Isotensor Component Research Articles

Masses of the Tz=−3/2 nuclei P27 and S29

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the $T_z=-3/2$ nuclei $^{27}$P and $^{29}$S. The new mass excess value $ME$($^{29}$S) $=-3094(13)$~keV is 66(52)~keV larger than the result of the previous $^{32}$S($^3$He,$^{6}$He)$^{29}$S reaction measurement in 1973 and a factor of 3.8 more precise. The new result for $^{29}$S, together with those of the $T=3/2$ isobaric analog states (IAS) in $^{29}$P, $^{29}$Si, and $^{29}$Al, fit well into the quadratic form of the Isobaric Multiplet Mass Equation IMME. The mass excess of $^{27}$P has been remeasured to be $ME(^{27}$P$)=-685(42)$ keV. By analyzing the linear and quadratic coefficients of the IMME in the $T_z=-3/2$ $sd$-shell nuclei, it was found that the ratio of the Coulomb radius parameters is $R\approx0.96$ and is nearly the same for all $T=3/2$ isospin multiplets. Such a nearly constant $R$-value, apparently valid for the entire light mass region with $A>9$, can be used to set stringent constraints on the isovector and isotensor components of the isospin non-conserving forces in theoretical calculations.

Isotensor hadronic parity violation

Weak interactions between quarks give rise to hadronic parity violation which can be observed in nuclear and few-nucleon systems. We study the QCD renormalization of the isotensor component of parity violation at next-to-leading order accuracy. The renormalization group is employed to evolve the interaction down to hadronic scales. As the results are renormalization scheme dependent, we compare various schemes, including 't Hooft-Veltman dimensional regularization, and several regularization independent-momentum subtraction schemes.

Electric Dipole Moments of Light Nuclei and the Implications for CP Violation

A definitive measurement of an electric dipole moment (EDM) would likely imply new physics beyond the standard model. Although the standard model strong interaction term could theoretically produce an EDM of any size, that it is constrained by the current neutron EDM limit to be some 10 orders of magnitude smaller than 1 suggests that the electroweak sector and CP violation will be the source of a measurable EDM. The weak interaction standard model EDM is itself orders of magnitude smaller than contemporary experiments can measure. Direct measurement of the neutron EDM lies in the next decade; measurement of the proton EDM could well come first. A BNL proposal for an electrostatic storage ring measurement lies in the offing. Unless the EDM proves to be an isoscalar, one will need other measurements to separate the isoscalar, isovector, and isotensor components. Measurement of a nuclear EDM will be required: 2H, 3H, or 3He being the simplest nuclear systems. A storage ring measurement of the triton EDM could be accomplished in a manner analogous to that proposed for the proton. However, the deuteron EDM measurement offers certain advantages, even though the experiment would be more complex, involving electric and magnetic fields, than that required for the proton and triton. The COSY facility in the Forschungszentrum Juelich is almost an ideal facility to house such an experiment; one could also measure in the same ring the EDM for the proton and He. The deuteron is the one nucleus for which exact model calculations can easily be performed. We briefly explore the model dependence of deuteron EDM calculations. Using a separable potential formulation of the Hamiltonian, we examine the sensitivity of the deuteron EDM to variations in the nucleon–nucleon interaction, including contemporary potential models, and we explore the dependence upon intermediate state multiple scattering in the 3P1 channel. We investigate the tensor force contribution to the model results and examine the effects of short-range repulsion that characterize realistic, contemporary potential models of the deuteron. Because one-pion exchange dominates the EDM calculation, separable potential model calculations appear to provide an adequate description of the deuteron EDM until such time as a measurement of better than 10 % is achieved.

Determination of the isotensor symmetry energy for the giant dipole resonance.

Comparison of giant dipole resonance energies measured in photonuclear reactions and in pion-induced double charge exchange allows determination of the isotensor component of the symmetry energy. Different models for the structure of the giant dipole and different assumptions for the mass dependence of the isotensor energy all produce the same qualitative results, with the isotensor energy about 1% of the isovector energy, and of the same sign.

Isospin dependence of Pauli corrections to the pion-nucleus optical potential.

We examine how the Pauli exclusion principle affects the pion-nucleus optical potential U in the region of the ..delta../sub 33/ resonance, i.e., for pion kinetic energies of 50--300 MeV. In our formulation the exchange effects enter through n-body correlation functions and contribute to U as higher-order corrections in a density expansion. We study the rate of convergence of the expansion and derive an integral equation that sums selected exchange terms to all orders. We calculate the isoscalar, isovector, and isotensor components of U for the cases of infinite nuclear matter and finite nuclei. We identify the limiting conditions under which our treatment of the Pauli effect reduces to that in more familiar approaches.

On isospin-forbidden transitions

In the continuum shell model, the width of the 3/2-, 3/2 resonance state at 15.1 MeV in the mirror nuclei 13C and 13N is calculated. If channel coupling and the Pauli principle are taken into account, the width calculated without external mixing and without isotensor components is of the correct order of magnitude. The calculations show that coupling to isospin-allowed channels which are closed plays an important role.

The [formula omitted] resonance at 6.44 MeV

The first T = 3 2 level in 13C has been studied as an isospin-forbidden resonance in the 9Be(α, n 0), 9Be(α, n 1) and 9Be(α, α 0) reactions at an incident energy of 6443.5±2.0 keV, corresponding to an excitation energy of 15108.9±1.5 keV in 13C. Total and partial decay widths were deduced as Γ = 5.49±0.25 keV and Γ α0/Γ ≧ 0.017 . The resonance parameters are discussed along the lines of similar investigations in 17O and 21Ne and with respect to the isobaric mass multiplet equation. Combining the present value for Γ with previous measurements of Γ γ0 / Γ yields the ground-state γ-decay width Γ γ0 = 22.8±1.9 eV, which compares well with the most precise value until now known from inelastic electron scattering data. The weighted combination of all available information on the ground-state-γ-decay of the first T = 3 2 states in 13C and 13N leads to B( 13 C)/B( 13 N)−1 = −0.15±0.07 for the reduced M1 transition strengths. The charge asymmetry of the reduced M1 transition strengths is discussed with respect to a charge-dependent mixing in the initial and final states and a possible isotensor component of the M1 transition operator.

An analysis of parity violating nuclear effects at low energy

We present an analysis of parity violating nuclear effects at low energy which attempts to circumvent the uncertainties due to the weak and strong nucleon-nucleon interactions at short distances. Extending Danilov's parametrization of the parity violating nucleon-nucleon scattering amplitude, we introduce six parameters: one for the long range contribution due to the pion exchange and five for the shorter range contributions. This choice gives an accurate representation of parity violating effects in the nucleon-nucleon system up to a lab energy of 75 MeV. For calculations in nuclei, we derive an effective two-body potential in terms of the parameters. The analysis of presently measured effects shows that they are consistent, and, in particular, that the circular polarization of photons in n + p → d + γ is not incompatible with the other measurements. It does not imply a dominant isotensor component.

Empirical parity-violating potentials and the coherent scattering of neutrons

The rotation of the spin in the coherent scattering of polarised thermal neutrons was calculated using empirical parity-violating potentials. The results obtained are up to an order of magnitude larger than expected on the basis of dimensional arguments. The sign of the effect depends on the isospin properties of the weak potential and can distinguish between potentials with large isotensor or large isovector components.

Isospin-forbidden particle decays in light nuclei: (II). Widths of [formula omitted] levels of 17O

The lowest four T = 3 2 levels of 17O have been observed as resonances in the 13C(α, n) 16O reaction. Excitation energies and widths obtained for these levels are 11.076 ± 0.005 MeV, Γ c.m. = 5.0 ± 1.1 keV; 12.458 ± 0.005 MeV, Γ c.m. = 8 ± 2 keV; 12.944 ± 0.006 MeV, Γ c.m. = 6 ± 2 keV; 12.993 ± 0.006 MeV, Γ c.m. < 3 keV. The total and partial decay widths for the lowest T = 3 2 level are much larger than those of the analogue level in 17F, implying significant isotensor components in the T = 1 2 admixtures.

Iso-Tensor Component in the S=0 Weak Process and I =1/2 Rule

The existence of large discrepancy be­ tween theoretical value and observed one -of !he circular polarization of emitted photon in n + p~d + r process is one of the most important problems.!) Recently, Mckellar has suggested2> 'that this discrepancy orig­ inates in not to take into account the large contribution of iso-tensor component in the .JS=O process. Further lie has emphasized that iso-tensot dominance is not contradic­ tory to other experiments; the a decay of 2- (8.88 MeV) state of 160 8> and the 110 ke V r-transition in 19 F. 4) However, iso-tensor dominance conflicts with the conventional model of ILl II = 1/2 rule; 8 enhancement and/or 27 suppression. Then, his conclu­ sion can be made_ use 'Df for a ch'ai.ce of model of I.Jll =1/2 rule. This note is con­ cerned with this proolem. In this note, we examine, as _a prototype, a few types of model of I.Jll =1/2 rule.*> In the model a) Bose statistics of quarks, 5> all ·27 components vanish automatically. Th.en, this model cannot allow the existence of iso·tensor component. Similar conclu­ sion also holds for the models based on b) *> Let us remember that in the usual Cabibbo theory, the non-leptonic interaction Hamiltonian has general' schematic form,

Parity non-conservation in the low energy nucleon-nucleon system:: Evidence for an isotensor weak interaction?

A phenomenological analysis of recent experiments on parity non-conservation in the low energy nucleon-nucleon system suggests that the ΔS = 0 non-leptonic weak interaction contains a large isotensor component. Further experiments to clarify the situation are suggested.

Inverse pion photoproduction in the vicinity of thep33(1232)resonance and a test of time-reversal invariance

Differential cross-section measurements are presented for ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\gamma}n$ at five energies around the ${p}_{33}(1232)$ resonance. A detailed comparison is made with $\ensuremath{\gamma}n\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}p$ deduced from $\ensuremath{\gamma}d$ experiments. In general, the results are in support of detailed balance. Using the Christ-Lee-Donnachie-Shaw model, our new data indicate that the $T$-violating phase in the isovector part of the ${M}_{{1}^{+}}$ multipole is less than 2\ifmmode^\circ\else\textdegree\fi{}, which is a very sensitive test of time-reversal invariance. No evidence is found for a possible isotensor component of the electromagnetic current. Our data are compared to various multipole analyses. In general, the agreement is poor.

Differential cross-sections for π−p → γn in the first resonance region

Differential cross-sections for negative pion radiative capture on protons at c.m. angles of 60°, 90°, and 120° have been measured at nine incident laboratory energies between 110 and 270 MeV. Comparison with measured cross-sections for pion photoproduction and with conventional multipole analyses shows neither evidence for a violation of time reversal invariance nor for an isotensor component of the electromagnetic current of hardrons.

Nuclear charge-asymmetry effects from isotensor currents

Departures from mirror symmetry in γ- and β-decays of nuclear states are investigated by postulating the existence of isotensor components in electromagnetic and weak currents of hadrons. Their effects are described by means of two-body exchange current operators. These are found to exhibit simple properties which enable one to set useful bounds on the isotensor admixture. The consideration of resonance-exchange contributions in the processes γ (or β) + N → π + N suggests the N → Δ transition current as the most favorable candidate. We discuss a numerical application to M1 and Gamow-Teller transitions of the isoquartet of states in the isobars A = 13 (B, C, N, O). It is found that the test is reasonably model-independent but very insensitive. Therefore, low-energy phenomena may be not too promising. Results are also given for other isoquartets in the p-shell.

Lifetimes of the Lowest52+and92+States of the Mirror PairNa23-Mg23

The lifetimes of the ${\frac{5}{2}}^{+}$ and ${\frac{9}{2}}^{+}$ members of the ${K}^{\ensuremath{\pi}}={\frac{3}{2}}^{+}$ ground-state rotational band in the mirror pair $^{23}\mathrm{Na}$-$^{23}\mathrm{Mg}$ have been measured via the Doppler-shift attenuation method. The reactions $^{12}\mathrm{C}+^{12}\mathrm{C}=^{23}\mathrm{Na}+ p$ or $^{23}\mathrm{Mg}+ n$ were used and line shapes of the decay $\ensuremath{\gamma}$ rays were observed in singles for recoil into Al and Ni backings. The results are 1.63\ifmmode\pm\else\textpm\fi{}0.20 and 2.00\ifmmode\pm\else\textpm\fi{}0.24 psec for the $^{23}\mathrm{Na}$ and $^{23}\mathrm{Mg}$ ${\frac{5}{2}}^{+}$ first excited states and 0.11\ifmmode\pm\else\textpm\fi{}0.02 and 0.08\ifmmode\pm\else\textpm\fi{}0.02 psec for the $^{23}\mathrm{Na}$ and $^{23}\mathrm{Mg}$ ${\frac{9}{2}}^{+}$ states. These results are compared to the predictions of the Nilsson model. The separation of the isoscalar and isotensor components of the $M$ 1 rates is discussed as well as the separation of the spin and orbital contributions to the isotensor component. Incidental to this work, the energies of the first excited states were determined as 439.80 \ifmmode\pm\else\textpm\fi{} 0.15 keV ($^{23}\mathrm{Na}$) and 450.70\ifmmode\pm\else\textpm\fi{}0.15 keV ($^{23}\mathrm{Mg}$), and the ${\frac{9}{2}}^{+}\ensuremath{\rightarrow}{\frac{7}{2}}^{+}$ transition energies were measured with comparable accuracy.

Analysis of the reaction γ+n→p+π− in the first and second resonance regions

The final results of an experimental investigation of the reaction γ+n→p+π− performed with a deuterium bubble chamber at the 1 GeV Frascati electrosynchrotron are presented. Total and differential cross-sections on neutrons are extracted by means of the spectator model, the reliability of which has been checked by numerous tests and is extensively discussed. The problems of a possible isotensor component in the electromagnetic current, the time-reversal invariance of the electromagnetic interactions and the photoproduction of the Roper resonance are considered in detail.

Upper limit of the isotensor contribution to the electroproduction cross section in the region of the Δ(1236) - resonance

The cross section for electroproduction on protons and neutrons was investigated. For momentum transfers 0.1 (GeV/ c) 2< q 2<0.5 (GeV/ c) 2 and invariant masses W < 1.4 GeV of the final hadronic state the measured cross section was split up into a resonant and a nonresonant contribution. The transition form factor G ∗ M n (q 2) of the γnδ-vertex is derived from the resonant cross section. The equality of the resonant part of the excitation cross section on neutrons and protons permits us to derive an upper limit of 7% for the isotensor component to the electroproduction process.

Isotensor dipole sum rules in nuclear electroexcitation

Electric dipole sum rules are analysed in electron-nucleus scattering for different isotensor components, in the limit q 2 → 0 . As an application the giant resonance form factor of several nuclei is studied. In particular the relation between the dip at 60 MeV/ c and the existence of the 〈Σ i l i · s i〉 SU 4 symmetry-breaking term is analysed.

Measurement of Inverse Pion Photoproduction Near theP330(1236)Resonance

The differential cross sections for ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}\ensuremath{\gamma}+n$ at ${40}^{\ensuremath{\circ}}&lt;{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\theta}}}_{\ensuremath{\gamma}}&lt;{150}^{\ensuremath{\circ}}$ at c.m. total energies 1245 and 1337 MeV are presented. The results do not agree with predictions based on the $\ensuremath{\Delta}I&lt;~1$ rule for the electromagnetic current and fixed-$t$ dispersion relations. Our data can be fitted with the Sanda-Shaw model, in which the electromagnetic current contains an isotensor component. Our angular distribution at 1337 MeV agrees with the inverse reaction as deduced from $\ensuremath{\gamma}d$ work; however, there is some disagreement at 1245 MeV.