P. Mühlemann

Coherent production of pions on nuclei determination of unstable boson-nucleon total cross sections

Abstract Measurements of the differential cross section and of the effective mass distribution of three- and five-pion coherent production on nuclei from Be to Pb are presented. The data are interpreted with an optical model and the total cross section of the unstable multipion system with nucleons is derived as a function of the effective mass.

A study of ππ → KK using an experiment on π−p → KS0KS0n at 8.9 GeV/c

We present an analysis of the KS0KS0 system, based on 6380 events, produced in the reactions π−p → KS0KS0n. Using a one-pion exchange model with absorption we determine the S- and D-wave amplitudes of ππ → KK. Several A2 production amplitudes were also included in the fits and we conclude that an unnatural parity exchange amplitudes which does not interfere with pion exchange, seems to present. A branching ratio Γ(f → KK/ Γ(f → ππ) = 0.029 ± 0.006 is determined. With this and other new values of branching ratios we have fitted the 2+ → 0−0− decay rates of the SU(3) relations resulting in (36.2 ± 1)° for the tensor nonet mixing angle.

A Study of pi pi --> K anti-K Using an Experiment on pi- p --> K0(s) K0(s) n at 8.9-GeV/c

We present an analysis of the KS0KS0 system, based on 6380 events, produced in the reactions π−p → KS0KS0n. Using a one-pion exchange model with absorption we determine the S- and D-wave amplitudes of ππ → KK. Several A2 production amplitudes were also included in the fits and we conclude that an unnatural parity exchange amplitudes which does not interfere with pion exchange, seems to present. A branching ratio Γ(f → KK/ Γ(f → ππ) = 0.029 ± 0.006 is determined. With this and other new values of branching ratios we have fitted the 2+ → 0−0− decay rates of the SU(3) relations resulting in (36.2 ± 1)° for the tensor nonet mixing angle.

Evidence for resonance behaviour of A1 and A3 mesons coherently produced on nuclei

Abstract An analysis of about 60 000 events produced in the reaction π − A → ( π + π − π − )A on nine different nuclear targets A at 8.9, 12.9, and 15.1 GeV/ c has been performed using the Illinois PWA program. In the A 1 region our “coherent” sample of events (which contains about 11% of incoherently produced events) is described by only four partial waves: the 1 + S and 1 + P amplitudes, which both show a peak, and the O − S and O − P waves, which are rather constant. We observed variations of both 1 + S and 1 + P phases relative to the O − P phase by about 90° across the A 1 peak. This observation suggests the presence of resonant 1 + waves in the A 1 mass region (∼1.1 GeV). In the A 3 region we observe that the 2 − state shows resonance behaviour and that fπ and ϱπ are two decays modes with amplitudes of opposite signs.

Energy dependence of coherent production of three pions on nuclei between 9 and 15 GeV and comparison with diffractive three-pion production on hydrogen

A total of 34 000 coherent events, produced on different target nuclei and at incident pion momenta from 9 to 15 GeV/c, allows us to discuss the energy dependence of the differential cross section and of the three-pion mass spectrum. The comparison with the optical model is satisfactory. This model is then used to determine the total cross section of the unstable three-pion system on nucleons; the result is a value of about 25 mb. The value of the forward differential cross section on nucleons obtained by application of the optical model shows a tendency to increase with incident energy. A similar increase with energy is found for the forward diffractive cross section of three-pion production on hydrogen; the absolute value of this cross section is somewhat higher than the cross section determined from nuclear targets. Also the mass spectrum on nucleons derived from the present experiment is compatible with that produced on hydrogen. Furthermore, there is no evidence for abundant coherent production of A2 and A3. On the other hand, these two resonances seem to be present in the incoherent background.

Differential cross section for coherent production of three-pion systems on nuclei at 15.1 GeV/c☆

Abstract The measured differential cross sections d 2 σ /d t ′ d m 3 π of three-pion production on nine different nuclei (based on 40 000 events) are presented. The corrections due to the background of incoherent reactions and due to the limited angular resolution of the experiment are discussed. The comparison of the differential cross sections of coherent three-pion production with the optical model is satisfactory.

Coherent production of K∗+ (890) on nuclei and determination of an upper limit for the radiative decay width Γ (K∗+ → K+γ)☆

We have observed the coherent production of K∗+ (890) on nuclei ranging from C to Pb at 10, 13 and 16 GeV/c. Fitting a model of coherent production, including strong and electromagnetic interactions and their interference, to the measured coherent production cross section we find the strong contribution to the dominant and determine the upper limit of the radiative decay width Γ(K∗+ → K+γ) < 80 keV with 95% confidence.

The charge exchange pp → nn at 5.0 and 7.76 GeV/c near the forward direction

The differential cross section for the charge exchange pp → nn has been measured with high statistics at 7.76 GeV/c and at 5.0 GeV/c. The 7.76 GeV/c data show a very narrow [Δt ⪅ 0.01 (GeV/c)2] forward peak superposed on a slow exponential fall-off.

Angular distribution and polarization in the backward peak of π−p → ΛKO at 4 and 6.2 GeV/c☆

Abstract Data are presented on the backward peak in the reaction π−p → ΛKO. 2950 events were observed at 4 GeV/c and 1080 at 6.2 GeV/c. The Λ-polarization rises very fast with decreasing u; it is already high at u = 0.

Evidence that absorption in the π+ π− π− system in nuclear matter is stronger in the JP = 0− than in the JP = 1+ state☆

Abstract In the reaction π− + A → π+ π− π− + A at 15.1 GeV/c the coherently produced 3π mass spectrum has a broad peak around 1.1 GeV which consists mainly of a JP = 1+ state and of a smaller fraction of 0-. Comparing the observed dependence of the production of these diffractive states on the atomic weight A of the target nuclei with the optical model, we have studied the absorption of the produced states in nuclear matter. We have found the following values of the absorption cross-sections: σ2(0-) = 49−7+9 mb and σ2(1+) = 15.8−1.3+1.5 mb.