Douglas Davis

The identification of a double hyperfragment

Abstract The detailed analysis is presented of an event which is interpreted as the mesonic cascade decay of a double hyperfragment produced by the capture of a Ξ− hyperon on a light emulsion nucleus. The most likely interpretations of the double hyperfragment are those in terms of either ΛΛBe10 or ΛΛBe11.

A new determination of the binding-energy values of the light hypernuclei (A ⩽ 15)

Abstract A new determination of the binding-energy values of the light hypernuclei ( A ⩽ 15) is reported, based on the analysis of some 27 000 π − mesonically-decaying hypernuclei produced by stopping K − mesons in nuclear emulsion. The results are combined with previous data obtained using similar experimental and analysis procedures.

Some properties of the charged Σ hyperons

Abstract A sample of some 7500 K − meson captures at rest on hydrogen in emulsion giving rise to charged Σπ pairs has been used to determine r , the decay branching ratio of the Σ + hyperon, λ, the ratio of Σ − to Σ + hyperon production, τ Σ − , the lifetime of the Σ − hyperon, and C , the mean orbital capture time of Σ − hyperons in emulsion. The results obtained are r= σ + → n +π + all σ + decay modes = 0.484 ± 0.015, γ = σ − π + σ + π − = 2.34 ± 0.08, γ σ − = 1.41 +0.09 −0.08 × 10 −12 sec , and C 10 −12 sec.

The non-mesonic to π- mesonic decay ratio and Λ-neutron stimulation fraction for p-shell hypernuclei

Abstract Kinematic analysis of simple hypernuclear production reactions has produced a sample of hypernuclei of ΛB, ΛC and ΛN with negligible background, and a much smaller sample of ΛBe. The values of the non-mesonic to π− mesonic ratio Q− for the above samples are 5.5 ± 0.5 and 4.3 ± 1.1, respectively. A sub-sample of 11ΛB hypernuclei was separated on the basis of production via an excited state of Λ12C, giving Q− = 4.8 ± 1.1 for Λ11B. Assuming the Fermi gas model is applicable to nuclei of mass A ≈ 11, two independent means of analysis of the non-mesonic decays give values for the Λ-neutron stimulation fraction n of 0.41 ± 0.09 and 0.34 ± 0.07 for hypernuclei of charge 5 ≦ Z ≦ 7.

The interaction of K − -mesons with photographic emulsion nuclei

SummaryThe emission of Σ-hyperons from 3 037 K− capture stars at rest in nuclear emulsion has been studied. The identification of the hyperons is discussed. Charged Σ-hyperons are emitted from (17.6±1.0)% of all K−-stars while (9.6±0.6)% of them lead to the emission of both Σ-hyperons and π-mesons. The Σ−/Σ+ ratio for all stars is 0.86±0.12 while for stars in which a π-meson is also emitted it is 0.45±0.10. The difference between this latter value and the markedly different value (2.0) obtained for K−-interactions at rest in a hydrogen bubble chamber is attributed to the Fermi motion of nucleons in the nucleus, and a dependence of the relative K− transition amplitudes on the energy of relative motion of the K−-meson and nucleon. The branching ratio,R, of the number of Σ+ decays to π+ and proton respectively is 1.23±0.27. The stable prong distribution of the stars in which a hyperon is emitted, as well as the energy distribution of the Σ-hyperons at emission has been studied. About 16% of all identified charged Σ-hyperons had an emission energy above 60 MeV, and had to be attributed to multi-nucleon interactions of the K−-meson. It is estimated that the proportion of all multi-nucleon primary capture prccesses may be as high as (30÷40)%. The interaction of a K−-meson with a pair of neutrons seems to occur rarely, if at all. From the fraction of Σ+ emitting stars which also emit a π−-meson it is concluded that only about 10% of the π−-mesons fail to escape from the nucleus in which capture occurs. This could be understood if K−-capture occurs predominantly in the peripheral region of the nucleus. From a similar study of Σ− emitting stars it is found however that the proportion of π+ or π0-mesons, or both, that are absorbed in the nucleus is much higher. Estimates are made of the mean free path in nuclear matter of Σ-hyperons and π0-mesons. Most of the one-nucleon interactions leading to Σ-hyperon production take place with protons and the transition amplitudes corresponding to theT=1 state of isotopic spin is smaller than theT=0 transition amplitude.RiassuntoÈ stata studiata l’emissione di 3 037 stelle di cattura K− a riposo in emulsione nucleare. Si discute l’identificazione degli iperoni. Il (17.6±1.0)% di tutte le stelle K− emette iperoni Σ carichi mentre il (9.6±0.6)% di esse emettono sia iperoni Σ che mesoni π. Il rapporto Σ−/Σ+ per tutte le stelle è 0.86±0.12, mentre che per le stelle che emettono anche un mesone π è 0.45±0.10. La differenza tra quest’ultimo valore e il valore nettamente differente (2.0) ottenuto per le interazioni K− a riposo in una camera a bolle a idrogeno si attribuisce al moto di Fermi dei nucleoni nel nucleo ed alla dipendenza delle ampiezze relative delle transizioni K− dall’energia del moto relativo del mesone K− e del nucleone. Il rapportoR del numero dei decadimenti Σ+ a quelli π+ e protonici rispettivamente è 1.23±0.27. È stata studiata la distribuzione dei rami stabili delle stelle in cui si emette un iperone, come pure la distribuzione dell’energia degli iperoni Σ all’emissione. Circa il 16% di tutti gli iperoni Σ carichi identificati avevano energia di emissione superiore a 60 MeV ed hanno dovuto essere attribuiti alle interazioni multinucleoniche del mesone K−. Si stima che la proporzione di tutti i processi multinucleonici primari di cattura raggiunga il (30÷40)%. L’interazione di un K− con una coppia di neutroni sembra occorrere, se mai, raramente. Dalla frazione di stelle che emettono Σ+ emettendo anche un mesone π− si conclude che solo circa il 10% dei mesoni π− non abbandonano il nucleo in cui la cattura avviene. Ciò sarebbe comprensibile se la cattura K− avvenisse prevalentemente nelle regioni periferiche del nucleo. Da un simile studio delle stelle che emettono dei Σ− si trova, tuttavia, che la proporzione dei mesoni π+ o π0, o delle due specie, assorbiti nel nucleo è molto maggiore. Si valuta il cammino libero medio degli iperoni Σ e dei mesoni π0 nella materia nucleare. La maggior parte delle interazioni di I nucleone che conducono alla produzione di iperoni Σ avvengono con protoni e le ampiezze di transizione corrispondenti allo statoT=1 dello spin isotopico sono minori dell’ampiezza di transizioneT=0.

A determination of the binding-energy values of light hypernuclei

Abstract The results of an experimental study of the binding energies of light hypernuclei are presented, based on a sample of 2008 uniquely identified π − mesonic decays. The problems of the calibration procedure and the selection criteria are discussed. Various aspects of the Λ -N interaction are reviewed in the light of these new results.

The decay of heavy hypernuclei

In a sample of 38238 interactions found in photographic emulsions exposed to K− mesons of 1.3 and 1.5 GeV/c momenta, a total of 1385 heavy hypernuclei (40 < A < 100) were observed, of which 11 decayed mesonically. The following results were obtained: an upper limit of 27±3 MeV for the Λ0 nuclear potential well depth, a value of Q− in the range 100 to 200 and a fraction, n, in the range 0.6 to 0.9 of non-mesonic decays stimulated by the weak Λ0-neutron interaction.

The production of heavy hyperfragments by K--meson captures at rest in emulsion nuclei

SummaryIn this study of hyperfragment production by K--meson absorptions at rest in emulsion nuclei, an attempt has been made to estimate the contribution of heavy spallation hyperfragments to the total observed number. In a sample of 24 640 K--meson captures, 1612 secondary events have been attributed to the production and subsequent decay of hyperfragments. From a study of the characteristics of both the production and decay disintegrations, it is concluded that about one half of the observed hyperfragments consists of λ0-hyperons trapped in the heavy spallation products of silver and bromine and these are almost entirely confined to the (0/3) μm range interval. The consequences of their presence aie discussed.RiassuntoIn questo studio della produzione di iperframmenti nell’assorbimento di mesoni K-in quiete nei nuclei dell’emulsione, si fa un tentativo di valutare il contributo dei frammenti pesanti di demolizione al numero totale osservato. In un campione di 24 640 catture di mesoni K-, 1612 eventi secondari sono stati attribuiti alla produzione e successivo decadimento degli ipeiframmenti. Da uno studio delle caratteristiche delle disintegrazioni sia di produzione che di decadimento, si conclude che circa la metà degli iperframmenti osservati consiste di iperoni Λ0 catturati nei prodotti pesanti di demolizione dell’argento e del bromo ed essi sono quasi interamente conflnati nel-Pintervallo di percorso (0/3) μm. Si discutono le conseguenze della loro presenza.

The interaction and decay of K- mesons in photographic emulsion - Part I. General characteristics of K--Interactions and analysis of events in which a charged π-meson is emitted

SummaryA total of 3 480 K−-mesons has been observed. Of these 445 interact in flight, at a mean energy of 40 MeV and only their general characteristics have been studied. Little difference has been observed between the number of unstable charged particles (Σ±-hypersons and π±-mesons) in these interactions and those in the 3 035 interactions at rest. The 3 035 K−-interactions at rest have been studied in detail. In this paper (Part I) are given the results on those interactions in which π-mesons are emitted. An estimate of the Σ-hyperon nuclear potential is based on the energy spectrum of π-mesons emitted in the K−-interactions in which the only charged particles emitted are a π-meson and a Σ-hyperon. A detailed examination of the angular distribution of 2-pronged events, and of the sign ratio of the π-mesons, the prong distribution and energy release in all events has led to an estimate of the probabilities of absorption of positive and negative Σ-hyperons in the parent nucleus, and to a partition of the events between the various primary reactions.RiassuntoÈ stato esaminato un totale di 3 480 mesoni K−. Di essi 3 035 interagiscono a riposo, 445 interagiscono in volo con un’energia media di 40 MeV. Delle interazioni in volo sono state studiate solo le caratteristiche generali. Si osserva poca differenza nel numero di particelle instabili (iperoni Σ e mesoni π) emesse nelle interazioni in volo e nelle interazioni a riposo. Le 3 035 interazioni a riposo sono state studiate particolareggiatamente. In questo lavoro (Parte I) sono presentati i risultati relativi alle interazioni nelle quali sono emessi mesoni π carichi. Dallo spettro di energia dei mesoni π emessi nelle interazioni in cui le sole particelle cariche emesse sono un mesone π e un iperone Σ è stata ricavata una stima del potenziale nucleare degli iperoni Σ. Un’analisi particolareggiata della distribuzione angolare delle stelle a due rami, del rapporto dei segni dei mesoni π, della distribuzione del numero di rami delle stelle e dell’energia totale visibile ha condotto a una stima della probabilità di assorbimento degli iperoni Σ positivi e negativi nel nucleo di origine, e a una ripartizione degli eventi tra le varie reazioni primarie.

On the lifetime of the 3ΛH ypernucleus

The mean lifetime of the 3ΛH hypernucleus has been estimated in nuclear emulsion using the (π−+1H+2H) decays both at rest and in flight. The result, τ(3ΛH) = (1.28−0.26+0.35) · 10−10sec, is significantly lower than another recent experimental value [5] and the theoretical predictions. A possible explanation of the discrepancy with the theoretical predictions is briefly discussed.