J.-M. Perreau

Search for neutrino decay

Abstract A search to observe neutrino decays has been performed. Its results are used to set limits on the couplings of a hypothetical massive neutrino with the ordinary v e and v μ . An improvement of about two orders of magnitude is obtained over previous limits in a wide range of neutrino masses.

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*.

K+p elastic scattering at 3.0 GeV/c

SummaryIn the course of a systematic study of K+p interactions at 3.0 GeV/c, the elastic-scattering reaction has been investigated. A total of 1720 events were identified as elastic scatters, giving a cross-section of (4.8±0.4) mb. The angular distribution shows characteristic diffraction peaking and was fitted using dσ/d|t|=(dσ/d|t|)0 exp [αt+βt2], in the momentum-transfer region (0.05÷1.14) (GeV/c)2. The best fit gaveα=(4.55±0.39) (GeV/c)−2 andβ=(0.64±0.42)(GeV/c)−4. The extrapolated experimental cross-section at 0°, (dσ/dt)0, is found to be (19.5±2.3) mb/(GeV/c)2, and exceeds the optical-theorem prediction by (3.8±2.3) mb/(GeV/c)2, implying that there is a contribution from the real part of the K+p scattering amplitude at 3.0 GeV/c.RiassuntoNel corso di uno studio sistematico delle interazioni K+p di 3.0 GeV/c si è esaminata la reazione di scattering elastico. Un totale di 1720 eventi è stato identificato come scattering elastici che danno una sezione d’urto di (4.8±0.4) mb. La distribuzione angolare presenta un caratteristico picco di diffrazione ed è stata approssimata da dσ/d|t|=(dσ/d|t|)0 exp[αt+βt2] nella regione dei momenti trasferiti (0.05÷1.14)(GeV/c)2. Un calcolo di « best fit » dàα=(4.55±0.39)(GeV/c)−2 eβ=(0.64±0.42)(GeV)−4. Si trova che la sezione d’urto sperimentale estrapolata a 0°, (dσ/d)0, è (19.5±2.5) mb/(GeV/c)2, e supera le predizioni del teorema ottico di (3.8±2.3) mb/(GeV/c)2, implicando che si ha un contributo della parte reale dell’ampiezza di scattering K+p a 3.0 GeV/c.

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.

Spin, branching ratios and production characteristics of the K∗(1420)

Abstract The reactions (1) K + p → K ∗ (1420) p and (2) K + p → K ∗ (1420) N ∗ (1236) are studied at 5 GeV/c incident K+ momentum. The simultaneous fitting of the K π and K ∗ (892)π decayy modes of the K∗ (1420) in the same production reaction makes it possible to eliminate 1− as the spin of the K∗(1420). The most likely K∗(1420) spin-parity assignment is 2+ although 3− cannot be excluded. The various decay branching ratios of the K∗(1420) are determined. The ratio of the K ∗ (892)π to the Kπ decay modes is found to be 0.9 ± 0.2. The peripheral production of the K∗(1420) seems to be due predominantly to vector-meson exchange in reaction (1) and to pion exchange in reaction (2).

Evidence for three Kππ resonant states below 1.4 GeV in K+p reactions at 5 GeV/c

Abstract The study of K+p reactions at an incident momentum of 4.97 GeV/c suggests that the wide enhancement in the (Kππ)+ mass spectrum may be split into several resonant states. In particular, examination of the K ∗ (892)π decay mode indicates the existence of the states K∗(1230), K∗(1280) and K∗(1320), in addition to the well known K∗(1420). The K∗(1230) and K∗(1320) have the properties expected from 1+ states produced by 0+ exchange.

Study of multi-body final states in K+p reactions at an incident momentum of 5 GeV/c

Abstract Results are given on the comparison of the predictions of the multiperipheral model of Chan, Loskiewicz and allison with the experimental data obtained from a study of the interactions of 5 GeV/c K+ mesons with protons. Comparisons have been made in the three-, four-, five- and six-body final states containing a proton, kaon and pions. With the addition of resonance production to the model, adequate predictions of experimental distributions are obtained. An attempt is made to describe the energy variation of the three- and four-body final-state cross sections.

Study of the exchange mechanism in K+p → K+π-N*++ at 5 GeV/c incoming K+ momentum

We present results on the analysis of the quasi three-body reaction K+p → K+π-N*++ obtained from bubble chamber exposures at 5 GeV/c incident K+ momentum. We have applied a method of analysis based on the study of the longitudinal phase space plot recently proposed by Van Hove. Evidence for the existence of π- mesons with a four-momentum transfer peaked at small values is shown. (Events contributing to this peak correspond to the cross section of about 80 μb.) The difficulties in explaining this result in terms of the double-particle exchange model are discussed.

Search for neutrino oscillations

A search for νμ→νe and νμ→νe oscillations has been conducted with the AGS wide-band neutrino beam at the Brookhaven National Laboratory. We find more νe (νe) interactions than were expected on the basis of the number of incident νe (νe) calculated as part of the neutrino beam. The excess is about a factor two over the expectation, the statistical significance being about two and a half standard deviations for νe and weaker for νe.

Search for baryonic resonance with S = +1 in K+p production experiments

Abstract In about 30 000 identified events produced in K + p reactions at incident momenta 3.0, 3.5 and 5.0 GeV/ c we have searched for the production of Z ∗ s. i.e. S = +1, T = 0 or 1 baryonic resonances. No indication has been found in the examination of the various available production reactions of Z ∗ decaying with visible modes. Upper limits for the corresponding cross-sections are generally less than 30 μb at 3.0 and 3.5 GeV/ c . 10 μb at 5.0 GeV/ c . Summing for a given production reaction over all possible Z ∗ decay modes. i.e. performing a missing mass type of analysis, does not revel any further evidence.