Observation of forward peaks in π+n → K°Y*+(1385) and in related single-meson-exchange-forbidden reactions

We have studied the production of decuplet-baryon resonances [Y^*( 1385), E*(1530), and AI1236)] in reactions which cannot proceed by the exchange of any single known meson.We have examined these exotic exchange reactions at incident momenta ranging from 1.0 t9 "i GeV/c, using data from several bubble chamber exposures.In all but one of the reactions studied, there Is significant "forbidden" forward peaking.At roughly 2 to 3 GeV/e, the cross section in the forward direction is 2 to 7 u.b/sr for «~p -• K + Y*" and K"p -* K + E*~.=20 u.b/s.» for K"p -• ir + Y*~, and slOO u.b/sr for it~p -» ir + a~; no peak Is seen in K"p -» K°H*°.A decrease with energy is evident.The data Is presented at length, to establish the existence of the forbidden peaking in final states which are dominated by other features: production and decay distributions are given.Various possible explanations of forbidden peaking are reviewed.We argue that Berger's model does not explain away the peaking as a kinematic reflection.Recent work suggests that two-meson exchange may provide an explanation.We also consider the oossible roles of exchange of a single (exotic) meson and of direct-channel resonance formation.P-1 GeV/c Tf-p-K + n"A (fa n*n-»K°TT*A) (Sample 3) i a w O O M(Ati~) JG<^ In Y*" Sand iT" .-.-• --(b) »• * » 9 I"" V. ts ir -, a (S-iC*)>0.7M«(An-) D a e o (0 ( Subtracted) Cos OPRDD (p -^"~) WjL 729-K15 Fifl-?.T '*"•*'&* * • ( Subtracted) of high nit -mass and cos 9pr s-1 to the luw-msss, forward production region.This enhancement is a kinematic reflection of peripheral iri production; in the forward region it results in broad bands in the A region which look quite a bit wider than the A. [The A band, (m0 2 ± m0D, is (1.B3 ± 0.1%) GeV 2 .]We discuss below (Sections 3.2<i, 3.321) our claim that there is in fact A production on top of this enhancement.From Fig. l + (a) and (c), we see that the production distribution of events in the B* band has forward peaking in all cases except E*° production (Samples (8)-(9)).and perhaps s*~ production at 2.1 SeV/c (Sample (6)).Note that in V* and Z*~ production there are backward peaks comparable in size to the forward peaks; in A production the backward peaks are small; in E*° production there are backward peaks with a pronounced dip in the extreme backward direction.The forward Dalitz plots show competing M* production in the tin system (that is, p or K*) in most cases.For Samples (l)-(3) and (10)-(12), M* production in this region is stronger than B* production; for Samples Ct)-(5) it is comparable in size.In Samples (6)-(9), K* production is OME-forbidden and is not evident in the B*-forward region.In all cases, Fig. l + (b) shows that the forward B s production is not predominantly connected with the B*-H* overlap region.In Figures 3.13 and 3.14 we give our best estimates for the shapes and sizes of the production angular distributions.They were obtained in various ways which, we hope, take into account the peculiarities of each sample.The.distributions for Samples (l)-(S) come from maximum-likelihood fits for the amount of B* production made In each region of production cosine.The E* distributions come from the data with a simple mass cut.There is no distribution shown for Sample (10).Those for Samples (11) and (12) come from an eyeball estimate of the number of A events in the Nn spectrum for each cosine interval.These distributions were normalized to total B* cross sections obtained from a variety of sources.We feel that the uncertainties in our procedures, especially for n ± N-*KY* and for A production, have given us good estimates for the size of the forward peak rather ihan careful measurements.Our primary interest was in studying the existence of authentic forward peaking, not in determining its magnitude.The error bars do not include any of the systematic uncertainties in the normalization.We see that the differential cross section in the forward bin ranges in siLe from about 1 u.b/sr to about 150 u.b/sr.(The forward cross sections, with normalization uncertainties included, will b<!summarized in Table 3.12 below.)In K 'P -K*H*~ (a) 2.1 GeV/c (b) 2.6 GeV/c -O 3-6: -1.0 0.0 \ ' •a b 6: -1.0 0.0