G. Bellini

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.

Electronic measurement of the lifetime of D± mesons

Abstract Charmed meson pairs have been photoproduced coherently on an active silicon target. Ninety-eight decays have been analyzed and the lifetime of charged Ds has been measured.

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.

Properties of the A1 studied in 16 GeV/c π− interactions on nuclei

magnetic field 20.3 KG) was filled with C2F3C1 (density 1.2, radiation length 25 cm). This liquid was chosen mainly because: — its short radiation length permits > 95% efficiency for detection of at least one of the two y rays from a decaying jt°; — it contains no hydrogen, and thus offers no possibility of reactions on free protons which could be confused with the reactions on nuclei which we wanted to study.

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.

The (3π)-nucleon collision in coherent production on nuclei at 40 GeV/c

Abstract Coherent 3 π production on nine different nuclear targets has been studied using a 40 GeV/ c π − beam at the Serpukhov accelerator (CERN-Serpukhov experiment no. 5). The absorption in nuclear matter of the produced system has been measured, analysing the data on the different nuclear targets. Identica results are obtained from the differential cross sections and from the coherent nuclear cross sections. The 1 + waves show a very weak absorption, definitely smaller than 0 − and 2 − waves. No influence on the absorption comes from the spin-flip amplitudes, which have been found to be negligible in the coherent region.

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 5-pion final states from nuclei at 16 GeV/c

Coherent production of 2π+3π occurs with cross section (0.1 ± 0.03) mb/nucleus in 16 GeV π- C2F5Cl collisions, compared with (4.0 ± 0.6) mb/nucleus for π+2π- coherent production. Comparison of 2π+3π- and π+2π− yields 90% confidence upper limits (γ0→2π+2π-)< 0.2% (γ0→π+π-) and (A1→2π+3π−)< <0.2% (A1→πγ0). The 5π mass peaks in the 1.9 GeV region. Indications of γ0 and A1 substructure are seen.

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.

Experimental results on 8 GeV/c π−-p interactions with two, three and four pions

Summaryπ−-p interactions at 8 GeV/c have been produced in a liquid-hydrogen target placed in 1 m heavy liquid bubble chamber of the Ecole Polytechnique. The following resonances have been observed:N32/*32/++ and ρ0 in the π+π−π−p channel, a small proportion of ω0 in the π+π−π0n channel; and ρ0 in the π+π−π+π−n channel. In particular in three-charged-pion production, there is no evidence for A1 and A2 states and quasi-two-body final states are very rare. The Deck-model predictions, without nuclear absorption and ρ-ρ symmetrization, are in fair agreement with π+π−π− mass andΔρ2 experimental distributions; however the angular predictions do not fit our data well. The total and differential cross-sections are given for each channel; for all reactions, the 4-momentum transfer distribution shows a slope comparable to that for elastic scattering.RiassuntoSi sono studiate le interazioni π−-p prodotte in un bersaglio di idrogeno liquido posto all’interno della camera a bolle a liquido pesante dell’Ecole Polytechnique Si è osservata la produzione diN32/*32/* e ρ0, nel canale π+π−π−p; di ω0 (in piccola quantità) nel canale π+π−π0n; di ρ0 nel canale π+π−π+π−n. In particolare negli eventi con tre mesoni π carichi in uscita, non vi è evidenza di A1 e A2 e, più in generale, le reazioni a due corpi sono praticamente assenti. Un confronto fra i risultati sperimentali ottenuti in questo canale e le previsioni teoriche del modello di Deck, calcolate senza assorbimento e senza simmetrizazione per l’interferenza ρ-ρ, dà un buon accordo per le distribuzioni della massa invariante π+π−π− e del quadriimpulsoΔρ2; tale accordo peggiora notevolmente quando si prendono in considerazione le distribuzioni angolari. Per ogni tipo d’interazione vengono riportate le sezioni d’urto totali e differenziali; queste ultime al variare nel quadriimpulso trasferito al bersaglio, mostrano un andamento simile a quello della diffusione elastica.РезюмеИсследованы π−-p взаимодействия при 8 Гэв/с в жидкой водородной мишени, помешенной в 1 м пузырьковой камере на тяжелой жидкости в Политехнической Школе. Наблюдались следуюшие резонансыN32/+32/++ и ρ0 в канале π+π−π−p; маленькая доля ω0 в канале π+π−π0n и ρ0 в канале π+π−π0n. В частности, при рождении трех заряженных пионов, не наблюдались состояния A1 и A2, и квазидвухчастичные конечные состояния являются очень редкими. Предсказания «Дэк» модели, без ядерного поглощения и р-р симметризации, находятся в довольно хорошем согласии с экспериментальными распраделениями масс π+π−π− иΔρ2; однако, угловые предсказания не полностью соответствуют нашим данным. Для каждого канала приводятся полные и дифференциальные поперечные сечения для всех реакций, распредение четырехмерного передаваемого импульса обнаруживает наклон сравнимый с наклоном для упругого рассеяния.