D.W.G. Leith

The KN interaction from 800 to 1200 MeV/c (I). K−p cross sections

K−p reactions have been studied at 23 different momenta between 777 and 1226 MeV/c. This interval corresponds to a mass range of the KN system from 1688 to 1897 MeV. About 350 000 photographs were taken in the 81 cm Saclay hydrogen bubble chamber, exposed to a separated K− beam at the CERN proton synchrotron. A total of about 100 000 events were analysed and partial and differential cross sections were determined; polarisations were also obtained for those reactions where the decay of the Λ or Σ+ hyperon allowed their measurement. The data for all the channels are presented here except those corresponding to two-prong configurations, which make the subject of a separate investigation.

The reaction K+p → K0pπ+ at 3 GeV/c

SummaryThe reaction K+p → K0pπ+ has been studied at 3.0 GeV/c. The two dominant reactions in this final state are K+p → K*p, K* →c K0π+ and K+p →N*K0,N* → pπ+ accounting each for (38±3)% of the events and corresponding to a cross-section of (0.8±0.1) mb. The production angular distributions for both K* andN* are suggestive of a peripheral mechanism of production. Within the framework of a simple one-mesonexchange model, the decay angular distributions for K* are consistent with the assumption that these events are predominantly produced by the exchange of a vector meson with only (7±6)% of the events being produced by the exchange of a pion. TheN* decay distributions agree with the prediction of the model of Stodolsky and Sakurai. For both K* andN*, a form factor of the type exp [-δ2/t2], witht equal to about 0.6 GeV/c, is needed to fit the very sharply peaked production distributions. A modified one-meson-exchange model, taking into account absorption effects due to competing processes, derived by Gottfried, Jackson and Svensson, gives a reasonable account of the production angular distribution and of the decay correlations for both K* andN*.RiassuntoLa reazione K+p → K0pπ+ è stata studiata a 3 GeV/c. Le due reazioni dominanti nello stato finale sono K+p → K*p, K* → K0π+ e K+p →N*K0,N* → pπ+. Queste due reazioni sono prodotte nel (38±3)% dei casi e corrispondono ciascuna ad una sezione d’urto di (0.8±0.1) mb. Le distribuzioni angolari di produzione sia per il K* che per l’N* suggeriscono un meccanismo di produzione di tipo « periferico ». Nel limite di un semplice modello a scambio di un solo mesone, le distribuzioni angolari di decadimento del K* sono in accordo con l’ipotesi che questi eventi sono prodotti prevalentemente tramite lo scambio di un mesone vettore; solo il (7±6)% degli eventi possono essere attribuiti allo scambio di un pione. Le distribuzioni angolari di decadimento dell’ N* sono in accordo con le predizioni del modello di Stodolsky e Sakurai. Le distribuzioni angolari di produzione sia del K* che dell’N* presentano un forte picco ed è necessario fare uso di un fattore di forma del tipo exp [-δ2/t2] cont ⇏ 0.6 GeV/c per poterie deserivere. Usando un modello di scambio di un solo mesone modificato da Gottfried, Jackson e Svensson per tenere conto dei processi di assorbimento doTuti a reazioni in eompetizione, è possibile ottenere un buon accordo fra le distribuzioni angolari di produzione e le correlazioni di decadimento del K* e dell’N*.

The reactionππ at 3 GeV/c and the production mechanism of

SummaryIn a study of the four-particle states K+p→ππ at 3 GeV/c, it is found that the quasi-two-body process K+p→* occurs in large proportion. The cross-sections for the various charge and decay modes of K** are in good agreement with the predictions based on isotopic-spin invariance. The sharply peaked c.m. production angular distribution suggests peripheralism. Within the framework of a simple one-meson-exchange model the decay angular distributions of K* and* are consistent with the assumption that the reaction proceeds predominantly through the exchange of a single pion. Such a model is unable to explain the nonzero value of the « interference » parameter Reϱ1, 0 (−0.13±0.02). In addition, the model requires the inclusion of a drasticad-hoc form factor, similar to the one used at 1.96 GeV/c, in order to fit the observed production angular distribution. A modified one-meson-exchange model taking into account the absorption due to competing processes and developped by Jackson, Donohue, Gottfried, Keyser and Svensson, does not explain the absolute value of the cross-section, but predicts, with a minimum of assumptions, a production angular distribution and decay correlations in reasonable accord with experiment.RiassuntoIn uno studio degli stati a quattro particelle K+ → a 3 GeV/c si è trovato che il processo di due quasi-corpi K+p→* avviene in grande proporzione. Le sezioni d’urto per i vari modi di carica e di decadimento del K** sono in buon accordo con le predizioni basate sull’invarianza dello spin isotopico. Il forte picco della distribuzione angolare della produzione nel centro di massa suggerisce il periferalismo. Entro lo schema di un semplice modello a scambio di un mesone le distribuzioni angolari del decadimento del K* e del* sono in accordo con l’ipotesi che la reazione proced a tramite lo scambio di un solo pione. Tale modello non è in grado di spiegare il valore non nullo del parametro di « interferenza » Reϱ1,0 (−0.13±0.02). Inoltre il modello esige che siano introdotti drastici fattori di formaad hoc, simili a quello usato a 1.96 GeV/c per approssimare la distribuzione angolare di produzione osservata. Un modello modificato a scambio di un mesone che tiene conto dell’assorbimento dovuto a processi in competizione, sviluppato da Jackson, Donhue, Gottfried, Keyser e Svensson, non spiega il valore assoluto della sezione d’urto, ma predice, con un minimo di ipotesi, una distribuzione angolare di produzione e delle correlazioni di decadimento in ragionevole accordo con l’esperienza.

Resonances and other partial waves in K−p → Σπ between 780 and 1220 MeV/c

Abstract A partial wave analysis of the reaction K − p → Σ + π − and Σ − π + is presented. Results are given on resonance formation in the mass region from ∼1.7 to ∼1.9 GeV.

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.

The KN interaction from 800 to 1200 MeV/c (III) a study of Λ(1520)

Abstract The resonant parameters of Λ(1520) have been determined from an analysis of the reaction sequence K − p → Σ (1760) → Λ (1520) π 0 with subsequent decay of Λ(1520) into K − p, Σ ± π ∓ and Λπ + π − . The best values for the Λ(1520) mass and width are: E R = 1517.2 ± 0.6 MeV. Γ = 14.7 ± 1.8 MeV. Its decay branching ratios are Γ(Σπ)/Γ( K N ) = 0.82 ± 0.08 and Γ(Λππ)/Γ( K N ) = 0.22 ± 0.03 and its branching fractions Γ( K N )/Λ = 0.49 ± 0.02, Γ(Σπ)/Γ = 0.40 ± 0.02. Γ(Λππ)/Γ = 0.11 ± 0.01 .

Formation of resonant states in between 780 and 1220 MeV/c

K−→K0n, K−p and K−p total cross section data are analyzed in terms of resonant and non-resonant partial wave amplitudes. Results are given on resonance formation in the mass region 1.7 to 1.9 GeV.

Formation of resonant states in K−p→KN between 780 and 1220 MeV/c

K−→K0n, K−p and K−p total cross section data are analyzed in terms of resonant and non-resonant partial wave amplitudes. Results are given on resonance formation in the mass region 1.7 to 1.9 GeV.

FORMATION OF RESONANT STATES IN K

K−→K0n, K−p and K−p total cross section data are analyzed in terms of resonant and non-resonant partial wave amplitudes. Results are given on resonance formation in the mass region 1.7 to 1.9 GeV.