D. Sotiriou

Partial-wave analysis of the 3π system produced in the reaction π+p → (π+π+π−)p at 8, 16 and 23 GeV/c

Abstract A partial-wave analysis has been made of the (3π) system produced in the reaction π + p → ( π + π + π − )p at 8, 16 and 23 GeV/ c using the Illinois partial-wave analysis program. The (3π) systems is in about 93% of the cases in the unnatural spin-parity states 0 − , 1 + , 2 − and 3 + and is produced in about 100% of the cases by natural parity exchange. For all J P states, the differential cross section d σ /d t ′ peaks at small tt ′, except for 2 + D which has a dip at t ′ ≈ 0. The cross sections for J P = 1 + , 2 + and 2 − in the A 1 , A 2 and A 3 regions, respectively, all have similar energy dependence, p lab − n , with n = 0.3 ± 0.2. The weak variation of the 1 + S phase across the 1.0–1.2 GeV mass region, suggests that A 1 cannot be considered as a single resonance, while the phase variation of 2 + D (ϱπ) in the A 2 mass region is consistent with resonance behaviour. In the A 3 region, the behaviour of the 2 − S and 2 − P phases is complex and further work is needed to understand the mechanisms involved there.

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.

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.

Multiplicity cross sections for 100 GeV/c π−p interactions

Abstract Charged particle multiplicity cross sections for 100 GeV/ c π − p inelastic interactions are presented and compared with results at other energies. The results for multiplicities n ⪖ 4 show a consistent trend in their energy dependence. The two-prong cross section curve differs as it tends to flatten off at the higher energies. The zero-prong cross section values decrease steeply as p lab −1.1 . The results for n ⩾ 2 for π − p and pp reactions appear to lie on a “universal” curve if nσ n / σ inel is plotted against 〈 n 〉/ n . This “scaling” rule is equivalent to KNO scaling, but the plot suggested here is more useful in studying low multiplicities. An interpretation in terms of a two-component model and a possible extrapolation to higher energies are proposed.

K∗(890) production in K−p → NKπ at 16 GeV/c

Results on K∗(890) production in K−p → NKπ reactions at 16 GeV/c are presented. Total cross sections, differential cross sections and density matrix elements are determined. It is found that: (i) The cross section for K∗−(890) p is consistent with containing two components, one due to natural parity exchange, falling slowly with increasing energy, the other, due to unnatural parity exchange, vanishing rapidly with increasing energy. For K∗o(890)n, the cross section decreases steadily as the energy increases. (ii) Natural spin-parity exchange dominates in the K∗−(890) p channel, the unnatural exchange contribution being approximately (5±4)%. In K∗o(890)n, natural spin-parity exchange is only about (30±10)%. (iii) The reacton K−p → K∗−(890)p is dominated (95±5)% by isoscalar exchange. (iv) The effective trajectory involved in the I = 0 natural parity exchange is consistent with the ω-f trajectory. (v) Interference terms between exchanges with positive (f, πo,…) and negative (ϱo, ω,…) C-conjugation amount to (23±12) μb at 16 GeV/c.

How well does an LPS analysis separate production mechanisms in π+p interactions at 8 and 16 GeV/c?

Abstract A longitudinal phase space (LPS) analysis has been performed on the reaction π+p→π+π+π−p at 8 and 16 GeV/c. Special attention has been given to the question of the separation of the various reaction mechanisms. The conclusions are: (i) Dissociation of the pion can be clearly isolated, both at 8 and 16 GeV/c. Baryon dissociation is fairly well separated at 16 GeV/c, but at 8 GeV/c it is strongly contaminated by the reactions δ++ϱ0 and even more by δ++f; (ii) The cross section for diffraction dissociation dominated regions of LPS is constant or even slightly increasing between 8 and 16 GeV/c.

Tests of s-and t-channel helicity conservation in pπ+ π−, K−π+π− and π±π+π− production in 16 GeV/c π±p and 10 GeV/c K−p interactions

Abstract Necessary conditions for s-and t-channel helicity conservation are tested for pπ+π−, K−π+π− and π±π+π− states produced in 16 GeV/c π±p and 10 GeV/c K−p interactions by means of a method which is independent of specific spin-parity assignments to these states. It is found that the production of pπ+π− states is generally consistent with t-channel helicity conservation for masses up to 2.4 GeV, but that for π±π+π− and K−π+π− some inconsistency with t-channel helicity conservation is found, primarily in the A1± and Q− mass regions. s-channel helicity conservation is excluded for all states.

Inclusive Δ++ production and a test of the triple regge limit

Abstract Results on the inclusive reaction π+ p→Δ++ X0 at 8, 16 and 23GeV/c are used for quantitative tests of the triple Regge formula. As predicted, the slopes ofthe missing mass (M) spectra at fixed t are independent of s. Furthermore the trajectory α(t) obtained from the s-iependence at fixed M appears consistent with that obtained from the M-dependence at fixed s. However, the pomeron intercept α (0) is found to depend on t, suggesting that no single triple Regge diagram dominates at the M and s values studied in this reaction. Provided such a t-dependence is allowed, a single term triple Regge formula gives a very satisfactory parametrisation of our data.

Study of the exchange mechanisms in the rho production reactions π±p→ϱN at 16 GeV/c

Abstract Results are presented of a study of the reactions π + p→ ϱ + p, π − p→ ϱ − p and π − p → ϱ 0 n, all at 16 GeV/ c . For ϱ + and ϱ − , the d σ /d t ′ distributions have a wide forward dip which results from ω exchange. For ϱ 0 , the d σ /d t ′ distribution is consistent with having a narrower forward dip as expected for π-exchange. The ω-exchange contribution is isolated from a linear combination of the three differential cross sections. It is found that the variation of the ω-exchange contribution with the energy s is consistent with s 2 α ω ( t )−2 , where α ω ( t ) = 0.5 + t . Using the Harari absorption model, the radius of interactions is found to be independent of energy. Evidence that exchange of both spin-parity series occurs was obtained from the density matrix elements. The values of ( ϱ 11 + ϱ 1−1 ) indicate that natural parity exchanges become more important at t ′ increases. The study of ( ϱ 00 GJ d σ /d t ′) allows us to isolate an exchange having the quantum numbers I =0, G =−1, P (−1) J =−1 which may represent the contribution of a “cut”, as no particle carrying these quantum numbers is known.