Evidence for aT=1Pion-Pion Resonance at 575 Mev

Abstract

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...

Isobar Model Analysis of Single Pion Production in Pion-Nucleon Collisions Below 1 BeV

“…13 Here the agreement with our model is striking. The low value for the ratio for small Q(?r,7r) is a consequence of the antisymmetry in Eq.…”

“…The shape of the Q(7r + , rr°) curve calculated from this model is quite similar to one which would be obtained from OPE diagram assuming a constant 7r -77 cross section. 14 (v) We have compared the Q(77 + , 77 0 ) curves at 820, 13 900, 13 910, 14 1050, 13 and 1090 14 MeV with our model. We find no statistically significant evidence for the reported £ meson of mass 575 MeV.…”

π−NScattering with Inelastic Contributions

“…(26) and (33) we see that F22M is denned in a cut plane: an elastic cut coming from ^2(^,^2) and extending from W2 to <*>, and an inelastic cut coming from Gz(w,W2) and extending from wz to 00. Clearly, F 2 z and Fzz are denned in planes cut in exactly the same manner, but, as seen from Eq.…”

“…(26) and (16). Such integrals are difficult to perform by machine calculation, since they involve principal-value integrals and imaginary parts.…”

Note on boson resonances

“…Thus, the (co, <f>) can be associated at once as radial doublets and the (K*, k ) by the plausible assumption of spin 1~ for the k ; where is the doublet partner for the p? Earlier experimental literature offered two possibilities: the ( [8] with a mass around 575 MeV on the double-humped p [9] in which the two components were separated by the order of a half-width and therefore hardly resolvable. Further experiment seems to have disposed of the first, but the second is intrinsically much more difficult to rule out and in fact appears to draw support from quite recent measurements [10].…”