H.J. Schreiber

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

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 polarization in inclusive K−p interactions at 10 and 16 GeV/c

A strong negative transverse polarization Pz is found for forward produced lambdas observed in 10 and 16 GeV/c K−p interactions. This indicates that exchanges of natural spin-parity are dominant in the production process. Using the polarization results, the dσdu′ distributions for natural and unnatural spin-parity exchanges are derived. For unnatural exchanges, a dip is observed at u′≅0.3 GeV2, which can be explained as a nonsense-wrong-signature zero of the Nβ trajectory. The value of Pz for forward producted lambdas is constant with energy. This is in agreement with the triple-Regge model prediction, as is the fact that Pz is constant as a function of M2s. The two non-transverse polarization components, Px and Py, have been measured and are found to be consistent with zero for all x values, unlike Pz.

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.

Study of the reactions K−p → K+K−Λ and K−p → ppΛ, at 10 and 16 GeV/c

Abstract The reactions (1) K − p → K + K − Λ and (2) K − p → p p Λ have been studied on samples of 109 and 64 events, respectively, at 10 GeV/ c and 125 and 69 events at 16 GeV/ c , reasonably free from contaminations. The investigation of the first reaction uses also 84 events of the K 0 K 0 Λ final state at 10 GeV/ c . Analysis of the Van Hove plots indicates that the K + K − Λ and p p Λ final states are produced by two main mechanisms: (i) a ΔQ = 0 process, with a strong diffractive component near threshold, involving the dissociations p → K + Λ in reaction (1) and K − → p Λ in reaction (2) and (ii) a ΔQ = 1 process involving hypercharge exchange, and producing K + K − and p p systems in reactions (1) and (2), respectively, recoiling off the Λ. With increasing energy, this hypercharge exchange process decreases slowly when K + K − is produced, but fast where the production of p p , violating the Zweig rule, occurs.

Spin-parity structure of the (Kπ) system produced in the reaction K−p → (K−π+)n

Abstract A spin-parity analysis is presented of the (K−π+) system produced in the reaction K−p → (K−π+)n at 10 and 16 GeV/c for M(Kπ)

Two-body hypercharge-exchange reactions in K−p and π+p interactions at 10 and 16 GeV/c

Cross-section values or upper limits are presented for twenty-five two-body hypercharge-exchange reactions in K−p and π+p interactions at 10 and 16 GeV/c. The 16 GeV/c results are compared with some predictions of line-reversal plus exchange-degenerate Regge poles, of SU(3) and of the additive quark model. Agreement is found in all cases.

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.

Inclusive production of Σ+ and Σ− in K−p interactions at 10 and 16 GeV/c and comparison with Λ production

Abstract The inclusive production of Σ+ and Σ− hyperons is studied in K−p interactions at 10 and 16 GeV/c. The total cross sections are 1010 ± 105 μb and 910 ± 95 μb for Σ+, and 690 ± 70 μb and 700 ± 75 μb for Σ−, at 10 and 16 GeV/c, respectively. The excess of Σ+ over Σ− is found to originate mainly in the proton fragmentation region, from low-multiplicity events of the hypercharge annihilation reactions K−p → Σ++pions. The production of Σ± is ∼2.6 times more abundant than that of Σ ∗± (1385) . The channels corresponding to the hypercharge non-annihilating reactions, K − p → Σ ± K K +X , represent ∼25% and ∼35% of the total, at 10 and 16 GeV/c, respectively. Evidence is found for the production of Σ ∗0 (1385), Λ ∗ (1405) and Λ ∗ (1520) , whose decay account for (7 ± 1)% of the Σ± observed at both 10 and 16 GeV/c.

Study of proton dissociation into ΛK+ in 3-body final states at 10 and 16 GeV/c

Abstract Results are presented for p → (ΛK+) dissociation in the reactions K−p → ΛK+K− and and π±p → ΛK+π±at 10 and 16 GeV/c. The cross sections for the low-mass ΛK+ enhancement are compatible with the energy dependence σ ∝ plab−0.3. The t′ spectra or the (ΛK+) threshold enhancement are exponential in shape. Its decay angular distribution reveals neither s-channel nor t-channel helicity conservation. The relative probabilities of the processes p → p, p → ( N π) I= 1 2 and p → (Λ K + ) dissociation are in the ratios 100 : 10 : 0.2, independent of the nature of the incident particle.