative manner, a forward peaking of the ir°'8 in the cm. system for all values of QTTTJ-and a much smaller asymmetry for the 7T +, s. This is in fact the case, except in the vicinity of 300 Mev, where the two asymmetries are equal within the experimental errors.To have a better idea of the Q^ value at the maximum of the peak, we have plotted on Fig. 1 a "densitogram." Each point in the diagram is proportional to the probability of finding an event from reaction (1) in a Q^ band of 10 Mev, assuming an experimental error of ±30 Mev for each determination of Q^. The continuous curve in Fig. 1 is a smooth curve drawn through the points of the "densitogram." The maximum of the peak is located at 300 Mev with an estimated error of ±20 Mev taking into account the error in incident energy. The experimental total width is not larger than 70 Mev.These results indicate the existence of a dipion which we call £.No similar peak was observed in the reaction, 7r + +/>-*7r + + 7r + + ?i,which favors T= 1 isotopic spin for £. If the production of the £ is due to peripheral collisions, one may determine its coupling to pions (assuming a coupling of the formwhere cp is the pion field, B^ is the £ vector field, *',./»& = 1,2,3 are isotopic spin indices, €{jk is the antisymmetric tensor, and g is the ff-7r-£ coupling constant) by considering the number of £'s produced in a momentum transfer band near to zero momentum transfer. 11 This has been done at 900 Mev and leads to a value of g too l...