Search

The total cross section of the pd→pdη reaction has been measured at two energies near threshold by detecting the final proton and deuteron in a magnetic spectrometer. The values are somewhat larger than expected on the basis of two simple theoretical estimates.

A set of anticommuting multivectors in Clifford algebras can be taken as orthonormal basis set. The Clifford algebra generated by this basis is isomorphic to the original algebra. The non linear transformations between orthonormal basis sets form a group. In the four dimensionnal case six sets of five anticommuting multivectors are found. These sets yield 30 matrices defining basis sets. These matrices are representatives of left cosets, members of these cosets are related by permutation of rows. From the equivalence of all basis sets of multivectors it can be concluded that there is no canonical set of basis vectors in Clifford algebras.

A fast algorithm to compute irreducible integer representations of the symmetric group is described. The representation is called tilted because the identity is not represented by a unit matrix, but a matrix β satisfying a reduced characteristic equation of the form ( β − I ) k = 0. A distinctive feature of the approach is that the non-zero matrix elements are restricted to ±1. A so called natural representation is obtained by multiplying each representation matrix by β −1 . Alternatively the representation property of the matrices is mantained by inserting the matrix β −1 between two representation matrices.

The [ital pp][r arrow][ital pp][eta] reaction is studied at energies near the [eta] production threshold. The total cross sections at nominal machine energies of 1260, 1265, and 1300 MeV are 90[plus minus]15, 790[plus minus]120, and 3460[plus minus]690 nb, respectively. None of the existing perturbative model calculations reproduces the energy dependence, which deviates strongly from phase space. This suggests that the cross section is enhanced in the near vicinity of the production threshold by a large [eta]-[ital pp] scattering length.

The reaction p + d → π − + X has been studied at Saturne at 1.45, 2.10 and 2.70 GeV, using the spectrometer SPES III. The analysing power and the differential cross sections show no evidence for narrow structures which could be interpreted as a resonance in the three baryon system. At the lowest incident energy, the shapes of the experimental cross sections are well reproduced by phase-space distributions.

The analyzing powers of the {sup 1}H({ital {rvec d}},{ital pp}){ital X} reaction have been measured using polarized deuteron beams at {ital T}{sub {ital d}}=2.1 GeV, {theta}=17{degree} and {ital T}{sub {ital d}}=1.722 GeV, {theta}=0{degree}. In both cases a small oscillatory pattern has been observed in the tensor analyzing power at the same two-proton invariant mass and width {ital M}{sub {ital p}{ital p}}=1.945 GeV, {Gamma}{sub 1/2}{approx}8 MeV. The number of standard deviations are equal to 2.5 and 3.9, respectively.

Orthonormal basis sets define isomorphisms and automorphisms in Clifford algebras. Orthonormal basis sets (ONB) are defined as sets of multivectors satisfying scalar product relations. A FORTRAN program determining ONBs is described. It is shown that any simple Clifford algebra is isomorphic to the tensor product of a Clifford algebra Cl m,m and a Clifford algebra isomorphic to ℝ, ℂ or ℍ. From the construction of matrix algebras isomorphic to Cl m,m given by the second FORTRAN program, matrix algebras with entries in ℝ, ℂ or ℍ can be used to construct isomorphisms to all simple Clifford algebras.

The complete reduction of simple Clifford algebras is spanned by matrix bases. Considered as generators of an algebra the basis elements pij satisfy the product relations pijpkl= δjkpil. It follows that subsets of pij with subscript or superscript fixed span minimal left or right ideals. The expression of the basis elements pij in the usual multivector basis is derived from isomorphic matrix algebras. Spinors and Hermitian conjugate spinors are defined as members of minimal left and right ideals. Spinor scalar products are defined, these scalar products are in fact scalar multiples of an idempotent.

In order to search for isospin-2 dibaryons and to study the inelastic nucleon-nucleon channels producing at least two pions, the differential cross sections and analyzing powers have been measured for the {ital p}({ital {rvec p}},{pi}{sup {minus}}){ital X} reaction with polarized proton beams of 1.45, 2.1, and 2.7 GeV. The pions were detected at an angle of 13.8{degree} by the SPESIII spectrometer located at Laboratoire National Saturne. No statistically significant evidence for structures corresponding to {pi}{ital NN} bound states or six-quark states could be established. However, upper limits to the cross section for such processes are deduced. The data analysis in terms of phase space has shown that as the bombarding energy increases, {ital N}{sup *}{ital N} final states have to be taken into account besides the usual {ital N}{Delta} and {Delta}{Delta} processes in order to adequately describe our results.

For masses larger than 2 % + MT, narrow peaks in two proton invariant (missing) masses spectra : M (M ) are now well established. The existence of such PP x narrow peaks is not so unquestionable below 2 $ + Mr. Tensor analyzing power T20 + of the invariant mass M has been studied using the p(d,pp)~ reaction. Depending PP on the assumption done for background substraction -a hble or an oscillation mayappear at M = 1941 MeV. Additional data are under analysis and will improve the statistics. PP It is well known that narrow structures have been obsenred in two nucleon invariant masses, or missing mass experiments/l/. These structures have been tentatively attributed to isovector dibaryons although no proof has been advanced to associate them with quark degrees of freedom. This assumption relies only on the fact that such structures are not present in classical nuclear physics using meson exchanges and nucleon or delta degrees of freedom. The experimental certainty depends mainly on the quantum numbers and the mass of studied dibaryons. Up to now, none has been observed in T = 0/2/ or T = 2/31 isospin states. In case of T = 1 isospin, for the range of masses larger than 2 $ + M* (M > 2.016 GeV) there is a good agreement for narrow peaks between results obtained at Saturnel41 : measurement of d2a/dQdMx for 38e(p,d)~ reaction, and Lanpf/5/ where Ay was measured. These data correspond to the second generation of experiments, since they have used electronical detectors. ('1 Present address : Ben Gourion University of the Negev, P.O. Box 653 84105Beer Sheva, Israel. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990631 C6-372 COLLOQUE DE PHYSIQUE Below 2.016 GeV, many structures have been presented, corresponding usually -but not alwaysto the first generation of experiments studying bubble chamber pictures. Here the masses of experimentally observed structures do not concentrate around a few values, but are more or less distributed from 2 % to 2 % + M?/1/. The necessity of a precise second generation experiment close to the two nucleon mass appears then naturally. 3 Such an experiment has been carried out at SATURNE with the p(d,pp)X reaction. Tensor polarized deuteron beam of 1.722 GeV has been used and the two protons detected at 0" using the SPES3 facility. The properties of the spectrometer : pmax/pmin z 2.5, and As2 = sr allow the study of the following two proton invariant mass range : 2 % < M < 1.963 GeV, PP the counting rate decreasing regularly from small to larger masses M . This result is mainPP ly due to pp FSI giving rise to a narrow maximum a few MeV above 2 M . Due to the granulaP rity of the detection, when two trajectories intercept a same scintillator of one among the two major trigger detection plans or give rise to two mixing tracks on the focal pla~e, the event is lost. This produces a loss of efficiency in the first three MeV of the spectra.This explains that our FSI is located around 1880.7 MeV, that is ?. 3 MeV higher than the peak maximum from low energy experiments. The detection has been described elsewhere/3/. The empty target accounts for about 5 % of the counting rate. The FWHM of the experimental wcdth for M is 1.2 MeV. The data have been therefore binned into 2 MeV intervals. The M hisPP PP togram presented in Fig. 1 is quite continuous, without any discontinuity having a number of standard deviations (S .D .) larger than 2.2. id = 1.722 GeV epp =0° 0.2 188

An investigation of the γ-decay of the 1 g 9 2 analogue state in 59Cu has been performed using the 58Ni(p, γ)59Cu reaction. The (p, γ) excitation function has been taken in the range Ep = 3450–3650 keV. The decay schemes of the (p, γ) resonances at Ep = 3483, 3532 and 3547 keV, measured with Ge(Li) detectors, lead to eight new levels in 59Cu with excitation energies between 1.8 and 4.7 MeV and to spin assignments of several states. The spins of the first two resonances are found to be ( 1 2 , 3 2 ) and ( 5 2 ). The spin of the Ep = 3547 keV resonan is, from angular distributions, uniquely determined to be J π = 9 2 + and this state is found to be the unfragmented analogue state of the E ∗ = 3.062 MeV , J π = 9 2 + parent state in 59Ni. The measured complete decay scheme of this resonance shows that its isovector M1 decay is in disagreement with all existing theoretical predictions.

High-spin states in $^{34}\mathrm{Cl}$ were populated in the $^{27}\mathrm{Al}(^{12}\mathrm{C}, \ensuremath{\alpha}n)^{34}\mathrm{Cl}$ fusion-evaporation reaction between 27 and 45 MeV incident energy and in the $^{31}\mathrm{P}(\ensuremath{\alpha}, n)^{34}\mathrm{Cl}$ reaction at 14.4 MeV. The decay scheme is obtained from $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ and $n\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements. The ${J}^{\ensuremath{\pi}}$ assignments for the levels at 4743, 4824, and 5315 keV result from angular distributions, directional correlation ratios, and excitation functions. The lifetime of the 5315 keV level was measured in a recoil distance experiment.

The investigation of analogue-to-antianalogue M1 γ-transitions has been extended to g 9 2 and d 5 2 analogue states in As isotopes. The strongly hindered transitions in 73,75As do not seem to fit into the general systematics.

The differential cross section and analyzing power have been measured for the p( p , π − )X reaction at an incident energy of 1450 MeV and a pion angle of 13.8° in order to study eventual dibaryonic T =2 structures in the vicinity of the πNN threshold. No statistically significant evidence for such structures corresponding to πNN bound states or six-quark states could be established. Upper limits to the cross section are deduced. Analyzing power data are discussed.

The ..gamma.. decay of resonances in the reaction /sup 62/Ni(p,..gamma..)/sup 63/Cu in the energy range E/sub p/ = 3766 to 3865 keV has been investigated. For a number of prominent resonances that seemed to decay to the 2509 keV 9/2/sup +/ level in /sup 63/Cu angular distributions were taken. These angular distributions show that the multipolarity of the transition gamma rays is not of pure M1 character as one would expect from a T/sub greater-than/ to a T/sub less-than/ transition. Further investigation with a thick target did not alter the overall results, which show that the g/sub 9/2/ analog state is most likely fragmented in such a way that none of the fragments has an observable strength contrary to a previous report.

The deexcitation of high-spin states in $^{37}\mathrm{Cl}$ was studied via the $^{27}\mathrm{Al}(^{19}\mathrm{F}, 2\ensuremath{\alpha}p)$, $^{27}\mathrm{Al}(^{12}\mathrm{C}, 2p)$, and $^{34}\mathrm{S}(\ensuremath{\alpha}, p)$ reactions. The decay scheme and spin assignments were deduced from $p\ensuremath{-}\ensuremath{\gamma}$ coincidences, $\ensuremath{\gamma}$-ray angular distributions, directional correlation from oriented nuclei ratios, and excitation function measurements. The lifetime of a new yrast level at 7020 keV ($J=\frac{15}{2}$) was determined in a recoil distance experiment. New levels are reported at 6001 keV ($J=\frac{9}{2}\ensuremath{-}\frac{13}{2}$) and at 8177 keV. The observed levels are compared to the predictions of the weak-coupling model.

The 3He(3He, π+)6 Li reaction has been studied as a function of energy using the 3He beam at SATURNE. Cross sections for the 6Li ground state (1+) and 2.18 MeV (3+) levels have been obtained at 350,420,500 and 600 MeV incident energies at angles θπ(lab) = 15° and 40°. These results are compared with two theoretical predictions.

The γ-decay of 26 resonances in the reaction 60Ni(p, γ)61Cu in the range Ep = 3690–3790 keV has been investigated. The g 9 2 isobaric analogue state (IAS) corresponding to the parent state at E ∗ = 2114 keV in 61Mi has been resolved into several fine structure components. The absolute strength for the transition to the antianalogue state is found to be Γγ = 0.24 ± 0.07 W.u. The measured M1 strength distribution for the decay of the g 9 2 . IAS is compared to theoretical predictions.

The 8.113 and 8.130 MeV states of 45Sc have been identified as the main components of the split isobaric analogue of the lowest 3 2 − state of 45Ca. A group of 45Sc levels around 4.5 MeV is strongly excited in the γ-decay. The total M1 strength has the expected order of magnitude.

Les courbes d'excitation des réactions 18O(p, p1) 18O* et 18O(p, α1,2,3)15N* ont été mesurées pour les énergies incidentes de 3,2 à 5,4 MeV. Les courbes montrent, pour certaines énergies, des anticorrélations entre les voies de sortie proton et alpha. Si cet effet peut être attribué à la conservation du spin isobarique dans les réactions, il détermine le spin isobarique de quelques états excités du noyau composé 19F.

The ( 3 He, π ± X ) reaction has been studied at 910 MeV using the 3 He ++ beam of the CERN SC. Complete π − spectra on CH 2 , CD 2 , 9 Be, 12 C, 27 Al, Cd and Pb, and π + spectra on CH 2 , CD 2 and 12 C targets have been measured at 0°, up or close to the kinematic limit x F = 1. The results are compared to various theoretical calculations.

A simple automatic system for measuring excitation functions is described below. The biasing of the target is done by the incident particles stopped in the target. The excitation curves are automatically normalized.