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STRANGE THEORY IN THE COSMIC RAY PERIOD R. Dalitz

To cite this version:

R. Dalitz. THEORY IN THE COSMIC RAY PERIOD. Journal de Physique Colloques, 1982, 43 (C8), pp.C8-195-C8-205. ￿10.1051/jphyscol:1982811￿. ￿jpa-00222371￿

HAL Id: jpa-00222371 https://hal.archives-ouvertes.fr/jpa-00222371 Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C8, suppldment au no 12, Tome 43, ddcembre 1982 page ~8-195

STRANGE PARTICLE THEORY IN THE COSMIC RAY PERIOD

R.H. Dalitz

Department of Theoretical Physics, 2 KebZe Road, Oxford OX1 3NP, U.K.

What role did theoretical physicists play concerning elementary in the cosmic ray period? The short answer is that it was the nuclear forces which were the central topic of their attention at that time. These were considered to be due primarily to the exchange of between , and the study of all aspects of the - interactions was the most direct contribution they could make to this problem. Of course, this was indeed a most important topic, and much significant understanding of the pion-nucleon interaction resulted from these studies, although the precise nature of the nucleonnucleon force is not settled even to this day. My own entry into the field of strange particle physics came about as a result of working at Bristol University for the academic year 194819 as a research assistant to Prof. N.F. Mott. Much impressed by the fundamental discoveries being made in the attic ('the fourth floor') of his department at Bristol, by Prof. C.F. Powell and his highly international group of co-workers, Mott had formed the intention of wor~ingagain in the field of , as he had done earlier in the 30's. Although not one of them, my contact with many of the younger people on the fourth floor was quite close and I was well informed about the new discoveries being made there. After moving to work with Prof. R.E. Peierls at Birmingham University in 1949, I remained in touch with them and with this work at Bristol in the subsequent years.

The contributions from cosmic radiation studies dominated strange particle physics until about 1954. The first strange produced in laboratory experiments using accelerator beams were observed in 1953 and the contributions from accelerator experiments then grew rapidly, especially after the began operation at the Radiation Laboratory, Berkeley, late in 1954. The Bagneres-de-Bigorre conference of 1953 was the last physics conference where the data came entirely from cosmic ray work. Bearing in mind the title of this Session, I have decided to review the major conferences in the field which took place about the years 1953 and 1954, and to discuss what theoretical work concerning new particles was presented ac them.

The first conference to consider is the Third Rochester Conference, held 18-20 December 1952. Of its seven sessions, three gave some attention to new particles; session III was on VO Particles, session IVA was on Superheavy , and the latter half of session V was given to Megalomorphs. It is worth mentioning Amaldi's report on r- events, in session IVA, which described two events from Bristol, three events from Imperial College, London, and one each from Padua and from Rome, their mean mass being 500-1_2 MeV. Not all of the secondary particles could be identified but those identifiable were pions, and the consistency between all eight decay events pointed to their most probable identification as

There was no theoretical discussion reported, concerning the new particles. There were a number of theoretical sessions, of course, devoted to the nucleon- nucleon interactions and to pionnucleon scattering and related phenomena.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982811 JOURNAL DE PHYSIQUE

The next meeting to mention was a Discussion Meeting held on 29 January 1953 at the Royal Society in London, organised by Powell to enable a thorough review to be made of all of the evidence available on new particlesL11. It was primarily a U.K. meeting, most of the papers being from Bristol, London and Manchester, but there were reportsalso from Paris (Ecole Polytechnique) , Milan, Padua and Rome, while Butler presented a report on the M.I.T. work on S particles. The powerful analyses which were made of these events, especially by the emulsion group from Bristol, were very impressive, and it became abundantly clear that the identification of the T-meson decay (1) was on a solid foundation. At last the complete decay mode had become established for at least one new meson. Now there were 11 decay events known and the time was ripe to give some serious consideration to their characteristics, in terms of the T-meson spin and parity. There were also decay events giving only a single a+ secondary, whose energy varied from event to event but lay in the energy range allowed for the decay mode

which would necessarily have a close relationship with the decay mode (1). There was no theoretical work presented at this meeting; the time was still too early for that. Its special value lay in the coming together of experimental groups working on a common problem in widely separated places and with different techniques or different approaches, and the importance of this cannot be over- estimated. Questions can be raised and explored, and often settled there and then. The conclusions reached in this way, through open presentation of the data from a wide range of experiments and its comparison and evaluation, have a force much greater than that resulting from the publication of a number of separate papers, each necessarily somewhat tentative and all scattered through a variety of scientific journals.

The International Cosmic Ray Conference for 1953 was arranged to concentrate on the new particles, and this was a major event in the lives of all the physicists who took part in it. It was held 6-12 July at Bagnsres-de-Bigorre, in the Basque country on the northern slopes of the Pyrenees. During this conference, it became clear that there was a substantial consensusconcerning the subject of all this widespread cosmic ray work. Previously it had seemed as if a new decay mode, or perhaps a known decay mode for a new parent mass, was being reported almost every month, but now it was seen that the most frequent decay nodes were quite limited in number and were associated with fairly definite mass values. Previously, the V: -t pn- particle was the only well established VO particle, known to us as the A(1115) ; now, the pain- stakingly precise work of Thompson had established the existence of the V: -+ a+a- particle, also known as the e0 meson, with mass 496f5 MeV, comparable with that for the meson. Of course, the cloud chamber work still left some further neutral events, labelled v!,v~,. ., and some charged V* events to be sorted out, the latter still to be related with the at-rest decay events reported from the emulsion work. The new techniques for using layered emulsion blocks were being perfected and it was becoming ~ossibleto follow charged secondaries through many layers and so to identify them uniquely and to measure their energies from their range. In committee, rules were drawn up for the formal specification of any new particles or new decay modes. It was an exciting time, as if the mists were lifting and we could at last look ahead.

There were five theoretical papers presented. A paper by Cheston and Primakoff, neither of whom are listed among the participants, gave a quantitative discussion of the possibility that a A hyperon bound to nucleons might undergo non-mesonic de-excitation, releasing the full 176 MeV available as kinetic energy for the final nucleons. The first identified A hypernucleus was reported by Danysz and Pniewski 121 only in early 1953, so this was a remarkably quick response to a new phenomenon. Bhabha discussed multiple production of mesons in high energy nuclear collisions, relating it with the localisation of field energy within a nucleon, following the theoretical ideas put forward by Fermi C31 and by Heisenberg C41. Michel gave an important survey of the absolute selection rules governing particle decay processes, as understood at that time. He provided a large table showing all the two-body and three-body decay modes conceivable for the e0 and rf mesons, together with their forbidden- ness (or otherwise) according to the selection rules of angular momentum, parity and charge-conjugation, noting (as an example) that they could be 'corresponding particles' if they had spin-parity (2+). Haber-Schaim, Yasin and Yekutieli reported on a calculation of K-meson production in nucleon-nucleon collisions, based on Fermi's statistical theory L3 1.

My own paper was concerned with the analysis of the T-meson decay process (1) in terms of its spin-parity, work which had been done C51 following the Discussion Meeting mentioned above. It was my opinion that the amplitude for the decay mode (1) should be largely calculable in form (although not in magnitude) in terms of angular momentum barrier considerations, apart from a few parameters necessary when the total angular momentum and parity could be apportioned to the internal orbital motions within the three-pion system in more than one comparable way. If so, it would then be possible to deduce the values of these internal angular momenta from the distribution of events and from them to reach some conclusions about the total spin-parity,at least toexclude some possibilities. First, a representation was needed to display the distribution of events pictorially. This was provided by the equilateral triangular plot ~hownin Fig. 1, each T+ event being represented by a point P within the triangle. Since the sum of the perpendiculars PA, PB and PC is constant for all points P within an equilateral triangle, we may choose P such that the length PC gives the n- kinetic energy and the lengths PA and PB the two IT+ kinetic energies; this requires that the triangle has side chosen to give (216) times the total kinetic energy. Since n+ mesons are indistinguishable, we may choose PA 2 PB and so plot all events on the right half of Fig. 1. The constraints of momentum conservation in the decay limits physical events to points P lying within the figure inscribed lightly within the circle. This plot has the advantage that a decay rate independent of 3n configuration corresponds to a unif orn distribution of events over this allowed region and it may therefore be termed 'a phase- space plot'.

The earliest data were obtained using single emulsion N C layers. Since the pions do not + ++- normally stop within a single Fig.1. Phase-space plot for T +n n IT events. layer, the pion charges were not JOURNAL DE PHYSIQUE

generally known. There are then three possible locations for the event on Fig. 1, depending on which of the pions is a-. Besides P, the points P' (the reflection of P in the diagonal ROL) and P" (the reflection of P in the diagonal MOS) are equally possible. If we plot a11 three points for each event, the distributions in the sectors TOL and SON will simply be the reflections of the distribution in the sector LOS in the diagonalsROL and MOS, respectively. Hence, in this situation, we can confine attention to the 60° sector LOS. This has been done in Fig. 2 for the 13 T-decay events available in July 1953.

The key question was whether the 8°-particle and the r+-particie could be closely related. Both decayed to give pions only, and their masses were quite comparable. The T+IT- state resulting from 'a0-decay necessarily has parity (-I)~, where J is their total angular momentum. Could the T+T+T- state resulting from T+-decay have spin and parity bearing the same relationship? It was easily proved that this could be the case only if the density of events on Fig. 1 vanished on the boundary of the allowed region, or equivalently, that the density of events on Fig. 2 vanished on its almost-circular boundary LS'. Clearly the 13 events plotted show no indication of such a behaviour. For particular spin values, other characteristic L features could be predicted. For example, with J = 1 and negative parity for the 3i7 state, the density of events was predicted to vanish at 0, falling quadratically as P approaches the centre0, for fixed direction OP. These data showed no evidence of this behaviour, but the statistics were rather small. For J = 0, the 3~ state necessarily has negative parity, whereas the 2r state has positive parity. The density of T+ decay events was predicted to be essentially uniform, for J = 0, and these data were certainly compatible with this possibility. It was also pointed out that, if the final 38 states had unit , the ratio r'/r must lie between 114 and 1, r' referring to events Fig. 2. July 1953 plot of 13 of the type (2). The value predicted was ?, 114 T-decay events disregarding for the case 3 = 0, and the few events available pion charges(se1domKnown then). did not disagree with this ratio.

There was a further Cosmic Ray Conference, held at DurhawNorth Carolina, and sponsored by Duke University and the N.S.F., at the end of November 1953. Only one of its six sessions was devoted to the new unstable particles, and this included a report by Whittemore, Shutt, W. Fowler and Thorndike on six hyperon production events observed in a hydrogen diffusion chamber exposed to the 1.5 GeV T- beam from the Cosmotron at Brookhaven National Laboratory. These included one clear example of associated new particle production (A+o') in a a- interaction most probably on hydrogen, and a possible example of (A-+K+) associated production, the A- particle having mass corresponding to the C- hyperon known today. There were no theoretical contributions to this session.

The Fourth Rochester Conference on High Energy Nuclear Physics was held 25-27 January, 1954. The new particles were discussed in Session 3, under the heading of Cosmic Rays, although now three of the experimental contributions were concerned with new particles produced by the Cosmotron. The report on hyperon production events in the 1.5 GeV a- beam was updated to ten events by Thorndike. M. Goldhaber reported on a star produced in emulsion exposed to the negative beam from the Cosmotron by a beam particle with mass about 560270 MeV and therefore possibly a negative . Crussard summarized the searches made by three groups for heavy unstable particles produced in nuclear emulsions exposed to the 1.5 GeV a- beam, which had yielded three and one A-hyper- fragment with charge +2. There were several theoretical contributions in this session. Primakoff reported on his calculations with Cheston concerning the rate of non-mesonic decay for a A hyperon bound to a nucleus. Wheeler reported on an attempt by Treiman to account for the energy spectrum observed for kaon decay in cloud chambers by Leprince-Ringuet and by Thompson. Treiman observed that 0 -t 271 and T -t 3a decay modes could be from the same parent particle if this had spin J = 1 and negative parity, and calculated the p+ spectrum to be expected from the decay of this particle, assuming the decay mode to be p'vy. Although his calculated muon spectrum peaked at almost %c2/2, it was still incompatible with Leprince-Ringuet's conclusions that the p+ energy was practically unique and that there was no y-ray emission associated with these . Also, as remarked above and again below, the data on T+ -t 3a were difficult to reconcile with spin-parity (I-).

R During 1953, the use of 'stripped emulsions' made into large emulsion stacks became Completely identified the norm and a pion track could emulsion event. be followed through many layers until it came to rest and 0 Cloud chamber event. revealed its charge as positive or negative. In quite a large 0 o Event completely fraction of T + 371 events, all identified if +ve three pions stopped in the charge assumed for T. emulsion, and in all such cases, the r-meson was found to have S X Event with one T' positive charge, as was in accord identified, ambiguous with expectation that negative even if +ve charge T kaons coming to rest in emulsion assumed. would undergo nuclear absorption N much more rapidly than decay. With all three pion charges Fig. 3. Phase-space plot for the 13 r+ -t 3a known, the event could be plotted events with one pion stopping in the on the full phase-space plot emulsion, as reported in January 1953. Fig. 1). By January 1954, this was possible for eight events, including those where only two pion charges were established (when both were a+, the third was necessarily a-; when only a+ and a- were established, the third was necessarily a+, if we accept that decay occurs in emulsion only from T+) and these were plotted on Fig. 3 C91. For three further events, where only one a+ was identified in charge, there was a two-fold ambiguity in the point P, and this is indicated for them on Fig. 3. With the full plot, further features of the distribution become informative. For example, the density of events must vanish as the a- kinetic energy approaches zero, i.e. as P approaches the lowest point N on Fig. 1, unless the 3~rsystem has even spin J and odd parity. This is illustrated by Fig. 4(L), for stationary a-. The total angular momentum J is then carried by the two a+ mesons, but Bose statistics for them limits J to even values, and so requires total parity (-1) for this 371 configuration. This

Configuration L Configuration N

a- a+ R+ xTI+ 71- IT+ f-. c, f. .+ stationary stationary

Fig. 4. Tne 3r configurations corresponding to the points L and N on Fig. 1. JOURNAL DE PHYSIQUE

argument holds even if the s- meson has a small kinetic energy; all that it requires is that non-zero orbital angular momentum for the s- meson in the r rest frame should have negligible probability for the configuration considered. The distribution on Fig. 3 did not suggest that low energies were unfavoured for the r- meson, but rather that the data were consistent with 3s spin-parity (0-1, (2-), etc. It is of interest to note in passing that later on, in 1956, a T+-decay event was observed C61, for which the s- kinetic energy was only 0.1 MeV, a configuration very close to that of Fig. 4(L). As a second example, it was shown C51 that, with spin-parity (I-) for the 3s system, the distribution must vanish all along the disgonal RON, falling quadratically as P approaches this axis perpendicular to it, a feature special to this spin-parity value. Again, the distribution on Fig. 3 shows no hint of such behaviour. To conclude this section, we refer to Fig. 4(N), the configuration in which one s+ meson is stationary (or at least has zero angular momentum in the T+ rest frame). The total spin J is then carried by the oppositely directed T+ and s- mesons, so that this 2s system has the parity (-I)~appropriate to a s+s- system resulting from the decay of a e0 meson with spin J. Since the stationary s+ meson contributes parity (-I), it is clear that this 37~configuration necessarily has parity opposite to the s+s- system with the same J. We note directly from Fig. 3 that a substantial fraction of r+ + 3s events gave a low-energy s+, even at this early stage. The T-0 puzzle was already quite apparent by the beginning of 1954.

In retrospect, it is surprising not to find in the 1954 Rochester Conference Report any discussion, nor even mention, of Gell-Mann's scheme for understanding the new particles and their interactions in terms of unconventional isospin assignments, although his Letter proposing this C71 had been published in November 1953. He was present at the conference, for he discussed the validity of pseudoscalar coupling following Goldberger's talk on theory in session 2. It is much less surprising, of course, that there was also no mention of the Letter published by Nakano and Nishijima in November 1953, since that issue of Progr. Theor. Phys. had probably not reached U.S.A. by the date of the conference and theremay have been no theoretical physicists who had come to the conference from Japan.

Shortly after this Rochester Conference, my paper on the analysis of T+ decay when the pion charges are known was accepted for publication C97. Besides discussing the general features of the distribution on the full phase-space plot, this paper also gave the form of the decay amplitude for arbitrary spin- parity for the 3s system, and discussed the effects that strong, or even resonant, ns interactions could have on the distribution. A parallel paper was submitted at about the same time by Fabri C101, which also gave a formal derivation of the decay amplitudes I had written down earlier [5! for the spin- parity values (0-), (I?), (2+). and (3-) in my discussion of the analysis of T decay when the pion charges are not known.

At Padua, during 12-15 April 1954, there was held Congress0 Internazionale sulle Particelle Instabili Pesanti e sugli Eventi di Alto Energia nei Raggi Cosmici C111. This conference was oriented particularly towards the cosmic ray studies going on in Europe, especially the work using the new technique of stripped emulsions and following the Sardinian balloon flights of the summer of 1953, but there were also sessions on new cloud chamber results, and several reports on the cloud chamber and high-energy interaction studies going on in U.S.A. Amaldi, Baroni, Cortini, Franzinetti and Manfredini presented an over-all report on the T-ESO~ investigations, using 20 fully identified events (assuming r+ only) and 6 events where only one s+ meson came to rest, more than half of which had resulted from the Sardinian flights. The statistical procedure used was based on Fabri's calculations, which were described briefly in a separate paper, the only theoretical paper of this conference. No accelerator data was presented or used, either.

The 1954 Glasgow Conference on Nuclear and Meson Physics C121 was held 13-17 July of that year. In meson physics, the emphasis was naturally on the pion-nucleon interaction, but Part VIII dealt with Heavy Mesons and . Besides surveys of the cosmic ray work, the Cosmotron work on heavy mesons was updated again by Thorndike, with special attention to the associated production events. This was particularly appropriate since it was followed by a joint paper presented by Gell-Mann and Pais surveying theoretical views on the new particles, since this concluded that associated production of new particles was an element both essential and informative in relation to any acceptable theoretical scheme. The possibility that the new particles might all have high spin was rebutted by the absence of evidence for the reaction NN -t AA at the Cosmotron. It was emphasized that the data required the existence of at least three different classes of interaction, strong interactions which conserved isospin, the electromagnetic interactions already known, and weak interactions which violated isospin conservation and were responsible for the new particle decay processes. Pais's earlier theory C131 which introduced isoparity was inadequate to account for E- stability, while Pais's later theory El41 based on a four- dimensional isospin space required the existence of particles (e.g. A+ at the A mass, and X++) for which there was no evidence. Gell-Mann's scheme L71 was in accord with all of the known facts and had led to some qualitative predictions (e.g. the absence of NN + AA) which were successful. It was noted that (Q-I,) varied from multiplet to multiplet. The authors had not found any inter- pretation of this variation, in terms of some enlarged group of symmetry operations, but they made it clear that this was their prime purpose. [Today, of course, this variation is written in the well-known form

where B = +I(-1) for a (antibaryon) and B = 0 for a meson, and s is the now known as '', characteristic of the multiplet; the symaetry group sought turned out to be the unitary symetry SU(3), in the octet form, but even this has become extended and mdified, more recently.] However, Gell-Mann's scheme did require that, however the r and 0 particles might be related, go must differ from e0 and yo must differ from TO, from which observation there emerged the remarkable theory of (KO,KO) mixing and the empirical area of (K~,K!) phenomena, to become much later a major chapter of elementary particle physics.

At the Fifth Rochester Conference, held at the end of January 1955, cosmic ray work was still the major contributor to strange particle physics. Of the 25 contributions to session IV, on Elementary Particles (Experimental), only six were from groups working at the Cosmotron with one further from the nuclear emulsion work at the Bevatron of the Radiation Laboratory at Berkeley, which had just come into regular operation. In session V, on Elementary Particles, Rossi surveyed our empirical knowledge of the strange particles, as derived from the cosmic ray work, and Pais discussed current views on 'the source of their remarkable stability', covering the ground of the Gell-Mann-Pais paper at the Glasgow conference, but more extensively and up-dating it. The relation (3) had still not been written down (although eq. (3) of the paper by Nakano and Nishijima C81 is equivalent to it) and the term 'strangeness' was not used (on the contrary, "Pais ..imadel.. a plea not to use words like strange or peculiar .. .Ii). My report on the r-meson situation was based on 53 fully identified T+ -t n+n+n- decay events, of which 42 had come from cosmic ray work. JOURNAL DE PHYSIQUE

G. Goldhaber had brought 10 events from the emulsion work at Berkeley, while Harris reported one event from the emulsion work at the Cosmotron; the advantage of higher energy for strange particle research became abundantly clear. By this time, the evidence on the 3n spin-parity from r+ -t ~IIdecay was very strong. This 37r spin-parity was incowatible with the 27~ spin-parity from e0 decay, at least for spin values J 2 5, and the only possibilities still acceptable for it were (0-) , (2-) and (4-), within this range for .I.

The last conference we shall discuss was Conferenza Internazionale sulle Particelle Elementari, the Centerary Celebration of I1 Nuovo Cimento, held 12-18 June 1955 at Pisa C151. About 90% of the papers on strange particles were still from the cosmic ray groups, largely as a result of the Sardinian flights of 1953 and the G-stack flight in October 1954, the latter having been organised by a collaboration between the Bristol, Milan and Padua groups. However, the papers on K* properties from the Bevatron groups showed that conclusions depending on large statistics and/or accurate measurements were more likely to come from accelerator experiments in the future. For example, using a momentum-selected Bevatron beam of positive charge, directed into nuclear emulsion, Fung, Mohler, Pevsner and Ritson used the range measurement and type of decay mode for each meson to deduce the value (2.522.5) MeV for the difference between the T+ mass and ihe mean mass of all K+ mesons, a most important item of information in considering the relationship between the r+ meson and any other K+ mesons. Much valuable information was coming from the G-stack collaboration but this was to do more with clarification and consolidation of earlier work or with of detail, rather than with further discoveries of new particles. Amaldi's r-meson report to the conference was based on 106 emulsion events, of which 35 had come from Bevatron work and 2 from Cosmotron work. His conclusion also was quite firm that the spin-parity of the 371 state from r+ decay was most probably (0-), although other even spin-odd parity possibilities ((2-), (4-), ...) could not be ruled out; all other possibilities were highly improbable, in view of these data. It is interesting to point out here that this conclusion did not depend crucially on the inclusion of the accelerator data, although the latter were a reassuring addition to the cosmic ray data, which had necessarily been collected in a much less systematic way.

The most important theoretical paper at the Pisa conference was that of Gell-Mam, entitled "The Interpretation of the New Particles as Displaced Charge Multiplets", in which he set out his scheme for the new particles in a coherent and rather settled form, giving the relation (3) and introducinn the name 'strangeness' for the quantum number s. We shall say no more about this paper here. since Gell-Mann himself will speak about the history of the concept of strangeness in a later session of this colloquium.

This 1955 Pisa conference was the last major conference on elementarv particle physics at which the data presented were dominated by cosmic ray contributions. This is well illustrated by reference to the Sixth Rochester Conference, held in April 1956, at which there were only fourpapers contributing to strange particle physics on the basis of cosmic ray studies.

To conclude, it is of interest to follow a line of argument i221 which was open already in 1955, by whj.ch time the T-8 puzzle, already apparent in 1954, was rather firm.' The straightforward solution to this puzzle was simple, to accept that K-m40ns apoeared as charge doublets. in accord with Cell-Flann's scheme, but to ccept further that thereexisted oniy one K-meson charge-doublet. As seen aboukhe 8O + Zn and r' + 3n modes for the decav of this (Ku.Kf)doublet. a state with some definite spin-parity (JP), then necessarily lead td final states' withborhparities +I. The natural expectation for (JP) was (0-), in accord with the spin-parity known for the *-meson, and with the r+ + 31r distribution. The main objection was that this implied parity nonconservation, a failure of invariance with respect to space reflections. How was it possible that reflection invariance should not hold, people asked - was not left-right invariance inherent in our most fundamental conceptions about space-time? The only answer available was that the occurrence of both K + 2n and K + 3n decays actually did demonstrate this, but this answer did not have compelling forcebecauseit could not point to any explicit empirical demonstration of parity failure. It led to the conclusion of parity nonconservation only when one insisted on the simplest possible interpretation of the complex K-meson story. It required much less faith to suppose that a substantially more complicated situation held true, namely that there existed two distinct K- meson charge doublets, labelled T and 0, close in mass but with different spin- parities, these being perhaps (0-) and (0+), respectively, although other spin- parity possibilities, or a larger number of K-meson doublets, were also considered. Since the states r+ and 0+ (0+ + afaO was not really well estab- lished until 1954 r16-181) would naturally differ in some degree in mass, lifetime, branching ratios, production cross sections, scattering properties, etc., much empirical effort in the years 1955-57 was devoted to the measurement of these quantities for T+and 0'in order to demonstrate these differences, but the results were always compatible with zero difference.

Even after these observations, the natural line of phenomenological argument r201 was not followed. This was to accept the hypothesis of a single K-doublet and to calculate the A + pn- amplitude which would follow from the observed and well-established processes KO + n'n- and K' + n'n+a-, neglecting all other effects. These two K-decay amplitudes may be represented by coupling amplitudes gg, and ggn, whose values could be deduced from the KO lifetime and the K+ lifetime and branching ratio T+/K+. These data were known correctly in order of magnitude by 1955. The A hyperon would have been assumed to have spin-parity (1/2+), as for the nucleons, with the APK coupling pseudoscalar, as was known for NNa. All other hyperons could have been ignored, also any +- hyperon couplings. The two perturbation-theory graphs are shown in Fig. 5. The (0) graph gives a convergent integral. the integral for the (T) graph diverges, but only logarithmically, so that it is insensitive to a cut-off, imposed at mass A. The Apa- decay interaction thus calculated has the form

where a and b are the coefficients calculated for Figs. 5(0) and 5(r), respectively. Since the pion momentum is small (q 2 100 E?~V/C),the non- relativistic limit for the amplitude,

M(A + pa-) = {a - b(rq/(M +M )} = s + A P is sufficient for our purpose. We shall not give expressions for s and p - -

Fig. 5. A + pa- decay graphs implied by the 0O and .r+ decay modes for the K-meson doublet. JOURNAL DE PEYSIQUE

separately, since the quantity needed for our purpose is the ratio C201

We note that the coupling parameter GANK has dropped out, and that ~&~~/4r2 14. The numerical value for p/s is about 0.3 for A % M and about 1.8 for A % 10 M P' P' The use of reaction amplitudes given in a spin-dependent form was well known [I91 in nuclear physics. With the form (4) for the A + pn- amplitude before us, the penny would have dropped sooner, and we would have realized that the key to explicit observable effects characteristic of parity violation lay in the study of polarized hyperon decay, since the amplitude (4) leads to the angular distribution

with ff = 2~e(s*p)/((s(~+!p('), for the decay of a h particle with polarization vector 3. The values just calculated for p/s give a running from 0.55 for A = M to 0.85 for A = 10 Mp, hence a rather large effect. How to obtain polarizedP A hyperons, then? By 1956, the reaction

was beginning to be well studied, by Walker C211 and by Steinberger 1221 in particular. With hindsight, we know that those A hyperons had a large polarization along the normal fi = p,xa/lETx&[ to the production plane. The A decay pions following reaction (8) would have shown a substantial up/down asymmetry relative to the A8 production plane, if this had been examined then, as would have seemed natural from this argument, and if the statistics could have been improved.

This rough calculation for the A + pn- amplitude would certainly have been severely criticized. It omits many possible effects and is only perturbative. However, an accurate calculation was not the purpose. The point is that it would have allowed us to see in a familiar context, explicit effects character- istic of parity violation, to free our minds from unfounded 2 priori miscon- ceptions and have shown us thatparityviolating effects in A decay need not necessarily be miniscule. The mental obstacle arose from the fact that the 1-0 puzzle did not provide an explicit demonstration of parity nonconservation. Any calculation along the above lines would have made it clear what the experimental physicist had to look for. This was not appreciated at the time and accurate experiments on A-particle production and decaydeveloped only slowly during the period 1955-7. Definitive A-decay asymmetry experiments were not carried out until about 1957, although they could certainly have been done earlier, if their potential importance had been understood at the time. Although the 1-8 puzzle emerged in the Cosmic Ray Era, and stemmed primarily from cosmic ray data, it is implausible that this second phase, the observation of A-decay asymmetry, could ever have been achieved through cosmic ray experiments. Its observation actually came relatively late in the story of the strange particles, and accelerator beams were essential to achieve the controlled conditions necessary for measuring it reliably.

The actual path followed by history was quite different, of course. Chen it was clear that parity conservation was in question for the K-mesons, it was the genius of Lee and Yang to relate this question, not just with K-meson decays, but with the whole class of weak interactions, so that the first empirical demonstration of P-nonconservation for weak interactions was not carried out with strange particles in 1956 but with nuclear beta-decay, a process apparently far removed from the topic of my discussion above, in 1957. REFERENCES

Proc. Roy. Soc. E,issue No. 1146, 27 January 1954. DANYSZ M. and PNIEWSKI J., Phil. Mag. 46 (1953) 348. FEW E., Phys. Rev. 81 (1951) 683; Progr. Theor. Phys. 5 (1950) 570. HEISENBERG W., ZS. f. Phys. 133 (1952) 65. DALITZ R.H., Phil. Mag. 46 (1953) 1068. see OREAR J, Phys. Rev. 106 (1957) 834. GELL-MANN M., Phys. Rev. 92 (1953) 833. NAKANO T. and NISHIJIMA K: Progr. Theor. Phys. 10 (1953) 581. DALITZ R.H., Phys. Rev. 96 (1954) 1046. FABRI E., Nuovo Cimento 1_1 (1954) 479. Suppl. Nuovo Cimento, series IX, 12 (1964) 161. Proc. 1954 Glasgow Conference on Nuclear and Meson Physics (eds. E.H. Bellamy and R.G. Moorhouse, Pergamon Press, London & New York, 1955). PAIS A., Physica 2 (1953) 869. PAIS A., Proc. Nat. Acad. Sci. 40 (1954) 484; ibid, 835. Suppl. Nuovo Cimentc, series X, 4 (1956) 135. BRIDGE H., DeSTAEBLER H., ROSS1 B., SREEKANTAN B. and CALDWELL D., Nuovo Cimento 1 (1955) 874. HODSON A., BALLAM J., ARNOLD W., HARRIS D., RAU R., REYNOLDS G. and TREIYAN S., Phys. Rev. 96 (1954) 1089. KEEFE D., ref. C111, p.412. see LEPORE J., Phys. Rev. 2 (1950) 137; DALITZ R.H., Proc. Phys. Soc. 64 (1951) 667; WOLFENSTEIN L., Ann. Revs. Nucl. Sci. 6 (1956) 43. DAETZR.H. , Atti del Convegno Mendeleeviano 'Periodicita e Simmetrie nella Strutture Elementare della Materia' (ed. M. Verde, Accad. Sci. Torino & Accad. Naz. Lincei, Torino, 1971) p.341. WALKER W.D., Proc. Sixth Ann. Rochester Conf. (ed. J. Ballam, V. Fitch, T. Fulton, K. Huang, R. Rau & S. Treiman, Interscience, New York 1956) p. VI-19. STEINBERGER j., ibid, p.VI-20.

Ch. PEYROU - The Bagnhres-de-Bigorre Conference was certainly a very good conferen- ce and a very decisive one but all problems were not solved. For instance there was a great puzzle, the small proportion of K- captured in nuclei and giving nuclear stars. Many people thought it proved that there were at least five types of K par- ticles, one of them having no interaction with nuclei (like the u). It is only the work of the Ecole Polytechnique at the Pic du Midi which demonstrated that there was in fact a huge positive excess in K particles and that was established only a few months later that positive excess was of course a consequence of associated production.

M. CONVERSI - As the organizer of the 1955 Pisa Conference I am still proud of (and still recall with deep admiration) the conclusive talk given there by the then very young Murray Gell-Mann, uncovering to a big audience the "strange world" ... But I feel the important contribution given by Nijishima to the concepts underlying the strangeness theory should also be mentioned here.

Perhaps also Elio Fabri (then a young theoretician linked in Rome to two Bruno's, Ferretti and Touschek) might be mentioned for this contribution to the development of the type of analysis first introduced masterly by Professor Dalitz (I mean the Dalitz plot).