G. Zobernig

Production of ρ0 and f in π−p interactions at 16 GeV/c and separation into fragmentation and central components

The production of ρ0(770) and f(1270) is studied in π−p interactions at 16 GeV/c. By comparison with inclusive K∗0 production in the reaction K−p → K∗0 + anything, and with inclusive ρ0 production in the reaction pp → ρ0 + anything, it is found that the data can be interpreted in terms of two production processes: the central production of resonances and the fragmentation of the beam particle. For the π−p reaction, the inclusive ρ0 beam fragmentation cross section is 3.1 ± 0.3 mb while that for central production is 1.6 ± 0.5 mb. The ρ0 central production cross section is consistent with increasing with energy as ln s behaviour. The ratio of ρ0 to π− inclusive cross sections (excluding the leading π−) is ∼0.2, independent of energy. The ρ0 to π− ratio increases as a function of pT to a constant value of ∼12 above 1 GeV/c. The ρ (charged and neutral) and f decays account for (25 ± 4)% and (1.4 ± 0.3)%, respectively, of all pions produced.

A comparison of πππ, Kππ, πKKKK system produced in the reactions meson +p→3 mesons + p

Abstract A comparison is made of the low-mass three-meson systems (πππ), (Kππ), (π K K ) and ( K K K ) diffractively produced in the reaction meson + proton → three mesons + proton. Several striking similarities and a few important differences are observed: (i) the reactions are consistent with the assumption that the three mesons decay entirely into a 0− meson and a 0+, 1− or 2+ resonance; (ii) the three-meson mass spectra have a peak ≈ 250 MeV above the effective threshold Meff of the dominant decay mode and then fall off approximately as (mass)−3;(iii) the average spin 〈J〉 = 0.55 + 1.1 Qeff, where Qeff = M - Meff; (iv) the average orbital angular momentum 〈l〉 increases according to 〈l〉 = 0.75 Qeff; (v) the three-meson states are produced dominantly in unnatural spin-parity states and no evidence for their being resonant is found; (vi) the only natural spin-parity states found are the well-established 2+ resonances A2 and K∗ (1420); they have similar properties to the non-resonant unnatural parity states except for a dip at t = 0 in the dσ/dt distributions; (vii) both the unnatural and natural spin-parity states are produced mostly by an exchange of natural parity; (viii) there is evidence for two types of production mechanism with different polarization properties, one approximately conserving helicity in the t-channel and the other in the s-channel.

Inclusive strange particle production in π+p interactions at 16 GeV/c

The production of KS0, Λ and Λ is measured in π+p reactions at 16 GeV/c. The total strange particle cross section is found to be 4.0 ± 0.3 mb, about 20% of the inelastic cross section. Cross sections for single strange particles and for strange particle pairs are determined, both inclusively and as functions of the charged multiplicity. Relative production rates for different strange particle combinations are compared with the prediction of the isospin statistical model. Inclusive spectra for single particles are studied and it is found the KS0 are produced mostly in the forward hemisphere, most probably by fragmentation of the incident pion into KKπ. The Λ are mostly backwards, probably deriving from fragmentation of the proton into ΛK pairs. The Λ tend to be produced forwards, but evidence is found for central ΛΛ production. Distributions in rapidity of the Λ particles from π+p interactions are compared in terms of the factorisation hypothesis. Results are given on the Λ transversal polarisation.

Eta and omega meson production in medium energy π+-p and K−p interactions

The production of η(550) and ω(780) mesons is studied in π±p interactions at 16 GeV/c and K−p interactions at 10 and 16 GeV/c. Cross sections for exclusive channels are presented, and for the π+p data differential cross sections are given for quasi-inclusive production where the η or ω is required to be accompanied by charged particles only. Close similarities are observed between η, ω and also ϱ0(770) production in terms of longitudinal and transverse variables. By a rough estimate, the η, ω and ϱ0 inclusive yields are found to be in the ratio 0.32 : 0.85 : 1, respectively, for 16 GeV/c π+p collisions. For non-peripheral production we estimate this same ratio to be 0.34 : 0.9 : 1.

A comparative investigation of low mass π+ω and K−ω systems at various energies

A comparison is made of the properties and production mechanisms of the π+ω and K−ω systems produced in the reactions π+p → π+ ωp at 4, 5, 8 and 16 GeV/c and K− p → K−ωp at 10 and 16 GeV/c. In the π+ω case apeak is observed at 1.23 GeV (the B meson), while the K−ω mass distribution has a threshold enhancement. The cross section of the low mass (<2.0 GeV) π+ω system falls as plab−2, while that of the low mass (<2.0 GeV) K−ω system is almost constant with energy, indicating diffractive production of the K−ω system, but not of the πω system. Using a modified version of the Illinois partial-wave analysis program, it is found that the K−ω system is dominantly produced in the JP = 1+ state with small contributions of 0− and 2+, mainly by natural parity exchange - as is found for reactions such as K−p → (K−π+π−)p which are predominantly diffractive. For the π+ω system in the B mass region, JP = 1+ states, produced mainly by natural parity exchange are found; the contributions of 0−P, 1−P, 2−P and 2+D are consistent with zero. The 1+D state occurs in the π+ω case but not in the K−ω system, nor in the Kππ− system produced in the K−p → Kππp reaction.

Prism plot analysis of the reaction π+p → pπ+πo at 16 GeV/c

Abstract A study of the reaction π + p → p π + π o at 16 GeV/ c incident momentum has been made using the prism plot analysis to reject background events arising from elastic and multineutral contaminations and to separate different reaction channels ( ϱ + p, g + p, Δ + π + , Δ ++ π o , π + (p π o ) DD ). Cross sections, invariant mass distributions and production and decay angular distributions are presented. For the channel corresponding to proton diffraction dissociation strong violation of both s - and t -channel helicity conservation is found for low values of the (p π o ) mass. We demonstrate that the prism plot method provides a better separation of background events than conventional methods using kinematic cuts.

Charge transfer in πp interactions at 8 and 16 GeV/c

The charge structure of the final states in 16 GeV/c π±p and 8 GeV/c π+p inclusive interactions is studied. For each event the total charge of particles with rapidities larger than y is measured and the distribution of this charge is analyzed for various values of y. The results are shown to agree with predictions of the neutral isotropic cluster model.

Study of the reaction π+p → ϱ0 Δ++(1236) at 16 GeV/c

Abstract The reaction π+p→ωΔ++(1236) is studied at 16 GeV/c. Cross section, differential cross section, single and joint spin-density matrix elements are given and the correlations between the e and Δ++(1236) decay angular distributions are investigated. Natural and unnatural spin-parity exchanges contribute to this reaction in roughly equal amounts. Natural exchanges lead predominantly to Δ++(1236) with helicity ± 3 2 , while unnatural exchanges lead predominantly to Δ++(1236) with helicity ± 1 2 and to e with helicity zero. Furthermore, unnatural exchanges are small at t′≅0.2 GeV2 compared to other t′ values, which may be due to the nonsense wrong-signature-zero of the B-meson exchange. Quark model relations are found to be satisfied by the data.

Study of s- and t-channel helicity conservation in the diffractive part of the reaction π±p→π(Nπ) at 16 GeV/c

Abstract By means of an isospin analysis of the reaction π±p→π(Nπ) at 16 GeV/c we have determined the decay angular distributions of the Nπ system with I= 1 2 produced by isospin zero exchange. Helicity conservation is not observed in the t-channel for the Nπ mass region below 1.6 GeV, where diffraction dissociation of the proton is supposed to dominate. There are indications for approximate t-channel helicity conservation for N∗ (1690) production. In the helicity frame, the experimental data are not in agreement with s-channel helicity conservation over the whole Nπ mass range investigated. Thus the diffractive process N→Nπ differs both from the process N→Nππ (or π→πππ and K→Kππ) which approximately conserves t-channel helicity and from the elastic scattering N→N which conserves helicity in the s-channel.

Spin-parity analysis of the diffractively produced low-mass Nπ system in the reaction π±p → π± (Nπ)+ at GeV/c

The dominant partial waves of the diffractively produced Nπ system at low Nπ masses (⩽ 1.4 GeV) are determined in the reactions π±p → π(Nπ) at 16 GeV/c. A satisfactory description of our data can only be obtained by strong contributions of both a 12− S-wave and a 32+ P-wave, violating the Gribov-Morrison rule. Spin and parity of the diffractively produced states are found from the interference between diffraction and Δ (1236) production. The interference term is obtained by an isospin analysis.