Using data on the coherent production of TT + 7T + TT~ systems in TT + collisions with nuclei, we have extracted an estimate for the radiative partial width of the A \. The rate for A\ -• 7r + y is 640 ± 246 keV, which is a factor of about 2-3 below the value expected on the basis of predictions from quark models and from vector dominance ideas. PACS numbers: 14.40.Cs, 13.40.Hq, 13.75.GxFrom its initial discovery, 1 the A x was thought to be the isovector member of the / />c =l + + antiquark-quark octet. The pioneering partial-wave analysis of Ascoli and co-workers 2 provided the first convincing evidence for a J p =l + assignment for the A\. More recently, several experiments have provided strong support for a resonant interpretation of the A i as observed in diffractive, 3 charge exchange, 4 baryon exchange, 5 and r-lepton decay 6 processes. Nevertheless, the estimates of the A x mass and width are still rather uncertain. 7 In this Letter we present evidence for the Coulombic production of the A u from which we extract the first estimate of the radiative decay width for A\-+ rry. Theoretical estimates for this partial width, based on the quark model and on vector dominance ideas, have been typically in the range of 1.0-1.6 MeV. 8 The data were obtained by using a 202.5-GeV/c incident 7r + beam on copper and lead targets. The specific reaction involved coherent production of three pions:This reaction is dominated by nuclear diffractive production, in which the presence of a large nonresonant Deck contribution 9 has always obscured the extraction of the A \ resonance parameters. The diffractive mechanism favors production of helicity M = 0 states in the t channel, particularly at small values of momentum transfer. Coulombic production, on the other hand, involves |M| = 1 states, which carry the helicity of the mediating (q 2 -0) photon. Despite the spin-flip nature of the electromagnetic process, and unlike the case in hadronic spin-flip reactions, the cross section for Coulomb production of the A i is expected to peak very close to the forward direction. This is because the presence of the photon propagator in the long-range Coulomb interaction provides a cross section that peaks at t == 2f 0 , where t 0 is the minimum momentum transfer needed to produce any three-pion mass. The maximum for the strong spin-flip cross section occurs at the much larger values of t that characterize the nuclear form factor [t ~ (12^2 /3 ) _1 GeV 2 ]. 10 A discussion of the apparatus and the data-taking procedure can be found elsewhere. 11 Also, the general features of the data and the methods used in the analysis of Reaction (1) have already been published. 12 In the present analysis we restrict ourselves to 85 200 events on Cu and 30 100 events on Pb, selected to be within the coherent forward peaks (t < t* = 0AA~2 /3GeV 2 ), with three-pion masses m 3n < 1.5 GeV. The \M\ = 1 component of the data is isolated by means of a partial-wave analysis (PWA), with use of the density-matrix formulation in the Gottfried-Jackson frame. 13 Fo...

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Coherent production of Kir systems observed in the excitation of 200-GeV/c positive kaons on nuclear targets has been analyzed, including both electromagnetic and strong contributions, to yield a new value for the radiative width for the process K* + (89Q)-+K + y of 51±5keV.PACS numbers: 13.40.Hq, 14.40.EvRadiative decays of vector mesons present one of the simplest regimes for testing unitary symmetry schemes and quark models of hadrons. In particular, the decay (vector meson) -(pseudoscalar meson) + y, which can be interpreted as a magnetic dipole transition between two quark levels, provides a rather sensitive probe of the symmetry of quark-antiquark systems. Much theoretical work has been devoted to the phenomenology of such decays from the viewpoint of unitary symmetry, simple quark models, and vector-dominance ideas.