Spin-parity Combinations Research Articles

Electromagnetic transition form factors and Dalitz decays of hyperons

Dalitz decays of a hyperon resonance to a ground-state hyperon and an electron-positron pair can give access to some information about the composite structure of hyperons. We present expressions for the multi-differential decay rates in terms of general transition form factors for spin-parity combinations J^P = frac{1}{2}^pm , frac{3}{2}^pm of the hyperon resonance. Even if the spin of the initial hyperon resonance is not measured, the self-analyzing weak decay of the “final” ground-state hyperon contains information about the relative phase between combinations of transition form factors. This relative phase is non-vanishing because of the unstable nature of the hyperon resonance. If all form factor combinations in the differential decay formulae are replaced by their respective values at the photon point, one obtains a QED type approximation, which might be interpreted as characterizing hypothetical hyperons with point-like structure. We compare the QED type approximation to a more realistic form factor scenario for the lowest-lying singly-strange hyperon resonances. In this way we explore which accuracy in the measurements of the differential Dalitz decay rates is required in order to distinguish the composite-structure case from the pointlike case. Based on the QED type approximation we obtain as a by-product a rough prediction for the ratio between the Dalitz decay width and the corresponding photon decay width.

Parity dependence of nuclear level densities

High resolution proton resonance data have been examined for a possible parity dependence of nuclear level densities. Five spin-parity ${\mathrm{combinations}\mathrm{---}1/2}^{+},$ ${1/2}^{\ensuremath{-}},$ ${3/2}^{\ensuremath{-}},$ ${3/2}^{+},$ and ${5/2}^{+}$---have been analyzed for three ${\mathrm{datasets}\mathrm{---}}^{44}\mathrm{Ca}(p,p),$ ${}^{48}\mathrm{Ti}(p,p),$ and ${}^{56}\mathrm{Fe}(p,p).$ Much of the uncertainty in the level density arises from the determination of the fraction of levels that are not observed. The missing fraction of levels was determined by (a) the standard width analysis method that assumes a Porter-Thomas distribution for the reduced widths and (b) a recently developed method that analyzes the spacing distribution and assumes the Wigner distribution for the nearest-neighbor spacings. There are indications of parity dependence of the level densities for several of the spin-parity combinations.

Nuclear spectroscopy of 26Al

The tensor analyzing powers T 20 and the cross sections for the reaction 28 Si( d , α) 26 Al to levels in 26Al with excitation energies up to 6 MeV have been measured at θ lab = 175°. The bombarding energy was varied in steps of 0.5 MeV from 10.0 to 12.5 MeV. Comparison of the measured T 20 values with theoretical predictions, taking into account the finite-angle effect, has led to assignments of natural or unnatural parity to some fifty levels in 26Al. Two levels with J π = 0 − could be identified on the basis of such a comparison. A (probable) assignment of J π = 0 + could be made to two levels in 26Al on the basis of a very low observed cross section. The information on spin-parity combinations found in this work was combined with spectroscopic data from other reactions to deduce new unambiguous J π assignments for 33 levels and J π restrictions for another three levels.

Energy levels of 56Co studied by the 58Ni( [formula omitted], α) 56Co reaction near 0°

Tensor analyzing powers, T 20, of outgoing α-particles in the 58Ni( d , α) 56Co reaction at detection angles near 0° have been measured for excited states in 56Co for beam energies of 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. Thirty-seven spin-parity combinations for 56Co excited states have been deduced. Previous J π ambiguities for 11 of these states have been eliminated, and results in conflict with existing assignments for the levels at 3.235 and 3.378 MeV have been obtained. A search for 0 + states was carried out from angular distribution measurements at forward angles of the unpolarized (d, α) reaction. The combined results from this and previous experiments were found to be in reasonable agreement with calculated level schemes.

Spin-parity determinations in 42K from the 44Ca( [formula omitted], α) 42K reaction near 0°

The reaction 44Ca(d, α) 42K has been studied, using a tensor-polarized deuteron beam, at incident energies of 7.00, 7.25, 7.50, 7.75, 8.00 and 8.25 MeV. From measurements of the tensor analysing power T 20 of the reaction when the α-particles are emitted at 4°, spin-parity combinations have been deduced for levels in 42K up to 2.3 MeV excitation. These results, together with previous work, lead to a number of new and unambiguous spin and parity assignments. The experimental information on the low-lying levels of 42K is assessed and compared with the predictions of shell-model calculations.

Spin-parity combinations in 16N from the [formula omitted] reaction

The reaction 18O(d, α) 16N has been studied using a tensor-polarized deuteron beam at incident energies of 8.50, 9.25, 10.00, 10.30, 10.75 and 11.30 MeV. From measurement of the tensor analysing power of the reaction when the α-particles are emitted near 0°, spin-parity combinations have been deduced for thirteen levels in 16N up to 6.2 MeV excitation. These results, together with previous work, lead to new and unambiguous spin and parity assignments for four of these levels.

Spin-parity combinations in 32P from the 34S( [formula omitted], α) 32P reaction

The reaction 34S( d , α) 32P has been studied using a tensor-polarized deuteron beam at bombarding energies of 8.25, 8.50, 8.75, 9.25 and 9.50 MeV. From measurement of the tensor analyzing power, T 20 when α-particles are emitted near 0°, spin-parity combinations have been assigned to 23 levels in 32P in the excitation energy range 1.0 to 5.6 MeV. These results add new information for the majority of these levels and in one case are in conflict with a previously accepted assignment.

Spin-parity combinations in 28Al

The tensor analyzing powers T20 and the cross sections for the reaction 30Si(d, α)28Al to levels in 28Al with excitation energies up to 5.3 MeV have been measured at θlab = 175° for bombarding energies from 8.0 to 12.0 MeV in steps of 0.5 MeV. Comparison of the measured T20 values with theoretical predictions has led to assignments of natural or unnatural parity to some forty levels in 28Al. In a number of cases unique spin-parity assignments could be deduced by combining the present information with published spectroscopic data. A (probable) assignment of Jπ = 0+ could be made to three levels in 28Al on the basis of a very low observed cross section. No states with Jπ = 0− have been observed for Ex < 5.3 MeV. The distribution of T20 values measured in cases where the final state has unnatural parity has been found to be in agreement with the prediction of the statistical theory for nuclear reactions. The spectroscopic information is compared with existing shell-model calculations.

Spin-parity combinations in 30P determined with A t20 polarized deuteron beam

Abstract The tensor analysing power T20 around θ = 180° has been measured of the 32 S( d , α 30 P reaction to final levels with excitation energies up to 7 MeV. This has been done for six bombarding energies between Ed = 8 and 12 MeV. Comparison of the measured T20 values with the theoretical value for the case that the final state has natural parity [T20 (0° or 180°) = 0.707 for (-)J = π] has led to certain assignments of unnatural parity and probable assignments of natural parity for some forty levels in 30P. In a similar way some probable assignments of Jπ = 0− and Jπ = 0+ could be made. The distribution of T20 values measured in cases where the final state has unnatural parity has been found to be in agreement with the predictions of the statistical theory for nuclear reactions.

Spin?Parity Combinations in 32S

Inelastically scattered IX-particles from the reaction 32S(IX, 1X')32S have been studied with solid state counters at extreme backward angles in order to determine spin-parity combinations for levels in 32S at excitation energies Ex up to 7 �15 MeV. The results confirm the well-established spin and parity values, show that the 5� 798 MeV spin 1 state has negative parity, and provide narrow limits for the possible spin and parity values of the 6'410,6' 666,6' 762, and 6� 854 MeV levels. A previously unreported natural parity level was found at Ex = 6�58 MeV. Magnetic analysis of the reaction 32S(p, p')32S confirmed the existence of this level and established its excitation energy as 6�581�0�003 MeV. Particle-y-ray coincidence studies showed that this level decays predominantly by y-ray transitions to the 2�23 MeV 2 + state.

Spin–Parity Combinations in 22Ne

Detection of inelastically scattered alpha particles near 180° from the reaction 22Ne(α,α′)22Ne has been used to identify natural parity levels in 22Ne. Energy spectra were recorded for ten incident energies from 16.5–21.0 MeV in steps of 500 keV. Levels at 4.46, 5.52, and 5.92 MeV have been identified as having natural parity, leading to definite assignments of 2+, 4+, and 2+ respectively. Levels at 5.14 and 5.64 MeV have been identified as having unnatural parity, implying assignments of 2− and 3+ respectively. Tentative assignments are suggested for other levels, and the implications of the present assignments with regard to the identification of band structure in 22Ne are discussed.

Lifetimes and branching ratios of energy levels of 28Si. II

The gamma rays from the 25Mg(α,nγ)28Si reaction have been studied for alpha-particle energies in the range 6.0 MeV to 7.2 MeV. A 25-cm3 Ge(Li) gamma-ray counter was used to observe the complex high energy gamma-ray spectra at angles between 0° and 130° to the alpha-particle beam. The lifetime ofthe7798-keV level was found to be 0.30 ± 0.10 ps by studying the Doppler broadening of the spectrum lines. The gamma-ray decays of the 8260, 8328, 8411, 8543, and 8587-keV levels were observed. The gamma-ray decays of the 8260 and 8543-keV levels had not been observed previously. The 8260-keV level decays to the first excited level at 1780-keV and the 8543-keV level decays only to the 4617-keV level. Information on the spin-parity combinations of these levels in 28Si was obtained by a measurement of the yield of alpha particles at zero degrees to the beam in the reaction 16O(16O,α)28Si.

Zero-Mass Bosons inS-Matrix Theory

We describe the soft coupling of zero-mass bosons to other particles, by considering the limit of a theory with a massive boson. With the standard $S$-matrix assumptions of analyticity and crossing for four-body helicity amplitudes, we demonstrate generally that in the limit of zero mass, a vector boson (${1}^{\ensuremath{-}}$) couples to a conserved charge and a ${2}^{+}$ boson couples to the inertial mass. Bosons of other spin-parity combinations (with the exception of zero spin) have no zero-mass soft coupling. With this technique, we not only give a pedagogically interesting solution to gauge invariance and the kinematics of zero-mass particles, but suggest new applications to small-mass integral-spin systems. We speculate on the application of this technique to such problems as $\ensuremath{\rho}$ universality, the Adler-Weisberger relation, and the universality of leptonic couplings in a vector or axial-vector state.

Remarks on the analysis of τ-meson experiments

The results from the analysis of 75 stopped τ-meson decays, found in a stack of emulsions exposed to the Berkeley Bevatron, are discussed. Special attention is paid to the effects of the finite dimensions of the stack, and a brief analysis of the problem is given. The experimental results are compared with those from other laboratories, and the authors conclude that there is no good evidence that the spectrum of the π-mesons from τ-decay is in accord with either of those which have been predicted for the two lowest spin-parity combinations: 0- and 1+.

Mass and spin-parity character of the Τ-meson

In this paper we present analyses of 41 Τ- and Τ′-meson decay events. Since the emulsion density was accurately known, and an improved range-energy curve is available, aQ-value for the Τ-decay was obtained which is more precise than that available heretofore. It is 75.08 ± 0.20 MeV. Analysis to determine the character of the Τ-meson was made by comparing the experimental data with the 5 spin-parity combinations that lead to unique energy and angular distributions. It is found on employing a χ2 test that there is a probability of 18% that the χ2 for the (0,−) choice may exceed that observed, whereas the next most probable choice, (3, −), yields a probability of only 2.8%. The data could not distinguish between the (0,−) and (2,−) states of the Τ-meson. The Τ′/Τ ratio observed is 0.25 ± 0.12, which is consistent with the hypothesis of charge independence. The energy distribution of the π+-meson in the Τ′-decay is consistent with a decay scheme Τt → π+ + 2π0, and an associated electron pair also confirms this supposed mode of decay.