W. Kittel

Single and double dissociation in π−p collisions at 11 and 16 GeV/c

The reactions π−p → 2π−π+p, π−p → 2π−π+πop and π−p → 2π−2π+n are analysed at 11 and 16 GeV/c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π−p → 2π−π+p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π−p → (2π−π+)p, and of the proton, π−p → π−(π−π+p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π−p → (2π)(πp) decrease with increasing energy. n nIn the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π−p → π−(π−π+πop); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π−p → π−(π−2π+)n. Another strong maximum corresponds to the pion dissociation π−p → (2π−π+πo)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π−p → (2π−2π+)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π−p → (2π−π+)(πop) and π−p → (2π−π+)(π+n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed. n nAn appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy.

Structure in differential cross section distributions of two-body reactions in 8 GeV/c π+p interactions

Abstract From 60 000 two- and four-prong interactions of 8 GeV/c positive pions in hydrogen, total cross sections have been determined for elastic scattering, for 13 inelastic two-body reactions which represent about 7% of the total cross section, and for the various many-body channels. For elastic scattering and the other two-body reactions, the differential cross section dσ/dt′ is studied in detail, especially in the forward direction. It is found that (a) in the region near t′ = 0 a dip is observed in the channels N ∗++ η, N ∗++ ω and p ϱ , and possibly also in N ∗++ π o and N∗++A2o; however, the forward dips predicted by the Regge-pole conspiracy model with factorisation for the channels N ∗++ ϱ o and N ∗++ f o are not observed, on the contrary N ∗++ ϱ o presents a steep rise near t′ = 0; (b) at larger t′ values, a dip is observed for the pϱ+ channel at about 0.55 GeV2 and for the N∗++A2o channel at about 0.45 GeV2; (c) in general, the dσ/dt′ distributions can be fitted in the small |t′| region by the function exp (−At′), while over a larger t′ range, a fit of the form exp (−at′ + bt′2) is more satisfactory. However, for the channels N ∗++ ϱ o and N∗++fo a significantly better fit is obtained between 0 and 2 GeV2 with the sum of two exponentials, K1 exp (−Ct′) + K2 exp (−Dt′), with the break point at |t′| = 0.2 GeV2; C is about 15 GeV−2, while D is about 3 GeV−2. The latter is about the same value as obtained for elastic scattering in the t′ range from 0.7 to 2.0 GeV2.

Charged particle multiplicity distributions for 33.8 GeV/c K−p and 50 GeV/c π−p interactions

Abstract Topological cross sections and average charge multiplicities 〈 n c 〉, are presented for 33.8 GeV/ c K − p and 50 GeV/ c π − p interactions observed in the 4.5 m Mirabelle hydrogen bubble chamber at Serpukhov. Values for 〈 n c 〉 of 5.16 ± 0.08 and 5.71 ± 0.13 were found for the K − p and π − p reactions, respectively. The two quantities 〈 n c 〉 and the correlation term ƒ; 2 have been tabulated for π ± p, K ± p, and pp reactions at various energies. It is shown that the energy variation of each of these two quantities is described by a universal curve when plotted against the Q -value of the reaction. The ratio 〈 n c 〉/ D , where D is the dispersion of the multiplicity distribution, is found to approach the constant value of 2 from above as the energy increases.

Isospin analysis of the reactions π± p → π (N π) at 8 GeV/c and comparison with the results at 16 GeV/c

Abstract An isospin analysis of the reaction π ± p → π (N π ) at 8 GeV/ c has been made and the results are compared with an analogous analysis at 16 GeV/ c previously published, to determine the energy dependence of the effects studied. It is found that isospin-zero-exchange processes are dominant in both cases and practically energy independent, isospin-one-exchange processes are less abundant and are decreasing with increasing energy. Interference effects are, at both energies, very small. The broad enhancement between 1.2 and 1.7 GeV in the I = 1 2 ( N π) mass spectrum due to isospin-zero-exchange, previously reported, is now studied in various mass-bins. It is found that (a) all mass intervals have approximately the same weak energy dependence, (b) all mass intervals have exponential d σ /d t distributions and (c) the slope of these d σ /d t distributions decreases sharply (from ∼12 to ∼3 GeV −2 ) as the mass increases. The results are consistent with the interpretation of this enhancement as predominantly due to diffraction dissociation of the target proton.

Longitudinal phase space analysis of three-body final states in meson-nucleon collisions

Abstract The longitudinal phase-space analysis is applied to various three-body final states of meson-nucleon collisions at 5–16 GeV/ c primary momentum. The distributions of the angle ω along the hexagonal phase space plot and the dependence of average transverse momenta on ω are investigated in some detail. From the very rich structure of the ω distributions it is possible to conclude that, whenever allowed, the diffraction mechanism (single Pomeranchuk exchange) dominates the three-body reactions at high energy. The study of the transverse momentum distributions reveals the existence of strong correlations between longitudinal and transverse momenta which cannot be explained by phase-space effects. Some consequences of our analysis for the Regge-exchange models are briefly mentioned.

Spin parity structure of the Q and L enhancements in K−p → (K−π−π+)p at 10 and 16 GeV/c

Abstract A parital wave analysis of the (Kππ) system produced in K−p → (K−π−π+)p at 10 and 16 GeV/c has been performed. It is found that in 88% of the cases the Kππ) system is in unnatural spin-parity (JP) states 0−, 1+, 2− …, with contributions from many decay modes, κπ, K ∗ (890)π, K ∗ (1420)π, K ϵ, K ϱ and Kf . More than one decay m and JP state are required to describe the Q and L enhancements, suggesting a composite structure of these enhancements. The (Kππ) system is produced dominantly (∼95%) by natural parity exchange. Overall, the production mechanism of the (Kππ) system is remarkably similar to that of the 3π system in π−p → (π−π−π+)p, even i detailed spin-parity structure.

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.

Study of lambda production in 10 GeV/c K−p interactions

Lambda production is studied in K−p interactions at 10.1 GeV/c, where the dominant reaction is K−p → Λ + pions. General characteristics such as the distributions of the double differential cross section in the lab system, of the variable x = pL∗pmax∗, of p⊥2 and of the missing mass to the lambda are presented. Total cross sections for Λ production and for the various channels are given. Differential cross sections dσdt, dσdt′ and dσdu′ are presented. Forward and backward peaks are observed in the dσdt′ and dσdu′ distributions, respectively. It is found that the exponential slope of these distributions decreases with increasing missing mass to the lambda and, for dσdt′, also for increasing multiplicity in the final state. The polarization of the lambdas is studied as a function of multiplicity, pL∗, (Λπ±) effective mass, t′ and u′. The forward lambdas show

Dominance of isospin zero exchange in π±p → π(Nπ) at 16 GeV/c and production by it of a low mass (Nπ) enhancement

An isospin analysis has been made of the production of (Nπ) states in the reactions π±p → π(Nπ) at 16 GeV/c. The three independent amplitudes and their three interference terms have been determined as functions of the (Nπ) mass. It is found that at 16 GeV/c isospin-zero exchange is the dominant process, and isospin-one occurs much less frequently, mostly producing the I = 32 Δ (1236) isobar. Interference effects do not seem very important. The I = 12 (Nπ) mass spectrum due to isospin-zero exchange presents a broad enhancement from 1.1 to 1.8 GeV, with a broad maximum between 1250 and 1500 MeV. The N* isobars found by phase shift analyses at 1470, 1520, 1690, etc. MeV cannot account for the whole enhancement observed and in particular for its low-mass part. These results are consistent with the interpretation that diffractive phenomena are important at low Nπ masses and may cause mass enahancements not revealed byphase-shift analyses.