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Boson and Baryon Resonances Boson and Baryon Resonances The number of reported particle resonances keeps increasing year by year, but more important the properties of previously discovered resonances become more firmly established. The detailed presentation of these findings was presented in six parallel sessions and reviewed in three raporteur reports by Professor R. Plano, Professor A. Astier, and the author at the XVth International Conference on High-Energy Physics at Kiev, USSR. In the ensuing comments I will attempt to touch on what I consider the more interesting as well as controversial aspects of the above noted sessions. The main interest concerning bosons, that is, states with nucleon number N = 0, was centered on the A 2 meson. Such a resonance with mass of 1320 MeV and with I'~ 80 MeV has been a bona fide member of accepted resonances for many years. The first indication of possible peculiarities in this mass region was reported by the CERN group examining boson resonances with a reasonably high resolution I'~ 15 MeV. They observed a split peak for the A2 in the reaction n-p--> A2 p whose significance keeps increasing year by year. This experiment has been repeated by the same group under slightly different geometrical conditions where the total effect (all their data) corresponds to a dip fluctuation of ~ 8 standard deviations. Investigation of the A2 --> K- K~ decay mode, by the same group with more limited data, also shows a dip but agrees (or disagrees depending on your point of view) with a single Breit-Wigner with a 1 % probability. On the other hand, a bubble­ chamber study of the Ai produced in the reaction n+ p-> Ai p by Group A at Berkeley does not confirm this previous result. In particular, the p0 n+, n°n+, and K+ K~ decay distributions are well described by a single Breit­ Wigner shape, probability of fit ~ (10-20) % and strongly disagree with a particular form of splitting, namely that of a dipole with a probability of fit ~ 0.3 %. However, if one kinematically restricts these latter data to the same kinematical region as that examined by the CERN missing-mass experiment, their statistical validity is sufficiently reduced so that there is no disagreement. Two new results concerning the A 2 were presented at the Conference, both favoring the splitting hypothesis but neither conclusive. The Bologna-CERN­ Strasbourg collaboration studying the A~ produced in the neutral-missing­ mass experiment n-p--> nA~ obtain a 1 % fit to a single Breit-Wigner while 177 the Goldhaber-Trilling group at Berkeley exammmg Ai production via n+ p ~ pAi get 9 % for a single peak. Most experiments agree on the spin­ parity assignment of 2 + for either the whole peak or both halves. There seems to be consensus that the dipole fit foas been overemphasized, two Breit­ Wigners being more than adequate to explain the data. Needless to say, the phenomenon is not completely understood, the data, taken as they stand, indicate a double structure for the Ai with the Ai ruling out a dipole shape but otherwise in agreement with a single or double Breit-Wigner shape. Further experiments with good resolution, high statistics, and wide kine­ matics acceptance are certainly needed. These are difficult requirements to satisfy since the counter experiments to date have had limited momentum transfer acceptances, while the bubble-chamber experiments have been more limited in their data accumulation. As has previously been noted, the naive quark model (in which bosons are conjectured to be composed of quark-antiquark pairs) adequately describes the observed spectra of mesons. Such a scheme restricts the possible rep­ resentation to {I} singlet and {8} octet. Over the years several states lying outside these classifications have been reported (i.e. I= f, S = ± 1 K states); however, repeat experiments with more data have invariably negated the original evidence. The search for such "exotic" resonances has continued but has yet to produce a believable candidate. The split A 2 , discussed above, produces the main difficulty with this simple quark model in that all evidence suggests JP= 2+ for both halves. Such a closeness in mass at ~ 1300 MeV is difficult to understand since the two lowest 2 + levels occur with orbitals of I = 1 and 3, and the intermediate I = 2 level has candidates with mass of ~ 1500-1600 MeV. The existence of several states in this higher mass region is now well established. In particular, there is the n(l640) which decays mainly into j 0 n, the cp(l660) observed to decay into pn, two I= 1 resonances at a mass ~ 1660 (g meson) and the L(l 750) with several decay modes. In the lower mass region there was new evidence presented for the existence of the [J meson decaying into 17n final state, confusion concerning the uniqueness of any resonant nn or Kn system as derived from dynamical analyses, and no new information concerning the A 1 ( 1080) and numerous possible K*(l 300)'s. In addition, several more very high mass peaks> 3 GeV were also reported from the missing-mass experiment. In all, lots of new information, but no break­ throughs. The new information concerning baryons, with nucleon number N = I and strangeness S = 0, ± I, - 2, and - 3, was as extensive but only slightly less controversial than in the boson case. One of the most interesting develop­ ments was that involving the existence of Z*, N = 1, S = + 1 baryons. Again such resonances cannot be accommodated by the simple quark model in 178 which baryons are composed of three quarks. This is due to the fact that the only strange quark has S = - 1 so that an antiquark is needed in order to achieve a state with S = + 1, thereby requiring a minimum of four quarks and one antiquark. The two structures deduced from a study of K+ p and K+ d total cross sections, an I= 1 at a mass of 1900 MeV and I= 0 at 1870 MeV have been verified with a new bump uncovered in I= 0 state with mass 1780 MeV. The former two enhancements are clearly associated with the onset of single pion production in contrast to the latest bump which is mainly elastic. In the past year very extensive experimental work has been conducted with respect to the I= I state. This has involved differential cross section, polarization, and partial cross section measurements conducted over a wide momentum range. A partial-wave analysis of all the available data by several of the groups contains among the numerous solutions one which has the P 312 amplitude behaving in a resonant manner. The non­ uniqueness of the solution as well as disagreement on the detailed behavior of this particular amplitude, i.e. as to whether there is a rate change as it approaches the resonant energy, leaves the question of its being resonant unresolved. The situation appears to be less complex in the case of the newest candidate, the elastic I= 0 bump at 1780 MeV, and increased experimental activity over the next year or so may lead to a definitive conclusion. Needless to say, if these three states were regular N = I, S = 0, - 1 candidates, they would already have been accepted and catalogued. The situation with respect to the nucleon states N, with I= 1, and LI, with I= 1-, has remained essentially unchanged since the 1968 Vienna conference. Numerous such states with well-known quantum numbers have been tabu­ lated, one of the most interesting features being the existence of higher­ massed low-spin states such as the N(l470) with JP = 1+ and N(l 700) with JP = -! - . Although first detected by partial-wave analysis of total, partial cross section and polarization measurements, such enhancements are also observed in effective mass distributions derived from production experiments. In fact, a large number of reports were presented concerning bumps in the 1700 mass region, measuring their mass, width, and decay modes with spin­ parities not presently deciphered. These seem to be replicas of the nucleon N(939) and N(l535), respectively, in all their quantum numbers and must be accommodated within any conjectured cataloguing scheme. In addition, two new high-mass resonances were also reported, N(2220) with JP = t + and width of 240 MeV and Ll(2320) with JP= !,} + and width 300 MeV. Progress continues in uncovering new A(J = 0, S = -1) and E(J =I, S = - I) resonances as well as verifying the existence of and determining the properties of previously reported states. Such is the case for the E(l 915) f +, E(2030) f +, A(2100) r, and E(2280) r where spin-parity assignments and detailed partial decay rates are now well established. The evidence for two 179 1:(1660) resonances, one decaying mainly into l:n and the other into l:nn final state, first presented at Vienna, has since been confirmed. Although not conclusive, the most probable spin-parity for both states is-! - . This situation of resonances with the same mass and spin-parity does show some resemblance to the previously discussed A 2 question among the bosons. The possible existence of numerous other resonances in both the intermediate mass range 1600-1800 MeV and in the 2000-2100 MeV region of assorted spin-parities has been proposed. If verified, then the experimental particle spectrum is much more complex than previously suggested. The exploration of S states (N = I, S = - 2) continues to be hampered by their small production cross sections and their inaccessibility via formation experiments. In spite of this, the existence of a 5(1820) and 5(1930) is well established with a 5(2030) and 5(2430), already observed by two groups, being quite reputable.
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