M. Deutschmann

K∗(890) and K∗(1420) production in quasi two-body K−p reactions at 10 GeV/c

Abstract Results are presented for six reactions induced by K−p collisions at 10 GeV/c, where K∗(890) and K∗(1420) resonances are produced in association with a proton, a neutron, a Δ+(1236) or ΔO(1236) isobar. Partial and differential cross sections as well as density matrix elements are compared to one another and to previous results at lower energies. The following observations are made: (i) There is a first indication of a difference in the energy dependence of the pK∗− and nK∗O cross sections. (ii) In the pK∗−(890) channel the ratio of π to ω exchange decreases with increasing energy. (iii) The ratio of π to ω exchange increases with the mass produced, the pion exchange contribution to the pK∗−(1420) channel being larger than that to the pK∗−(890) channel. (iv) Natural spin-parity exchange dominates in the pK∗−(890) channel, but there are significant contributions of both natural and unnatural parity exchanges in the nK∗O(890) channel. (v) There exists a clear hierarchy of exchanges such that when ω or fO-exchanges are possible, they dominate over pion-exchange; pion exchange dominates over ϱ or A2-exchange in ΔQ = 1 or ΔI = 1 reactions, at least up to 10 GeV/c. (vi) Differential cross sections dσ/dt′ of reactions dominated by pion exchange are better fitted by a two-exponential formula of the type P[exp (−Qt′)+R exp (−St′)], than by the pion propagator. (vii) At least ≈ 25% of the unnatural exchange in the nK∗O(890) channel at 4–10 GeV/c must be due to mechanisms other than simple pion exchange.

Observation of t-channel helicity conservation in diffractive reactions producing A1 and Q-mesons

Abstract It is shown that helicity is conserved in the t-channel for A 1 and Q-meson production by pions of 8 and 16 GeV/ c and kaons of 10 GeV/ c , respectively. This is in contrast to evidence for s-channel helicity conservation reported for elastic scattering and rho photoproduction. This difference in behaviour of diffractive processes may be due to the fact that there is a change in spin in the former reactions but not in the latter, or, alternatively, to different mechanisms operating in the two cases.

Mass dependence of the small angle differential cross section for high energy interactions

Abstract A study of differential cross sections for 8 GeV/ c π + p and 10 GeV/ c K − p reactions, treated as two-body processes, has indicated that as the mass M of one of the “bodies” varies, the small angle differential cross section is described by d σ /d t ′ = K exp (− At ′) for all values of M , whether corresponding to a resonance or to a “background” region. In general, similar values of A are obtained for a resonance region and for its neighbouring mass bands. In most channels, as M increases, the angular distribution becomes broader, namely, A ∝ M − α , with 0 α

Experimental evidence from π+p reactions at 8 GeVc against the hypothesis of pion regge conspiracy plus factorisation

Abstract It has been predicted that with the hypothesis of a Regge trajectory conspiring with the pion trajectory and applying the factorisation theorem, the differential cross section for the reaction π + p → N ∗ ϱ 0 should have a dip in the forward direction near t = 0. It is shown that no such dip is observed in 8 GeV c π+p interactions.

Resonance production in 8 GeV/c π+ interactions in hydrogen

From the measurement of 30,000 two-prong and 30,000 four-prong events of 8 GeV/c π+-p interactions, resonance production has been studied. A peak is observed in the (π+πo) mass distribution near 1630 MeV. No enhancement is observed in the (π+π+) mass distribution in the reaction π+p → nπ+π+. A broad enhancement from 1000 to 1400 MeV is observed in the (π+π+π−) effective mass distribution in the reaction π+p → pπ+π+π−; no clear evidence is seen for a splitting of this enhancement into the A1 and A2 mesons. Alternative decay modes of the A enhancement into πη, πη′ and KK are discussed. Evidence is presented for a (3π) resonance observed near 1660 MeV which appears to decay partly into (π+fo. A broad enhancement in the (π+ω) mass distribution is observed near 1260 MeV.

Limiting properties in inclusive π+p reactions at 8 and 16 GeV/c with exotic and non-exotic particle combinations

Abstract The distributions in the Feynman variable x for the pions produced in the inclusive reactions (1) π+p → π− + anything and (2) π+p → π+ + anything are compared at 8 and 16 GeV/c. The distributions are similar in first approximation, but second order differences are observed. In particular, at x ≈ 0 the distributionsfor π+ overlap at the two energies for π− they are different, and at x ≈ −2 they are similar for π− but not for π+. Comparison of the lonigtudinal momentum spectra in the lab. (target) system gives results consistent with limiting fragmentation for reaction (1), but not for reaction (2). Since the combination (abc) is (π+pπ+), exotic, for reaction (1) and (π+pπ−), non exotic, for reaction (2), these results are consistent with the suggestion of Chan et al. that when (ab c ) is exotic, limiting fragmentation may set in already at moderate (accelerator) energies.

Charge exchange and charge distributions in K−p interactions at 10 and 16 GeV/c

Abstract For each K − p event, the charge exchange, ΔQ , is calculated, as the difference between the total charge of the forward going particles and of the incident K-meson. Taking all inelastic events, it is found that zero charge exchange does not dominate, occuring in only 40% of the cases. The proportion of ΔQ = + 1 is also 40%, while ΔQ = − 1 and ΔQ = + 2 both represent about 10% of the events and other values of ΔQ are not important. The average charge exchange and the dispersion of ΔQ values are compared with the predictions of the multiperipheral and fragmentation models.

Quark-model predictions for joint decay distributions in π+p double resonance production reactions at 8 GeV/c

Abstract The quark-model predictions of Bialas and Zalewski are compared with the experimental results for the joint decay distributions in the double resonance production reactions π + p → N ∗++ ϱ 0 , π + p → N ∗++ ω at 8 GeV /c . The relations fall into three categories, which differ in the assumptions made. Those of class A, which depend only on the additivity assumption, are found to be well satisfied for both reactions, within large errors. Similar agreement is found for relations of class B. A number of class C relations are badly violated in Jackson and helicity frames but are found to be valid in the Donohue-Hogaasen system.

The “true” multiplicity distribution in K−p interactions and the two-component model

Abstract A method is presented for deriving channel cross sections for all multiplicities, considering both charged and neutral particles, for K − p reactions of pion production, Thus the “true”, rather than charged, multiplicity is obtained. The input data are the channel cross sections measured with four- and one-constraint fits. The model employed is the two-component one, so that for each reaction channel, separate cross sections are derived for kaon diffraction, proton diffraction and for non-diffractive processes. Results are obtained up to the highest energies at which data of channel cross sections are available, i.e. up to 16 GeV/ c incoming momentum, and some extrapolation is also made towards higher energies. It is found that the cross section for single kaon diffraction dissociation is equal to that for single proton diffraction dissociation. In the energy range considered, the sum of kaon and proton single diffraction processes has a cross section of (2.4 ± 0.3) mb, i.e., about 75% of the elastic cross section. The cross section for kaon diffraction dissociation into (Kπ) is consistent with zero.

Evidence for a new Ξ resonance at 2500 MeV in 10 GeV/c K−p interactions

Abstract Baryon states with strangeness S = −2 have been studied in 10 GeV/c K−p interactions. The existence of the Ξ (2050) resonance is confirmed and also its decay into (Λ K π) is observed. Evidence is presented for a new Ξ state of mass 2500 MeV decaying into (Λ K π) , (σ K π), (Ξππ) and possibly into (Y K ). The possibility of there being a Ξ resonance near 2240 MeV is discussed.