S. Humble

An impact parameter description of single and double diffraction dissociation

Abstract An s -channel impact parameter model of inelastic diffraction dissociation is constructed which describes quantitatively the momentum transfer distributions of both pion and nucleon induced reactions. It is stressed that the impact parameter profile is peripheral and is approximately the same for all helicity amplitudes. From the fits to the data, the concept of a universal limiting strength (ULS) for each amplitude is deduced and its consequence for other diffractive processes, and the approximate TCHC hypothesis are discussed. Finally, detailed predictions are presented the momentum transfer distributions in the double diffraction process NN → (N π )(N π ), and compared with a simple t -channel picture of diffraction dissociation.

Determination of the fire-ball dimensions from second-order interference between two pions

Abstract Positive correlations are observed for pion pairs of equal charge and similar momentum vectors, produced in the reactions π p→p+5 π and π p→p + 7 π at 4 to 25 GeV/ c . The results are consistent with being due to the second-order interference effects expected for pairs of identical bosons and lead to the determination of the average dimension of the “fire-ball’ from which the pions are emitted (〈 R 〉 = 1.0 ± −0.2 +0.4 fm). Attempts are made to study the fire-ball shape by selecting pions pair emitted in various polar and azimuthal directions.

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.

The peripheral impact parameter description of inelastic diffraction dissociation at very high energies

Abstract A simple peripheral impact parameter model for inelastic diffraction dissociation is used to describe both the very high energy exclusive process pp → p π + n and the inclusive process pp → pX. It is found that the width of the impact parameter profile has a small dependence on s / M 2 which is compatible with a Regge shrinkage factor of the form ( s / M 2 ) 0.17 t . It is suggested that the profile is independent of the number of particles produced, and that the non-vanishing of the triple pomeron coupling and the sharp t -behaviour of the inclusive distributions at small M 2 values is the generalization of the slope-mass correlation effect observed in exclusive processes.

Forward-backward multiplicities in π+p interactions between 8 and 23 GeV/c

Abstract We study the multiplicity distributions and the correlations of charged particles produced forward and backward in the centre-of-mass system. The correlations are small but significant. This type of analysis shows clearly the presence of the diffractive component of the inelastic cross section, amounting to about 15% of it. It is also apparent that KNO multiplicity scaling is not satisfied.

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.

Partial-wave analysis of the (KKK) system in the reaction K−p→K−K−K+p at 10 and 16 GeV/c

The reaction K−p→K−K−K+p is shown to have properties similar to those of the diffractive reaction K−p→K−π−π+p. This is true, at least in first approximation, for (a) the energy dependence of the cross sections, (b) the shape of the three meson spectra and (c) the differential cross sections dσ/dtpp which have the same slope for the same mass of the three-meson system. Furthermore, the partial-wave composition of the (KKK) system is similar to that of the (Kππ) system, both systems being produced mostly in unnatural spin-parity states by natural parity exchange. In the KKK system, the JP = 1+ state with S-wave decay into Ko(1020) is dominant, (72 ± 11) % of all events with (KKK) masses below 2 GeV.

Proton diffractive excitation in the reaction K−p→pπ+π− at 16 GeV/c

By means of kinematic cuts, we select 1 421 events of the reaction K−p→K−pπ+π− at 16 GeV/c which involve the diffractive excitation of the proton. Decay of the diffractive cluster is most simply described in the Gottfried-Jackson reference frame, where a highly anisotropic decay is observed. The cluster, containing spin components J ⩾ 52, has a dominant decay mode δ++ (1236) π−.

Study of cross-over of dσ/dt′ for π± p → π± (π− Δ ++) as a test of deck model predictions

To check predictions of the Deck model, dσ/dt′ distributions are compared for the reactions π+p→π+(π−Δ++) and π−p→π−(π−Δ++) at 16 GeV/c, for M(π−Δ++) ⩽ 1.9 GeV. No cross-over in dσ/dt′ is observed for the total sample of events. When the data are divided into two subsets, according to whether the absolute value of the azimuthal angle φS of the Δ++ in the s-channel helicity system is greater than or less than 12π, then cross-over effects are observed for each subset. For |φS|>12π the “normal” result is obtained that slope b− of the dσ/dt′ distribution for negative beam particles is greater than that for positive beam particles, i. e. b− > b+. For |φS| b− is obtained, a situation observed here for the first time in high-energy reactions. These results are consistent with predictions of the Deck 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.