The production of unstable particles in peripheral collisions at high energies is considered, with emphasis on the connection between the production mechanism and the angular correlations in the decay of the unstable particles By means of the crossing relations for helicity amplitudes a direct relation (10) is established between thet-channel helicity amplitudes and the spin-space density matrix (relative to a suitable system of co-ordinate axes) of one of the reaction products The extent to which the decay angular distributions determine the density matrix is fully discussed As an example, various quasi-two-body chan of KN collisions are considered, especially KN → K*N It is shown that, while the decay correlations of K* at 3 GeV/c are consistent with dominantly vector-meson exchange, they are also consistent with the exchange of systems with any J ⩾1 provided the parity is (-1)J Advantage is taken of this observation to generalize the one-mesonexchange model by means of Eegge-pole exchange in a way which gives a natural explanation of the very peripheral nature of the production process, while maintaining agreement with the decay data

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On the connection between production mechanism and decay of resonances at high energies

The authors survey the current status of light-meson spectroscopy, beginning with a general introduction to meson spectroscopy and its importance in understanding the physical states of quantum chromodynamics. Phenomenological models of hadron spectroscopy are described with particular emphasis on the constituent-quark model and the qualitative features it predicts for the meson spectrum. The authors next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids and glueballs, as well as multiquark states. The established meson states are compared to the quark-model predictions and most meson states are found to be well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. The review ends with a brief description of future directions in meson spectroscopy.

Light meson spectroscopy

Nucleon--antinucleon potential from single-meson exchanges.

The physics of nucleon-antinucleon annihilation

The cosmic γ-ray spectrum below 1 GeV arising from cosmic ray p-p interactions is calculated. Its characteristics are determined by the properties of secondary neutral pion production occurring at accelerator energies. A model is chosen for numerical calculations in which the two dominant modes of neutral pion production at accelerator energies are the production of the Δ (1.238) isobar and one fireball. The effect of α-p and p-α interactions on the cosmic γ-ray spectrum is also calculated. The final results are given in terms of both differential and integral γ-ray energy spectra.

The cosmic gamma-ray spectrum from secondary particle production in cosmic-ray interactions

Approximately 20 000 pionic annihilations on neutrons of antiprotons brought to rest in a deuterium bubble chamber have been measured and analysed 1, 3 and 5-prongs events were studied in detail and the rates into the various multipion final states were determined Strong production of the pionic resonances ρ, ω and f0 has been observed, and their production rates are reported A comparison of some two-body annihilations on protons (in hydrogen) and neutrons (in deuterium) has been accomplished finding agreement between the production rates The ratio of the number of annihilations on the proton ans the neutron of the deuterium has been determined

Annihilation into pions of the\(\bar p\)n system from antiprotons at rest in deuterium

In the study of the annihilation\(\bar pn \to \pi ^ + \pi ^ + \pi ^ - \pi ^ - \pi ^ - \) at rest in deuterium, we have found a deviation of the π+π+π-π- effective-mass distribution with respect to the prediction of phase space. We show that this deviation could be satisfactorily explained in terms of a ρρ resonance withm=1.410, γ=0.090 GeV/c2JPG=0++ andI=0 or 2.

Evidence for a strong, possibly resonant, scalar ρρ interaction

We have analysed the three-body annihilations at rest \(\overline p n \to K\overline K \pi \), from a total sample of 2.22·105 annihilations. The annihilation at rest \(\overline p n \to K\overline K \pi \) has been studied by Conforto et al. The « multiple final-state interaction model » developed by them has been used to analyse the Open image in new window annihilation; an overall consistent picture is obtained in this way.

The annihilation at rest → K\overline K \pi π

We have analysed 2.22·105\(\bar pn\) annihilations at rest into final states with two K-mesons and pions. The annihilation rates into three-body (\(K\bar K\pi \)) and four-body (\(K\bar K\pi \pi \)) final states are found to be comparable, whereas the rates for the five-body states (\(K\bar K\pi \pi \pi \)) are one order of magnitude lower, with the exception of the rate for the quasi-three-body state K−K10ω. No signal for the production of the Kππ resonance C or for the\(K\bar K\pi \) resonance E is observed. These results are compatible with the production of C and E mesons from\(\bar pp\) annihilations at rest if the 0216 isospin of C is 1/2 and that of E is 0, but not ifI(C)=3/2 andI(E)=1.

Annihilations\(\bar pn\) n at rest into final states containing K-mesons

From an exposure of the CERN 2 m HBC to a separated\(\bar p\) beam at 12 GeV/c we got ∼90 000 pictures. The topological cross-sections are reported. A fairly systematic investigation of 1098 events fitting the reaction\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is presented. We looked also for the channels\(\bar pp \to \pi ^ - \pi ^ - \pi ^ + \pi ^ + \) and\(\bar pp \to \bar p\bar ppp\). The cross-section for\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is (2.35 ± 0.25) mb. Various experimental distributions are well described by two versions of the one-pion exchange model. In the first one off-shell corrections are applied by introducing form factors (OPEM), whereas in the other model the exchanged pion is Reggeized (RPEM). Large production of Δ++(1236) and\(\overline {\Delta + + } (1236)\) is observed. dσ/dt and\(\sigma (\bar \Delta \Delta )\) for the reaction\(\bar pp \to \overline {\Delta + + } \Delta ^{ + + } \) have been determined by a maximum-likelihood fit which takes into account the peripheral nature of the interaction:\(\sigma (\bar \Delta \Delta ) = (0.64 \pm 0.07)\). There is evidence for ρ0 production ((40 ÷ 120) µb) and a suggestion of a peak at 1.72 GeV in the combined mass distribution of the systems pπ+π− and\(\bar p\pi ^ + \pi ^ - \). For the 302 events in the double-isobar region 12 independent statistical tensors connected with the joint decay angular distribution have been determined. They have been compared with the prediction of a simple OPE model with absorptive corrections. The relations among statistical tensors, which follow from the assumption of additivity in quark models, have been tested in the Jackson frame. Within the large experimental errors they are found to be satisfied. Since the inclusion of the peripheral effect, neglected in previous experiments, affects the determination of the\(\bar \Delta \Delta \) cross-section,\(\sigma (\bar \Delta \Delta )\) has been re-evaluated for two experiments at 3.6 and 5.7 GeV/c and is (2.4 ± 0.2) mb and (1.45 ± 0.2) mb, respectively. The dependence of\(\sigma (\bar \Delta \Delta )\) on the primary momentum agrees with the prediction of the OPE model.

R. Santangelo

Papers

Annihilation into pions of the\(\bar p\)n system from antiprotons at rest in deuterium

Evidence for a strong, possibly resonant, scalar ρρ interaction

The annihilation at rest → K\overline K \pi π

Annihilations\(\bar pn\) n at rest into final states containing K-mesons

The reaction\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) at 12 GeV/cat 12 GeV/c