B. Bouquet

Evidence for two narrow pp resonances at 2020 MeV and 2200 MeV

Abstract From the study of the reaction π − p→p F p p π − using a fast proton (p F ) trigger device in the CERN Omega spectrometer, we find evidence for two narrow p p states produced mainly in association with a Δ° (1232) and a N° (1520). The statistical significance of each peak is greater than 6 standard deviations. Masses and natural widths of these resonances are respectively M = 2020 ± 3 MeV , Λ 1 = 24 ± 12 MeV and M 2 = 2204 ± 5 MeV, Λ 2 = 16 −16 +20 MeV. Our data are consistent with a small production of the narrow ∼ 1935 MeV resonance already reported. Production cross sections for these new p p resonances are given.

Backward production in π−p→pfπ+π−π− on at 9 and 12 GeV/c

We have analyzed backward meson production in the reaction π−p → pfπ+π−π− at 9 GeV/c and 12 GeV/c incident π− momenta, from an experiment performed at the CERN Ω Spectrometer using a fast proton (pf) trigger device. We find strong production of quasi-two-body processes N∗ϱ and N∗f with a production mechanism consistent with u-channel nucleon exchange. At a lower level, we observe N∗π processes with a 3-body baryon decay through Δ(1232)π. In the (3π)− system, we find evidence for A1− and clear A2− backward production with similar cross sections (≈0.5 μb).

Search for non-strange exotic mesons produced via baryon exchange

Abstract Negative results on backward production, via baryon exchange, of exotic non-strange mesons are presented. The reactions π − p→p forward X − and π − n→ p forward X −− have been studied with a 12 GeV/ c π − beam in the Omega spectrometer at CERN. No resonant peak in X → p p π − , p p π − π − , p p π − π 0 ,π + π − π − π 0 has been seen. The upper limits obtained on cross sections for exotic meson production X → N N π, N N π, 4π are lower than the ϱ − backward production cross section in the π − p→p ϱ − reaction; this result seems to contradict the predictions of the two-component duality model. Compared to already published experiments in the search for exotics produced via baryon exchange, the sensitivity of this experiment is increased by an order of magnitude.

Results on π−p backward elastic scattering and search for π−p→dp AT 9 and 12 GeV/c

Abstract From an experiment done with the CERN Omega spectrometer, triggered by a fast forward proton device, we present results on the differential cross section d σ d u for π−p backward elastic scattering. The d σ d u distribution agrees with an AeBu law. The compilation of existing results shows a discrepancy between results but the ( d σ d u ) u=0 data fit perfectly an s2α0−2 dependence, as predicted by a single Δδ Regge trajectory exchange. A search for the reaction π − p → d p , with a fast forward deuteron, which can be produced by a double-baryon exchange mechanism, gives cross-section upper limits of ∼1% of the backward elastic cross section.

Further results on the narrow pp̄ resonances at 2020 and 2204 MeV

Abstract An unsuccessful search has been made for the reaction π − p → p F p n to find an M− state in pn corresponding to the M0 (2020) and M0 (2204) found earlier in the baryon exchange process π−p → Δ0 (or N0 (1520)) + M0. From this negative result, it is concluded that the previous reaction proceeds mainly by nucleon exchange and it is suggested that the NN coupling for the M0 is more important than the ΔN coupling. Some indications are also given for the NN mode as the main decay mode for these M0 states.

Backward enhancement in the A1 region and A2 production in the reaction π-p→pfπ+π-π- at 9 and 12 GeV/c

Abstract Results on backward (3 π ) - system produced in π - p→p f π + π - π - reaction at 9 and 12 GeV/ c are given. The ϱ 0 π - mass spectra show two clear signals at 1050 MeV (A 1 region) and 1303 MeV (A - 2 ). The width of the enhancement in the A 1 region (195±32 MeV) is narrower than found in diffractive experiments. Total backward cross sections for those signals are of the same order of magnitude (∼0.5 μb).

Baryon exchange in the reaction π−p → N∗0π0 at 9 GeV/c and 12 GeV/c

Abstract In π−p interactions at 9 GeV/c and 12 GeV/c, the forward production of N∗1680 and N∗1520 has been observed with features of nucleon exchange. The production of Δ1232 is strongly suppressed.

Backward ω0 and η0 production in πp interactions at 9 and 12 GeV/c

We have studied the backward production of ω0 mesons in the u-channel Iu = 12 exchange reaction π−p → N0(1680)ω0 at 9 GeV/c and 12 GeV/c incident momenta. The data come from an experiment performed at the CERN Omega Spectrometer using a fast proton trigger device. The backward production of the η0 meson has also been observed and the coupling constant ratio gηNN/gπNN has been estimated.

Impact parameter analysis of the reaction π−p→pπ+π−π− at 9 and 12 GeV/c with very fast secondary proton

The results on the impact parameter for the reactions π−p→pπ+π−π−, pK+K−π−, with a fast secondary forward proton are given at 9 and 12 GeV/c. For the reaction π−p→pπ+π−π− our results corresponding to the small |u| region (|u| < 1 GeV2) are compared to the existing data for the small |t| region. We show that small |u| events are more central than small |t| events.

Backward K∗0 production in the reaction π−p→Λ0K+π− at 9 and 12 GeV/c

Abstract We have analysed about 85 000 fast Λ0 events, obtained in a fast proton triggered experiment performed at the CERN-Ω spectrometer at 9 and 12 GeV/c incident π− beam. Nearly 2500 Λ0K+π− events have been isolated. We find strong production of quasi-two-body processes Λ 0 K ∗0 and ∑ ∗− K + consistent with u-channel hyperon exchange. Results on Λ0 polarization, K∗0 decay parameters and differential cross sections are given for Λ 0 K ∗0 (892) and Λ 0 K ∗0 (1430) final states. A comparison is made with the associated backward Λ 0 (1520) K ∗0 production seen in the four-prong reaction π−p→pK−K+π− obtained in the same experiment.