H. Drevermann

General characteristics of particle production in 16 GeV/cπ−p interactions

Abstract Partial cross sections are presented for the various topologies and the various reaction channels in 16 GeV/ c π − p interactions. Their energy dependence is discussed. The distributions of events as a function of the squared four-momentum transfer are presented, as well as the transverse and c.m. longitudinal momenta distributions. The predictions of the Chan, Łoskiewicz and Allison model describe some of the gross features of these experimental results, but important disagreements are found, mostly in the low multiplicity channels.

Dominance of isospin zero exchange in π±p → π(Nπ) at 16 GeV/c and production by it of a low mass (Nπ) enhancement

An isospin analysis has been made of the production of (Nπ) states in the reactions π±p → π(Nπ) at 16 GeV/c. The three independent amplitudes and their three interference terms have been determined as functions of the (Nπ) mass. It is found that at 16 GeV/c isospin-zero exchange is the dominant process, and isospin-one occurs much less frequently, mostly producing the I = 32 Δ (1236) isobar. Interference effects do not seem very important. The I = 12 (Nπ) mass spectrum due to isospin-zero exchange presents a broad enhancement from 1.1 to 1.8 GeV, with a broad maximum between 1250 and 1500 MeV. The N* isobars found by phase shift analyses at 1470, 1520, 1690, etc. MeV cannot account for the whole enhancement observed and in particular for its low-mass part. These results are consistent with the interpretation that diffractive phenomena are important at low Nπ masses and may cause mass enahancements not revealed byphase-shift analyses.

Separation of isospin 12 and 32 contributions in single pion production in π+p interactions at 8 and 16 GeV/c

The isospin I = 12 state of the nucleon pion system in π+p reactions leading to single pion production has been separated from the I = 32 state. The contribution of the former to the total ππN cross section is found to decrease much less with increasing energy than that of the latter. This is interpreted in terms of pomeron-exchange (or diffractive) processes responsible for at least 70% of the I = 12 contribution at 16 GeV/c and 50 % of the ππN cross section. For I = 12, the (Nπ) mass spectrum has a broad unresolved enhancement which reaches its maximum value at ≈ 1300 MeV, well below the first known I = 12 isobar. The production characteristics of this low mass (Nπ) system are different from those of the Δ (1236).

How well does an LPS analysis separate production mechanisms in π+p interactions at 8 and 16 GeV/c?

Abstract A longitudinal phase space (LPS) analysis has been performed on the reaction π+p→π+π+π−p at 8 and 16 GeV/c. Special attention has been given to the question of the separation of the various reaction mechanisms. The conclusions are: (i) Dissociation of the pion can be clearly isolated, both at 8 and 16 GeV/c. Baryon dissociation is fairly well separated at 16 GeV/c, but at 8 GeV/c it is strongly contaminated by the reactions δ++ϱ0 and even more by δ++f; (ii) The cross section for diffraction dissociation dominated regions of LPS is constant or even slightly increasing between 8 and 16 GeV/c.

Interference between (P, f0…) and (ϱ0, B0,…) exchanges in pion diffraction dissociation at 16 GeV/c

Abstract It is found in the reactions π±p→(π±π+π−)p, believed to be dominated by diffraction dissociation, that the d σ d t′ distributions show a “cross-over” effect at t′ ≈ 0.15, similar to the effect observed in elastic scattering. This gives evidence for the interference of (ϱ0, B0,…)-exchanges with ( P , f 0 , …) -exchanges in pion diffraction dissociation reactions. No such evidence is found for baryon dissociation, π±p→π±(pπ+π−), at the same energy.

Three- and four-pion angular correlations in high-multiplicity πp interactions

Abstract A new set of parameters is introduced to describe charge dependence of three- and four-pion correlations. An effect similar to the GGLP effect observed for pion pairs, but stronger, is found for three- and four-pion systems. This effect, however, can be interpreted as a reflection of the GGLP effect.

Analysis of pπ+π− enhancements produced in the reaction π+p → π+pπ+π− at 5 GeV/c

Abstract The p π + π − effective mass spectrum is studied in the mass range from 1.2 to 2.3 GeV in the reaction π + p→ π + p π + π − at 5 GeV/ c . A resonant-type-enhancement is observed at 1.71 ± 0.01 GeV whose position is consistent with that of the S 11 pion-nucleon resonance rather than the D 15 or F 15 states, but whose width 66 ± 26 MeV is considerably narrower than that of the S 11 . No evidence for an intermediate two-body decay is found. From the small value of the four-momentum transfer distribution slope 3.2 ± 0.4 GeV −2 a diffractive mechanism can be excluded. The p π + π − spectrum also shows a rapid increase near threshold, which, after elimination of peripheral Δϱ events appears as a broad diffractive-like enhancement (average slope 11.0 ± 1.0 GeV −2 ; the shape of this threshold effect has been obtained from a study of the variation of the four-momentum transfer distribution along the mass spectrum.

Longitudinal phase-space analysis of 5 GeV/c π+p reactions

The longitudinal phase-space analysis is applied to various three-body configurations produced in interactions of 5 GeV/c positive pions on protons. Distributions of the polar angle ω and of the average transverse momenta are presented. The dependence of cross sections upon incident momentum in different ω intervals is investigated for the final states π+πO and π+π+n. It is shown that in the ω-region where pomeron exchange is expected these cross sections remain constant with energy and are dominated by the isospin 12 amplitude.

Observation of the four-pion decay mode of the D(1285) meson in π±p reactions at 16 GeV/c

Abstract A narrow peak is observed at 1283 ± 6 MeV in the (π + π + π − π − ) mass distribution in the reactions π ± p → π ± p(4π) 0 at 16 GeV/ c . This is interpreted as evidence for a four-pion decay mode of the D-meson. It is shown that this enhancement is mainly ϱππ. The cross section for π ± p → π ± pD 0 , with D 0 → π + π + π − π − is 16 ± 5 μb and 6.0 ± 1.5 μb for incident π + and π − , respectively. An upper limit of 1.8 is established for the ratio of (ηπ + π − ) to (π + π + π − π − ) decay modes of the D-meson.

The four-momentum transfer distributions of nucleons in π−p interactions at 16 GeV/c

Abstract Distributions of four-momentum transfer from target proton to nucleon, d σ /d t , for 16 GeV/ c π − p reactions giving in the final state 2 to 8 pions, are analysed in terms of the F ( t ) function, which gives the average t -dependence of the matrix element squared for the reaction in question. It is shown that this function has a shape similar to the squared pion propagator for the reactions containing 3 to 8 pions in the final state. The data for 8 GeV/ c π + p reactions give results qualitatively similar to those for 16 GeV/ c π − p.