Electromagnetic and weak interactions with nuclei: One-body densities in 6LI

Eleastic and inelastic magnetic electron scattering, and other electromagnetic data, are used to accurately determine one-body densities (and wave functions) for the valence nucleons in ⁶Li. Semi-leptonic weak processes are then analyzed in an essentially model-independent way.     

The electron-nucleon weak neutral current and 0− -0+ nuclear isomerism

Since one photon exchange is completely forbidden in processes of the type ?0^? → ?0⁺ (? is a charged lepton) or any crossed version of such a process, weak neutral current effects are considerably enhanced. It is shown that 0̄?0⁺ nuclear isomers, if they exist, will accordingly exhibit strikingly large neutral current effects. In particular, the monoenergetic conversion electrons will have helicities h of the order of 25% which should be easily measurable. In certain favourable cases, the nuclear weak current matrix elements may be measured experimentally via the analogue β decay or electron capture so that measuring h will constitute a measurement of the electron's neutral vector current coupling constant. In any event, detection of h would prove the existence of an as yet unseen class of neutral currents.     

Weak neutral currents in the U3(W)-gauge theory of weak and electromagnetic interactions

A discussion is given of the implications of the recently proposed U₃(W)-gauge theory of weak and electromagnetic interactions (Pandit 1976) for some phenomena resulting from its weak neutral currents: (1) neutrino-electron scattering, (2) neutrino-nucleon elastic and inelastic scattering, (3) coherent neutrino-nucleus scattering (4) weak interaction effects ine ⁺ e ⁻→μ⁺μ⁻ and (5) parity-violation in atomic physics. The theory agrees quite well with the available experimental results on neutrino processes. We find the coherent neutrino-nucleus cross-section for Fe⁵⁶ to be about 6 times larger than that in the WS-GIM theory giving some hope of accounting for supernova explosion by the resulting neutrino-radiation pressure.     

Charm production by weak neutral currents

We discuss quantitatively the production of charm in $\text{v}\text{N and}\text{v}\text{N}$ neutral current interactions, and the anomalous lepton events that follow from semi-leptonic charm decay. Diagonal neutral currents, in the Weinberg-Salam model and similar models, predict associated charm production with small cross sections: e.g. $\text{σ(v}\text{N}\text{→ vcc}{}^{\mathrm{?}}\text{X}\text{)/σ(v}\text{N}\text{→ μ}{}^{\mathrm{?}}\text{X}\text{) ? 10}{}^{\mathrm{?2}}$ at high energy. The meagre data on vN → ve⁺X are consistent with a rate of this order. Non-diagonal neutral currents, if present, could give larger cross sections via valence p → c transitions. It should be possible to distinguish diagonal from non-diagonal contributions by their x- or u-dependences, where u = x(1 ? y). We calculate the expected energy distributions of the leptons in characteristic vN → v?⁺X and vN → v?⁺?^?X charm decay events using simple models, and discuss some practical problems in neutral current measurements.     

The transition form factors for semi-leptonic weak decays of J/ψ in QCD sum rules

Within the standard model, we investigate the semi-leptonic weak decays of J/ψ. The various form factors of J/ψ making the transition to a single charmed meson (D^(*) _(d,s)) are studied in the framework of QCD sum rules. These form factors fully determine the rates of the weak semi-leptonic decays of J/ψ and provide valuable information on non-perturbative QCD effects. Our results indicate that the decay rate of the semi-leptonic weak decay mode J/ψ→D^(*)- _s+e⁺+ν_e is at the order of 10^-10. PACS 13.20.Gd; 13.25.Gv; 11.55.Hx     

层子模型中重子的半轻子衰变和高能中微子反应

本文从层子模型的结构观点出发,应用文献[1]中的层子与轻子的弱相互作用等效哈密顿量和文献[2]中给出的(1/2)⁺重子波函数及文献[3]中定出的参数,对(1/2)⁺重子的半轻子衰变和高能中微子的弹性和准弹性反应进行了讨论。文中得到的(1/2)⁺重子的半轻子衰变的理论结果与实验符合较好;得到的ΔS=0过程的轴矢形状因子和ν_μ+n→p+μ^-过程的截面在误差范围内均与实验相符合;得到的ΔS=1过程的轴矢形状因子和ν_μ+p→Λ+μ⁺过程的截面与目前实验给出的粗略结果也是符合的。特别需要指出的是,本文在讨论反应问题时,理论中所包含的参数全部从电磁过程定出,在给出的轴矢形状因子和截面公式中没有引进新的参数。     

Electrically weak asymmetry in the elastic scattering of electrons by nuclei with multipole moments

The differential elastic-scattering cross section of longitudinally polarized electrons by nuclei of arbitrary spin and isospin is calculated in the Born approximation, taking into account the interference of electromagnetic interaction and the interaction of weak neutral currents. For a nucleus of spin J 3/2, explicit expressions are obtained for the interference terms in terms of the electromagnetic multipole form factors F_C, f_Q, f_M1 and f_M3. Using the example of the³He,¹¹B,¹⁹F and⁴⁰Ca nuclei, the behavior of electrically weak asymmetry of the cross section is investigated in detail in the framework of three gauge models. It is shown that the asymmetry of a_Rl is 10^–5 for E - 100 MeV, and is sensitive to the nuclear structure: By taking into account the contributions not only of the charge and magnetic-dipole scattering but also of the quadrupole and octupole scattering, one can obtain additional information on the structure of weak electron-nucleus interaction.This paper was presented at a conference on Investigations of Spin Phenomena in High-Energy Physics held at Protvino, November 23–26, 1982.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 79–84, April, 1984.     

Analyses of electron and proton scattering to low-excitation isoscalar states in 20Ne, 24Mg and 28Si

The excitation of low-lying isoscalar 2⁺ and 4⁺ states in ²⁰Ne, ²⁴Mg and ²⁸Si by electron and proton scattering is studied. Large basis models of nuclear structure are used to determine both the electromagnetic and hadronic transition densities. The analyses of the longitudinal form factors obtained from electron scattering show that little or no effective charges are required with these nuclear structure models. Proton inelastic scattering to these states then is analysed to test effective forces based upon the Paris and Hamada-Johnston interactions. At intermediate energies (155 MeV) density-dependent t-matrices from both potentials were used with fits to data giving a clear preference for that based upon the Paris interaction. For lower energies only the Hamada-Johnston t-matrix is available and comparison of analyses of 24 and 49 MeV data made using this (complex) t-matrix with those in which the (real) Paris G-matrix is used as the effective force show a clear preference for the t-matrix. This is particularly the case with analyses of polarization data and suggests that the use of the G-matrix as an effective force in nuclear reaction calculations is inadequate even at low energies.     

Low-energy theorems for nuclear compton and raman scattering and 0+ → 0+ two-photon decays in nuclei

Low-energy theorems for elastic photon scattering (nuclear Compton scattering) from a nucleus of arbitrary spin are derived in the nonrelativistic approximation through terms quadratic in the photon frequency. The same derivation is made for the special case of 0⁺ → 0⁺ nuclear excitation by inelastic photon scattering (nuclear Raman scattering). Use is made of the general principle of gauge invariance, which bypasses the need to specify the form of the current operator explicitly. A general discussion of the contribution of mesonic exchanges is made and their effect is isolated. The center-of-mass correction to the nuclear diamagnetic susceptibility is calculated. The 0⁺ → 0⁺ two-photon decay amplitude is obtained from the nuclear Raman amplitude and the transition rate is calculated.     

Two-proton transfer reaction48Ca(18O,16C)50Ti at 102 MeV

Differential cross sections for¹⁸O elastic scattering and the (¹⁸O,¹⁶C) and (¹⁸O,¹⁷N) reactions on⁴⁸Ca were measured at 102 MeV using a Q3D magnetic spectrograph. The transitions to the 7/2^? ground state (g.s.) of⁴⁹Sc and the 0⁺ (g.s.), 2⁺ (1.554 MeV), 4⁺ (2.675 MeV), and 6⁺ (3.198 MeV) states of⁵⁰Ti were analyzed by DWBA calculations which include finite-range and recoil effects. Simple cluster-transfer calculations were done for all two-proton transfer transitions. For the 0⁺ (g.s.) transition a two-nucleon transfer code employing microscopic wave functions was also used. It was found that absolute cross sections for this kinematically well-matched transition were underrated by a factor of about 7 for a reasonable amount of configuration mixing in the nuclear states involved in this transition. This factor is very close to the value 5 derived for the similarly well-matched⁴⁸Ca(¹⁸O,¹⁶O)⁵⁰Ca reaction.     

Information on neutral weak currents from circular polarization of bremsstrahlung

Bremsstrahlung of unpolarized electrons is analysed for information on neutral weak interactions. The coupling of axial lepton and polar hadron neutral currents should cause a circular polarization of about 10^?6 at electron and photon energies of a few hundred MeV and forward angles. The specific nuclear structure is of modest importance.     

The axial isoscalar neutral current from inelastic electron-nuclear scattering

Parity violating effects due to neutral currents in isoscalar 0⁺→1⁺ nuclear transitions induced by electron scattering are enhanced owing to the small isoscalar magnetic dipole strength. A polarization asymmetry of the order 10⁻⁴ is expected at q 100 MeV for the ¹²C transition to the 1⁺(T = 0) state at 12.71 MeV. It would allow to single out the (electron vector current) × (hardonic axial isoscalar current) quantum numbers and couplings of the neutral current interaction.     

The isospin forbidden 1− → 0+ transition in 40Ca

The isospin forbidden transition 1^? (6.95 MeV) → 0⁺ (g.s.) in ⁴⁰Ca is explained within a model that mixes isospin through single-particle energy differences and the two-body Coulomb interaction. There is no need to introduce an isospin non-conserving part of the nucleon-nucleon interaction to explain this anomalously fast transition.     

Auger electron emission spectra from foil and gas excited carbon beams

Auger electron emission spectra from 2 MeV C⁺ ion excited by collisions with thin carbon foils and Ne gas are presented. The similarity of qualitative features for the C⁺ → C (foil) and C⁺ → Ne spectra indicates the similarity of ionization mechanisms for beam foil and beam gas excitation. The spectra were normalized to the lowest lying Li-like quartet state (1s 2s 2p)⁴P⁰ in carbon by comparison with time delayed foil excited electron decay-in-flight spectra. Comparison to Hartree-Fock calculated transition energies indicates that transitions in three and four electron carbon ions dominate the prompt spectra.     

An improved impulse approximation treatment of the “allowed” weak and analogue electromagnetic transitions in 12C

Weak and electromagnetic transitions between the ¹²C ground state and 1⁺ members of the T = 1 isomultiplet are studied in the framework of the impulse approximation. Uncertainties due to the nuclear configuration mixing and radial effects are examined, the former in the framework of the Cohen-Kurath and Migdal models, and the latter in the density-dependent Hartree-Fock approach. Within the uncertainties of the models, the weak and electromagnetic observables at low momentum transfer (≦ 0.5 fm^?1) are reasonably well reproduced. The inelastic electron scattering from factor at higher momentum transfer shows strong deviations from the impulse approximation estimates.     

Tests of the space-time properties of neutral currents in electron-positron annihilation

Electron-positron processes are very valuable for investigating neutral currents. Interference between neutral currents and the electromagnetic current provides unambiguous tests of the space-time properties of neutral currents. We discuss the structure of this interference with particular reference to the process e⁺e^? → hadron + anything.     

Elastic and inelastic scattering of protons from Li between 25 and 45 MeV

The elastic and inelastic scattering of protons from ⁶Li has been studied at incident energies of 25.9, 29.9, 35.0, 40.1 and 45.4 MeV. The 2.18 MeV (3⁺, T = 0) first excited state of ⁶Li was found to be strongly excited, but the 3.56 MeV (0⁺, T = 1) second excited state was quite weakly excited. Angular distributions for excitation of the 2.18 MeV level were measured at all five energies, while angular distributions for excitation of the 3.56 MeV level were extracted only at 25.9 and 45.4 MeV. To test the applicability of the optical model for the scattering of protons from such a light nucleus the elastic scattering angular distributions have been analyzed using the eleven-parameter search code SEEK. Available polarization angular distributions were included in the analysis. Reasonable fits to the data have been obtained with an average geometry potential. Theoretical estimates of the real part of the optical potential and the inelastic scattering differential cross sections have been made using the microscopic model for proton-nucleus scattering. Both phenomenological and realistic forces have been considered and the necessary nuclear transition densities have been extracted from experimental elastic and inelastic electron scattering data. An estimate of a possible spin-spin term in the optical potential has also been made.     

A weak coupling calculation for the A = 14 nuclei

A calculation of the A = 14 levels is performed by using the weak coupling approach. Very good agreement with experiment is obtained as all known levels below 13 MeV in ¹⁴N are satisfactorily explained. A few additional states are predicted to be found. The electromagnetic transition strengths are calculated and reproduce the experimental values in almost all cases. In particular, a simple calculation of the isospin mixture of the negative-parity states provides an explanation of the known T-forbidden E1 transitions. Also the reduced proton widths are considered and are found to be consistent with (p, p) scattering experiments on ¹³C.