H. Schiller

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.

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.

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.

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.

Interference between (P, f0…) and (ϱ0, B0,…) exchanges in pion diffraction dissociation at 16 GeV/c

Abstract It is found in the reactions π±p→(π±π+π−)p, believed to be dominated by diffraction dissociation, that the d σ d t′ distributions show a “cross-over” effect at t′ ≈ 0.15, similar to the effect observed in elastic scattering. This gives evidence for the interference of (ϱ0, B0,…)-exchanges with ( P , f 0 , …) -exchanges in pion diffraction dissociation reactions. No such evidence is found for baryon dissociation, π±p→π±(pπ+π−), at the same energy.

Density matrix elements and Donohue-Høgaasen parameters of resonances produced in π+p and γp reactions

Abstract We present, in the first part of this paper, density matrix elements for several quasi two-body processes produced in π + p interactions at 8 GeV/ c . In the second part, we study the decay of vector mesons and of Δ ++ (1236) isobar in terms of the Donohue-Hogaasen (DH) spin population parameters α, β and γ and the rotation angle Θ. The data used are from π + p scattering at 4 and 8 GeV/ c and from γp interactions in the photon energy region below 5.8 GeV. It is found that the DH parameters have the tendency to be less dependent on momentum transfer than the density matrix elements in the Jackson or in the helicity systems. The stronger variation of the matrix elements in the latter two systems is shown to be a kinematic consequence of the rotation of the frames. This effect is particularly well seen in the reaction γ p → Δ ++ π − where the rotation angle strongly depends on the photon energy. The suggestion of Donohue and Hogaasen, that the DH reference system is more natural frame than the others is well confirmed by our experimental results.

Lambda production in 10 GeV/ c K − p interactions

Abstract Lambda production in 10 GeV/c K− interactions in hydrogen is studied. Cross sections are given for resonance production in quasi-two-body and many-body reactions, and their energy dependence is studied. It is found that the cross sections for quasi-two-body reactions requiring exchange of a strange meson decrease faster than that for the reaction K − p → pK ∗− (890) which requires exchange of a non-strange meson. The cross section for Y∗−(1385) production decreases faster than that for Y∗+(1385) in quasi-two-body reactions. The distributions of the lambda c.m. production angle, transverse momentum and c.m. longitudinal momentum as a function of the particle multiplicity are studied.

The effect of resonance production on angular correlations

Abstract Angular correlations between pairs of secondary particles emitted in 4- and 6-prong events of π + p interactions at 16 GeV/ c are studied as functions of the effective mass M of the two particles involved. It is argued that this is superior to the usual technique of studying correlations as functions of Δy (the difference in the rapidities of two particles), because effects of different origins such as resonance production and Bose-Einstein interference, which all occur predominantly at Δy ≈ 0, are well separated when plotted in terms of the mass. It is shown that the angular correlations are strongly affected by the production of two-particle resonances such as ϱ 0 , f, and g mesons for ( π + π − ) pairs, and Δ ++ (1232) and Δ ++ (1890) isobars for the (p π + ) pairs. A variable D ( M ) is proposed, related to the difference in angular correlations of pairs of like and unlike charges, which is shown to be effective in enhancing resonance signals and could be useful also at higher energies.

A study of the reaction K−p → pKoπ− with the Veneziano model

Abstract The five-point function Veneziano model has been applied to the reaction K − p → p K 0 π − in the energy range from ≈ 3 to 10 GeV/c. The predictions of the model are shown to be sensitive to the choices made for the assumptions and the parameters involved in the expression of the amplitude. Several choices can be made that give a satisfactory general description of this reaction. However, the mass spectra are not described in detail and the differential cross sections do not have the correct t-dependence, especially at the lower energies. These difficulties derive mostly from the fact that the simple model used does not consider the possibility that more than one trajectory is exchanged between a pair of particles.